Oidium mangiferae is a plant pathogen that infects mango trees causing powdery mildew. Powdery mildew of mango is an Ascomycete pathogen of the Erysiphales family that was initially described by Berthet in 1914, using samples collected from Brazil. O. mangiferae is found in all areas where mangoes have been raised long term, but is particularly widespread in India where both the host and the pathogen are native. Currently no teleomorph stage has been identified, but due to certain morphological characteristics it has been suggested that O. mangiferae belongs in the Erysiphe polygony group. Mango is the only known host for this pathogen, though O. mangiferae appears to be identical to fungi responsible for powdery mildew diseases on various other plant species, particularly oak, though some differences may be observed. In particular, the number of cells in conidiophores varies from 2 on mango to 3-5 on oak. O. mangiferae has been known to infect oak leaves in the laboratory, however due to the lack of a known teleomorph stage O. mangiferae is still considered to only be a pathogen of mango. Recent analysis of its ribosomal DNA suggests it is conspecific with Erysiphe alphitoides, the causative agent of powdery mildew in European oaks.
Host
Mangoes are a commercial fruit crop of the cashew family (Anacardiaceae) which are an important crop in many countries including India, China, Pakistan, Mexico and Thailand. There are many species of mango, but the only one raised for commercial fruit production is Mangifera indica, as the fruit it bears is the most appealing and edible. Originally cultivated in India beginning more than 5,000 years ago, mangoes currently represent up to 50% of international fruit production. Mangoes are susceptible to a large variety of pathogens, including root rot, anthracnose, die back, and leaf spot, among others. However, one of the most prevalent and damaging diseases of this crop is powdery mildew of mango, caused by Oidium mangiferae; crop losses between 20 and 90 percent have been reported from various regions due to powdery mildew infections. Most mango cultivars in use today were released from 1949–1967, with continuous cultivation since that time. Long term cultivation may affect disease resistance, and very little breeding for resistant varieties has been undertaken; due to this, almost all cultivars show susceptibility to O. mangiferae and it is extremely widespread. The most popular variety of mango, the Tommy Atkins, was developed in the 1920s in Florida and is favored because of its long shelf life and anthracnose resistance, though it is still somewhat susceptible to powdery mildew and control strategies must still be enacted. Diseases of perennial crops such as mango are devastating due to the long time period to maturity for the plants, as a tree grown from seed will not produce fruit until it has reached three to six years of age. In contrast, the mango tree may live and produce fruit for several hundred years if it remains healthy and is well cared for. In addition, perennial crops can allow for buildup of inoculum and widespread dispersal of pathogens such as powdery mildew because the host is present in all seasons and through multiple years.
Disease Oidium 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.
Control
As with most pathogens of plants, there are three main methods of preventing or controlling infection of mango by powdery mildew: exclusion, evasion, and eradication.
… excerpt ends here. Continue reading the full article.
