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2000 Southern Division Meeting Abstracts

March 5-7, 2000 - Gainesville, Florida

Posted online March 23, 2000

Mapping a symptom determinant of Cowpea chlorotic mottle virus. F. M. ASSIS FILHO, O. R. Paguio, J. L. Sherwood, and C. M. Deom. Department of Plant Pathology, The University of Georgia, Athens, GA 30602. Publication no. P-2000-0001-SOA.

The type strain of Cowpea chlorotic mottle virus (CCMV-T) produces an intense and extensive chlorosis in cowpea (Vigna unguiculata cv. California Blackeye - CB), while the attenuated variant (CCMV-M) induces mild symptoms. The symptom difference was shown to reside in RNA 3 (Wyatt and Kuhn, 1979. Phytopathology 69:125-129). Infectious RNA 1 and RNA 2 transcribed in vitro from RNA 1 and RNA 2 cDNA clones of CCMV-T along with RNA 3 transcribed in vitro from RNA 3 cDNA clones of the T and M strains inoculated onto cowpea induced symptoms that were indistinguishable from those of the parent strains. Comparison of the nucleotide sequence of both strains revealed four base changes. Chimeric CCMV RNA 3 cDNA clones were constructed by exchanging restriction fragments. Infectivity studies using in vitro transcripts showed that the genetic determinant of symptom expression is located in the 3’ portion of the coat protein gene. Specifically, amino acid residue Ala 151 in CCMV-T is changed to Val 151 in CCMV-M.

Post infection control of citrus scab and melanose using systemic fungicides.
P. M. BUSHONG and L. W. Timmer. Citrus Research and Education Center, University of Florida, Lake Alfred, FL 33850. Publication no. P-2000-0002-SOA.

Efficacy of the systemic fungicides azoxystrobin, benomyl and fenbuconazole for post infection control of citrus scab caused by Elsinoe fawcettii on rough lemon and melanose caused by Diaporthe citri on grapefruit was assessed under greenhouse conditions. Potted plants were cut back to stimulate new shoots of uniform size and new young shoots were inoculated with conidial suspensions. Fungicides were applied at the recommended rates as single sprays prior to inoculation or 16 hr, 24 hr, 48 hr or 72 hr afterward. The total number of scab lesions on selected leaves on the two largest shoots of rough lemon plants was counted and melanose severity was rated on all leaves of grapefruit plants. Data from post infection treatments were transformed into percentage of control lesions and analyzed versus time by linear regression. Azoxystrobin, benomyl and fenbuconazole to a lesser extent provided control of citrus scab when compared to untreated controls. The level of control usually decreased with time but applications up to 72 hr after inoculation reduced disease significantly. Azoxystrobin provided some postinfection melanose control but benomyl and fenbuconazole were not effective as post infection treatments.

Primary and secondary disease gradients of Monilinia vaccinii-corymbosi from point sources of inoculum.
K. D. COX and H. Scherm. Department of Plant Pathology, University of Georgia, Athens, GA 30602. Publication no. P-2000-0003-SOA.

Little is known about the mechanisms and distances of dispersal of ascospores and conidia of Monilinia vaccinii-corymbosi, the causal agent of mummy berry disease of blueberry. In 1999, the spread of primary (shoot blight) and secondary (mummified fruit) infection was recorded at two locations with no history of the disease. Point sources of ascospores (apothecia on pseudosclerotia) and conidia (blighted shoots on potted plants) were placed in separate rows of blueberries. Primary disease gradients were steeper upwind than downwind, with 95% of the blighted shoots occurring <8 m upwind and <30 m downwind of the ascospore source. Compared with the primary gradients, secondary gradients were steeper downwind and flatter upwind of the conidial source, suggesting a reduced importance of wind in conidial dispersal. The Pareto and exponential disease gradient models, differing in the tail of the distribution, were fitted to extrapolate to dispersal distances beyond the row lengths. This will allow for risk assessment of infection from outside inoculum sources, e.g., neighboring fields or wild blueberries.

Activity of acibenzolar-S-methyl and imidacloprid on TSWV in tobacco.
A. S. CSINOS (1), H. R. Pappu (1), R. M. McPherson (2), and M. G. Stephenson (3). (1) University of Georgia, Plant Pathology Dept., Tifton, GA 31794; (2) University of Georgia, Entomology Dept., Tifton, GA 31794; (3) University of Georgia, Crops and Soils Dept., Tifton, GA 31794. Publication no. P-2000-0004-SOA.

Actigard (acibenzolar-S-methyl), a plant activator, and imidacloprid, an insecticide, were evaluated for activity against Tomato Spotted Wilt Virus (TSWV) in Georgia. Plots were established at four locations using K326 tobacco. The treatments were: nontreated control, Actigard, imidacloprid, and Actigard plus imidacloprid. Applications of Actigard at 2gai/7000 plants and imidacloprid at 9.9gai/1000 plants were made on float trays 5-7 days prior to transplanting. In the field, Actigard at 25gai/A and Imidacloprid at 22.7gai/A were applied weekly either three or four times. Counts for incidence of TSWV were made every 2 wks. Ten root and leaf samples were taken every 2 wks from plots and evaluated for TSWV using ELISA. Incidence of TSWV was 8, 19, 26 and 45% in nontreated plots in the four locations. In three of the four tests, all treatments reduced the incidence of TSWV. The percent of leaf and root tissue positive for TSWV by use of ELISA mirrored percent incidence in all tests. In one test, imidacloprid treated plants were significantly larger than the other treatments at 7 wk post-transplanting. In one test, a light epidemic caused by Peronospora tabacina occurred and only plots treated with Actigard reduced incidence of blue mold.

Response of C-99R and Georgia Green peanut cultivars to chlorothalonil applications for leaf spot control.
A. K. CULBREATH (1), D. W. Gorbet (2), and T. B. Brenneman (1). (1) Coastal Plain Expt. Stn., Tifton, GA 31793; (2) North Florida Res. and Ed. Center, Marianna, FL 32446. Publication no. P-2000-0005-SOA.

