GENETIC AND PHENOTYPIC VARIATION IN VARIOUS SUB-POPULATIONS OFPHYTOPHTHORA INFESTANS FROM TOMATOES
GENETIC AND PHENOTYPIC VARIATION IN VARIOUS SUB-POPULATIONS OF PHYTOPHTHORA INFESTANS FROM TOMATOES
KENNETH L. DEAHL
USDA, ARS, Vegetable Laboratory, Rm. 224, Bldg. 010A, BARC-West, Beltsville, MD 20705-2350, USA
Background and objectives
Late blight, caused by the fungus Phytophthora infestans, has reemerged as the single most important pathogen of the cultivated potato on a worldwide basis. The recent occurrence of metalaxyl-resistant P. infestans in North America has resulted in sharply increased late blight problems in several production areas and devastating disease control failures in most production areas in the United States. Each year since 1990, when insensitive strains were first detected in the Pacific Northwest, the distribution of these strains has become broader and control has become increasingly more difficult . Most growers were unprepared for the severity caused by metalaxyl-resistant strains. This may be due to the finding that these strains are more aggressive than the metalaxyl-sensitive strains that previously constituted the P. infestans population in the United States. Disease control and suppression may now be inherently more difficult with these recently migrant strains that are resistant to metalaxyl. The possible loss of metalaxyl as an effective component of disease management necessitates an increase in management activities required to suppress late blight. Therefore, growers in areas which contain resistant strains are evaluating various methods to combat late blight. These include: more judicious timing of effective fungicides; scouting and disease forecasting; use of resistant varieties; eliminating sources of inoculum (seed tubers and cull piles); and monitoring to detect whether or not metalaxyl-resistant strains were active in the immediate area.
Materials and methods
Spread of late blight in commercial and home garden tomatoes has also been projected as an important source of inoculum that has the potential to aid in the spread of damaging genotypes to new locations. Therefore, continued monitoring of the pathogen population in tomatoes and other bridging hosts is warranted. In order to develop more effective control strategies for late blight disease management in potatoes, we attempted to characterize genetic and phenotypic variations in various sub-populations of P. infestans in tomato crops. Phenotypical analyses included mating type and sensitivity to metalaxyl. Cellulose-acetate electrophoresis was used to resolve allozyme genotypes at the glucose-6-phosphate isomerase locus . Sixty-four isolates were collected over four years of sampling: 1990, 1992, 1994 and 1997.
Results and conclusions
The vast majority of the samples were the US-6 and US-7 clonal lineage, but there were also a few US-1 and a few US-8. In 1997, several US-11 genotypes and few US-17 genotypes were isolated. Both were resistant to metalaxyl. Both physiological races, Tl and T0, were determined from the reactions on a set of differential genotypes. Most of the isolates were Tl. The results of this study indicated that major changes occurred in the genetic diversity of populations of P. infestans from tomatoes in the U.S. from 1990-1997. Future monitoring and sampling is necessary to determine whether sexual recombination is another potential source of novel genotypes in the United States.
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2. Goodwin SB, Schneider RE, Fry WE, 1995. Plant Disease 79:1182-1185.