Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/25/2003
Publication Date: 4/11/2004
Citation: Kull, L.S., Pedersen, W.L., Hartman, G.L. 2004. Mycelial compatibility and aggressiveness of Sclerotinia sclerotiorum. Plant Disease. 88:325-332. Interpretive Summary: Sclerotinia stem rot is an important yield-reducing disease for soybean in the United States. Isolates of the fungus that cause this disease vary in several ways including how fast they can colonize soybean tissue (aggressiveness) and whether or not mycelium from one isolate can fuse with another isolate. This is called mycelial compatibility and is one measure of genetic relatedness. In this study, isolates from two soybean fields in Illinois, a soybean field in Argentina and isolates from hosts other than soybean were compared. Among 305 isolates tested, 42 mycelial compatibility groups were identified indicating that there are genetic differences in the pathogen even within one field. There was large variation among isolates for aggressiveness within one field, over locations and within those derived from hosts other than soybean. The ranking of isolates in terms of aggressiveness was similar on both resistant and susceptible soybean varieties. This study is important in documenting how different isolates from various geographic regions group together based on several attributes which is important to further understanding of the genetic variability that exists within this pathogen. This study is useful to soybean pathologists, mycologists, and soybean breeders who have an interest in Sclerotinia stem rot and finding better ways to control it.
Technical Abstract: Population variability of Sclerotinia sclerotiorum, the causal organism of Sclerotinia stem rot of soybean, was determined by mycelial compatibility grouping (MCG) and isolate aggressiveness comparisons. MCG and aggressiveness of S. sclerotiorum isolates from diverse hosts and geographic locations (Diverse Set, 24 isolates), from a soybean field in Argentina (Argentine Set, 21 isolates), and from soybean fields in DeKalb and Watseka, Illinois (DeKalb and Watseka Sets, 125 and 135 isolates, respectively) were assessed. Among 305 isolates tested, 42 MCGs were identified, and 61 percent were represented by single isolates observed at single locations. Within the Diverse Set, 17 MCGs were identified; one MCG consisted of six isolates, and 16 MCGs consisted of one isolate each. Each Illinois field was a mosaic of MCGs, but MCG profiles differed between the two fields. Nine MCGs were identified within the Argentine field with two MCGs composed of either five or six isolates, two MCGs composed of two isolates, and the remaining composed of one isolate each. MCGs were shared among the Diverse, DeKalb, and Watseka Sets, but MCGs within the Argentine Set were not shared with other sets. MCGs within the DeKalb and Watseka Sets were spatially aggregated based upon sampling location within the field. MCG 8 was the most frequently sampled and widely distributed MCG and occurred at a frequency of 29, 36, and 62 percent in the Diverse, DeKalb, and Watseka Sets, respectively. Variation in isolate aggressiveness was assessed using a limited-term, plug inoculation technique and area under the disease progress curve (AUDPC) was calculated for each isolate. Isolate aggressiveness varied (P equals 0.001) within the Diverse, DeKalb, Watseka, and Argentine Sets and varied (P probability 0.10) within MCGs composed of isolates from different locations, but did not vary within MCGs composed of isolates from a single field. Additionally, MCGs within the DeKalb and Watseka Sets differed in aggressiveness. A cultivar x isolate interaction was not detected. Pathogen population structure and isolate variability may be important considerations in disease management systems.