Interpretive Summaries

Lateral Clipping of Canopy Influences the Microclimate and Development of Apothecia of Sclerotinia sclerotiorum in Carrots. Cezarina Kora and Mary Ruth McDonald, Department of Plant Agriculture, and Greg J. Boland, Department of Environmental Biology, University of Guelph, Guelph, Ontario, Canada N1G 2W1. Plant Dis. DOI: 10.1094/PD-89-0549, 2005 (online). Accepted for publication 8 January 2005.

Sclerotinia rot, caused by the pathogenic fungus Sclerotinia sclerotiorum, is an important disease of carrots. The disease affects carrots in the field and in storage, and currently available management practices often do not provide effective control in commercial crops. Previous epidemiological studies revealed that the structure and growth stage of carrot contribute to development of Sclerotinia rot. The dense and closed canopies of carrots provide favorable conditions for the development of the pathogen and disease, and senescing leaves in contact with soil are most susceptible to infection. Therefore, a study was initiated to evaluate the effects of reducing the canopy width and removing the senescing leaves of carrot on the crop, microclimate, apothecia (specialized structures containing spores produced by this pathogen), and disease development. Carrot tops were laterally clipped to 60% of the original width by removing 20% of overlapping leaves above the furrow and senescing leaves lying on soil on both sides of the carrot bed after first apothecia were observed in the field. Maximum air and soil temperatures were lower and relative humidity was higher under the closed canopies than in clipped canopies. The number of apothecia in clipped plots was up to 76% lower than in unclipped plots. In addition, clipping completely controlled Sclerotinia rot where it developed, without affecting foliar and root weights. Clipping can suppress the production of apothecia by increasing the temperature and reducing moisture in the crop. In addition, clipping can suppress disease by removing susceptible tissue and minimizing plant-to-soil and plant-to-plant contact. This study offers a cultural option for the management of Sclerotinia rot through the reduction of inoculum and potential infection sites. Incorporating canopy clipping into an integrated system may substantially improve the control of the disease in both conventional and organic carrot production.

Epiphytic Survival of Xanthomonas axonopodis pv. allii and X. axonopodis pv. phaseoli on Leguminous Hosts and Onion. David H. Gent, National Forage Seed Production Research Center, USDA-ARS, Corvallis, OR 97331; and Jillian M. Lang and Howard F. Schwartz, Department of Bioagricultural Sciences and Pest Management, Colorado State University, Fort Collins 80523-1177. Plant Dis. DOI: 10.1094/PD-89-0558, 2005 (online). Accepted for publication 17 January 2005.

Xanthomonas leaf blight of onion (caused by Xanthomonas axonopodis pv. allii) and common bacterial blight of dry bean (caused by Xanthomonas axonopodis pv. phaseoli) are perennial problems in the Central High Plains of the United States. Onion and dry bean are commonly grown in rotation in Colorado, but it is unknown if X. axonopodis pv. allii and X. axonopodis pv. phaseoli survive epiphytically or pathogenically on dry bean and onion, respectively. Under high humidity growth chamber conditions, epiphytic X. axonopodis pv. allii populations increased on alfalfa, chickpea, dry bean, lentil, and soybean, but the epiphytic populations were at least 10-fold greater on onion. When artificially inoculated under field conditions, epiphytic populations of X. axonopodis pv. allii were recovered from dry bean, lentil, and onion, but the bacterium did not persist on chickpea or soybean. Epiphytic X. axonopodis pv. phaseoli was recovered from symptomless onion plants in fields cropped to dry bean the prior year, but not from fields cropped to a host other than dry bean. In this study, we have established, for the first time, that X. axonopodis pv. allii and X. axonopodis pv. phaseoli are capable of epiphytic survival on both onion and dry bean. Close rotation of onion and dry bean may allow X. axonopodis pv. allii and X. axonopodis pv. phaseoli to persist epiphytically. The implications of epiphytic survival of these pathogens on crops commonly grown in close rotation remain unclear, but cropping systems that avoid close rotations of onion and dry bean and encourage rapid breakdown of crop residues should reduce potential inoculum sources of both pathogens in the Central High Plains.

