Interpretive Summaries

Characterization of Capsicum chinense Cultigens for Resistance to Meloidogyne arenaria, M. hapla, and M. javanica. J. A. Thies and R. L. Fery, U.S. Vegetable Laboratory, Agricultural Research Service, U.S. Department of Agriculture, 2875 Savannah Highway, Charleston, SC 29414. Plant Dis. Publication no. D-2001-0105-01R, 2001 (on-line). Accepted for publication 7 November 2000.

Several species and races of root-knot nematodes are serious pests of Scotch Bonnet and Habanero-type peppers belonging to Capsicum chinense. The development of cultivars with genetic resistance to root-knot nematodes is the most efficient and environmentally benign way of controlling this production problem. Host resistance also provides the most viable and easily implemented alternative to the use of methyl bromide soil fumigation to control root-knot nematodes. The studies described in this paper were conducted to determine if reported resistance to southern root-knot nematode, Meloidogyne incognita, in three Scotch Bonnet cultigens is effective against other species and races of root-knot. The experiments showed that in addition to M. incognita, the resistance was effective against M. arenaria race 1 (peanut root-knot nematode), M. arenaria race 2, and M. javanica (tropical root-knot nematode), but was not effective against M. hapla (northern root-knot nematode). The resistance has substantial value to the development of resistant cultivars (especially Habanero types that have become popular in the United States) because of its effectiveness against three species of root-knot nematodes. Evaluations of other plant germplasm resources, however, are needed to find a separate source of resistance to the fourth species, M. hapla.

Resistance of Sugarcane Cultivar R 570 to Puccinia melanocephala Isolates from Different Geographic Locations. C. Asnaghi, A. D’Hont, J. C. Glaszmann, and P. Rott, Centre de coopération internationale en recherche agronomique pour le développement, Cirad, TA 71/09, Avenue d’Agropolis, 34398 Montpellier Cedex 5, France. Plant Dis. D-2001-0102-01R, 2001 (on-line). Accepted for publication 12 November 2000.

Common rust caused by the fungus Puccinia melanocephala is one of the major diseases of sugarcane, and the disease exists in most locations where sugarcane is grown. The pathogen attacks leaves on which it produces rust colored pustules. The presence of numerous pustules results in reduced growth of sugarcane and yield losses. Rust can be controlled by the production and cultivation of resistant cultivars. We investigated the potential value of a major resistance gene identified in sugarcane cultivar R 570 to improve resistance to rust. For that purpose, we developed a detached leaf technique that allowed us to inoculate sugarcane with isolates of the pathogen from different geographic locations. Cultivar R 570 was found to be resistant to isolates of the rust fungus from Brazil, Colombia, Florida, Guadeloupe, Réunion Island, and Zimbabwe. It was also shown with a progeny of R 570 that the resistance of this cultivar to all isolates was probably due to a single gene or a single chromosomic region. This major gene can therefore be used by sugarcane breeders to improve resistance to rust in different geographic areas.

Management of Tomato spotted wilt virus in Flue-Cured Tobacco with Acibenzolar-S-Methyl and Imidacloprid. A. S. Csinos and H. R. Pappu, Department of Plant Pathology, R. M. McPherson, Department of Entomology, and M. G. Stephenson, Department of Crop and Soil Sciences, University of Georgia, Coastal Plain Experiment Station, Tifton 31793. Plant Dis. D-2000-1215-01R, 2001 (on-line). Accepted for publication 21 November 2000.

Tomato spotted wilt virus (TSWV) causes enormous losses in tobacco each year in Georgia. The disease can cause losses from transplanting time until harvest. Symptoms include stunting, leaf necrosis, stem necrosis, and death. Both Admire (imidacloprid) and Actigard (acibenzolar-S-methyl) have demonstrated activity in reducing the incidence of TSWV on tobacco. This study evaluates the treatment scenarios that most effectively reduce incidence of TSWV and evaluates plants for presence of TSWV by the use of enzyme-linked immunosorbent assay (ELISA). Our findings show that Actigard used on tobacco transplants prior to transplanting and in the field as a posttransplant treatment significantly reduced the number of symptomatic plants in flue-cured tobacco in Georgia. The use of Admire and Actigard together further reduces the number of symptomatic plants. As the rates of Actigard are increased, phytotoxicity is likely to occur.

Vegetative Compatibility and Pathogenicity of Verticillium dahliae from Spearmint and Peppermint. L. I. Douhan and D. A. Johnson, Department of Plant Pathology, Washington State University, Pullman 99164-6430. Plant Dis. D-2000-1220-02R, 2001 (on-line). Accepted for publication 21 November 2000.

