Oidium mangiferae attacks the young tissue of all parts of the inflorescence, leaves and fruit. Infection shows initially as small patches of white powdery mycelium, which may later coalesce to cover large areas. On older leaves and fruit, infected tissue has a purplish-brown cast as the white growth weathers away (Mossler & Crane, 2009).
Young infected leaves may become distorted. Grey necrotic lesions appear on the upper side of the leaf, and leaves tend to curl downwards (Verma and Sharma, 1999). Leaves may become brown and dry, and drop from the plant if disease is severe.
Flowers are most susceptible to infection once they have begun to open on the panicles (3-5 weeks after budbreak) (Lonsdale and Kotze, 1993). Flowers and stalks can become severely infected. Sepals are more susceptible than petals. Infected flowers may fail to open and may drop from the panicle. Often inflorescences become completely covered by the mildew and eventually blacken (Burchill, 1978), or become brown and dry. Severe blossom infection can result in complete loss of fruit (Akhtar and Alam, 2000).
As infected newly set fruit develop, the epidermis of the infected area cracks and corky tissue is formed (Joubert et al., 1993). The entire fruit may become covered by mildew. It is also reported that infected fruit may become yellow and misshapen (Verma and Sharma, 1999).
Initial infection with O. mangiferae is promoted by warm temperatures and moderate relative humidity, although development of the disease is favoured by cool, dry conditions (Gupta, 1989; Mossler and Crane, 2009).
The disease is spread by wind-borne conidia from other mango trees or from within an infected tree’s canopy (Nelson, 2008).
Oidium mangiferae survives from one season to the next as mycelium in dormant buds and as haustoria on old infected leaves (Akhtar & Alam, 2000).
Many countries have brought in quarantine procedures against a number of mango pathogens, including Oidium mangiferae. Measures include washing, fungicide application prior to export and hot water treatment.
Cultural control and sanitary measures
Fallen infected leaves should be picked up and destroyed, as conidia can survive for a period of time on fallen debris. Severely infected panicles should be removed. Pruning of mango trees to improve air circulation and removal of tall weeds are useful measures to help reduce the incidence of the disease in an orchard. Intercropping with other fruit trees or forestry species reduces the ease with which the disease can spread (Nelson, 2008).
Foliar applications of potassium dihydrogen phosphate, a potassium-phosphate fertilizer, have proved effective in suppressing the development of powdery mildew in South Africa, and can be combined with fungicide application to reduce the cost of disease control (Oosthuyse, 2000).
In field trials in India, incidence of powdery mildew decreased with increasing rates of NPK application (Cheema and Malhi, 1986).
A small number of potential biological control agents for Oidium mangiferae have been investigated, including pathogenic fungi and predatory mites. Ampelomyces quisqualis, which is capable of parasitizing O. mangiferae (Sztejnberg et al., 1989; Srivastava, 2000), is registered as a biopesticide.
The disease can be controlled effectively with proper use of fungicides. Spray applications needs to well-timed, with the first application by 50% flowering or when the first signs of disease are observed (e.g. Nelson, 2008; Joubert et al., 1993); poor timing of treatments can result in unnecessary crop losses. Three spray applications are often used.
Sprays of wettable sulphur or sulphur dusts provide reasonable control and are commonly used, but may cause phytotoxicity to flowers and young fruit if applied during sunny conditions (Nelson, 2008).
Predictive models have been developed to improve the management of O. mangiferae, based on the relationship between temperature and relative humidity and the onset of the disease (e.g. Sinha et al., 2001; Jain et al., 2009). When chemical control is employed, the use of predictive management can maximise its efficacy.
Numerous studies have been conducted to compare cultivar resistance to O. mangiferae. There are no totally resistant cultivars. Those most susceptible bear no fruit if infection is left untreated. Worldwide, the most susceptible cultivars are Zill and Kent (Joubert et al., 1993). Tommy Atkins is one of the more resistant cultivars, still producing fruit following infection, although yield is lower as the fruit tend to be smaller. The cultivars Rocha and Regina have also been evaluated as having good resistance (Galli et al., 2008).
In field trials in Egypt, biological control agents and a number of natural compounds (potassium dihydrogen phosphate, kaolin and ascorbic acid) were applied alone or in combination to mango trees infected with Oidium mangiferae (Nofal and Haggag, 2006). The greatest reduction in disease severity was obtained with mixtures of all 4 agents, suggesting that their application could be used in an integrated management system to control mango powdery mildew.
Oidium mangiferae is a serious disease of mango and can cause extremely high reductions in yield, mainly as a result of blossom infection (Joubert et al., 1993). Powdery mildew is widespread in India, where it causes significant crop losses; Prakash and Srivastava (1987) reported losses of 30-90% in Lucknow. In China the disease reduces yields by 20-30% (Li GuiZhen et al., 2003). Losses of 80-90% have been reported in South Africa (Brodrick, 1971; Kotze, 1985), while losses of up to 20% have been recorded in Florida in some seasons (Cook, 1975).
Oidium mangiferae is not reported to affect any other crops.