Interpretive Summaries

Characterization of California Isolates of Fusarium oxysporum f. sp. vasinfectum. Y. Kim, Department of Plant Pathology, University of California, Davis 95616; R. B. Hutmacher, Department of Agronomy and Range Science, University of California, Davis 95616; and R. M. Davis, Department of Plant Pathology, University of California, Davis 95616. Plant Dis. DOI: 10.1094/PD-89-0366, 2005 (online). Accepted for publication 8 November 2004.

Fusarium wilt of cotton is a widespread disease in the United States and most cotton-growing areas of the world. Until a few years ago, the disease caused mild symptoms on widely grown cotton cultivars. Recently, two developments have raised concerns about Fusarium wilt of cotton in the San Joaquin Valley of California. The first was the emergence of extremely virulent strains of the causal agent, Fusarium oxysporum f. sp. vasinfectum, in Australia. When the California dairy industry began importing hundreds of thousands of metric tons of Australian cotton seed for feed in the late 1990s, cotton growers worried about the possible introduction of these strains into local fields. The second concern was the alarming occurrence of serious economic losses to Fusarium wilt in some Pima cotton fields in California. Because of these issues, an investigation was needed to determine the diversity and locality of California strains. Although eight races of F. oxysporum f. sp. vasinfectum have been described throughout the world, only race 1 and race 2 were known to occur in the United States. In this study, we identified the existence of four races of the pathogen in California. One of the races, race 4, was responsible for the extensive crop losses in the infested Pima fields. Australian isolates were not found in California. This is the first report of the occurrence of races 3, 4, and 8 in California.

Greenhouse Evaluation of Binucleate Rhizoctonia for Control of R. solani in Soybean. F. U. Khan and B. D. Nelson, Department Plant Pathology, and T. C. Helms, Department of Plant Sciences, North Dakota State University, Fargo 58105. Plant Dis. DOI: 10.1094/PD-89-0373, 2005 (online). Accepted for publication 8 November 2004.

The fungal pathogen Rhizoctonia solani attacks soybeans and other crops and causes damage to seedlings and adult plants. This pathogen lives in the soil and is difficult to control. One method of controlling the diseases caused by R. solani is to use other microorganisms that interfere with the activity of the pathogen on the roots of the plant. This greenhouse study investigated the use of strains of binucleate Rhizoctonia, another soil fungus that lives on soybean roots, for the control of R. solani. Results showed that binucleate Rhizoctonia can reduce the amount of disease if they grow on the roots prior to the pathogen attacking the plant. Field studies are needed to determine if the binucleate Rhizoctonia have any practical use in reducing diseases in soybean production.

Genetics of Chickpea Resistance to Five Races of Fusarium Wilt and a Concise Set of Race Differentials for Fusarium oxysporum f. sp. ciceris. Kamal Dev Sharma, Weidong Chen, and Fred J. Muehlbauer, USDA-ARS Grain Legume Genetics and Physiology Unit, Washington State University, Pullman 99164-6434. Plant Dis. DOI: 10.1094/PD-89-0385, 2005 (online). Accepted for publication 9 November 2004.

Fusarium wilt of chickpea caused by Fusarium oxysporum f. sp. ciceris is an important disease in many chickpea production regions. There are at least eight different races of the pathogen. Resistance to the races of Fusarium wilt is generally conditioned by major resistance genes. The genetics of resistance to race 5 in chickpea line WR-315 is not known, and the genetics of resistance to race 2 is controversial. This study using 100 F7 recombinant inbred lines derived from a cross between WR-315 and C-104 showed that resistance to race 5 in WR-315 is conditioned by one gene. It also showed that resistance to race 2 in WR-315 is conditioned by a single recessive gene. Knowing the number of genes involved in resistance is important in devising strategies to move the resistance genes to elite breeding lines and cultivars. We also observed a phenomenon termed slow wilting, which is different from previously reported late wilting in three aspects: latent period, disease progress rate, and final disease incidence. Identification of races is usually through using chickpea differentials. Previously reported differentials are imprecise in distinguishing some races, particularly in separating between races 2 and 3. We identified a recombinant inbred line that can be used to unambiguously separate races 2 and 3. After evaluating previously used differentials, we developed a concise differential set of eight chickpea lines that will offer important improvements over previous differential sets and will provide more precise and unambiguous identification of the races.