Plant Disease magazine, January 2005 issue, Volume 89, Number 1
Selection of interpretive summaries of articles relevant to seed professionals

Illinois study finds transmission of soybean dwarf virus to U.S. soybean crops unlikely

Incidence of Soybean dwarf virus and Identification of Potential Vectors in Illinois.
Barbara Harrison and Todd A. Steinlage, Department of Crop Sciences, University of Illinois, Urbana 61801; Leslie L. Domier, USDA-ARS, Department of Crop Sciences, University of Illinois, Urbana 61801; and Cleora J. D’Arcy, Department of Crop Sciences, University of Illinois, Urbana 61801. Plant Dis. DOI: 10.101094/PD-89-0028, 2005 (online). Accepted for publication 26 July 2004.

Soybean dwarf virus (SbDV) is an aphid-transmitted virus that regularly causes severe yield losses in soybean in Asia. In the United States, SbDV is commonly found in clovers, but rarely infects soybean plants. Because of its persistent manner of transmission, aphids retain the ability to transmit the SbDV for days after feeding on an infected plant. The inability of SbDV to move from clovers to soybean plants in the United States has been attributed to the absence of aphids that colonize soybeans and have the ability to transmit the virus. In the summer of 2000, the Asian soybean aphid was found in North America for the first time. These highly mobile aphids colonize soybeans and have been reported to transmit several viruses. To gauge the potential of endemic SbDV strains to cause disease in Illinois, we determined the distribution of SbDV in Illinois and evaluated five aphid species, including the Asian soybean aphid, to transmit SbDV from infected clover plants to soybean. We found that almost half of the red clover plants tested were infected with SbDV and that an aphid species native to North America that does not colonize soybean transmitted SbDV poorly from clover to soybean, but SbDV was not transmitted by the Asian soybean aphid. These findings suggest that the Asian soybean aphid will not be an important vector of SbDV in the United States unless new virus strains or aphid biotypes appear.

Fungus that causes sorghum disease can survive season to season in Texas

Survival of Claviceps africana Within Sorghum Panicles at Several Texas Locations
Louis K. Prom, USDA-ARS, Southern Plains Agriculture Research Center, 2765 F & B Road, College Station, TX 77845; Thomas Isakeit, Department of Plant Pathology and Microbiology, Texas A&M University, College Station 77843; Gary N. Odvody, Texas Agricultural Experiment Station, 10345 Agnes Street, Corpus Christi 78406; Charlie M. Rush, Texas Agricultural Experiment Station, P.O. Drawer 10, Bushland 79012; Harold W. Kaufman, Texas Cooperative Extension, Route 3, Box 213AAA, Lubbock 79403; and Noe Montes, Department of Plant Pathology and Microbiology, Texas A&M University, College Station 77843. Plant Dis. DOI: 10.101094/PD-89-0039, 2005 (online). Accepted for publication 13 August 2004.

In the United States, sorghum ergot was first observed in 1997 in Texas. This new fungal disease can threaten sorghum used for production of hybrid seed. The use of cultural practices may be one way of controlling the disease. At present, there is no information on the ability of the fungus to survive from season to season in Texas. Therefore, this study was conducted to determine the survival of the fungus in different climates at several Texas sorghum production areas. The results indicate that the fungus can survive in all major sorghum production areas in Texas from one growing season to the next. We found that survival of the fungus spores decreased when they were buried in soil, as compared with spores left on the soil surface or above the soil on crop residue. Based on our results, plowing the infected debris into the soil after harvest could be a useful control measure for this disease.

Pathogenic fungi causing symptoms similar to Phaeosphaeria Leaf Spot of maize

Pathogenic Fungi Causing Symptoms Similar to Phaeosphaeria Leaf Spot of Maize in Brazil
A. L. Do Amaral, M.Sc., Departamento de Plantas de Lavoura; F. K. Dal Soglio, Ph.D., Departamento de Fitossanidade; M. L. De Carli, Departamento de Plantas de Lavoura; and J. F. Barbosa Neto, Ph.D., Departamento de Plantas de Lavoura, Faculdade de Agronomia, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS, 91501-970, Brazil. Plant Dis. DOI: 10.101094/PD-89-0044, 2005 (online). Accepted for publication 13 August 2004.

In the last decade, Phaeosphaeria leaf spot (PLS) of maize became one of the most important diseases of this crop in Brazil. Phaeosphaeria maydis has been considered to be the causal agent of this disease, and the symptoms attributed to PLS in Brazil include round foliar lesions, from 0.1 to 2.0 cm in diameter, initially chlorotic, later becoming necrotic, white to straw colored, with or without brown margins. The lesions may coalesce and reproductive structures of fungi may be found in the center of the lesions. Because of some differences with typical PLS symptoms as described in the literature, and the lack of stability of resistant cultivars, which present different resistance reactions in different environments, it has been suggested that different causal agents may be associated with PLS-like symptoms. To identify and characterize pathogens associated with PLS-like symptoms in different environmental conditions in Brazil, we collected leaves from two locations at normal and late growing seasons. Three species of fungi, other than P. maydis, were demonstrated to cause PLS-like symptoms. Among them, Phoma sorghina was the most widely distributed; the presence of the other two depended on location and growing season. These results indicate the need for a more accurate diagnosis of causal agents of diseases of maize with PLS-like symptoms in different environments in Brazil, and suggest that breeding programs should adopt selection of resistant cultivars based on artificial inoculation.

