Mycelial Compatibility Grouping and Aggressiveness of Sclerotinia sclerotiorum

A pernicious pathogen, Sclerotinia sclerotiorum (Lib.) de Bary has a wide geographic distribution and a diverse host range, including many agronomic crops (16). On soybean, S. sclerotiorum causes Sclerotinia stem rot (SSR), a disease recognized as an important yield-reducing disease in the United States (41) and a major disease on soybean in Illinois (15). Partial resistance associated with either escape mechanisms or physiological resistance to S. sclerotiorum has been reported in soybean (17). Extensive field and greenhouse evaluations to assess soybean resistance to S. sclerotiorum resulted in low correlations (18,32,39), and different experiments to assess cultivar response to S. sclerotiorum frequently were inconsistent (9,10). Problems associated with evaluating and breeding for resistance may be due in part to lack of consideration for pathogen population structure and variability in isolate aggressiveness. S. sclerotiorum can spread by sexually and vegetatively produced propagules (16). Sclerotia, the over-wintering vegetative form of the fungus, reside in the soil and germinate during the cropping season to produce apothecia which release millions of sexually produced, airborne ascospores. Up to 90% of the ascospores, the primary inoculum source that infects soybean flowers, remain within 100 m of the dispersal site (3). Additionally, mycelia-infected seed can provide a soil inoculum source for continuance of the pathogen’s life cycle (15,31). Although mycelia-infested seed can be dispersed widely throughout soybean-growing regions, no reports are available to suggest that the thin-walled ascospores are viable after long-distance wind dispersal. Mode and range of pathogen dispersal are important considerations potentially impacting population structure and disease control. Naturally occurring S. sclerotiorum populations exist as a mosaic of clones, which tend to be genetically isolated from each other (2,11,25). Phylogenetic analysis of clones using DNA sequence data from four regions suggests a predominately clonal mode of evolution with no evidence of contemporary genetic exchange and recombination between individual genotypes (6). Clones of S. sclerotiorum can be identified by DNA fingerprinting using a dispersed, repetitive probe and/or mycelial compatibility groups (MCGs; 21,23,25). The S. sclerotiorum mycelial compatibility-incompatibility grouping system is a macroscopic assay of the self-nonself recognition system common in fungi and is determined using a side-by-side pairing system (23). Although the MCG system is similar to vegetative compatibility grouping systems, in which nuclei may be mobile within compatibility groups, self-self pairings may vary regarding movement of nuclei between hyphal filaments that appear compatible (27). Movement of nuclei (via sexual or vegetative means) is not known to occur between MCGs (27). Although MCG systems are propagated mitotically, they are not necessarily clonal, as indicated by MCGs associated with one or more DNA fingerprints (11,14) which has been attributed to (i) parallel gain or loss of fingerprint fragments resulting from transposable element activity or (ii) rare episodes of genetic exchange (6). Factors for MCG or clone designations were shown to be stable and unchanged through successive sexual generations and after serial culturing, and the correlation between an MCG and a DNA fingerprint or fingerprints supports the synonymous relationship between MCGs and clones of S. sclerotiorum (21). A system for identifying and naming S. sclerotiorum clones has been established (14,20,24) and currently utilized to report the global distribution of clonal genotypes. Both MCGs and clones ABSTRACT Kull, L. S., Pedersen, W. L., Palmquist, D., and Hartman, G. L. 2004. Mycelial compatibility grouping and aggressiveness of Sclerotinia sclerotiorum. Plant Dis. 88:325-332.