Mycologia, MASTIGOMYCOTINA, PERONOSPORALES, PERONOSPORACEAE .
Identification, Description, Biology, Epidemiology, Treatement, Possible misleading, Images
Early in their life cycle plants are stunted, leaves show discoloration and lesions.
Elongated, slightly yellowish lesions occur on the leaves and the flowers which are covered by a pale purplish down.
The affected parts become distorted, wilt and, on windy days, may fall to the ground.
The fungus survives on onion debris in the soil, as oospores.
Oospores germinate in spring and produce sporangia which release zoospores ; these are the source of the primary infections.
Mycelium can also survive in the bulbs and extend to the leaves in spring.
The attacked plants form sporangia on the lesions.
The sporangia can attack other plants.
In autumn, oospores form in the infected tissues.
Mildew first occurs as foci in the field. The outbreak is favoured by rainy weather (fog) and temperatures between 4 and 25 °C. (11 - 13 °C. being the optimum for sporulation, sporangial germination and host penetration).
Over 3 years rotation is recommended.
Avoid tight sowing.
Avoid excessive use of nitrogenous fertilizers.
Destroy infected debris.
If a preventive fungicidal treatment is recommended locally, practice alternating sprays of different products in order to avoid formation of resistant strains.
HYP3 on line : Species (scientific name), Diseases (common names), Glossary, Crops.