Powdery mildew is a disease of young tissue and only grows on green parts of the vine. However, not all stages of development of the powdery mildew fungus are found on green material.
Sexual and asexual structures
Asexual spores are called conidiospores. These form on specialised hyphae on the surface of the tissue. The hyphae grow vertically from the plant surface and bear chains of conidiospores, also called conidia (refer to Figure 3). The conidia produced during this process are clones of one another.
Sexual spores are called ascospores. These are produced from sexual fruiting bodies called chasmothecia (previously cleistothecia) that develop when the hyphae of two compatible isolates make contact. Chasmothecia are 0.1mm in diameter, just visible with the naked eye and form mid to late summer on leaves, shoots and bunches.
They are white when young and change to yellow, orange, brown then black as they mature (Figure 10). Chasmothecia turn yellow after about seven days from initiation, begin to form the bristle like appendages after three weeks and after 4-5 weeks are disconnected from the surviving colony. Sexual variation in the ascospores may lead to strains of the powdery mildew fungus that are more resistant to fungicides or that are more virulent.
The fungus survives the winter months in two ways:
- Infected buds – the fungus grows down between the bud scales on infected shoots in early spring and remains in the buds through the winter. It remains dormant, with the bud scales providing protection from the elements, particularly in regions such as WA that experience mild winters compared to other growing regions in the northern hemisphere.
- Chasmothecia – mature chasmothecia are washed into bark crevices and other sheltered places such as leaf litter and remain over the winter. Chasmothecia located in the bark have a higher rate of survival than those in the soil or leaf litter.
Infection and spread
Powdery mildew primary infections occur by either flag shoots or chasmothecia.
Infected buds produce shoots called flag shoots (described earlier under ‘symptoms’) in spring. Depending on how much powdery mildew was present early the previous season to cause infected buds there will generally be one flag shoot per 1000 shoots. Flag shoots produce conidiospores that spread the disease early in the season and are thought to be the main source of carryover in Australia.
Chasmothecia produce ascospores after a minimum of 2.5mm of rain and when temperatures are 10-30°C. This occurs mostly between budburst and flowering (late winter and early spring). Ascospores infect the lowest leaves and shoots as these are closest to the over wintering chasmothecia, as a result it is usually the underside of these leaves that are infected first. The ascospores germinate and produce powdery mildew colonies that then produce conidiospores.
Conidiospores are spread by wind dispersal. Conidiospores landing on the green parts of the vine germinate and infect the vine by sending haustoria (root-like appendages) into the epidermal (surface) cells. The fungus absorbs nutrients from the grapevine for its development. The absorption of nutrients by the fungus eventually leads to death of the infected tissue.
Both conidiospores and ascospores can infect the vine within 24 hours of dispersal. Germination, infection and development of ascospores to conidiospores and of conidiospores take around 5-12 days depending on temperatures. Several infection cycles can occur through the growing season and the incidence of infection increases rapidly if controls are not applied or are ineffective after infection.
Powdery mildew is favoured by:
- mild cloudy weather;
- low to moderate light such as sheltered parts of the canopy or vineyard;
- optimum temperatures 22-28°C with a range of 6-33°C; and
- humid conditions (enhances sporulation).
Powdery mildew is reduced on exposed leaf surfaces by:
- air temperatures of 35°C or higher; and
- direct sunlight (UV radiation reduces colony expansion and germination of conidia).
Unlike most other grapevine diseases, powdery mildew does not require free moisture for infection (except for the production of ascospores from chasmothecia as discussed above). Free water from rain, dew, irrigation or high volume spraying can cause poor or abnormal germination of conidiospores or wash them from the vine surface. However, established colonies repel water and those that are sheltered by the vine canopy will probably survive. Water on vines may also reduce canopy temperature and increase humidity, thus encouraging sporulation and more infection.