Downy mildew of grapevines

Page last updated: Tuesday, 4 May 2021 - 11:52am

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Disease cycle

Downy mildew is an obligate parasite (meaning it requires a living host) and therefore it grows on all green parts of the vine. However, there is one overwintering stage of downy mildew development that is not found on green tissue.


Oospores (resting bodies) are formed in late summer or autumn from the mycelium within leaves, shoots or berries (Figure 16). These resting bodies fall to the ground when leaves and bunch parts fall in autumn. There they overwinter in infected leaves and litter in the soil for 3-5 years (possibly up to 10 years).

Oospores are the sexual structures of downy mildew. An oospore has a thick wall that makes it less susceptible to fungicides and adverse weather conditions, such as exposure to the sun, than zoospores or sporangia (see below) (Figure 17).

Oospores in a leaf
Figure 16 Downy mildew oospores develop within grape leaves in Autumn. The leaf veins highlight the size of the oospores.
Downy mildew oospores
Figure 17 Close up of downy mildew oospores showing the double wall, making oospores resistant to fungicides and the weather.

Primary infection (soil to vine)

The 10:10:24 ‘rule of thumb’ refers to the conditions required for primary (first) infection to occur. At least 10mm rainfall (and irrigation) is required while the temperature is 10°C or more over a 24-hour period. Not all 10:10:24 conditions are suitable for a primary infection but this 'rule of thumb' provides a guide to monitor for favourable primary infection conditions when no other options are available.

More specifically the conditions required for oospores to germinate are:

  • soil needs to be wet for at least 16 hours;
  • usually achieved by 3-5mm rainfall (and/or irrigation); and
  • temperature also needs to remain above 10°C.

The germinated oospores then release zoospores (that swim in free water) which then need to be splashed by rain or irrigation to the vine canopy before the end of the 24 hour period. This process usually requires another 3-5mm of rain (and/or irrigation) to ensure sufficient splash and leaf wetness for infection on the underside of the leaves. For this, the foliage must remain wet for at least 2-3 hours at 20°C (or 4-5 hours at 10°C) for the spores to infect the leaf and complete the primary infection cycle.

Soil moisture levels are important for the germination of oospores. A combination of rainfall and irrigation together can be sufficient for germination to occur.

Oil spots

The zoospores released during primary infection that establish on the underside of the leaf begin to grow hyphae. These hyphae grow inside the leaves to form oilspots that appear 5-17 days (but more often 5-10 days) after infection has occurred. The development of oilspots is quickest in warm weather (18-27°C). At warmer or cooler temperatures the incubation period is longer.

Primary infection levels are usually low with only 1-3 oilspots developing per 50m of vine row per germination event. Hence, primary infections are very difficult to find if only a few germination events have occurred at the beginning of the season.

Secondary infection (leaf to: leaf, shoot, inflorescence, berries, stalk)

Active oilspots need to be present before secondary infections can occur. These oilspots (and surfaces of other diseased tissue) produce sporangia (seen as white down) on suitable warm wet nights. Oilspots are known to survive the high summer temperatures in WA. During these periods the oil spot may remain inactive, by not producing sporangia, but when conditions become suitable again sporulation will resume.

Sporulation requires at least four hours of darkness to develop, during which time the temperature is 13°C or more and humidity is 98% or more.

To then cause infection the foliage must also be wet for at least 2-3 hours once sporulation has occurred. The wet foliage can be the result of rainfall, overhead irrigation or occasionally from heavy dew.

Secondary infections can occur any time during the growing season whenever oil spots are present and conditions are favourable.

Sporangia are the asexual structures of downy mildew. Sporangia are produced on tiny treelike structures known as sporangiophores. A single sporangium can in turn produce between 1-10 zoospores. Zoospores are able to move (swim) through water but are spread mainly by wind and rainsplash.


Primary infections begin the disease cycle by providing a source of oil spots. Overseas experience has shown that primary infection events occur throughout the growing season and are important for disease development and spread.

Secondary infections produce spores that can be spread by wind and rain to establish new infection sites within the local vicinity of the primary oil spot. Secondary infections can drive the disease to epidemic levels rapidly if conditions are favourable.