Effect of Planting Date, Cultivar, and Stage of Plant Development on Incidence of Fusarium Wilt of Lettuce in Desert Production Fields. Michael E. Matheron, The University of Arizona, Yuma Agricultural Center, 6425 West Eighth Street, Yuma 85364; James D. McCreight, U.S. Department of Agriculture, Agricultural Research Service, U.S. Agricultural Research Station, 1636 East Alisal Street, Salinas, CA; and Barry R. Tickes and Martin Porchas, The University of Arizona, Yuma Agricultural Center, 6425 West Eighth Street, Yuma 85364. Plant Dis. DOI: 10.1094/PD-89-0565, 2005 (online). Accepted for publication 11 January 2005.

Fusarium wilt of lettuce, first recognized in Japan in 1955, has since been discovered in the United States (California in 1990, Arizona in 2001), Iran (1995), Taiwan (1998), and Italy (2001). In Arizona, the causal agent, Fusarium oxysporum f. sp. lactucae, has been recovered from lettuce plants in 27 different lettuce fields during the 2001 to 2003 production seasons. Disease symptoms include wilting and yellowing of leaves as well as a red-brown to black necrosis of internal taproot and crown tissue. Studies were initiated to examine the impact of planting date, cultivar, and stage of plant development on the resultant incidence of disease in the field. In 2002 and again in 2003, tested lettuce cultivars were sown in at least one of the following planting windows: early-season (September), mid-season (October), and late-season (December). Within each planting date, significant differences in disease incidence were noted among lettuce cultivars. The mean incidence of Fusarium wilt on cultivars sown in September, October, and December in 2002 and 2003 was 92.3, 15.1, and 2.0%, respectively, in 2002 and 74.2, 5.1, and 0.7%, respectively, in 2003. The mean soil temperature at the 10-cm depth during the September, October, and December plantings for both years was 26, 14, and 14°C, respectively. Initial symptoms of Fusarium wilt were apparent as early as 14 days after seeding, with increasing incidence of disease noted as the crop developed and reached maturity. Among different types of lettuce, crisphead (iceberg) was most susceptible, whereas romaine was least affected by the disease. Selection of appropriate lettuce cultivars and planting times should allow successful production of lettuce with minimal or no incidence of disease in fields infested with F. oxysporum f. sp. lactucae.

Crop Damage Estimates for Crown Rot of Wheat and Barley in the Pacific Northwest. Richard W. Smiley, Professor, and Jennifer A. Gourlie, Sandra A. Easley, Lisa-Marie Patterson, and Ruth G. Whittaker, Faculty Research Assistants, Oregon State University, Columbia Basin Agricultural Research Center, Pendleton 97801. Plant Dis. DOI: 10.1094/PD-89-0595, 2005 (online). Accepted for publication 3 February 2005.

Crown rot affects yield of wheat and barley worldwide. Several fungal pathogens causing similar symptoms are associated with the disease. Effects on grain yield and quality were poorly defined for the most important pathogens in the Pacific Northwest: Fusarium culmorum, F. pseudograminearum, and Bipolaris sorokiniana. We collected whole-plant samples of mature soft-white winter wheat from commercial fields in Oregon and Washington, separated the tillers into groups according to disease severity symptoms on lower internodes, and evaluated yield components for tillers in each disease severity class. We then determined actual yield and calculated potential yield, assuming all tillers were inherently capable of producing as much grain as healthy tillers. Crown rot reduced winter wheat yield as much as 1,550 kg/ha (23 bushels/acre), equivalent to a 35% reduction valued at $219/ha ($100/acre). The average yield reduction for 13 fields was 9.5% (421 kg/ha, $51/ha or 6 bushels/acre, $23/acre). Crown rot also increased grain protein content and reduced test weight and straw production, each of which can have further negative effects on the value of soft-white wheat and on compliance with soil conservation provisions of federal farm programs. Damage from crown rot in the Pacific Northwest is more widespread and damaging than previously recognized.

Effect of Pruning on Apple Scab in Organic Apple Production. I. J. Holb, Department of Plant Protection, University of Debrecen, Centre of Agricultural Sciences, P.O. Box 36, H-4015 Debrecen, Hungary. Plant Dis. DOI: 10.1094/PD-89-0611, 2005 (online). Accepted for publication 10 January 2005.

