Variation in severity of cashew powdery mildew (Oidium anacardii Noack) disease in Tanzania: Implications for research and extension

Abstract

Cashew trees in farmers’ fields in two contrasting areas in southern Tanzania were surveyed for growth, powdery mildew development, amounts and applications of sulphur, pest damage and nut yield by village‐based extension workers in 1993. There was a wide range of powdery mildew levels on cashew flowers, apples and nuts at both the Tunduru (inland) and Newala (western Makonde plateau). In both Newala and Tunduru application of sulphur dust was associated with reduced severity of blossom powdery mildew. Sulphur usage commenced later in Newala than Tunduru though this was associated with earlier onset of blossoming in the latter district. Farmers in both areas applied similar numbers of rounds of sulphur, however in Newala more sulphur was applied per tree reflecting the bigger trees found in this district. There was a highly significant (P < 0.001) negative association between use of sulphur and severity of blossom powdery mildew irrespective of nut yield. There was a strong positive association (P < 0.01) between use of sulphur and more than one nut per panicle from harvests in October, no significant association in November and a significant negative association (P < 0.05) between use of sulphur and more than one nut per panicle in December. The survey highlighted areas in Newala where there were higher powdery mildew levels, through either lack of sulphur application or poor control by late application of sulphur, with relatively high numbers of nuts per panicle. Conversely, areas were identified where despite control of cashew powdery mildew severity by application of sulphur nut yield remained relatively low. The results suggest that final yield results from an interaction of factors of which cashew powdery mildew is only one. There was a highly significant (P < 0.01) association between the number of sulphur rounds and severity of cashew powdery mildew (Newala data only) but no significant association between number of rounds of sulphur and yield taken during October. Pest damage estimates from cashew nuts showed no consistent pattern in either region. Survey results suggest that there is considerable potential in the optimization of sulphur usage as a strategy for control of powdery mildew.