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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.

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Sclerotinia minor (Jagger) Kohn is serious and increasingly prevalent pathogen of peanut (Arachis hypogaea L.). Peanut stem tissues were reported to differ in their resistance to S. minor, but field performance is not always correlated with laboratory evaluations of resistance to Sclerotinia diseases in other crops. Differences in genotype performance in field and laboratory results may reflect differences in mechanisms of resistance. The objective of this study was to characterize mechanisms of resistance to S. minor in selected peanut genotypes by using agar culture tests, wounded and nonwounded stem inoculations, and field trials. For the culture test, sap was expressed from five genotypes with different levels of field‐resistance toS. minor. Each extract was incorporated into an agar medium, which was overlaid with a dialysis membrane. The fungus produced distinctive infection hyphae on the media. Genotype extracts differentially affected size of terminal and secondary hyphae and the number of hyphae per organized cluster. Nine genotypes were evaluated for resistance to S. minor in two stem inoculation tests. Inoculation sites were wounded in the first method, and were not wounded in the second method. Significant differences in lesion size were found with both methods, but more differences were found among genotypes in the nonwounded inoculation. Genotype performance in culture and stem inoculation tests was not correlated with performance in the field. These studies demonstrated that although some genotypes had resistance to stem colonization by S. minor, other mechanisms account for most of the resistance expressed in the field.

Twenty peanut (hachis hypogaea L.) populations in F, generation from an M x N mating design involving five late leafspot (Cercosporidium personaturn)-resistant female parents and four adapted male parents were evaluated for late leafspot resistance with a detached leaf culture technique. Agronomic traits were evaluated in the field. Objectives were 1) to identlfy the best parent for agronomic traits and the best source of resistance to late leafspot, 2) determine the correlations among components of resistance, 3) determine the correlations of resistance and agronomic traits, and 4) estimate heritability of late leafspot resistance. General combining ability was highly significant for agronomic traits and for most measurements of late leafspot resistance. Specific combining ability was significant for pod length and seed size. Of the male parents, NC 6 and NC 7 produced the best progenies for both agronomic traits and late leafspot resistance. Components of resistance to late leafspot among resistant female parents were not significantly different. NC 17090 produced the best progenies for pod yield and seed yield. NC 17135 produced progenies with good agronomic traits. Correlations among components of resistance to late leafspot indicated that lines with increased latent period, decreased lesion number, lesion size and defoliation, and reduced spore production can be selected. However, high yielding plants tended to be susceptible to late leafspot. Broadsense heritability for components of resistance was low to moderate (0.13-0.68). Narrow-sense heritability for parameters of 'Paper no.Pepnut science (1987) 14:M-W resistance was consistently low (0.0-0.128). Selection for late leafspot resistance in the F, populations was not effective.

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