For a successful and economical integrated control program aimed at a particular disease, pertinent information, regarding the environmental conditions prevailing in the growing area, the crop itself and the pathogen, must be available. Recently, the control of powdery mildew disease on cashew has moved from the use of non-systemic fungicides with a wide range of action, to highly specific systemic ones. Such a shift requires a more effective integrated control system, whereby tolerant varieties in combination with fungicide unaffected biocontrol agents are timely used to ensure disease control and reduce the hazards associated with excessive fungicide applications. The purpose of this study was to understand the relationship between the disease epidemic and some climatic factors over time. Appropriate periods for management interventions were determined. The cellular host reaction to infection by Oidium anacardii Noack was studied with a view to rapidly identify disease tolerant host types. Potential antagonists were isolated, screened and compared with commercial biocontrol products using in vivo techniques and chemical control programs were finally evaluated. Electron microscopy elucidated that the powdery mildew tolerant cashew variety (H1) had a relatively higher consistency of cytoplasmic aggregates upon infection by O. anacardii when compared to the susceptible clone. Based on conidia and conidiophore morphology, conidial germination and conidiogenesis processes observed indicated that O. anacardii belongs to the subgenus Pseudoidium (Y.S. Paul&J.N. Kapoor) comb.Et. Stat. Nov. (Holomorph Erysiphe Sect. Erysiphe U. Braun). There was no direct relationship between the progress of the cashew powdery mildew epidemic and temperature, relative humidity or dew point over time. However, the epidemic did not start until conditions of average temperatures under the tree canopy were below 30°C, relative humidity was 80% and dew point was above 15. In vivo screening of 72 isolates, amongst them bacteria and fungi, from cashew leaves and florets showed that none were effective against O. anacardii, the causal agent of cashew powdery mildew. However, commercial antagonists, Candida saitoana, Bacillus subtilis and B. licheniformis, significantly reduced the growth and branching of primary hyphae. One antagonist, B. licheniformis, was as effective as the commercial fungicide triadimenol 25% EC (Bayfidan). Chemical fungicides were found to be effective against powdery mildew; however, the currently prevailing economic environment in Mozambique was found inappropriate for the use of expensive organic fungicides. Additional gain from the use of fungicides was found to be solely qualitative and thus did not represent a fair investment return ratio in terms of cashew nut prices and production costs. The use of integrated cashew management was finally recommended. Further studies should focus on development of integrated and cost effective disease management strategies.
Dissertation (MSc(Plant Pathology))--University of Pretoria, 2003.