Soil temperature and moisture impacts fungi that affect agricultural crops

Influence of Soil Temperature and Moisture on Eruptive Germination and Viability of Sclerotia of Sclerotinia minor and S. sclerotiorum.
M. E. Matheron and M. Porchas, The University of Arizona, Yuma Agricultural Center, Yuma 85364. Plant Dis. DOI: 10.101094/PD-89-0050, 2005 (online). Accepted for publication 29 August 2004.

Sclerotinia minor and S. sclerotiorum are soilborne fungi that can cause disease on a variety of different agricultural crops. These fungi overseason in soil as resistant structures called sclerotia, which germinate under favorable conditions in the presence of a host crop to initiate disease. Experiments were initiated to test the effect of soil temperature and moisture on the viability of sclerotia of S. minor and S. sclerotiorum in field soil. In laboratory trials at constant temperatures, the proportion of sclerotia of both pathogens that germinated in wet soil tended to decrease as soil temperature increased from 15 to 40°C, with no germination of sclerotia of S. minor and S. sclerotiorum detected after 1 and 2 weeks, respectively, at 40°C. In contrast, after 1 to 4 weeks in dry soil, germination of sclerotia of S. minor and S. sclerotiorum ranged from 28 to 55% and 42 to 77%, respectively. Field trials revealed that germination of sclerotia of both pathogens (i) was significantly lower in irrigated soil compared to dry soil, (ii) was significantly lower in irrigated soil with a mean temperature of 32°C compared to irrigated soil with a mean temperature of 26°C, and (iii) was completely inhibited after 3 weeks in flooded soil with mean soil temperatures ranging from 30 to 33°C. Results from these studies suggest that flooding fields with a history of lettuce drop caused by S. minor or S. sclerotiorum for at least 3 weeks with soil temperatures in the range of 30 to 33°C could significantly reduce the population of sclerotia that are viable and capable of causing disease.

Researchers test for leaf and stem rust resistance in Israel

Leaf Rust and Stem Rust Resistance in Triticum dicoccoides in Israel.
Y. Anikster and J. Manisterski, Institute for Cereal Crops Improvement, Tel Aviv University, Ramat Aviv 69978, Israel; D. L. Long, U. S. Department of Agriculture - Agricultural Research Service, Cereal Disease Laboratory, University of Minnesota, St. Paul 55108; and K. J. Leonard, Plant Pathology Department, University of Minnesota, St. Paul 55108. Plant Dis. DOI: 10.101094/PD-89-0055, 2005 (online). Accepted for publication 25 August 2004.

Leaf rust is one of the most serious diseases of wheat in the United States and worldwide. The most effective and economical way to control leaf rust is through the development of resistant wheat varieties. Unfortunately, most resistance to leaf rust is effective against only some races of the leaf rust fungus but not others. Such resistance usually remains effective for only a few years, because new rust races arise that are not affected by the resistance. The available supply of rust resistance genes in cultivated wheat varieties and breeding lines is nearly exhausted. We collected seed from 742 plants of wild emmer wheat from natural habitats in northern Israel and tested the wild emmer lines for resistance to leaf rust in Israel and the United States. Twenty-one of the lines had resistance in field tests that appeared to be at least moderately effective against all known races of the leaf rust fungus. These lines can be crossed easily to cultivated wheat varieties and may provide combinations of resistance genes with long-lasting protection against leaf rust in U.S. wheat production. This would prevent wheat leaf epidemics that periodically reduce yields by 15% or more in major wheat producing states in the United States.

Midday temperature an important factor in determining Downy Mildew of lettuce

Analyses of the Relationships Between Lettuce Downy Mildew and Weather Variables Using Geographic Information System Techniques.
B. M. Wu and K. V. Subbarao, Department of Plant Pathology, University of California, Davis, c/o United States Agricultural Research Station, Salinas, CA 93905; and A. H. C. van Bruggen, Biological Farming Systems, Wageningen University and Research Centre, The Netherlands. Plant Dis. DOI: 10.101094/PD-89-0090, 2005 (online). Accepted for publication 6 September 2004.

Downy mildew has long been one of the most destructive diseases of lettuce in the Salinas Valley, California. Previous studies in coastal California suggested that the length of time lettuce leaves remain wet in the morning, and the air temperature immediately after this leaf wetness period, greatly affect the downy mildew pathogen. In this study, we found that the higher the midday temperature, the lower the percentage of lettuce plants with downy mildew. High humidity and long leaf wetness periods also were associated with high levels of downy mildew. The Salinas Valley was divided into two areas with different disease risks, based on midday temperature and relative humidity in the morning. The northern part of the Salinas Valley tended to have higher morning relative humidity, lower midday temperatures, and more downy mildew than the southern Salinas Valley. These results confirmed that midday temperature is an important factor determining lettuce downy mildew, and its effects should be added into disease warning systems for coastal California.

Plant Disease is a leading international journal of applied plant pathology, published by The American Phytopathological Society. It publishes original research articles focusing on practical aspects of plant disease diagnosis and control. Each issue also includes a monthly feature article summarizing a significant topic in plant pathology. The Disease Notes section contains timely reports of new diseases, outbreaks, and other pertinent observations. For a full list of contents, visit http://www.apsnet.org/pd/current/.