The fungus Botrytis cinerea is a wound and opportunistic parasite and is capable of infecting nearly all plants. In wine production, the fungus can contribute substantially to quality improvement in certain white wine production. However, it can also cause severe yield losses and equally severe reductions in must quality.
Grey mould (grapes) [Botrytis cinerea (vites)]
Grey mold (grapes) - image 1
The fungus overwinters as a mycelium in grape wood (primarily in bunch stem fragments) as well as in fallen leaves and pruned wood. It develops resistant dormant bodies known as sclerotia on dead grape parts and poorly matured wood. The temperature requirements of Botrytis cinerea are few, temperatures around freezing and high humidity suffice for mycelium growth. The disease spreads rapidly during moist-warm weather conditions and temperatures above 20 °C, during which the fungus produces massive quantities of spores that are dispursed by the wind. The conidia survive on the host tissue for prolonged periods and do not become infective until they are in contact with dripping water in the form of dew or rain for 2 hours.
Sour rot, Botrytis (grapes)
Pattern of damage
Injuries to individual berries caused by hail, grape berry moth caterpillars, or wasps are frequently the starting point for Botrytis infection. So-called sour rot occurs on unripe berries with must weights below 50 °Oechsle. As a result of Botrytis (gray rot) infection, many thin-skinned grape varieties can collapse within a few days during early ripening (starting at around 50 °Oechsle). Only poor quality must with low sugar and high acid content can be produced from grapes infected with gray rot. In ripe bunches at 65 °Oechsle or more, however, Botrytis infection leads to a highly-prized enrichment of the berry juice during dry weather and nights with heavy dewfall. Many premium-quality wines are produced from bunches infected with "noble rot" in white wine varieties. In red wine varieties, the red pigment is broken down by the enzymatic activity of the fungus. Low color intensity and off-colors are characteristic of red wines produced from grapes infected with Botrytis. Newly planted grafted vines can be severly damaged by botrytus, which can infest the graft causing callouses to form. Essentially all parts of the vine can be infected. In winter, the mycelium can grow from year-old wood into the buds, which will cease to sprout and eventually die. Leaves and growing tips will turn brown and if humid conditions persist, the infection sites will become covered with a gray layer of fungus. Infected fruit sets die off, turn brown, and desiccate (botrytis bloosom blight) (Picture 1). High yield losses can also be expected if the fungus infects the stems. The ripening process of the bunches is interrupted, the stems rot and entire bunches drop to the ground. In contrast to stem shrivel (frequently secondary Botrytis infection), in stem-end rot the infection sites on the bunch are wet. Stem-end rot occurs especially in the variety Riesling. Botrytis cinerea, especially on the bunches, is responsible for economic losses in all varieties. The fungus can develop on blossom residues in unclean bunches to such an extent that it can also attack the tissue of green berries with the help of the enzymes produced.
The key starting point for indirect control measures against Botrytis cinerea is the high moisture requirement for the development of the fungus. All measures that create an unfavorable microclimate for the fungus and promote rapid drying of the canopy are effective. Removing the foliage from the bunch zone and exposing shaded bunches improves ventilation and speeds up drying. Furthermore, the more intensive exposure of the bunches to sunlight also results in better developed berry skin. These measures make it considerably easier to achieve a thorough spray coverage of the bunches, which is of paramount importance in Botrytis control.
The intensity of crop protection measures to control Botrytis is determined by weather conditions. In the event of wet-cold weather at bloom time, many blossom residues remain stuck to the bunches and are colonized by the fungus. Spraying with a fungicide effective against Botrytis during late bloom is an important standard control measure. The goal of crop protection measures against bunch Botrytis is to delay the infection of the berries for as long as possible and to control the course of infection according to the type of wine sought. Effective grape berry moth control is also essential. Existing Botrytis infection can no longer be stopped. Shortly before the grapes touch, the stems should be coated once more with a fungicide. The final spray application delays infection until the grapes ripen.