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Onion Downy Mildew P. destructor

Downy mildew (Peronospora destructor) attacks onions in many parts of the world. It can become severe on leaves of commercially grown onion plants and on leaves and seed stalks of onions grown for seed, especially when relatively cool, moist weather prevails. Other reported hosts are shallot, leek, garlic, and chive.

Symptoms and economic impact

Downy mildew is characterized by pale green, yellowish to brownish areas of oval to cylindrical size and shape on infected leaves or seed stalks. These areas may consist of alternating yellow and green layers of tissue. The causal organism of Downy Mildew produces fruiting bodies and spores called sporangia on the surface of the leaves and seed stalks. The masses of spores are at first transparent to greyish, and then rapidly become violet in colour. Leaves become girdled in the region where mildew develops and the leaves collapse. This results in dead leaf tips that usually can be seen within defined regions in a field. The dead leaf tissue is rapidly colonized by purple blotch, which is dark in colour. Downy Mildew seldom kills onion plants, but bulb growth may be reduced. Bulb tissue, especially the neck, may become spongy and the bulb may lack keeping quality.

Biology of Peronospora destructor

Dormant Period: It is believed that the Downy Mildew fungus overwinters primarily as mycelium in infected onions that remain in onion fields or in nearby cull piles. The pathogen also can overwinter in perennial varieties of onion in home gardens.

Infection: Spores are formed on systemically infected material form the last year or on mature lesions from the actual season at night when high humidity and temperatures of 4–25 C occur, with an optimal temperature of 13°C. The spores mature early in the morning and are disseminated during the day. Spores remain viable for about 4 days. Germination occurs in free water from 1–28 °C with an optimal range of 7–16 °C. Rain is not needed for infection if heavy dews occur continuously during the night and morning hours.

Incubation Period: The mycelium of Downy Mildew in leaves of infected onion plants in commercial bulb production fields produces a new crop of spores called conidia in cycles of approximately 11–15 days.

Milioncast - Peronospora destructor Model:

Sporulation: The greatest number of sporangia was produced at 100% relative humidity (RH), and sporulation decreased to almost null when humidity decreased to 93% Rh (modeling: about 4800°minutes needed for 100% sporulation; incerease of infection: average degree /hour * 60/ degreeminutes needed).

Infection: Downy mildew spores require temperatures between 6 to 32°C and anywhere between 4 to10 hours of leaf wetness to germinate. Spores survive 1- 3 days after sporulation, free water has to be available.

Incubation Period: The mycelium of Downy Mildew in leaves of infected onion plants in commercial bulb production fields produces a new crop of spores called conidia in cycles of approximately 11–15 days.

In Fieldclimate.com:

1.) Sporangia are producted during darkness and  calculated if leaf wetness >0 and temperature between 6 and 24°C (see graph)
2) If Sporangia are there and additional precipitation the infection starts to be calculated (also between a temperature regime of 6-24°C).
3) If infection is 100% we had optimal conditions for the fungal pathogen to infect the plant and so the incubation starts to calculate the time between infection and first symptoms will be seen in the field.