Split-plot studies were conducted on Georgia Green and C-99R peanut (Arachis hypogaea) cultivars in tests at Tifton, GA and Marianna, FL in 1999 for control of early (Cercospora arachidicola) and late leaf spot (Cercosporidium personatum). Cultivars were combined with three treatments: 1) no fungicide; 2) chlorothalonil, 1.26 kg/ha at 21 day intervals; and 3) chlorothalonil, 1.26 kg/ha at 14 day intervals. At Tifton, leaf spot was exclusively early leaf spot, but late leaf spot was predominant at Marianna. Final Fla. 1-10 scale leaf spot ratings were 8.4, 7.2, and 3.8 (LSD = 1.2) for treatments 1, 2 and 3 respectively on Ga. Green and 8.1, 6.5, and 3.2 (LSD = 1.1) on C-99R at Tifton, and 8.2, 7.9, and 5.6 (LSD = 0.7) on Ga. Green, and 8.5, 7.4, and 4.4 (LSD 0.6) on C-99R at Marianna. Yields for those respective treatments were 4127, 4575, 4810 (LSD = 542) kg/ha for Ga. Green and 4035, 4467 and 4829 (LSD = 619) kg/ha for C-99R at Tifton, and 2563, 3288, and 3857 (LSD = 922) kg/ha for Ga. Green and 3182, 3865, and 4524 (LSD = 428) kg/ha for C-99R at Marianna.

Protein phosphatase 2B (PP2B) affects dimorphism and pathogenicity in the corn smut pathogen Ustilago maydis.
J. W. DUICK and S. E. Gold. Dept. of Plant Pathology, Univ. of Georgia, Athens, GA 30602. Publication no. P-2000-0006-SOA.

Ustilago maydis
is a dimorphic basidiomycete and is the causal agent of corn smut disease. Dimorphism is controlled through signal transduction pathways involving cAMP and the mitogen-activated protein (MAP) kinase cascade. Protein phosphatases reverse the effects of protein kinase phosphorylation. Post-translational modification and specifically protein phosphorylation is a ubiquitous form of regulation in signal transduction pathways. Protein phosphatase 2B (PP2B, also known as calcineurin) is a serine/threonine protein phosphatase and, in other systems, has a role in reversing the phosphorylation of the substrates of cAMP-dependent protein kinase. Progress has been made in understanding the role of PP2B through the cloning and disruption of the PP2B gene, ucn1. Mutants in ucn1 are viable and have a multiple budding phenotype. In compatible mating reactions, pathogenicity of these mutants was severely reduced and galls never developed in innoculated maize plants. These results suggest that PP2B plays an important role in morphogenesis and pathogenicity in U. maydis.

Latent infection by Monilinia fructicola in Georgia peaches.
K. M. Emery and H. SCHERM. Plant Pathology, University of Georgia, Athens, GA 30602. Publication no. P-2000-0007-SOA.

Peach fruit are most susceptible to Monilinia fructicola, the causal agent of brown rot, during the final maturation stage. The sources of inoculum for preharvest infection have remained elusive. In particular, the role of latent infection of immature fruit as a means of carryover of M. fructicola from the spring (blossom blight phase) to the preharvest period is unknown. From 1997 to 1999, immature, non-wounded peach fruit were collected at 14-day intervals from orchards in middle and northern Georgia and washed in sterile water. An aliquot of the wash water was plated to determine incidence of surface contamination by M. fructicola. Fruit were then surface-disinfested and treated with paraquat (1997) or frozen overnight (1998 and 1999) to induce senescence and activate latent infections. There was a strong correlation between brown rot incidence at harvest and the incidence of latent infection 7 to 8 weeks (r=0.9763, P=0.0237) or 10 to 14 days (r=0.9966, P=0.0034) before harvest. Pending further validation, detection of latent infection during the earlier period may be a useful biological predictor of brown rot risk, thereby increasing lead time for disease management decisions.

Interactions of the tobacco downy mildew pathogen, Peronospora tabacina, with host roots.
E. P. HEIST, W. C. Nesmith, and C. L. Schardl. Department of Plant Pathology, University of Kentucky, Lexington, KY 40546. Publication no. P-2000-0008-SOA.

We used gnotobiotic systems to study interactions of 14 Nicotiana species with Peronospora tabacina, the oomycete that causes blue mold (downy mildew) of tobacco. Our focus was on interactions between the pathogen and host roots. In all host species studied the pathogen was capable of moving systemically from foliar infections to roots where its hyphae then emerged from root tips and epidermis. The pathogen was also found to produce sporangiophores, with asexual spores, from infected roots exposed to air. Structures resembling sexual propagules of the pathogen (oospores) were produced on hyphae emerging from roots of N. repanda, a wild tobacco species implicated in the overwintering of the pathogen. Root transmission of the blue mold pathogen was also shown to occur in both wild and commercial Nicotiana species, indicating an alternate mode of transmission, which to date has not been implicated in the epidemiology of this disease. The gnotobiotic system for studying P. tabacina/Nicotiana associations appears relevant to the natural interactions and may serve as a model for similar investigations of other downy mildews.

Efficacy of binucleate Rhizoctonia spp. for control of pre-emergence damping-off of bedding plants caused by Rhizoctonia solani.
E. W. HONEYCUTT and D. M. Benson. Dept. of Plant Pathology, North Carolina State University, Raleigh, NC 27695. Publication no. P-2000-0009-SOA.

