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United States Department of Agriculture

Agricultural Research Service

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Research Project: GENETIC VARIATION AND VIRULENCE OF S. SCLEROTIORUM IN THE UNITED STATES

Location: Sunflower Research

2010 Annual Report


1a.Objectives (from AD-416)
1. Determine genetic variation of S. sclerotiorum from crops grown outside the North Central region using DNA fingerprinting and mycelial compatibility groups. 2. Characterize the virulence of selected isolates on cultivars of soybean, sunflower, canola, dry bean, pea and lentil. 3. Prepare a set of isolates representing the genetic and virulence diversity of S. sclerotiorum in the United States for distribution to researchers on Sclerotinia.


1b.Approach (from AD-416)
Sclerotinia sclerotiorum is one of the most important pathogens of field crops in the North Central USA, causing economic losses in the numerous susceptible crops grown in the northern tier of states. The diseases caused by this pathogen have been very difficult to control. There is no published information on the genetic variation of S. sclerotiorum in the North Central region of the United States that includes information on isolate virulence across crops such as soybean, sunflower, canola, drybean, pea and lentil. Understanding the genetic variation and virulence of the current isolates of S. sclerotiorum in the region is a fundamental part of the overall strategy of using resistance or other controls for this pathogen. Such information may also help explain the mixed results of field screening for resistance that has been observed over the region and the lack of correlation between field and laboratory evaluations. This research will study genetic variation and virulence of a set of isolates from the current population of the pathogen on dry bean, canola, soybean, sunflower, pea and lentil in the northern tier of states in the North Central Region. This region has the greatest concentration of the numerous crops susceptible to white mold.

The primary approach is to make a collection of isolates of the pathogen from the six crops, identify mycelial compatibility groups, use microsatellites to characterize the genetic variation and then characterize virulence of isolates representative of the clones/genetic groups on all six crops using two different methods. Two techniques will be employed to evaluate virulence:.
1)a novel technique of measuring fungal biomass in infected tissue and.
2)a standard cut stem inoculating method which measures lesion length as criteria for virulence. This research will answer some fundamental questions on the biology of this pathogen as it pertains to six important crops grown in a region where Sclerotinia diseases commonly occur.


3.Progress Report

This project was initiated on June 1, 2008, research is ongoing, and the overall objective is to characterize the genetic variation and virulence of S. sclerotiorum on six crops in the North Central Region of the USA. ADODR monitoring activities to evaluate research progress included phone calls, meetings with the cooperator, and an annual meeting held each year in January.

Genetic variation and virulence of S. sclerotiorum. Sclerotinia sclerotiorum is one of the most important pathogens of field crops in the North Central USA, causing economic losses in the numerous susceptible crops. There is limited published information on the genetic variation of S. sclerotiorum isolates across the United States that includes information on isolate virulence across crops such as soybean, sunflower, canola, dry bean, field pea and lentil. Our first objective is to make a collection of isolates of the pathogen from these crops, identify mycelial compatibility groups (MCG’s), use microsatellite markers to characterize the genetic variation and then characterize virulence of isolates that have common and unique haplotypes on six crops grown in the North Central Region. We completed screening on 149 isolates which form 46 MCGs. A few MCGs appear to be quite common although most are relatively rare and represented by 1-2 isolates: four MCGs contain 10 or more isolates while 36 MCGs contain a single isolate. To date, no differences in MCGs have been detected across crops. The most common was MCG 9 found in nine states and on all four crops. Six of the most common MCGs represented 58% of the isolates and were found across crops. The frequency at which MCG’s occur varies with crop and geographic region. We also tested a selection of 30 isolates for virulence on the six crops in the greenhouse; these isolates represented the diversity of isolates across crops and geographic areas of the collection. The entire experiment consisted of 900 five week-old plants inoculated with the cut-stem/straw technique, and seventy two hours after inoculation all lesions were measured with electronic calipers. Analyses are ongoing, but there are obvious differences between isolates, and a repeat of the experiment is currently in progress. We have also been measuring growth rates of isolates at different temperatures to determine if there are differences among isolates in the adaptation to environmental conditions. Using this unique large collection of isolates from four crop species across the North Central Region we are beginning to document the extent of genetic variation with respect to both geography and crop and link this variation to measurements of virulence across the six crop species. Understanding the genetic variation and virulence of the current isolates of S. sclerotiorum in the United States is a fundamental part of the overall strategy of using resistance or other controls for this pathogen.


Last Modified: 8/21/2014
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