Rhizoctonia solani AG-5 is associated with root rot of field pea in North Dakota

Rhizoctonia solani Kuehn is known to cause stem and root rot of field peas (Pisum sativum L.) and the anastomosis group commonly related to this condition in peas is AG-4. However, disease surveys conducted during the summers of 2007 and 2008 in major pea growing counties in North Dakota led to the isolation of R. solani AG-5 isolates in addition to AG-4. These were obtained from the brown discolored lesions on the roots and lower base of the stems of the pea samples. AG groups of the isolates were confirmed using anastomosis pairing with testers and by comparisons of ITS sequences of these isolates with those from members of all common R. solani AG groups. Three AG-5 isolates were tested in the greenhouse for pathogenicity on two pea cvs, yellow 'Admiral' and green 'Striker', soybean (cv. Barnes) and dry bean (cv. Montcalm) using inoculated wheat grains for infection. Non-inoculated plants and those inoculated with wheat grains infected with an AG-4 tester isolate served as negative and positive controls respectively for all the hosts. When the roots were evaluated for disease severity, distinct reddish brown lesions were observed on the hypocotyls and roots in the infested pots for all three isolates and three crops. Koch's postulates were established and initial results of pathogenicity tests suggest that these AG-5 isolates are comparable to AG-4 in their aggressiveness on field peas. To the best of our knowledge this is the first report of AG-5 on field pea.