Pesta formulations (0.5%) of binucleate Rhizoctonia spp. (BNR621 and P9023) were compared with commercial formulations of Gliocladium catenulatum (0.14%), Trichoderma harzianum T-22 (3g/g seed, RootShield), T. virens GL21 (0.7%, SoilGard) and the fungicide thiophanate-methyl for biocontrol of pre-emergence damping-off of impatiens caused by Rhizoctonia solani. Amended potting mix was incubated in plug trays or black plastic bags for 1 or 3 days prior to seeding. Seeded trays were covered with soilless mix infested with R. solani. Pre-emergence damping-off of impatiens with BNR was only 3%-5% compared to 70% in the infested control (IC). In later experiments (1 vs. 3 day colonization period and bag vs. tray), disease levels with BNR and GL21 were 40%-48% and 41%-48% respectively compared to the IC with 58%-75% disease. However, isolate T-22 and G. catenulatum resulted in greater disease than the infested control. Colony forming units (cfu’s/g) of BNR in soilless mix at seeding was 2.2-15.2 cfu’s for BNR 621 and 16.4-34.2 cfu’s for P9023. Survival of BNR’s in formulation decreased sharply after 2 months with a gradual decline thereafter.

Seed transmission of Acidovorax avenae subsp. citrulli in cucurbits.
D. L. HOPKINS and C. M. Thompson. Mid-Florida Research and Education Center, University of Florida, Institute of Food and Agricultural Sciences, Leesburg 34748. Publication no. P-2000-0010-SOA.

The best control of bacterial fruit blotch of watermelon is to prevent the introduction of the bacterium into the transplant house or field, where it may spread rapidly. Watermelon seedlots are carefully screened for the presence of Acidovorax avenae subsp. citrulli, but other cucurbit seed have not been as carefully monitored for presence of the bacterium. In 1997 and 1999, seed transmission of the bacterium was evaluated for watermelon, cantaloupe, cucumber, honeydew melons, summer squash, winter squash, and pumpkin. Two to 3 weeks prior to harvest, fruit were inoculated by misting with a bacterial solution until runoff occurred. Fruit symptoms were observed on watermelon, honeydew melons, and cantaloupe. Harvested seed were washed and dryed, prior to a greenhouse grow-out assay to evaluate seed transmission. Seed transmission of A. avenae subsp. citrulli was observed in all cucurbits in at least one of the two seasons. Seed transmission was highest in watermelon, followed by honeydew melons, cantaloupe, and winter squash. Even though there were no fruit symptoms, seed transmission in butternut squash was as high as 11%. All cucurbits should be considered as potential sources for the introduction of A. avenae subsp. citrulli into the transplant house or field.

Control of aphid-transmitted viral diseases (ATVD) in yellow summer squash (YSS) in Florida with resistant cultivars.
T. A. KUCHAREK (1), D. E. Purcifull (1), J. H. Fletcher (2), W. E. Crawford (1), R. Hoover (1), and C. R. Semer (1). (1) Plant Pathology Dept., University of Florida, Gainesville, FL 32611; (2) 900 College Ave., Madison, FL 32340. Publication no. P-2000-0011-SOA.

In the springs of 1996, 1997, and 1999, field tests were established in northern Florida in Madison County to evaluate resistance to ATVD in select cultivars of YSS. A susceptible cultivar, ‘Dixie’, was included in all three tests as were ‘Prelude II’, ‘Destiny III’, and ‘Liberator III’, three transgenic cultivars (tcs) with resistances to watermelon mosaic virus II (WMV2) and zucchini yellow mosaic virus (ZYMV). ‘Meigs’, which possesses the precocious yellow gene (pyg), was included in 1997 and 1999 and ‘General Patton’ (pyg) was included in 1997. Based upon SDS- immunodiffusion with leaf extracts, WMV 2 predominated all three years with ZYMV and papaya ringspot virus type W (PRSV-W) occurring in 1996 and 1999, respectively. In 1996, two fruit of Dixie had viral symptoms. At the final harvest, the incidences of fruit with symptoms in 1997 and 1999, respectively, were 90 & 81% for Dixie, 0 & 14% for the tcs, and 31 and 7% for the pyg. Symptoms were produced in fruit of the tcs only when PRSV-W was present.

Evaluation of methods for long-term storage of Rhizoctonia solani AG 2-2 LP.
JIANFENG LI, S. N. Jeffers, and S. B. Martin. Department of Plant Pathology and Physiology, Clemson University, Clemson, SC 29634. Publication no. P-2000-0012-SOA.

Seventeen (17) isolates of Rhizoctonia solani AG 2-2 LP isolated from warm season turfgrasses were used to evaluate methods for long-term culture storage. Wheat seeds and plates of potato dextrose agar (PDA) were colonized by mycelia for about 7 days at room temperature (RT; 22-24°C). Colonized agar plugs (5 mm) and seeds were stored at three different temperatures and conditions: -80°C in 15% glycerol as a cryoprotectant, fresh frozen at -20°C, and at RT in sterile mineral oil. Experimental units were sampled randomly after 1, 2, 3, 6, and 9 months of storage and plated on PDA to determine viability. After 9 months of storage, recovery of the isolates was: seeds at -80°C in glycerol--100%, agar plugs at RT in mineral oil--92%, agar plugs at -80°C in glycerol--53%, seeds at -20°C--39%, agar plugs at -20°C--22%, seeds at RT in mineral oil--2%. To date, it appears that the best methods for long-term storage of isolates of R. solani AG 2-2 LP are colonized wheat seeds stored at -80°C in 15% glycerol and colonized PDA plugs in mineral oil at room temperature. Viability of these isolates will be tested again at 12, 24, and 36 months, which will provide additional information on these storage methods.

A viral suppressor of post transcriptional gene silencing functions downstream of the mobile silencing signal.
A. C. MALLORY and V. B. Vance. Department of Biological Sciences, University of South Carolina, Columbia, SC 29208. Publication no. P-2000-0013-SOA.

