Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 28, 1997
Publication Date: N/A
Interpretive Summary: Phytophthoras cause some of the most destructive plant diseases known. The Irish potato famine, dieback of eucalypt forests in Australia, black pod disease of cocoa, and root rots of soybeans and other crops are all caused by Phytophthoras. A better understanding of the population biology of these organisms may lead to improved disease management strategies. Therefore, the literature on the population genetics of Phytophthora was reviewed to identify general trends and evaluate the current status of the field. These analyses revealed that mutations both in nuclear and mitochondrial DNA occurred at a high enough rate to be detectable in historical times. Breakdown of resistance in some potato cultivars was probably the result of changes to increased aggressiveness in the pathogen. Migrations in most Phytophthora species appear to have been very limited, and in some cases a single introduction caused widespread disease. Secondary migrations containing individuals that were better adapted, and probably more aggressive, often had serious consequences for plant disease epidemics. For example, recent epidemics of late blight on potatoes and tomatoes in the United States were caused by new migrations from Mexico. Thus, plant quarantine restrictions to prevent new migrations may be useful even in areas where a pathogen already occurs. Analyses of the amount of inbreeding in field populations showed that it did not always correspond to what was predicted by behavior in the laboratory. Explanations for many of the observed phenomena were proposed, and testable hypotheses about unanswered questions were formulated to guide and stimulate future research.
Technical Abstract: There are more than 50 species in the genus Phytophthora, most of which are destructive plant pathogens. Despite extensive efforts to control them, Phytophthora diseases still pose an immediate and real threat to world agriculture. For example, in 1994 alone, losses of potato & tomato crops to late blight caused by P. infestans in the U.S. were estimated at over $100 million, & more than $100 million was spent to control the disease. A better understanding of the population genetics of the Phytophthora pathogens may increase the effectiveness of disease management strategies. This review concentrated on four main areas of population genetics: sources of variation; migration; selection; & mating systems. Analyses of the literature revealed that mutation & mitotic recombination both contributed to previously observed genetic & phenotypic variation within known clonal lineages. Migrations from the main centers of diversity for most species appear to have been extremely limited. Recipient populations generally contained very limited genetic variation and exhibited a high degree of clonal reproduction. Fitness in clonal populations may have been reduced due to the accumulation of deleterious mutations (Muller's Ratchet). Such a reduction in fitness could explain the observed rapid replacement of old clonal lineages of P. infestans by new ones following migration. Fixation index values were calculated from published data - 16 Phytophthora species. Most homothallic species showed high levels of inbreeding. Heterothallic species had low fixation index values indicating a mixed mating system. Based on this review, testable hypotheses were formulated for many of the observed phenomena in an effort to stimulate & guide additional research.