Oidium mangiferae Berthet (a fungus), causing powdery mildew of mango, is widely distributed throughout the Mango cultivated areas. It infects panicles, fruits, and leaves. Mango is the only known host of the mango powdery mildew pathogen (i.e., only mango can be infected by the fungus).
Symptoms on panicles
Infected panicles (flowers, flower stalks, and young fruits) become coated with the whitish powdery growth of the pathogen . Infected flowers and fruits eventually turn brown and dry. The dead flowers can easily crumble in one’s hand. Infection often causes flowers and small fruits to abort and fall off. Fruits that become infected after they have set have purple-brown blotchy lesions that crack and form corky tissue as the fruitlet enlarges. The full-bloom stage is the most susceptible to infection.
Symptoms on leaves
On some cultivars, new flushes of growth and younger leaves are highly susceptible and may curl up and become distorted. Older leaves are more resistant to infection. Grayish, necrotic lesions or large, irregularly shaped spots may form on leaves. On very susceptible cultivars, the youngest leaves may become completely covered with fungal spores and mycelium, and eventually die . On some cultivars, the whitish residue of the fungus tends to appear on the lower leaf surface, along the leaf midrib.
Conidia of O. mangiferae are wind-disseminated from other mango trees or from within an infected tree’s canopy. The environmental conditions for spread of powdery mildew occur across a broad daily range of temperature ( 10–31°C) and relative humidity (60–90%). These conditions are common in Middle East which explains why powdery mildew is such a widespread and entrenched problem. For infection (the penetration of host tissues by germ tubes of the pathogen) the optimum condition is approximately nbsp;23°C plus high relative humidity followed by an abrupt reduction of relative humidity. This can occur in Middle East where cool nights are followed by warm days. Airborne conidia of O. mangiferae land upon susceptible host organs, the spores produce germ tubes, and the germ tubes infect the tissues. The fungal mycelium of the pathogen ramifies within and upon host tissues, eventually killing host cells and absorbing their contents. The fungus produces spores abundantly all over the surface of affected tissues (it is the abundant fungal mycelia and the conidia and condiophores on the surface of host tissues that creates the typical “powdery mildew” appearance of mildewed plants). O. mangiferae survives on infected plants and for a period of time as conidia on fallen mango plant debris. Populations of O. mangiferae can build up rapidly during seasonal flushes of new mango leaves and flowers. The conidia can be transported long distances by winds.