Gene silencing in plants is a genetic control mechanism involved in virus resistance, developmental control and genome maintenance. A sequence at the 5’ proximal region of the potyviral RNA genome, initially identified as a mediator of synergistic viral disease, has recently been shown by our lab to suppress the establishment of both transgene-induced and virus-induced post transcriptional gene silencing. This sequence, termed P1/HC-Pro, encodes two complete proteins, P1 and helper component proteinase HC-Pro and also a small part of the protein P3. Previous experiments suggest a systemic signal molecule, thought to be a nucleic acid due to the sequence specificity of the silencing, mediates post transcriptional gene silencing. Although P1/HC-Pro clearly suppresses gene silencing, it is not known whether it acts upstream or downstream of the systemic silencing signal. However, preliminary data suggests P1/HC-Pro functions downstream of this mobile silencing signal.

A plant suppressor of post-transcriptional gene silencing does not suppress transcriptional gene silencing.
R. M. Marathe (1), T. H. SMITH (1), R. Anandalakshmi (1), H. Vaucheret (2) and V. B. Vance (1). (1) Department of Biological Sciences, University of South Carolina, Columbia, South Carolina 29208; (2) Laboratoire de Biologie Cellulaire, INRA, 78026 Versailles Cedex, France. Publication no. P-2000-0014-SOA.

Homology-dependent gene silencing is a regulatory mechanism that limits RNA accumulation from affected loci either by suppression of transcription (transcriptional gene silencing, TGS) or by activation of a sequence-specific RNA degradation process (post-transcriptional gene silencing, PTGS). The P1/HC-Pro sequence of plant potyviruses has been shown to interfere with PTGS. The ability of this viral suppressor of PTGS to interfere with TGS was tested using the 271 locus, which imposes TGS on transgenes under 35S or 19S promoters and PTGS on the endogenous nitrite reductase gene (Nii). P1/HC-Pro reversed PTGS of Nii genes in 271-containing tobacco plants, but failed to reverse TGS of 35S-GUS transgenes in the same plant. P1/HC-Pro expression from a transgene also failed to suppress either the initiation or maintenance of TGS imposed by the NOSpro-silencing locus, H(2). These results indicate that PTGS and TGS operate through unlinked pathways or that P1/HC-Pro interferes at a step in PTGS that is downstream of any common components in the two pathways.

Some factors affecting recovery of Phytophthora spp. from recycled irrigation water.
J. L. MCCRACKEN and S. N. Jeffers. Department of Plant Pathology & Physiology, Clemson University, Clemson, SC 29634-0377. Publication no. P-2000-0015-SOA.

spp. are important pathogens on numerous ornamental crops and are known to occur in irrigation waters at nurseries. We are investigating factors that affect detection and quantification of these fungi by filtration in water. To date, the effects of three factors on pathogen recovery have been studied: the volume of water passed through a filter and the duration and temperature for storing water samples before filtration. Water samples were collected from a naturally-infested irrigation pond at a local nursery. Samples were thoroughly mixed, and replicate subsamples of 25, 50, and 100 ml were passed through 5-µm membrane filters. Samples were stored for 24 and 48 h at 15 or 20°C before filtering to determine if propagule concentration remained constant over time. Estimates of propagule concentrations (cfu/ml) were significantly different in the three volumes. A 25-ml subsample constantly estimated a higher concentration of propagules than a 100-ml subsample. Using a standard volume to recover Phytophthora spp. in irrigation water will produce more consistent, meaningful results in future experiments. Storing samples before filtering did not affect propagule concentration significantly.

Reduction of soilborne pathogens of impatiens in west central Florida by soil solarization during autumn.
R. J. MCGOVERN (1), M. L. Bell (1), and R. McSorley (2). (1) University of Florida, GCREC, Bradenton, FL 34203; (2) University of Florida, Dept. Entomology and Nematology, Gainesville, FL 32611. Publication no. P-2000-0016-SOA.

An experiment was conducted during autumn, 1998 in west central Florida, to evaluate the effectiveness of soil solarization alone and in combination with the biocontrols Streptomyces lydicus or Pseudomonas aureofasciens, or the fungicide fludioxonil in reducing Rhizoctonia crown and stem rot, Pythium root rot, and densities of Meloidogyne incognita in impatiens (Impatiens wallerana). Naturally-infested, 9.9-m(^2) plots were solarized from 10 Sept to 27 Oct using a double layer of clear mulch. Plants were monitored for crown rot for 2 months. Solarization decreased the progress of Rhizoctonia crown and stem rot and final disease incidence, incidence of Pythium sp. in roots, and root discoloration, and increased plant biomass. The technique also reduced population densities of Meloidogyne incognita and other plant-parasitic nematodes. Rhizoctonia crown and stem rot was also reduced by fludioxonil, but unaffected by the biocontrols.

Distance Diagnostics through Digital Imaging: A key component to implementing landscape integrated pest management programs along the Georgia Coast.
J. A. MICHEL (1) and J. Williams-Woodward (2). (1) The University of Georgia Cooperative Extension Service, Glynn County; (2) Dept. of Plant Pathology, The University of Georgia. Publication no. P-2000-0017-SOA.

Accurate pest identification is essential in implementing integrated pest management programs in the landscape industry. The implementation of the Distance Diagnostics through Digital Imaging system in the county office offers homeowners and green industry professionals an option to quickly diagnosis many plant pests. Distance diagnostics increases landscape company scouting results by providing pest identification and selection for control strategies that are pest specific in a timely manner. With an accurate diagnosis, cultural practices, resistant varieties, chemical recommendations and biological control options can be discussed with landscape operators as well as procedures to reduce the risk of future disease occurrence. Distance imaging was applied to determine a difficult to control, root and crown disease of St. Augustine grass. Distance imaging equipment was essential to distinguish take-all root rot caused by Gaeumannomyces graminis var. graminis from the more common brown patch. These two diseases were not easily differentiated by pest control operators and lawn maintenance personnel. With accurate diagnosis of take-all, disease management programs specific for this disease were implemented in homeowner and commercial sites in a timely manner with high levels of reported success.

