|Crop Knowledge Master||Fungi|
|powdery mildew(Plant Disease Pathogen)|
Jeri J. Ooka
Extension Plant Pathologist
Department of Plant Pathology, Kaui Branch Office
University of Hawaii
Papayas are the only known host of this fungus.
Powdery mildew of papaya was initially described in 1898 from Brazil. It has since been recognized in many tropical and subtropical areas of the world including Hawaii, Australia, Bermuda, Florida, Taiwan, India, and New Zealand.
This common disease generally causes little damage or yield loss to bearing trees where lesions are usually found on senescent leaves. However, powdery mildew may severely damage young plants in environments with moderate rainfall and temperatures.
Diffuse mats of white mycelium commonly develop on the lower leaf surface, especially in areas adjacent to the leaf veins, but can occur occasionally on the upper surface of leaves. Initially, infected areas become light green and chlorotic and lesions may be surrounded by a dark green margin. Conidia on the mycelial mat gives it a distinct powdery appearance. Stems, flower pedicels, and fruits as well as leaves can become infected.
Although leaves of all ages are considered susceptible, infection is largely confined to the older leaves approaching senescence. Seedling plants are especially susceptible to attack and may be seriously affected. Defoliation, stem and fruit lesions on young plants may lead to measureable yield losses.
Oidium caricae does not have a saprophytic stage and can, therefore, grow and reproduce only on living papaya plants. Seedlings grown in greenhouses are especially susceptible, often killing the apical portions. Disease development appears to be enhanced by low light levels, high humidities, moderate temperatures (18 to 32ûC), and moderate rainfall (1500 to 2500 mm per year).
Mycelium of O. caricae is hyaline and septate. Haustoria develop in the host's epidermal cells to obtain nourishment for the pathogen. Conidia are hyaline, granular, measure 14-19 x 28-30 µm and are borne in chains of three to five or more. The conidial stage is the only form present in Hawaii and other tropical areas. The sexual stage has never been observed.
Conidia of O. caricae are dispersed by wind.
Although control measures are generally not needed applications of wettable sulfur, sulfur dust, or lime sulfur have proved helpful in controlling this disease. However, wettable sulfur may be toxic to the plant during hot weather and is not very effective during periods when the disease is severe.
Fungicides effective in controlling powdery mildew on other crops have generally been effective in controlling papaya powdery mildew. Benomyl, bupirimate, carbendazim, mancozeb, thiophanate-methyl, and triadimefon have demonstrated efficacy in the field.
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