Grape black rot attacks grape vines during hot and humid weather. The disease cycle begins with the overwintering structures. The grape black rot pathogen overwinters in many parts of the grape vine, especially mummies and lesions, and can overwinter on the vineyard floor. Spring rains release the ascospores and conidia contained in the overwintering structures and are spread by wind and rain to infect leaves, blossoms, and young fruit.

In the presence of moisture, ascospores germinate slowly, taking 36 to 48 hours, but eventually penetrate the young leaves and fruit stems. The infections become visible after 8 to 25 days. The second type of spore, conidia, are dispersed short distances by splashing rain drops throughout the summer. Infection occurs when either of the two spore types lands on green grape tissue and the tissue remains wet for a sufficient length of time, which is dependent on temperature.

Minimum temperature for infection is 8°C. At 10°C it takes 24 hours of leaf wetness for a spore to germinate but at temperatures between 15 – 32°C it takes less than 12 hours with an optimum around 22°C. A film of water on the vine surface is necessary for the infection to inoculate. This completes the disease cycle.

Botrytis cinerea is a fungus that affects many plant species, although its most notable hosts may be wine grapes. In viticulture, it is commonly known as ‘botrytis bunch rot’. The fungus kills the host plant cells and use the contents to support their own growth.

Botrytis hibernates as mycelium and fruiting bodies in dormant buds, under and in the racks of bark, on dead matter, on bunch stems, and on rotten berries. Sclerotia are hard black structures with a diameter of approximately 3 mm and constitute the most important structure involved in the survival of Botrytis. In spring, when conditions are favourable, millions of spores are formed by mycelium and sclerotium. These spores are produced throughout the growing season. Free water is required for at least 15 to 20 hours and may be in the form of rain, mist, dew, or irrigation. Spores can germinate at all temperatures between 1 and 30 °C, although temperatures between 15 and 20 °C are more favourable. The spores may infect young leaves, shoots, grape flowers, berry stems, stomata, and wounds, which in turn may cause further production of spores.

Downy mildew overwinters in leaf litter and the soil 3 to 5 years and possibly up to 10 years. In spring, spores germinate which under wet conditions are released and splashed and blown onto the new leaves and cause a primary infection. Irrigation practices are promoting epidemics. At least 10 mm rainfall (or irrigation) is required while the temperature is 10°C or more over a 24- hour period. The optimum temperature for disease development is 18 to 25°C.

Once inside the plant, the fungus grows and spreads through tissues. The development of is quickest in warm weather. At warmer or cooler temperatures, the incubation period is longer. Infections are usually visible as yellow lesions after about 7-12 days.

At night during periods of high humidity and temperatures above 13°C, the fungus grows out through the stomates of infected tissue and produces microscopic, branched, tree-like structures on the lower leaf surface. More spores are produced on the tips of these tree-like structures. The small sporangiophores and sporangia make up the cottony, downy mildew growth. Sporangia cause secondary infections and are spread by rain and wind.

If water is present on grape tissue, the sporangia quickly germinate to produce large numbers of motile zoospores that can result in secondary infections throughout a vineyard and thus rapidly spread the disease. Spore germination, penetration, and infection can occur in 30 minutes to 2 hours at an optimum temperature of 10° to 16°C with a minimum of about 4°C.

Powdery mildew overwinters as fungal fruiting structures that have become lodged in bark crevices on the grapevine. In spring airborne spores are released from the fruiting bodies and are carried by wind to susceptible tissues. Powdery mildew can infect all green parts of the grapevine.

The spores germinate on any green surface of the developing vine, resulting in primary infections. After infection, the fungus colonises the plant tissue and produces another type of spore on the surface. The spores and fungal mycelium give a powdery or dusty appearance to the infected plant parts (occurring in patches on fruit, leaves, and vines). Spores serve as secondary inoculum for new infections during the remainder of the growing season.

Late in the fall fruiting bodies form on infected plant surfaces. Many of these fruiting bodies wash into bark crevices where they overwinter, thus beginning the next cycle of the disease.

Secundary infection and disease development are favoured by temperatures of 20 to 25°C, but the fungus may be active from 15 to 32°C. Higher temperatures inhibit the powdery mildew fungus. A relative humidity of 40% to 100% is conducive to spore germination and infection. Unlike most other grape diseases that require free water for spore germination and infection, powdery mildew only requires high humidity. Free moisture, especially rainfall, is detrimental to the survival of spores. This means that powdery mildew can become a serious problem even when it is too dry for other diseases to develop. Low, diffuse light also favours powdery mildew development.