Global population genetics and phylogeography of the rice sheath blight, maize banded leaf, and soybean aerial blight pathogen Rhizoctonia solani anastomosis group 1 IA

^{“Rhizoctonia solani thus emerges as a fungus of ancient lineage, great versatility, and enormous capacity for destruction of man`s crops. The relatively simple structure and life cycle of this fungus, evidence of continuously evolutionary plasticity, may well be basic to its marked success in both time and space… Just how ancient, may eventually be determined” (K.BAKER, 1970).}

One of the most important groups within the Rhizoctonia solani species complex is AG-1, a major group of fungal pathogens infecting soybean, rice, maize and other important crops worldwide. The fungus causes leaf blight on soybean, sheath blight on rice and banded leaf and sheath blight on maize. These diseases are considered devastating in terms of yield loss where they occur. Control strategies available for these diseases are limited, relying mainly on fungicide applications, which have significant, deleterious environmental effects. Breeding resistance to leaf/sheath blight into these crops is difficult because resistance is expressed quantitatively. Increased knowledge of the population biology of the pathogen is needed to implement more effective control strategies, and this project is structured to provide significant new insights into the population biology of R. solani AG-1 IA.

R. solani AG-1 IA is still considered to be mainly asexual, especially the rice and maize-infecting populations, although we have presented evidence that recombination occurs (Rosewich et al. 1999; Linde et al. 2005) based on analysis of population structure in rice-infecting populations from Texas and India. Knowledge regarding the center of origin, migration patterns, and the possibilities of population subdivision is non-existent.

Although is some evidence suggesting host specialization between the soybean-infecting and rice-infecting populations of R. solani AG-1 IA, this is a question that needs a rigorous examination. Knowledge of the population biology and phylogeography of pathogens is essential to implement sustainable management strategies and to identify sources of infection.

Our key objectives for this project are:

1) To determine whether speciation driven by host-specialization is under way or has already occurred between Poaceae-infecting (rice/maize) and Fabaceae-infecting (soybean) populations of R. solani AG-1 IA.

2) To determine the intercontinental genetic structure of geographically distinct Poaceae-infecting (rice/maize) and Fabaceae-infecting (soybean) populations of R. solani AG-1 IA.

As part of this goal, the first objective will be to determine the degree of similarity and genetic relatedness among geographically distinct field populations of the pathogen. A second objective will be to determine the importance of recombination in populations of R. solani AG-1 IA originating from different hosts. These populations will be analyzed using a set of microsatellite loci recently developed in our lab as novel molecular marker tools for Rhizoctonia.

3) To determine the phylogeography of R. solani AG-1 IA using DNA sequence loci from the nuclear and mitochondrial genomes. This will provide us with information concerning the center of origin of the pathogen and the directionality of gene flow. We intend to test the hypothesis that these are native pathogens that were selected out of the local environment when the crops (maize, rice, or soybean) were introduced to new continents.