In a 3-year Hungarian study, the effectiveness of apple tree pruning as a management tactic for control of apple scab (Venturia inaequalis) was investigated on two susceptible (cvs. Jonagold and Mutsu), two moderately susceptible (cvs. Elstar and Idared), and two V(f) resistant apple cultivars (cvs. Liberty and Prima) in three high-density organic apple orchards treated with copper- and sulfur-based fungicide programs. The area under the disease progress curve (AUDPC) for leaf and fruit incidences was calculated to evaluate three winter pruning treatments (unpruned, weakly pruned, and strongly pruned). Strong pruning significantly decreased leaf scab on the susceptible and the moderately susceptible cultivars at all sites compared with unpruned ones. Moreover, for the cultivar Jonagold in all years and for the cultivar Mutsu in 2001, AUDPC values of strongly pruned trees were significantly lower at P = 0.05 compared with the weakly pruned trees. The effect of pruning was lower on the development of fruit scab compared with that of leaf scab in all years and at all sites. Only strong pruning treatments significantly decreased means of AUDPC for fruit incidence on scab-susceptible cultivars compared with unpruned treatments. The effects of pruning treatments on moderately susceptible cultivars were significant only in 2001 at all sites. Resistant cultivars showed no significant effect of pruning treatments on development of either fruit or leaf scab except for cultivar Liberty in 2001 at one site. Pruning resulted in no consistent difference in the apple tree canopy microclimate. However, strong pruning resulted in improved spray deposition in the tree canopy when applications were made with an airblast sprayer at midsummer.

Management of Xanthomonas Leaf Blight of Onion with a Plant Activator, Biological Control Agents, and Copper Bactericides. David H. Gent, U.S. Department of Agriculture-Agricultural Research Service, National Forage Seed Production Research Center, Oregon State University, Department of Botany and Plant Pathology, Corvallis 97331; and Howard F. Schwartz, Department of Bioagricultural Sciences and Pest Management, Colorado State University, Fort Collins 80523-1177. Plant Dis. DOI: 10.1094/PD-89-0631, 2005 (online). Accepted for publication 14 February 2005.

Xanthomonas leaf blight (Xanthomonas axonopodis pv. allii) is a yield-limiting disease of onion in the western United States. Frequent applications of copper-based bactericides amended with an ethylenebisdithiocarbamate fungicide (e.g., maneb or mancozeb, class B2 carcinogens) provide some disease suppression, but strategies to reduce conventional bactericide use are needed to minimize grower costs, environmental impact, and public exposure to class B2 pesticides. In growth chamber studies, applications of acibenzolar-S-methyl reduced in planta and epiphytic populations of X. axonopodis pv. allii as effectively as applications of copper hydroxide-mancozeb. Under field conditions, four weekly applications of acibenzolar-S-methyl reduced severity of Xanthomonas leaf blight as or more effectively than 9 to 12 weekly applications of copper hydroxide or copper hydroxide-mancozeb. Acibenzolar-S-methyl applications did not increase bulb yield or grade compared with copper bactericide treatments. However, bulb yield was reduced 22 to 27% when 10 weekly applications of acibenzolar-S-methyl were made in the absence of disease. Application of a commercial formulation of both Pantoea agglomerans strain C9-1 and Pseudomonas fluorescens strain A506 reduced severity of Xanthomonas leaf blight in field experiments. Weekly copper hydroxide applications starting 1 to 2 weeks before bulb initiation were as effective as weekly applications started 3 to 4 weeks before bulb initiation, irrespective of the maneb rate used. In this study, effective and economical means of reducing the use of copper bactericides and EBDC fungicides for suppression of Xanthomonas leaf blight were identified. The number of copper bactericide and EBDC fungicide applications may be reduced by applying them during the critical periods for disease control, or by relying upon alternative products such as acibenzolar-S-methyl and biological control agents. Integration of these chemical and biological control treatments into an integrated pest management program that emphasizes the planting of pathogen-free seed, moderate nitrogen fertility, sanitation of weeds, leguminous crops, infested crop debris, and volunteer onion, avoids reuse of irrigation water, and follows a 2-year or longer rotation to nonhost crops such as small grains, should reduce the need for class B2 carcinogens to manage Xanthomonas leaf blight in Colorado and possibly elsewhere.

