Interpretive Summaries

Prevalence and Severity of Nattrassia mangiferae Root and Stem Rot Pathogen of Cassava in Bénin. W. Msikita, Ohio State University, Department of Plant Cellular and Molecular Biology, 318 W. 12th Avenue, Columbus 43210; B. Bissang, Université du Bénin, Ecole Supérieure d’Agronomie, B.P. 1515, Lomé, Togo; B. D. James and H. Baimey, International Institute of Tropical Agriculture, B.P. 08-0932, Cotonou, Bénin; H. T. Wilkinson, University of Illinois, Department of Natural Resources and Environmental Sciences, 1102 S. Goodwin Avenue, Urbana 61801; and M. Ahounou and R. Fagbemissi, International Institute of Tropical Agriculture, B.P. 08-0932, Cotonou, Bénin. Plant Dis. DOI: 10.1094/PD-89-0012, 2005. Accepted for publication 12 July 2004.

Root rots are a major problem that hampers cassava production. Because root-rotting pathogens affect mostly the underground portion of the plant (and are hence out of sight), and because the cassava plant has an extensive root system (and thus can remain standing despite a significant portion of its roots being rotted), and due to lack of extensive diagnostic surveys designed to quantify cassava root rots, the nature and extent of cassava root rot problems are poorly understood in Africa. Diagnostic surveys were conducted to determine the extent of root rot problems in Bénin (West Africa). Root rot pathogens were found in all departments (regions) of the country, affecting 86 to 100% and 96 to 100% of cassava fields during the dry and rainy seasons, respectively. Disease incidence in individual fields ranged from 0 to 53% and averaged 16 to 27% per department. Nattrassia mangiferae, a cosmopolitan fungus that attacks a wide range of crop species (from bananas to woody ornamental and fruit trees) in the field and in storage, and some of whose isolates cause skin, nail, and eye infections in humans, was the most frequently isolated root rot pathogen (56% in the dry season and 22 to 52% in the rainy season). The fungus significantly reduced the number of roots and induced 3- to 15-cm-long lesions on the lower stem portion. In pathogenicity studies, cultivar TMS 30572 was found to have good resistance to the fungus, suggesting the existence of resistant cassava cultivars. Other root rot pathogens detected during the dry season were Macrophomina phaseolina (14.2%), Fusarium spp., Botryodiplodia theobromae (7.7%), and Pythium spp. (2.9%). Results of the study are discussed with a view to creating awareness of the destructive power of N. mangiferae, a hitherto poorly recognized root rot pathogen of cassava in Bénin and West Africa in general, and to highlighting the need for extended research into cultural practices and the dynamics of the fungus as a crop and human pathogen.

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: 10.1094/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.

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: 10.1094/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 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: 10.1094/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.

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: 10.1094/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.

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: 10.1094/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.

Effect of Simulated Wind-Driven Rain on Duration and Distance of Dispersal of Xanthomonas axonopodis pv. citri from Canker-Infected Citrus Trees. C. H. Bock, USDA-ARS-USHRL, 2001 S. Rock Rd., Ft. Pierce, FL 34945; P. E. Parker, Center for Plant Health Science and Technology Pest Detection Diagnostics and Management Laboratory, USDA-APHIS, Moore Air Base, Edinburg, TX 78541; and T. R. Gottwald, USDA-ARS-USHRL, 2001 S. Rock Rd., Ft. Pierce, FL 34945. Plant Dis. DOI: 10.1094/PD-89-0071, 2004 (online). Accepted for publication 30 August 2004.

Xanthomonas axonopodis pv. citri is a plant-pathogenic bacterium that causes citrus canker, a disease that can result in severe yield loss due to fruit and leaf drop. The pathogen occurs in many humid-wet tropical and subtropical citrus growing regions and was most recently discovered in the Miami area of Florida in 1995. Due to the threat posed to the Florida citrus industry by the disease, an eradication program was instigated. Our understanding of the epidemiology of the disease is limited, but a thorough knowledge of the factors that allow infection and dispersal of the pathogen will contribute to a fundamental understanding of the disease epidemiology and should pave the way to a better-informed and more rational approach to its management and eventual eradication. To ascertain effects of wind and rain splash/spray on the dynamics of dispersal of the bacterium, simulated wind/rain splash events were produced using electric blowers (15 to 20 m s(^–1)) and sprayer nozzles to produce water droplets entrained in the wind flow. The splash was blown at canker-infected trees 1 m downwind. The spray was collected downwind of the infected trees over different time periods and distances. Bacteria were dispersed up to 52 h after commencing the simulated rain splash event, and detailed study of dispersal duration over the first 4 h showed that the greatest quantity of bacteria were dispersed in the first 5 min of dispersal. Bacteria were collected at all distances sampled up to 12 m from the inoculum source, although the greatest numbers of bacteria were collected closest to the source. These data show that citrus canker bacteria are readily dispersed in wind-driven rain. They are produced in large quantities immediately after the stimulus for dispersal occurs, are dispersed over a prolonged period, and can be blown a substantial distance in the splash. This combined action of wind and rain has ramifications for the spread of the disease, likely leading to infection of susceptible citrus in rainstorm events in southern Florida where the disease is currently being eradicated.

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: 10.1094/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.