Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 21, 1996
Publication Date: N/A
Interpretive Summary: Late blight is the most destructive disease of tomatoes and potatoes worldwide. Recently the disease threatens the tomato crop in North America. This threat is rooted in the fact that new populations of pesticide resistant strains of the fungus are rapidly emerging. After suffering substantial losses running into millions of dollars in the 1994 growing season, tomato scientists wanted to know if some commercial varieties or wild germplasm sources had any resistance to the disease. In this study, when 7 tomato species and some popular varieties were tested against the attack of 2 strains of this fungus, all of the varieties and wild germplasm accessions suffered some degree of damage. All of the commercial varieties suffered extensive damage, however, some of the wild tomato accessions sustained only minor amounts of infection. Currently there are no U.S. varieties that show substantial resistance to the new races and types of the fungus. Thus, dollar savings in few pesticide applications could be realized if resistant varieties were available. Thus the development of germplasm which would stand up to the fungus would assure tomato production at lower cost to the farmer and the pocketbook of the consumer and less risk to the environment.
Fifty-two genetically diverse isolates of Phytophthora infestans from the United States, the Philippines and Taiwan were evaluated in vitro for their sensitivities to the fungicide metalaxyl, mating type and allozyme genotype. Significant differences in virulence and metalaxyl sensitivities among the isolates tested were found but these responses were not dependent on the country of origin or time in culture. Fifty-nine percent of the isolates had EC-50 values in excess of 100 ppm. Variation in sensitivity phenotype was lowest for US-1 genotypes and highest for US-7. Tomato and wild related species without described resistance to late blight were screened for their reactions to two representative stains of the fungus. When plants of 104 wild Lycopersicon accessions and 5 commercial varieties were tested in the laboratory for resistance to Phytophthora infestans, L. pimpinellifolium, L. peruvianum and L. hirsutum showed a high level of resistance to the pathogen but there were no symptomless reactions. It seems probably from the diverse origin of most of the accessions that different factors of resistance are involved.