Screening St. Augustinegrass Genotypes for Brown Patch and Large Patch Disease Resistance

Two diseases of St. Augustinegrass are caused by different anastomosis groups of Rhizoctonia solani.  Brown patch (BP) is a foliar disease of little economic importance, and large patch (LP) causes leaf sheath rot and death of affected turfgrass shoots and stolons.  St. Augustinegrass genotypes were inoculated with an isolate that causes brown patch and an isolate that causes large patch in repeated experiments.  Three disease severity parameters were calculated from data recorded from each genotype, including final disease severity, area under the disease progress curve (AUDPC), and the apparent infection rate (r_L).  The inoculation protocol reliably produced symptoms, and both r_L and AUDPC provided statistically significant separations between genotypes for their susceptibility to BP and LP.  A significant interaction between isolate and genotype was observed, and some genotypes that were very susceptible to LP were among the least susceptible to BP.  These data suggest that differences in resistance to LP can be quantified in St. Augustinegrass genotypes, but genotype response may be isolate dependent.