A weather-based scheme to advise on limited chemical control of groundnut leaf spot diseases in India

Infection of groundnut by pathogens causing early and late leaf spot diseas es is strongly affected by accumulated daily leaf wetness periods and, in t he rainy season, temperature is unlikely to severely limit infection. Earli er work relating patterns of leaf wetness to infection, was used to define a daily Wetness Index (WI) which was compared with infection on inoculated plants exposed in the crop for periods of 7 d. Infection was only severe wh en the 7-d WI total exceeded a value of 2.3. The proportion of leaves with one or more lesions on the main stem was used to assess the amount of inocu lum in the crop. When the proportion of diseased leaves exceeded 10% and th e WI total exceeded the threshold, application of a fungicide was advised. Successive sprays were separated by at least 14 d and a maximum of three sp rays were applied in the growing season. Field trials showed that three spr ays gave limited benefit where the disease pressure was severe, but substan tial increases in pod and haulm yield were possible with only one or two fu ngicide applications in locations with less disease pressure.