Correlation of early leafspot disease in peanut with a weather- dependent infection index

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Virginia Polytechnic Institute and State University


Development of early leafspot, caused by Cercospora arachidicola Hori, was monitored on' Florigiant' peanut (Arachis hypogaea L.) at two field sites in Suffolk, Virginia. In one study, plants in 27-cm-diameter plots were inoculated with 20,000 conidia and inoculation dates were replicated in five randomized complete blocks. At location one in 1985 and 1986, lesions/leaf at two weeks after inoculation correlated significantly (P ≤ 0.05) with infection indices (IND) developed by the Virginia leafs pot advisory and hours of relative humidity (RH) ≥ 95%. At location two, correlations between lesions/leaf and IND as well as hours of RH ≥ 95% were significant in 1986, but not in 1985. Certain site specific factors were believed to have altered plant susceptibility to leafspot at this site in 1985. In another study, pots with greenhouse-grown peanut were placed between unsprayed rows of field plants, heavily colonized by C. arachidicola. Plants were removed after 3, 5, and 7 days of field exposure for six consecutive weeks in 1986 and returned to the greenhouse. Lesions/leaf at two weeks after initial exposure were correlated with IND values computed by five versions of the leaf spot advisory. Significant correlations were found between lesions/leaf on plants with field exposures of 5 and 7 days and cumulative IND values and hours of RH ≥ 90% and 95%. The low incidence of lesions resulting with field exposures of only 3 days coupled with a lack of significant correlations between disease and cumulative IND values for 3 days after inoculation in both studies suggests that infection processes require several days, and that fungicides may be applied to achieve disease control during this time.