In early September 2012, symptoms similar to aerial blight were observed on runner peanut (cv. Georgia 09B) in a commercial field in Randolph County, Arkansas (3). Leaves within the canopy closest to the soil had water-soaked, gray to green lesions or tan to brown lesions. Localized areas of matted leaves with mycelium occurred on stems and hyphae extended along stems and newly affected leaves. Dark brown spherical sclerotia (1.5 to 4 mm diam.) were produced on the surface of symptomatic peanut tissue (3). Aerial blight symptoms were observed in two peanut fields (~4 to 6 ha) that were furrow irrigated. Symptomatic plants were localized in a single circular pattern (~20 × 25 m) near the lower end of each field with the final disease incidence of less than 5%. Isolations from surface-disinfected leaves on potato dextrose agar consistently yielded light brown to brown colonies with sclerotia typical of Rhizoctonia solani AG1-IA. The fungus was confirmed to be R. solani AG1 by anastomosis reaction (2) with known cultures of AG1-IA isolated from soybean and rice in Arkansas. Sequencing of the rDNA ITS region 5.8s with primers ITS1 and ITS4 (1) supported the identification of the R. solani isolates as AG1-IA. The BLAST search revealed that the sequence had a 96 to 97% maximum sequence identity to several R. solani AG1-IA isolates collected from rice sheaths in China and Arkansas. Eight-week-old peanut plants (cv. Georgia 09B) growing in pots were sprayed until runoff (2 ml/plant) with a solution containing approximately 1 × 105 hyphal fragments/ml. Five inoculated plants were placed in a humidity chamber within a greenhouse where temperatures ranged from 28 to 33°C. After 14 days, water soaked, gray to green or light brown lesions developed on all inoculated plants along with hyphal strands along inoculated sections of the peanut with dark brown sclerotia. None of the plants inoculated with sterile water expressed symptoms. Rhizoctonia solani was consistently reisolated from symptomatic tissue plated on PDA. Inoculations were repeated on peanut cv. Flavor Runner 458, Florida 07, FloRun 107, and Red River Runner with similar results. Although R. solani AG1-IA is a common pathogen on rice and soybean, causing sheath blight and aerial blight, respectively, to our knowledge this is the first report of aerial blight of peanut in the region. Currently, there is a renewed interest in peanut production in the state. Production practices include furrow irrigation, which can distribute floating sclerotia to peanut vines and the rotation practiced with soybean and, less frequently, rice, could potentially increase inoculum for the subsequent crop. Thus, this may be a significant disease problem in the region or Mid-South where peanut is planted after rice or soybean and furrow irrigated.
References: (1) S. Kuninaga et al. Curr. Genet. 32:237, 1997. (2) G. C. MacNish et al. Phytopathology 83:922, 1993. (3) H. A. Melouk and P. A. Backman. Management of soilborne fungal pathogens. Pages 75-85 in: Peanut Health Management. H. A. Melouk and F. M. Stokes, eds. The American Phytopathological Society, St. Paul, MN, 1995.