Verticillium wilt of mint, caused by the vascular pathogen Verticillium dahliae, reduces essential oil production by limiting plant growth or killing the plants. The disease is economically important for peppermint and Scotch spearmint. Verticillium dahliae isolates are thought to have a wide host range and lack host specificity, however, pathotypes have been reported for potato and cotton. This study examined the vegetative compatibility using nitrate nonutilizing mutants (nit) and aggressiveness of 128 V. dahliae isolates from spearmint and peppermint from the western and midwestern states. Using known nit testers strains (OARDC), all isolates were assigned to VCG 2B, except for one isolate was assigned to VCG 2A, and two isolates were assigned to VCG 4A. The VCG 2 isolates were found in all the mint growing regions while the two VCG 4A isolates originated from southern Idaho, demonstrating a widespread distribution of VCG 2B on mint. The predominance of the VCG 2B mint strain may be attributed to the asexual reproduction of V. dahliae, vegetative propagation of mint and the shift of mint production areas from the U.S. Midwest to the Pacific Northwest. Isolates from mint and other hosts were used for pathogenicity experiments were conducted on mint and potato, a susceptible crop to V. dahliae. Mint isolates were significantly more aggressive on mint than other host isolates, indicating that mint isolates have become host-adapted and are distinct pathotype. Additionally, when isolates from mint and potato belonging to VCG 4A were inoculated to potato, they were more aggressive on potato than mint VCG 2B and potato VCG 4A isolates. The low VCG diversity of mint isolates is likely due to the introduction single aggressive strain that proliferated in infected mint rhizomes, which were disseminated throughout the mint growing regions. Knowledge of different V. dahliae strains within mint rhizomes or the soil can be useful in selecting crop rotations involving mint or other host susceptible to V. dahliae.

Spatio-Temporal Analysis of Soybean Root Colonization by Fusarium solani f. sp. glycines in Fields. Y. Luo, S. K. Chong, and O. Myers, Department of Plant, Soil and General Agriculture, Center for Excellence in Soybean Research, Teaching and Outreach, Southern Illinois University, Carbondale 62901-4415. Plant Dis. D-2001-0112-01R, 2001 (on-line). Accepted for publication 21 November 2000.

Soybean sudden death syndrome (SDS), caused by Fusarium solani f. sp. glycines, is an important disease in some soybean production areas in the United States. Symptoms on leaves usually appear slightly before or after reproductive stages. Root colonization by the pathogen occurs prior to symptom appearance. Data of the spatial distribution and temporal development of root colonization, as well as overwintering of the pathogen in root residue, provide important information to help understand mechanisms governing disease epidemics, pathogen infection cycles, the effect of no-till practice on pathogen populations, and control of the disease using resistance. The objectives of this study were to characterize the developments of root colonization in space and time, to examine the correlations in root colonization between winter and growing seasons, and to determine pathogen population dynamics in relation to foliar disease development. Soybean roots were sequentially collected from two no-till fields from June 1997 through December 1998. Roots were ground to isolate and enumerate the fungus F. solani f. sp. glycines, to obtain colony-forming units (CFU) per gram of root. The log CFU [log(10)(CFU + 1)] versus sampling time was used to produce the pathogen population curve, and the area under the pathogen population curve (AUPC) was calculated for each plot. The average log CFU from all plots for each sampling date was used to fit the logistic equation. Plot data of log CFU at each sampling time of the growing season, AUPC, and foliar disease index (FDX) were correlated with each other. Correlations among the log CFU in root residue in the winter of 1997, the log CFU, and FDX in the 1998 growing season were also conducted. Geostatistics was applied to determine the spatial dependence in root colonization for different spatial lag distance in the fields using semiviograms. Spatio-temporal autocorrelations of root colonization were studied using a computer model STAUTO. During the growing season, pathogen population in roots followed logistic growth in both fields. Pathogen populations in root residue decreased during the winter of 1997 and increased slightly in the spring of 1998 prior to planting. AUPC significantly correlated with FDX in both fields in 1997. Pathogen populations in root residue at three sampling dates in the winter of 1997 significantly correlated with FDX of the 1998 growing season in one field. No spatial dependence in root colonization was detected early in the growing season. However, some spatial dependence in certain directions of the fields was detected later in the growing seasons. Spatial dependence in AUPC in the across-rows direction was detected in both fields in 2 years. Pathogen dynamics in roots performed a short-distance dispersal, and only the root colonizations between two sequential samplings were related. A practical finding emerges from this study, namely that accelerated decay or removal of root residue may help in disease control.

