Genetic Variation of Sclerotinia sclerotiorum from Multiple Crops in the North Central United States

Abstract

Sclerotinia sclerotiorum is an important pathogen of numerous crops in the North Central region of the United States. The objective of this study was to examine the genetic diversity of 145 isolates of the pathogen from multiple hosts in the region. Mycelial compatibility groups (MCG) and microsatellite haplotypes were determined and analyzed for standard estimates of population genetic diversity and the importance of host and distance for genetic variation was examined. MCG tests indicated there were 49 different MCGs in the population and 52 unique microsatellite haplotypes were identified. There was an association between MCG and haplotype such that isolates belonging to the same MCG either shared identical haplotypes or differed at no more than 2 of the 12 polymorphic loci. For the majority of isolates, there was a one-to-one correspondence between MCG and haplotype. Eleven MCGs shared haplotypes. A single haplotype was found to be prevalent throughout the region. The majority of genetic variation in the isolate collection was found within rather than among host crops, suggesting little genetic divergence of S. sclerotiorum among hosts. There was only weak evidence of isolation by distance. Pairwise population comparisons among isolates from canola, dry bean, soybean and sunflower suggested that gene flow between host-populations is more common for some crops than others. Analysis of linkage disequilibrium in the isolates from the four major crops indicated primarily clonal reproduction, but also evidence of genetic recombination for isolates from canola and sunflower. Accordingly, genetic diversity was highest for populations from canola and sunflower. Distribution of microsatellite haplotypes across the study region strongly suggest that specific haplotypes of S. sclerotiorum are often found on multiple crops, movement of individual haplotypes among crops is common and host identity is not a barrier to gene flow for S. sclerotiorum in the north central United States.

Fig 1. Geographic distribution of isolates of Sclerotinia sclerotiorum from the North Central United States.

There were 145 isolates from the north central area and several adjacent western states, and Manitoba, Canada. The map was produced with Ersi software ArcGIS 10.2 using a National Geographic basemap of North America.

Fig 2. Frequency of each mycelial compatibility group (MCG) within four crops for isolates of Sclerotinia sclerotiorum.

Distribution is almost identical for microsatellite haplotypes.

Fig 3. Geographic distribution of four mycelial compatibility groups (MCG) from the North Central United States.

The MCG were collected from the north central area and adjacent western states and Canada. MCG 9 was the most widely distributed MCG. The map was produced with Ersi software ArcGIS 10.2 using a National Geographic basemap of North America.

Fig 4. Principal component analysis of haplotype of Sclerotinia sclerotiorum within four crops.

Plot of the mean first and second principle component. Centroids and standard error estimates are indicated.