Plant essential oils as potential bio-fumigants for the management of soilborne pathogens of tomato.
M. T. MOMOL (1,3), D. J. Mitchell (1,4), P. A. Rayside (1,4), S. M. Olson (2,3) and E. A. Momol (3). Departments of (1) Plant Pathology and (2) Horticultural Sciences, Univ. of Florida, (3) NFREC, Quincy, FL and (4) Gainesville, FL. Publication no. P-2000-0018-SOA.

The efficacy of several essential oils was evaluated for management of the following soilborne pathogens of tomato: Ralstonia solanacearum, Fusarium oxysporum f. sp. lycopersici, Phytophthora capsici, Pythium aphanidermatum, and Athelia rolfsii. In a field experiment, the incidence of bacterial wilt in naturally infested soil was reduced by pre-plant soil application of thymol or by tea tree and palmarosa essential oils. In glasshouse tests using microwaved soil infested with fungal pathogens, infection of tomato roots and subsequent root rot caused by the four fungi was reduced or eliminated in soil treated before planting with palmarosa oil and, for P. capsici and A. rolfsii, by oils of wild marjoram and thyme. Root weights generally were greater in soil infested with the four fungi and treated with palmarosa oil than in the infested, nontreated soil. In conclusion, essential oils have potential as bio-fumigants for integrated management programs against soilborne pathogens of tomato.

Root disease and insects associated with longleaf pine mortality may be induced by current edaphic and silvicultural conditions.
W. J. OTROSINA (1), C. H. Walkinshaw (2), S. S. Sung (1), and B. T. Sullivan (3). (1) USDA Forest Service, 320 Green Street, Athens, GA 30602; (2) USDA Forest Service (Ret.), #2 Rilwood Circle, Anniston, AL 36207; (3) University of Georgia, Department of Entomology, Athens, Georgia 30602. Publication no. P-2000-0019-SOA.

Longleaf pine, Pinus palustris Mill., once occupied approximately 30 million ha in the southern United States but currently its range is only about 2.5 million ha. Preliminary studies indicated certain root infecting fungi such as Heterobasidion annosum and some Ophiostomoid species are associated with mortality following relatively cool temperature prescribed fires. We initiated a field experiment to determine effects of three different burn intensities (hot, medium, and cool plus unburned control) on mortality, root infecting fungi, and bark beetle populations. Significantly higher mortality was observed in the hot burn intensity plots, along with increased insect activity by Hylastes sp. Dendroctonus terebrans also exhibited higher populations in the hot burn plots. Fungi such as H. annosum and Leptographium species were isolated from symptomatic trees. Higher fine root damage was also observed in the hot burn intensity plots. These results and other observations suggest a complex of interacting factors may be involved in this mortality. Among these factors are inadequate fire regimes and past agricultural practices that have resulted in extensive soil erosion. These circumstances characterize an exotic ecosystem that emerged from such rapid edaphic and silvicultural changes.

Comparison of parachisels and standard chisels for application of 1,3-dichloropropene for control of root-knot nematodes.
C. RIEGEL (1), D. W. Dickson (1), L. N. Shaw (1), L. G. Peterson (2), and J. L. Nance (2). (1) University of Florida, Gainesville, FL 32611; (2) Dow AgroSciences, Indianapolis, IN 46268. Publication no. P-2000-0020-SOA.

In row and broadcast fumigation of 1,3-dichloropropene (1,3-D) applied 30 cm deep with standard chisels and parachisels, with and without sealing by disking, were compared for management of Meloidogyne spp. on spring squash grown in a deep sand soils. The rates applied were 0 and 84 liters/ha (78 ml/chisel/30 m of row). Cucurbita pepo was planted 7 days after fumigation. Number of fruit and yield of squash were determined and the number of plant-parasitic nematodes in soil and root-knot nematode gall indicies were determined at 34 (Pm) and 65 (Pf) days after planting. Plots treated with 1,3-D applied broadcast followed by disking produced the highest number of fruit (P < 0.1) and 1,3- D applied in row with standard chisels followed by disking produced the highest yield (P < 0.1). At Pm and Pf, the number of second-stage juveniles was lower in all fumigated plots except in plots treated with 1,3-D in row with parachisels compared with the untreated control (P < 0.1). Fumigation with 1,3-D regardless of method, resulted in a decrease in root galling (P < 0.1).

Efficacy of four rates of 1,3-dichloropropene for control of root-knot nematodes in deep sand soil.
C. Riegel (1), D. W. Dickson (1), L. G. Peterson (2), and J. L. Nance (3). (1) University of Florida, Gainesville, FL 32611; (2) Dow AgroSciences, Tallahassee, FL; (3) DowAgroSciences, Winter Haven, FL 33884. Publication no. P-2000-0021-SOA.

Two trials were conducted to determine the most effective rate of 1,3-dichloropropene (1,3-D) for the control of Meloidogyne spp. on spring squash. Rates tested included 0, 56, 84, 112, and 168 liters/ha of 1,3-D applied broadcast 30 cm deep with standard chisels. The chisel openings were sealed by disking immediately following fumigation. Cucurbita pepo cv. Sunex 9602 was sown 7 days after fumigation. Number of fruit and yield of squash were determined and the number of plant- parasitic nematodes in soil and root galling were determined at 34 (Pm) and 65 (Pf) days after planting. There was a numerical increase in the number of fruit and yields in all plots compared with the untreated control. In trial one, the number of second- stage juveniles (J2) were less than the untreated control in all fumigated plots at Pm (P < 0.05) and Pf (P <= 0.0001). In trial two, there was a decrease in the number of J2 at Pf in all fumigated plots compared to the untreated control (P < 0.01). Root galling was decreased in all fumigated plots in both trials (P <= 0.0001), however, fumigation with broadcast rates of 84, 112, and 168 liters/ha provided the best control of root-knot nematodes for squash grown in sandy soil during spring months when the soil temperature remained below 20°C.

