Potential sources for initial inoculum and dispersal pattern of cashew powdry mildew disease

Abstract

Investigations to identify major sources for initial inoculum of powdery mildew disease caused by the fungus Oidium anacardii were carried out in southern Tanzania. Sexual stages of O. anacardii were not observed. Surveys conducted in Mtwara, Newala, Nachingwea and Tunduru districts indicated that active infections occuring on immature shoots and flowers produced within tree canopies were the major sources for cashew powdery mildew inoculum. However, numbers of immature shoots varied between districts with more in Mtwara and Newala than Nachingwea and Tunduru. Dispersal gradients of O. anacardii showed that conidial concentrations decreased with increasing distance from a single tree and isolated block of cashew trees. Approximately 6.3%.of the initial conidial concentration was recovered 16 metres away froml a single tree source. Even from a block of infected cqshew trees, about 3.8% of initial conidial concentrations were trapped at a distance of 180 metres from the source. This shows that sanitary measures alone, can not be effective in managing the disease in southern Tanzania. At sites in Nachingwea and Tunduru districts; however, where initial inoculum appeared to be generally lower than other areas, sanitary practices during non-flowering season could delay the onset of cashew powdery mildew epidemics. With proper scouting on mildew development to determine the timing of first round of fungicide application, it may be possible to reduce the nuinber of applications from the recommended six rounds of sulphur to three or four at sites in these districts. Further studies to determine the impact of off-season immature shoots and flowers on the build-up of initial inoculum at different locations are necessary.

Keywords: Oidium anacardii, initial inoculum, conidia, cleistothecia, sanitation

Tanzania J. Agric. Sc. (1999) Vol. 2 No. 1, 63-70