Response of Heterodera glycines and Soybean Cultivar to Tillage and Row Spacing. S. Y. Chen, University of Minnesota Southern Research and Outreach Center, Waseca 56093; W. C. Stienstra, Department of Plant Pathology, and W. E. Lueschen, Department of Agronomy and Plant Genetics, University of Minnesota, St. Paul 55108; and T. R. Hoverstad, University of Minnesota Southern Research and Outreach Center, Waseca. Plant Dis. D-2001-0111-01R, 2001 (on-line). Accepted for publication 23 November 2000.

The soybean cyst nematode (SCN), Heterodera glycines Ichinohe, has a major impact on soybean yield in most soybean-growing regions of the United States and the world. Use of resistant soybean cultivars, along with crop rotation, is an effective tactic for managing the nematode. Farmers in the United States have shifted to conservation tillage to reduce soil erosion, improve soil moisture holding capacity, and reduce labor and fuel costs. In this study, the influence of five tillage treatments and two treatments of row spacing on SCN population dynamics and yield of SCN-resistant and -susceptible soybean cultivars was investigated in a corn-soybean rotation system in southern Minnesota from 1993 to 1996. No effects of tillage and row spacing were observed on nematode population density. As expected, the susceptible cultivar Sturdy consistently supported higher nematode densities than did the resistant cultivars Bell in 1993 to 1995 and Freeborn in 1996. Nematode reproduction varied among years. The effects of tillage and row spacing on soybean yield were inconsistent. The resistant cultivars yielded 653, 195, 435 kg/ha more than the susceptible cultivar in 1994, 1995, and 1996, respectively, but no yield difference between susceptible and resistant cultivars was observed in 1993. Planting resistant cv. Bell increased the yield of the following susceptible cv. Sturdy compared with continual planting of the susceptible cultivar. A sequence with continued resistant cultivar or cultivars, however, produced a higher overall yield and lower nematode density at the end of the 4-year rotation cycle than any sequence in which the susceptible cultivar was included. Yield of resistant and susceptible cultivars was negatively related to the SCN initial population density. This study suggests that conventional tillage and conservation tillage practices do not affect effectiveness of using resistant cultivars and crop rotation in SCN management in Minnesota.

Virulence of Aphanomyces euteiches Isolates from Iowa and Wisconsin and Benefits of Resistance to A. euteiches in Alfalfa Cultivars. G. P. Munkvold, Department of Plant Pathology, Iowa State University, Ames 50011; W. M. Carlton, Iowa State University Cooperative Extension, 107 E. Benton, Albia 52531; E. C. Brummer, Department of Agronomy, Iowa State University, Ames 50011; J. R. Meyer, 701 Ridge St., Madison, WI 53705; D. J. Undersander, Department of Agronomy, University of Wisconsin, and C. R. Grau, Department of Plant Pathology, University of Wisconsin, Madison 53706. Plant Dis. D-2001-0116-02R, 2001 (on-line). Accepted for publication 4 December 2000.

During the 1980s and 1990s, Aphanomyces root rot of alfalfa became recognized as an important disease in the United States, especially in poorly drained soils. This disease is caused by a soilborne fungus, Aphanomyces euteiches. Alfalfa cultivars with genetic resistance to Aphanomyces root rot were widely planted during the 1990s. The yield benefits of planting these resistant cultivars have been documented in several studies. However, it has become clear that in some fields with root rot problems, resistance to Aphanomyces root rot does not consistently result in higher yields, and in some states, such as Iowa, there has not been sufficient documentation to make conclusions regarding the performance of resistant cultivars. Meanwhile, researchers recorded the existence of a second race (race 2) of the fungus, which can cause disease on the resistant cultivars. The distribution of race 2 is not known. Therefore, we identified the race of several A. euteiches isolates from Iowa and Wisconsin by inoculating race 1–resistant alfalfa cultivars. We found that all 14 Iowa isolates (from different locations) and 21 of 59 Wisconsin isolates (all from one field) were race 2. Additionally, we found that race 1–resistant cultivars did not differ in yield from susceptible cultivars at two of three Iowa locations where we know A. euteiches is present. In the Wisconsin field from which the race 2 isolates were recovered, a field test showed that a race 2–resistant alfalfa breeding population yielded better than the race 1–resistant population and the susceptible alfalfa cultivar. Our conclusion is that the potential benefits of Aphanomyces root rot resistance are being hindered by the presence of A. euteiches race 2, and that incorporating race 2 resistance into alfalfa cultivars should significantly enhance their performance in Aphanomyces–infested soils.