Use of a Rotorod spore sampler to examine potential airborne dispersal of Fusarium avenaceum causing crown and stem rot of lisianthus.
T. E. SEIJO (1), R. J. McGovern (1), and R. H. Morrison (2). (1) University of Florida-IFAS, GCREC, Bradenton, FL 34203; (2) Sakata Seed America, Salinas, CA 93907. Publication no. P-2000-0022-SOA.

Fusarium avenaceum
causes a crown and stem rot of lisianthus (Eustoma grandiflorum), which resulted in losses of up to 70% for cut-flower growers in 1997. In the advanced stages of this disease, sporodochia (masses of macroconidia) are produced on the lower stem of plants. Sporodochia have been implicated in the aerial spread of F. oxysporum f. sp. lycopersici between tomato plants. To determine if F. avenaceum can be spread aerially, a Rotorod spore sampler was set up to collect airborne spores at a California lisianthus transplant production facility during a Fusarium crown and stem rot (FCSR) epidemic. Spores collected by the sampler were streaked onto a Fusarium selective medium. All Fusarium colonies were screened via PCR to identify F. avenaceum. Positive PCR reactions were confirmed morphologically and by pathogenicity to lisianthus. Only 3 cfu of F. avenaceum were recovered by the spore sampler, suggesting that airborne spread of the fungus is not essential for epidemics of FCSR in lisianthus transplants.

Variation among tomato spotted wilt tospovirus isolates from peanut in Georgia and Florida.
K. L. SHELTON (1), A. K. Culbreath (1), S. S. Pappu (2), H. R. Pappu (1), J. W. Todd (2), and C. M. Deom (3). (1) Dept. of Plant Pathology, (2) Dept. of Entomology, University of Georgia, Tifton, GA 31793, (3) Dept. of Plant Pathology, University of Georgia, Athens, GA 30602. Publication no. P-2000-0023-SOA.

Samples of peanut (Arachis hypogaea) suggestive of tomato spotted wilt virus (TSWV) infection were collected in 1997 and 1998 from three areas in Georgia and one area in Florida. Samples showing differences in symptoms (ringspots, severe stunting, oakleaf pattern, and bronzing) were selected and a portion of each sample was tested for TSWV by ELISA. Immunocapture reverse transcriptase polymerase chain reaction (IC-RT-PCR) was used to amplify the nucleocapsid (N) gene from these samples. Selected IC-RT-PCR products were cloned and sequenced. Sequence comparisons revealed that there was a high degree of sequence identity among the isolates tested. Phylogenetic reconstruction studies showed that the peanut isolates were closer to each other than to other known TSWV-N gene sequences. Three amino acid sites found to be conserved among all isolates from GA and FL were different in isolates from geographical areas outside the continental United States. No correlation could be drawn between symptom type and identity of the N-gene.

Identification of factors contributing to the spread and incidence of tomato yellow leaf curl virus in tomatoes.
T. SHERWOOD (1), J. E. Polston (1), and R. D. Berger (2). Univ. of Florida, (1) Gulf Coast Res. & Educ. Ctr., Bradenton, FL 34203, and (2) Dept. Plant Path., Gainesville, FL 32611. Publication no. P-2000-0024-SOA.

Tomato yellow leaf curl virus (TYLCV-Is), a whitefly-transmitted geminivirus native to the eastern Mediterranean, was observed for the first time in Florida in 1997. It has since been found in all tomato production areas of the state. The results of a study to identify factors contributing to the spread and incidence of TYLCV-Is in tomatoes in the Palmetto-Ruskin area are reported. The study was conducted in the spring and fall of 1999. Approximately 300 tomato fields farmed by 20 growers and spanning an area of 162 ha. were evaluated for the presence of TYLCV-Is each season. The presence and final incidence of TYLCV-Is were correlated with data on disease and insect management practices, location to known virus reservoirs, transplant origin, cultivar, planting dates, expected and realized yield, which were collected from growers. The data generated has increased our understanding of the mechanisms and speed with which TYLCV-Is spreads and establishes in new areas. Results will be used to improve TYLCV-Is management strategies.

Epidemiology and management of white rust of spinach.
M. J. SULLIVAN and J. P. Damicone. Dept. of Plant Pathology, Oklahoma State University, Stillwater, OK 74078. Publication no. P-2000-0025-SOA.

White rust, caused by Albugo occidentalis, is the foremost disease problem of spinach in Oklahoma. The influence of temperature (T) and wetness duration (W) on infection of spinach by A occidentalis was determined by exposing plants to post-inoculation temperatures of 6 to 28°C and interrupted W periods that totaled 3 to 84 hours. When accompanied by a minimum of 3 hours of W, 12 to 18°C favored infection. Weather-based advisory programs, based on the periods of T and W that favored infection, were developed, evaluated for timing fungicide applications to control white rust, and compared to a previously published program of 12 continuous hours of W, a 7-d program, and an untreated control in three field trials. The protectant EBDC and the systemic azoxystrobin fungicides were applied after 3, 6, 12, 24, and 36 cumulative hours of favorable T and W (T/W). Disease control was less effective for protectant fungicides for all spray programs. All T/W programs reduced the number of sprays compared to the 7-d. Based on spray reductions and disease control, the 6- and 12-h T/W programs most efficient for the EBDCs and azoxystrobin, respectively.

Seedling diseases and root rots of carrot in Georgia.
D. R. SUMNER (1), D. E. Carling (2), and S. C. Phatak (3). (1) Dept. of Plant Pathology, University of Georgia, Tifton, GA 31794-0748; (2) Palmer Research Center, University of Alaska, Palmer AK 99645-6629; (3) Dept. of Horticulture, University of Georgia, Tifton, GA 31794-0748. Publication no. P-2000-0026-SOA.

