The disease powdery mildew, caused by the fungus Oidium mangiferae, is a serious problem in all mango-growing areas in Kenya. Infections can result in complete crop loss and defoliation of trees. The disease favours cool and cloudy weather but also occurs in warm and humid climatic conditions. It attacks leaves, buds, flowers and young fruits. Infected tissues are covered with a whitish, powdery growth of the fungus. Mature spores are easily blown away by wind and produce a fresh infection, or they may remain dormant alive but not growing; a resting stage during the unfavourable season awaiting optimum germination conditions in the next season. Spraying 3–5 times at 10–14 day intervals from the onset of flowering until fruit set can control untreated subjects used for comparison with those given a particular treatment powdery mildew. Several chemicals are recommended and have been used to control the disease. These include Benomyl, Pyrazophos, Triadimefon, Bupirimate, Triforine and sulphur, all mixed with a spreader/sticker material added to a spray to increase retention on plant foliage.
Cultivars treated for powdery mildew in this way show remarkable increases in fruit set ranging from about 40% to more than 500%. The input costs of the spray applied per tree are justified as they are recovered fully by increased returns.
Finally, as already mentioned, all mango cultivars are susceptible not resistant to powdery mildew infestation to some extent. The range of resistance (with Sensation being the most resistant) could be: Sensation, Chino, van Dyke, Tommy Atkins, Sabine, Kent, Keitt, Gesine, Batawi, Apple, Ngowe, Haden, Maya.