Pod rot is a problem on peanut in West Texas and many other areas. Crop consultants scouting fields assume that pod rot will be more severe in high moisture areas of the field, and therefore bias the intensity of their sampling to those areas. The objective of this research was to evaluate the relationship between soil moisture and pod rot incidence in irrigated fields. Studies were conducted in four fields located in Yoakum County, TX in 2002 and 2003. Surface soil moisture (0 to 12 cm in depth) was measured at 5 min and 24 hr after the center pivot irrigation system passed from each sampling point (20 to 34 points/field). Three plant samples were collected at each point in August and October, and were evaluated for pod rot incidence. In one field where Pythium spp., but not Rhizoctonia spp. was isolated from rotted pods, pod rot incidence in August was negatively correlated with soil moisture at 5 min (r 5 20.41, P 5 0.04, n 5 25) and 24 hr (r 5 20.43, P 5 0.03, n 5 25). No correlations between disease and soil moisture in the other three fields was found. Results from these studies suggest that pod rot associated with Pythium spp., or Pythium spp. and Rhizoctonia spp., is not greater in wetter areas of irrigated peanut fields. A comprehensive approach to sampling, irrespective of soil moisture content, is essential for accurate estimation of pod rot.

Wheat (Triticum aestivum L.) is an important crop in Oklahoma and throughout the Central Plains of the United States. The soilborne fungus, Sclerotium rolfsii, is a major pathogen on peanut (Arachis hypogaea L.) but is not known to cause major damage on wheat. During September of 1998, damping-off and rotting of young wheat seedlings were observed in breeder plots in Payne County, OK. The occurrence of symptoms was sporadic with an estimated stand reduction of 10 to 15%. Symptomatic plants were collected from the field and brought to the laboratory. Sclerotia-like bodies from the symptomatic plants were surface disinfested in aqueous 1% NaOCl for 2 min and allowed to germinate at 25 ± 2°C on sterile filter paper moistened with a 1% aqueous solution of methanol. Aerial mycelia from germinating sclerotia were transferred to potato dextrose agar amended with 100 ppm of streptomycin (SPDA) to obtain pure cultures. Pure cultures had coarse, white mycelium distinctive of S. rolfsii and produced very small (0.05 to 0.1 mm), abundant, round, brown sclerotia on the surface of the medium after 15 days of incubation. Pathogenicity was tested on three hard red winter wheat cultivars commonly grown in Oklahoma (Jagger, 2137, and 2174). Four plants of each cultivar were inoculated at the two-leaf stage (Feekes' scale stage 1) by placing a 0.5-cm agar disk removed from a 3-day-old culture onto a 1-cm diameter filter paper that was then pressed to the base of the shoot. Noninoculated plants were used as a control. After inoculation, pots were covered with polyethylene sheets to maintain 95 to 100% relative humidity and incubated at 25 ± 2°C in the greenhouse. Lesions were initially superficial, yellowish, and water soaked. Lesions expanded and resulted in damping-off of seedlings. Noninoculated plants were free of disease and remained healthy. No significant difference (P ≤ 0.05) in disease severity was observed among the cultivars. To fulfill Koch's postulates, the fungus was reisolated onto SPDA where it had the same characteristics as the initial culture. To our knowledge, this is the first report of S. rolfsii on wheat in Oklahoma. Even though S. rolfsii is not expected to pose a significant risk to wheat production, infection of wheat may enhance survival of S. rolfsii and facilitate infection and losses in a following peanut crop. This is especially important in certain areas of Oklahoma where a wheat-peanut rotation is occasionally practiced.

Field studies from 2002 through 2004 examined effects of irrigation rate and fungicide on peanut kernel damage, yield and net return in Gaines County, TX. Irrigation rates, the main plots of a split-plot design, were applied with a center pivot system planted in concentric circles, and represented a base irrigation rate (B)(ca. 75% of evapotranspiration), B + 33%, and B 2 33%. Azoxystrobin or metalaxyl were applied to the subplots at 60 and 90 days after planting and there was a no-fungicide control. Irrigation rate had no effect on percent damaged kernels from harvested pods. Plots with B + 33% irrigation rate had higher pod yields (P , 0.05) than B or B 2 33% irrigation rates in 2002 and 2003, but had similar yields to the B rate in 2004, a relatively wet growing season. Irrigation rate did not result in an overall difference in net return in any year. Plots treated with azoxystrobin had higher peanut grades and lower percent damaged kernels in all 3 yr than nofungicide and metalaxyl treated plots. Azoxystrobin treated plots had higher yield and net return than metalaxyl treated and no-fungicide treated plots in 2 of 3 yr. Rhizoctonia spp. was the primary fungal pathogen isolated from peanut pods. Use of azoxystrobin resulted in an average net return of $170/ha. Azoxystrobin effects on yield were additive to water status across the entire spectrum of irrigation rates during the combined three years. Peanut producers from this region who want to maximize yield by increasing irrigation rate should use azoxystrobin if there is any history of Rhizoctonia pod rot.