Downy mildews

downy mildew

1. Downy Mildews

2. CONTENTS Introduction Symptoms Pathogen Causal organism Disease cycle Development of Disease Disease Control

3. What is downy mildew ? Downy mildews are primarily foliage blights . They attack and spread rapidly in young, tender green leaf, twig, and fruit tissues. They develop and are severe when a film of water is present on the plant tissues and the relative humidity in the air is high during cool or warm, but not, periods. Downy mildews can severe losses in short periods of time.

4. DOWNY MILDEWS OF GRAPES Downy mildew of grape occurs in most parts of the world where grapes are grown. Although the pathogen is native to north America, where it attacks native grape vines, it does not affect them very seriously. Downy mildew affects the leaves, fruit, and shoots of grapevines.

5. It causes losses through killing of leaf tissues and defoliation, through production of low – quality , unsightly , or entirely destroyed grapes ,and through weakening, dwarfing, and killing of young shoots. When the whether is favorable and no protection against the disease is provided, downy mildew can easily destroy 50 to 75% of the crop in one season.

6. Symptoms: At first, small, pale yellow, irregular spots appear on the upper surface of the leaves, and a white downy growth of the sporangiophores of the oomycete appears on the underside of the spots. Later , the infected leaf areas are killed and turn brown , while the sporangiophores of the oomycete turn gray. The spots often enlarge , coalesce to from large dead areas on the leaf frequently result in premature defoliation .

7. Fig. Fig :- Downy mildew symptoms on leaves of cantaloupe

8. Pathogen:- Plasmopara viticola. The mycelium diameter varies from 1to60 micrometers because the hyphae take the shape of the intercellular spaces of the infected tissues. Globose haustoria grow into the cells.

9. Causal organism The causal fungus overwinters in soil as M icrosclerotia which are clusters of thick walled cells that collectively are no larger then finally ground paper. Microsclerotia are produced abundantly In cortical tissues and rhizobium nodules of infected peanut roots. As infected tissues decompose M icrosclerotia are released in to soil. Moment of this structure in plant D ebreis and soil represent the primary avenue for disease spread.

10. Disease cycle :- Figure:

11. Development of disease : The pathogen overwinters as oospores in dead leaf lesions and shoots and , in certain areas, as mycelium in infected, but not killed, twigs. During rainy periods in the spring the oospores germinate to produce a sporangium.

12. The sporangium or its zoospores are transported by wind or water to the wet leaves near the ground, which they infect through stomata of the lower surface. Leaf hairs provide a basic protection barrier against the downy mildew pathogen , but in varieties lacking additional or different defense strategies it is overcome.

13. Fig. Fig :- Development of disease .

14. disease Control :- Serveral American grape varieties show considerable resistance to downy mildew, but most E uropean varieties are quite susceptible. Even the relatively resistant varieties, however , require protection through chemicals.

15. The most effective fungicides for the control of downy mildew have been copper-based products such as the bordeaux mixture, some broad- spectrum protective fungicides, and several systemic fungisides. The application begin before bloom and are continued at 7 - to 10 - day intervals or, depending on the frequency and duration of rainfall, during the growing season.

16. Disease prediction systems, based on the duration of leaf wetness, relative humidity ,and temperature, are used to identify infection periods and to time fungicide applications. In recent years, sprays of systemic fungicides in combination with copper or board- spectrum preventive fungicides have given excellent control of grape downy mildew.

17. Reference:- PLANT PHATHOLOGY by George N. A grios . Google.com.in Downy mildew symptoms on leaves of cantaloupe in image. Disease cycle in image.com. Development of disease in image. Com. in

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