From 1996-1999, the number of hectares of carrots in Georgia increased from 120 to 1800. To determine the incidence of seedling diseases and root rots, 1100 seedlings from 35 fields and 400 immature to mature roots from 24 fields of loamy sand or sand soils in different crop rotations were sampled in 9 different counties. Rhizoctonia solani anastomosis group (AG)-4, other unidentified isolates of R. solani, or binucleate Rhizoctonia spp. were isolated from seedlings in 20% and from maturing roots in 79% of the fields. Pythium spp. were isolated from seedlings and mature roots in 34% and 29% of the fields, respectively. R. solani AG-4 and AG-2-2, unidentified R. solani isolates, and P. irregulare were highly virulent on carrot seedlings. R. solani AG-4 and unidentified R. solani isolates caused a reddish-brown to black decay in mature carrot slices. Binucleate Rhizoctonia spp. were weakly virulent to avirulent. In field plots, a metalaxyl drench over the row at planting reduced population densities of Pythium spp. in soil and seedling infection with Pythium spp. (primarily P. irregulare), and significantly increased plant stand. Rhizoctonia solani AG-4, AG2-1, AG2-2, and other R. solani anastomosis groups, and P. irregulare are widely distributed in soils in Georgia, and may decrease yield of marketable carrot roots.

Differences among loblolly pine families in anatomical responses to fusiform rust.
C. H. WALKINSHAW. USDA Forest Service, Southern Research Station, 2500 Shreveport Hwy., Pineville, LA 71360. Publication no. P-2000-0027-SOA.

Millions of loblolly pines, Pinus taeda L., have been planted in the last 60 years in the southern forest. Fusiform rust incidence can be high in these plantations where climate favors infection and spread. However, mortality due to rust varies widely among families. To examine this variation, I measured gall growth and anatomy in 138 loblolly pine families in FL, GA, LA, and MS. Compartmentalization of diseased tissue, was a major defense reaction in loblolly pine, as was repression of infection in branches and stems. Trees with small stem galls often have higher dbh values than healthy loblolly pines. This even occurred in 75 – 100 year-old loblolly pines in Mississippi. It appears that several defensive traits in loblolly pines serve to limit fusiform rust in the stems. These observations expand our knowledge of resistance of sapling and adult loblolly pines.

Effects of cultural practices on Fusarium root and crown rot of hosta.
B. WANG and S. N. Jeffers. Department of Plant Pathology & Physiology, Clemson University, Clemson, SC 29634. Publication no. P-2000-0028-SOA.

Fusarium root and crown rot is a newly reported disease of container-grown hostas, caused by an unusual species of Fusarium that has yet to be identified. Cultural practices to manage this disease were investigated, including type of container mix and watering regimen. Hosta ‘Francee’ plants were inoculated with two isolates of Fusarium sp. by dipping wounded roots and crowns into suspensions of conidia. To evaluate the effect of container mix, plants were transplanted into 100% pine bark, 100% Canadian peat, or a mixture of 50% bark and 50% peat. To evaluate the effect of watering regimen, plants were transplanted into a commercial nursery mix (75% bark, 25% peat) and kept dry, moist, or wet. Plants were grown at 20 to 25°C for 5 weeks, and then disease severity was assessed and plant weights were measured. Fusarium root and crown rot was affected significantly by both container mix and watering regimen. Disease was more severe in bark than in peat or the bark/peat mixture, and disease was most severe under the dry regimen compared to the moist and wet regimens. These results indicate that modifying certain cultural practices can have a significant impact on Fusarium root and crown rot on container-gown hostas.

Zosteric acid efficacy in control of strawberry anthracnose (Colletotrichum fragariae)
D. E. Wedge (1), K. J. Curry (2), and B. J. Smith (3). (1) USDA-ARS, Natural Products Utilization Research Unit, The National Center for Natural Products Development, University of Mississippi, MS 38677; (2) University of Southern Mississippi, Hattiesburg, MS 39406-5018; (3) USDA-ARS Small Fruit Research Unit, 306 S. High Street, Poplarville, MS. Publication no. P-2000-0029-SOA.

Zosteric acid (ZA), a synthetic natural product, is being evaluated for its efficacy as a non-toxic fungal control agent for strawberry anthracnose caused by C. fragariae. In vitro trials showed that ZA reduced the growth of C. fragariae in culture. The anthracnose susceptible strawberry cultivar, Chandler, was used as the host in whole plant and detached leaf and petiole trials. Plant tissues were inoculated by applying a conidial suspension (1.5 × 10(^6) spores/mL) as a spray to the point of runoff. Using detached leaf and petiole assays the optimum effective concentration of the disodium salt of ZA was determined to be ca. 1-2% when the agent was applied in phosphate buffer (pH 7.0) containing 0.1% Tween 20. Greenhouse evaluations confirmed that the optimal concentration of ZA was 1-2% for C. fragariae. No phytotoxicity was observed on detached leaves or whole plants even at the 6% ZA level (the maximum solubility of the ZA salt in an aqueous solution). Microscopic analysis confirmed that ZA prevented fungal spore attachment and germination.

Differentiation of pathotypes of Xylella fastidiosa by SDS-PAGE of proteins.
R. L. WICHMAN and D. L. Hopkins. Mid-Florida Research and Education Center, University of Florida, Institute of Food and Agricultural Sciences, Leesburg, 34748. Publication no. P-2000-0030-SOA.

Xylella fastidiosa
produces leaf scorch and dieback symptoms in a wide range of plant hosts. Although all of these diseases are caused by this single species, there is host specificity among the strains, indicating different pathotypes or subspecies. Pathogenicity tests with some of the hosts requires 18-24 months; therefore, a rapid method to compare new strains of X. fastidiosa with previously-described strains is needed. Strains can be separated into two distinct groups based on nutritional fastidiousness and digestion of a polymerase chain reaction product with RsaI. Whole-cell protein SDS-PAGE was evaluated as a method of differentiating strains of X. fastidiosa obtained from elderberry, grapevine, lupine, oak, oleander, and sycamore. Different protein banding patterns were observed between 21,500 and 45,000 daltons. Based on the presence or absence, or difference in intensity, of one or two protein bands within that molecular weight range, the strains were separated into six groups: grapevine and lupine(1 strain); elderberry; oak; oleander; sycamore; and lupine(1 strain). SDS-PAGE of whole-cell proteins did differentiate among several pathotypes of X. fastidiosa and could be used to compare a new strain with known pathotypes.

