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Occurrence of Pod Rot Pathogens in Peanuts Grown in North Carolina

Pod rot diseases historically caused significant losses in peanut production in North Carolina. Advances in the understanding of pod rot diseases and changes in cultural practices minimized losses in the years since 1979. By the early 1990s, however, some peanut growers began to observe pod rot that apparently was not associated with infection by common soilborne pathogens. Incidence of pod rot also was high in research plots used to study conservation tillage methods. Selected farms were surveyed in the fall of 1994, 1995, and 1996 to identify the fungi associated with pod rot symptoms in North Carolina. Over the three years of the study, more than 6,000 symptomatic pods from 125 peanut fields were assayed for Rhizoctonia spp., Pythium spp., Cylindrocladium parasiticum, Sclerotium rolfsii, and Sclerotinia minor. All five pathogens were isolated during the field survey, with Pythium spp. and Rhizoctonia spp. isolated most frequently. Rhizoctonia spp. were the dominant pathogen in the majority of fields in 1994, whereas Pythium spp. predominated in 1995 and 1996. Combinations of pathogens were identified from 12 to 15% of pods; Rhizoctonia spp. + Pythium spp. and Pythium spp. + C. parasiti-cum were the most frequent combinations. The mean estimated incidence of pod rot was 6.6% in 1995 and 5.9% in 1996. The effects of cover crops and tillage on pod rot incidence were studied in microplots in 1995 and 1996. In 1995, winter cover crops (wheat, oat, rye, and fallow soil) did not affect pod rot incidence, but incidence was greater in no-till treatments compared to plots with conventional tillage. Pod rot incidence did not differ among infestation treatments and no interactions among pathogen, cover crop, or tillage treatments were significant. In contrast, significant (P = 0.04) interactions among winter cover crops and tillage occurred in 1996. Tillage did not affect pod rot incidence following wheat or oats, but incidence following rye was much greater in no-till than in tilled plots.