Influence of pH and NaHCO(3) on Effectiveness of Imazalil to Inhibit Germination of Penicillium digitatum and to Control Postharvest Green Mold on Citrus Fruit. J. L. Smilanick, M. F. Mansour, D. A. Margosan, and F. Mlikota Gabler, United States Department of Agriculture–Agricultural Research Service, San Joaquin Valley Agricultural Sciences Center, Parlier, CA 93648; and W. R. Goodwine, Janssen Pharmaceutica, Inc., Titusville, NJ 08560. Plant Dis. DOI: 10.1094/PD-89-0640, 2005 (online). Accepted for publication 15 February 2005.

Postharvest decay losses of citrus fruit due to green mold shorten the storage life of fresh citrus fruit. Green mold is commonly controlled by imazalil (IMZ), but resistance among isolates of Penicillium digitatum to this fungicide has become widespread and its effectiveness has declined. In this work, we showed a simple and economical method to manage IMZ-resistant isolates and increase the effectiveness of this fungicide. Immersion of citrus fruit in sodium bicarbonate (SBC), a common and inexpensive food additive with high pH, will partially control green mold. When sodium bicarbonate and IMZ were combined into one treatment, we obtained excellent control of both IMZ-sensitive and -resistant isolates. The synergistic activity we observed probably was not due to pH alone. Although IMZ was more toxic to P. digitatum spores in buffered liquid media at high pH, the control of green mold in wound-inoculated citrus fruit was only slightly improved by using IMZ with other alkaline buffers. The synergistic activity we observed with IMZ and SBC probably was not due only to the high pH, because pH in the wounds was only modestly elevated when measured soon after the treatments, and it had decreased markedly 24 h later.

A Member of the Closteroviridae from Mint with Similarities to All Three Genera of the Family. Ioannis E. Tzanetakis, Department of Botany and Plant Pathology & Center for Gene Research and Biotechnology, Oregon State University, Corvallis 97331; Joseph D. Postman, National Clonal Germplasm Repository, USDA-ARS, Corvallis 97333; and Robert R. Martin, Horticultural Crops Research Lab, USDA-ARS, Corvallis 97330. Plant Dis. DOI: 10.1094/PD-89-0654, 2005 (online). Accepted for publication 22 February 2005.

An ornamental mint, Mentha × gracilis ‘Variegata’, was determined to be infected with three viruses. Infection with these viruses resulted in a bright yellow vein-banding symptom that was graft-transmissible and could be eliminated by standard virus elimination procedures. One of the viruses was determined to be a unique member of the Closterovirus family. There are three genera in this family of viruses, and this mint virus has similarities with all three genera, suggesting it could be a progenitor of the virus family. The closterovirus was transmitted by the mint aphid and in this way could be separated from the other two viruses that were not aphid-transmitted. When this virus infects M. canadensis, M. spicata, or M. × piperita, it causes vein-banding symptoms in single infections; thus, the name Mint vein-banding associated virus (MVBaV) has been proposed. In single infections in M. × gracilis, this virus does not cause the vein-banding symptom, suggesting that the symptom in ‘Variegata’ is caused by infection with more than one virus. A method for rapid detection of the virus in mint has been developed and is being used to determine the distribution and significance of this virus in commercial mint production.

Relative Pathogenicity of Cryphonectria cubensis on Eucalyptus Clones Differing in Their Resistance to C. cubensis. S. W. van Heerden, Department of Genetics, Forestry and Agricultural Biotechnology Institute (FABI), University of Pretoria, Pretoria, South Africa; H. V. Amerson, Department of Forestry, North Carolina State University, Raleigh; and O. Preisig, B. D. Wingfield, and M. J. Wingfield, Department of Genetics, Forestry and Agricultural Biotechnology Institute, University of Pretoria. Plant Dis. DOI: 10.1094/PD-89-0659, 2005 (online). Accepted for publication 2 November 2004.