Fungal and mycotoxin contamination of stored pearl millet grain.
J. P. WILSON (1), Z. Jurjevic (2), D. M. Wilson (2), and H. Casper (3). (1) USDA-ARS and (2) University of Georgia, Tifton, GA 31793; (3) North Dakota State University, Fargo, ND 58105. Publication no. P-2000-0031-SOA.

Storage conditions affecting fungal colonization and mycotoxin contamination in pearl millet grain were evaluated. Grain from 1996, 1997, and 1998 were stored for 3 to 9 weeks at 20C or 25C, and at 85.6, 91, or 100% relative humidity (rh), under air or nitrogen, and three grain moisture regimes (9-11%, 17-20%, or 20-22%). The most common fungi were Fusarium chlamydosporum (19%). Curvularia (14%), F. semitectum (16%), Alternaria spp. (9%), Aspergillus flavus (8%), "Helminthosporium" spp. (6%), and F. moniliforme (3%). Year of grain production was the primary determinant of fungal contamination. Frequency of A. flavus isolation increased in the 17-20% and 20-22% grain moisture regimes, particularly at 25C. Aflatoxins averaged 174 ppb over all treatments, and increased up to 798 ppb at the high moisture regime at 25C. Frequency of F. chlamydosporum isolation increased at 85.6 and 91% rh. Nivalenol contamination averaged 0.07 ppm and generally increased in grain stored under 100% rh. Low levels of deoxynivalenol (0.00 - 0.13 ppm) and zearalenone (0.00 - 0.33 ppm) were detected.

Control of foliar citrus diseases in Florida with Flint™.
T. R. YOUNG. Novartis Crop Protection, Vero Beach, FL 32967. Publication no. P-2000-0032-SOA.

Important citrus diseases in Florida include greasy spot (Mycosphaerella citri) and melanose (Diaporthe citri). Of lesser importance, due to the limited acreage on which susceptible varieties are planted, are scab (Elsinoe fawcettii) and Alternaria brown spot (Alternaria alternata pv citri). Control of these diseases is necessary for both the economical production of fruit and to ensure the survival of citrus trees. The most widely used chemical control tools have been copper-based fungicides. Copper fungicides are not effective under all conditions, and they have limitations including adverse effects on the environment, phytotoxicity, and lack of eradicant activity. Flint, a strobilurin fungicide, was evaluated in the field using commercial airblast application equipment on bearing citrus of various varieties. It was found that 2 oz./acre of Flint 50WG provided excellent control of the above named diseases. Application methodology, crop tolerance, mixture compatibility and fungicide resistance management were studied.

SOVRAN® fungicide - Update on performance and resistance management guidelines.
H. L. YPEMA and R. E. Gold. BASF Corporation, 26 Davis Drive, PO Box 13528, Research Triangle Park, NC 27709. Publication no. P-2000-0033-SOA.

Sovran® fungicide (a.i. kresoxim-methyl) is a strobilurin fungicide developed by BASF in the NAFTA-region. The first registrations for Sovran fungicide on pome fruits, grapes and pecans in the United States were issued in May 1999. Sovran fungicide received registration on apples in Canada in December 1999. Sovran fungicide will be available for the entire growing season in the year 2000. It will offer growers excellent activity against many important plant pathogenic fungi by virtue of its high intrinsic activity and favorable biokinetic properties. The active ingredient, kresoxim-methyl, adheres strongly to the leaf surface under various weather conditions. Laboratory, greenhouse and field studies have demonstrated that kresoxim-methyl diffuses laterally over the leaf surface and translaminarly from one leaf surface to the other. As a result kresoxim-methyl exhibits both Surface Systemic Activity™ and translaminar activity. Sovran fungicide combines a strong inhibitory action against spore germination, surface mycelia and sporulation with a long residual activity and excellent crop safety. The presentation focuses on the management of pome fruit and pecan diseases in the Southeastern United States, and new use guidelines for the year 2000 to prevent the development of resistance to strobilurin fungicides.

Copper tolerance in strains of Pantoea ananatis, causal agent of center rot of onion, in Georgia.
L. Zolobowska (1), R. GITAITIS (2), H. Sanders (2), D. Langston (2), and A. Purvis (3). (1) Department of Virology and Bacteriology, Institute of Plant Protection, Poznan, Poland, (2) Department of Plant Pathology, Coastal Plain Experiment Station, University of Georgia, Tifton, GA 31793-0748 and (3) Department of Horticulture, Coastal Plain Experiment Station, University of Georgia, Tifton, GA 31793-0748. Publication no. P-2000-0034-SOA.

Center rot of onion, caused by Pantoea ananatis, has been a problem in Georgia since 1997. Current recommendations include the use of fixed-copper bactericides. A survey was conducted to determine if copper tolerance occurred in strains of P. ananatis in Georgia. Bacterial strains were collected from 15 different varieties of sweet onion (Allium cepa) in replicated (three) plots in three different fields (HortHill, Chula, and Tattnall) in southern Georgia. Dilute suspensions of P. ananatis were streaked on to nutrient agar (NA) or NA amended with copper sulfate pentahydrate (200 ug ml(^-1)). A total of 80, 60, and 73% of the strains from HortHill, Chula, and Tattnall, respectively, exhibited either confluent growth or partial growth on the copper-amended medium. There were no significant differences due to variety or location. However, all strains were sensitive to a mixture of copper sulfate pentahydrate (200 ug ml(^-1)) and maneb (40 ug ml(^-1)) in NA.