Management of common bean (phaseolus vulgaris) angular leaf spot (pseudocercospora griseola) using cultural practices and development of disease-weather models for prediction of the disease and host characteristics

Abstract

Bean seed collected from 82 households growing beans in Chinyika Resettlement Area indicated that sugar (sweet) bean type was the most preferred and cultivated by 84.1 % of farmers. The most common seed source was home-saved seed as indicated by 73.2 % of the farmers. Common seedborne fungi isolated from the different seed lots using the blotter method included; Fusarium oxysporum (73.2 %), Alternaria alternata (70.7 %) and Colletotrichum lindemuthianum (51.2 %). The major sources of inoculum identified for Pseudocercospora griseola were; infected seed, concomitant infected plants in the field, the soil at the end of the growing period, the air, rainfall and irrigation water. The north and south facing slides on the local trap were the most efficient in conidia trapping and the north conformed to the main wind direction which was north-north-east. A field study conducted on the control of common bean angular leaf spot (Pseudocercospora griseola) during two years (2002/3 and 2003/4) indicated that, as a consequence of unfavourable weather conditions for disease development, winter and early planting in summer associated with any irrigation method can be adopted. To target high yield, early planting can be complemented with any irrigation method, whereas the winter crop worked best with sprinkler irrigation. However, the high amounts of water received and high humidity conditions that prevail during maturity when beans are planted early might interfere negatively with harvesting operations and seed quality. This situation will require harvesting at physiological maturity, and putting in place facilities for drying plants/pods and seed after processing. The variables estimated at the start of the disease and leaf disease variables from 4 to 8 weeks after planting (WAP) [incidence, severity and defoliation], pod disease variables [incidence and severity], and weather variables from 4 to 10 WAP [duration of humidity and temperature, and daily mean humidity) were positively correlated with the main disease variables and negatively correlated with yield. Days to first disease infection and daily mean temperature were correlated negatively with the main disease variables and positively with yield. Duration of water and daily mean water were the most variable across evaluation periods, and correlations were specific to evaluation periods. For each one of the dependent variables, one most appropriate equation was developed and the number of predictors was reduced to 7 - 10. The most important predictors included in the equations developed; disease incidence and severity, mean daily humidity and water, duration of humidity and water, were associated with specific evaluation periods. Consequently, decisions can be made early enough to permit disease control measures and warn farmers. As soon as the conditions of water received and humidity favourable to the disease are recorded, farmers should be warned and control measures should be applied when necessary. The critical periods for disease infection were 4, 6 and 10 WAP