Cryphonectria canker is caused by the ascomycetous fungus Cryphonectria cubensis. This disease causes severe losses to Eucalyptus trees, which are widely planted as exotics in plantation in tropical and subtropical parts of the world. The most effective means used to reduce losses due to Cryphonectria canker is achieved through breeding and selection of disease tolerant clones and hybrids. For this reason, effective and reasonably rapid disease screening procedures are important. Our study focused on evaluating the commonly used screening procedure for resistance to Cryphonectria canker in South Africa. This procedure typically uses a single highly pathogenic isolate for inoculation and subsequent identification of disease resistant clones. Results of this study showed that disease resistance screening should preferably be undertaken using more than one isolate of C. cubensis. However, results also show that the isolate that has been used in the past produced relatively reliable results. Overall, results of this study have shown evidence suggestive of vertical resistance in the C. cubensis–Eucalyptus pathosystem, and they have contributed to an improved understanding of one of the most important Eucalyptus diseases in South Africa.

Epidemiology and Chemical Control of Soybean Rust in Southern Africa. C. Levy, Plant Pathologist, Commercial Farmers Union of Zimbabwe, Harare, Zimbabwe. Plant Dis. DOI: 10.1094/PD-89-0669, 2005 (online). Accepted for publication 8 December 2004.

Two fungal species are known to cause soybean rust: Phakopsora pachyrhizi, the very aggressive “Asiatic” form, and the inconsequential P. meibomiae, the “New World” form. Until fairly recently, the former was limited to several countries in Southeast Asia and Australasia, where it caused erratic but severe losses to small-scale soybean production. The latter species has been restricted to parts of South America and several Caribbean Islands during the twentieth century, where it was generally ignored, as it caused little or no losses. In 2001, however, Asiatic rust entered South America and severely depleted soybean yields in Paraguay and Brazil, and thereafter, in parts of northeastern Argentina and Bolivia. Thus, there is a growing interest in North America to know the reason for the sudden movement of the Asiatic form worldwide, and to adopt appropriate measures used elsewhere to limit its impact. P. pachyrhizi was first confirmed on the African continent in central Uganda in 1996, although several unsubstantiated reports suggest an earlier presence on other legumes. The very rapid spread of this pathogen down the continent between 1998 and 2002, and its devastation of large commercial soybean fields in Zimbabwe and South Africa, are highlighted. While an integrated control strategy was adopted by researchers in these countries to limit this impact, this paper chronicles the chemical studies done during this period. The close relationship between plant age and susceptibility to the disease was evident from the data, more-so than the prevailing environment. Therefore, several experiments were designed to determine the optimum number and timing of fungicidal sprays, and the persistence of the chemicals used. A preliminary analysis of meteorological conditions that favor initial field infection in Southern Africa is also presented.

Assessment of Epidemic Potential of Soybean Rust in the United States. S. Pivonia and X. B. Yang, Department of Plant Pathology, Iowa State University, Ames 50011; and Z. Pan, Department of Earth & Atmospheric Sciences, St. Louis University, St. Louis, MO 63103. Plant Dis. DOI: 10.1094/PD-89-0678, 2005 (online). Accepted for publication 8 December 2004.

Assessing the epidemic potential of soybean rust (Phakopsora pachyrhizi) to U.S. soybean production is critical to risk management. This article reviews previous studies of soybean rust risk assessment and further assesses the epidemic potential of soybean rust in the United States. We identified three critical components of uncertainty in risk assessment: (i) suitability of climate conditions in production areas for soybean rust epidemics; (ii) likelihood of establishment of the fungus in the North America; and (iii) dispersal potentials including the entry potential into North America and the seasonal dispersal potential of the pathogen from overwintering regions to major soybean production regions in North America. The three components independently contribute to rust epidemics, and epidemics occur only if all three components are suitable. Assessments of the first and second components suggest that conditions are suitable for soybean rust epidemic occurrence in the United States. The certainty of the third component, spread of the pathogen over the U.S. mainland during a soybean growing season, is yet to be determined. In this article, the assessments of climate suitability and establishment likelihood are reviewed, and the seasonal dispersal potential of P. pachyrhizi in North America is discussed.