Epidemiology

Oospores – sexual structure, resting bodies that form in late summer and overwinter in fallen leaves and prunings.Resistant to fungicide and harsh weather. May also overwinter as mycelium in buds.Oospores germinate in favourable conditions to release Zoospores.

10 10 24
10mm rain/irrigation 10°C min temp over a 24hr period

Zoospores are able to swim in water and must be splashed into canopy within 24 hours. 3-5mm rain sufficient to achieve this. Leaves must remain wet for 2-3 hours at 20°C or 4-5 hours at 10°C to complete infection process. Primary infection - Zoospores enter through stomata and grow fungal hyphae inside leaves creating oil spots, visible in 5-20 days. Low levels of primary infection 1-3 oil spots per 50m of canopy making detection difficult. Secondary infection - Leaf to leaf; berry, shoot, inflorescence, etc only once oil spots are present with sporulation occurring under suitable conditions.

98% humidity 13°C+ Temperature 4 hours of darkness Wet leaves 2-3 hours near dawn

The resulting Sporangia (conidia) are the asexual structures of downy mildew produced on tree like Sporangiophores. Sporangium germination must be in water and is temperature dependant.

Temperature °C 6 8 13 18 23
Leaf wetness, hours 17 14 1.5 1 0.5

The zoospores produced survive longer at lower temperatures, are able to move through water but are primarily spread by wind and rain splash. Outbreaks are most frequent in years with wet springs and warm summers with frequent rain events.