Disease
O. mangiferae appears in spring, from December through March, and primary inoculum comes either from wind borne conidia from other infection sites, or from surviving inoculum and mycelia on the tree. Powdery mildew is most prevalent in cool, dry years, and secondary infections also occur from wind dispersed conidia. The disease is highly polycyclic, with the pathogen life cycle from germination to conidiation taking only 9 days, and conidia infecting the plant tissues 5–7 hours after germination. Warm temperatures with cloud cover and heavy dew promote infection, though dry conditions favor disease development. The fungus overwinters in infected leaves and malformed inflorescences in the form of dormant mycelium, conidiophores or conidia. This fungus is ectophytic, existing primarily on the surface of plant tissues. It draws nutrients from the plant via haustoria that penetrate the epidermal layer. This pathogen is also biotrophic, surviving only on live plant tissues. As such, growth of the pathogen ceases when the tissue it occupies becomes necrotic. Conidia germinate between 9 and 30.5 degrees Celsius, with an optimal temperature of 22 degrees. Atmospheric temperatures of 10-31 degrees Celsius are the optimum for development of the disease on mango trees. Disease development is inversely proportional to the number of hours of sunshine per day, and moderate to high relative humidity also plays a role in the severity of infections. Powdery mildew infections show a characteristic white or gray powdery growth on various plant surfaces, including leaves, flower scales, buds, axils, stalks, and fruits. If the white outer growth is removed or disturbed, the disease appears as purple-brown blotches on older leaves and fruit. Infection may also cause distortion of young leaves, as the infection usually begins on the underside of the leaf. Necrotic lesions appear on the upper side of the leaf, and the leaves tend to curl downward. The disease is often of most concern and causes the most damage when it is present on flowers, inflorescences and fruit. When infections occur, Oidium mangiferae may cause premature shedding of flowers and fruit, often before fruits reach the size of peas or prior to flower fertilization. This causes a severe decline in fruit yield and crop commercial return. Blossoms are most susceptible to infection at the point that flowers are just beginning to open on the panicles. Infected fruits turn a yellow color and may become misshapen. The conidia produce germ tubes, which develop into hook-like appressoria that are used for cuticle penetration. Penetration pegs develop into tubular haustoria, which later swell and become globular. O. mangiferae produces septate, hyaline conidia that range from barrel shaped to elliptical. Germ tube length is dependent on relative humidity, and hyphal cells are 40-80 μm long.
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