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Phytophthora molecular genetics, epidemiology & management
Research
Our current research focuses on studying the biology and control of Phytophthora diseases affecting horticultural crops with a particular emphasis on the Sudden Oak Death pathogen, P. ramorum.
Our research includes the following three broad objectives:
1. Describe the epidemiology, evolution, and genetics of Phytophthora spp. affecting nursery crops;
Symptoms of P. ramorum on rhododendron
(Photo: N. Grunwald)
2. Determine gene expression profiles of P. ramorum in different developmental tissues and during host-pathogen interactions using functional genomics approaches; and
Petri dish culture and sporangia releasing
zoospores of P. ramorum
(Photos: M. Garbelotto and D. Rizzo)
3. Development and integration of cultural, biological, and chemical controls for management of Phytophthora diseases under commercial conditions.
Assessment of the variability of resistance to
P. ramorum in the genus Viburnum
(Photo: N. Grunwald)
Recent reports
Grünwald, N. J., S. B. Goodwin, M. G. Milgroom, and W. E. Fry. 2003. Analysis of genotypic diversity data for populations of microorganisms. Phytopathology 93: 738-746.
Estimation of genotypic diversity is an important component of the analysis of the genetic structure of plant pathogen and microbial populations. Estimates of genotypic diversity are a function of both the number of genotypes observed in a sample (genotype richness) and the evenness of distribution of genotypes within the sample. Currently used measures of genotypic diversity have inherent problems that could lead to incorrect conclusions, particularly when diversity is low and/or sample sizes differ. The number of genotypes observed in a sample depends on the technique used to assay for genetic variation; each technique will affect the maximum number of genotypes that can be detected. We develop an approach to analysis of genotypic diversity in plant pathology that makes specific reference to the techniques used for identifying genotypes. Preferably, populations that are being compared should be very similar in sample size. In this case, the number of genotypes observed can be used directly for comparing richness. In most cases sample sizes differ and use of the rarefaction method to calculate richness is more appropriate. In all cases, scaling either Stoddart and Taylor’s G or Shannon and Wiener’s H' by sample size should be avoided. Under those circumstances where it might be important to distinguish whether richness or evenness contribute more to diversity, a bootstrapping approach where confidence intervals are calculated for indices of diversity and evenness is recommended.
We tested the hypothesis that the population of Phytophthora infestans in the Toluca Valley region is genetically differentiated according to habitat. Isolates were sampled in three habitats 1) from wild Solanum species (WILD), 2) from land-race varieties in low-input production systems (RURAL), and 3) from modern cultivars in high-input agriculture (VALLEY). Isolates were sampled in 1988-89 (n = 179) and in 1997-98 (n = 389). In both sampling periods the greatest genetic diversity was observed in the RURAL and VALLEY habitats. Based on the Gpi and Pep allozymes, the subpopulations from the three habitats were significantly differentiated in both sampling periods. In contrast to allozyme data, for 1997-98 no differences were found among the three subpopulations for sensitivity to metalaxyl. Two groups of isolates identical for allozyme and mating type, were further investigated using RFLP fingerprinting; 65% of one group and 85% of another group were demonstrated to be unique. The genetic diversity data and the chronology of disease occurrence during the season are consistent with the hypothesis that populations of P. infestans on wild Solanum populations are derived from populations on cultivated potatoes in the central highlands of Mexico near Toluca.
The Mexican national potato program has produced several cultivars with high levels of field resistance. We evaluated durability of resistance to potato late blight of a selection of 12 such cultivars using data from 1960 to the present. Data were extracted from the field notebooks located in the archives of the Mexican National Potato Program in the John S. Niederhauser Library in Toluca, Mexico. There was a trend to indicate that field resistances to potato late blight of Mexican cultivars released between 1965-1999 were durable. At least two of the cultivars, namely Sangema and Tollocan, have been grown on at least 4-5% of the potato acreage and over long periods of time without decay in levels of field resistance. Pedigrees of the 12 cultivars indicate that most of the field resistance was introgressed from Solanum demissum. Field resistance might also be derived from commonly grown land-race cultivars such as “Amarilla de Puebla” and “Leona.” These have been grown in Mexico since about the 1780’s. They have the appearance of S. andigena-derived material but their genetic background is unknown.
Horticultural Crops Research Laboratory, USDA ARS
3420 NW Orchard Ave., Corvallis, OR 97330
Voice: (541) 738-4049 • Fax: (541) 738-4025
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grunwaln
<at> science.oregonstate.edu
Last Updated:June 22, 2007