Botrytis cinerea is a fungus that lives mostly on plant debris, but which can develop a parasitic mode under certain conditions. Unlike some other major vine diseases it is world-wide in distribution, and can affect a wide variety of plants. The symptoms caused by its infection of the vine not only compromise crop quality, but lead to dramatic reduction in quantity.

Symptoms

Symptoms can occur on leaves in humid, cold conditions. The disease is encouraged by factors causing leaf damage such as heavy winds, other diseases, spray scorching and friction against wires. The symptoms appear as stains 2 - 3 cm in diameter resembling burns, usually on the leaf edges. The stains exhibit grey felting on the underside and sometimes on the topside as well. This attack is usually limited in extent and not treated as serious: in any case, pesticide application has little effect.

Symptoms on shoots are rare but can occur in very wet springs or autumns, especially on damaged canes or those in contact with diseased foliage or berries. Shoots tend to weaken and break. Symptoms appear as darkish brown patches covered with a grey dusting. In autumn, black crusty patches 1 - 5 mm across can appear on badly ripened cane tips killed by early frosts.

Attacks on inflorescences are rare but can occur before or during flowering in very damp weather. It can lead to dramatic yield reduction and also to later outbreaks if infection of the flower ovary occurs or if contaminated flower debris is held within the bunch. The separate inflorescences appear brown, dry up and drop off.

Peduncular symptoms are reasonably common in cold climate vineyards with damp soils planted with vigorous vines. They appear as brown then black patches that dry up the stems. The berries stop ripening, shrivel up and sometimes drop off.

The infection of berries is the most serious manifestation of the disease and is called grey rot. The berries go brown in red cultivars and grey-yellow then light brown in white cultivars, the skin becoming very sensitive to the touch. If the weather is dry, the contaminated berries will shrivel and dry up. If it rains, dense tufts of conidia appear. Often the infection will start from one berry near the centre of the cluster and spread rapidly in damp weather, particularly if bunches are tight and berries split releasing juice.

Botrytis cinerea infections will often be associated with other micro-organisms such as Penicillium, Aspergillus and Acetobacter. These secondary infections have very serious effects on wine quality.

Berry infection after maturation is the manifestation of Botrytis cinerea known as 'Noble rot'. The symptoms are very similar to those for grey rot, but the berries are a lilac colour and the velveting less apparent in the early stages. In dry, hot weather conditions the berries shrivel and cause the concentration of sugars desirable for the production of dessert wines. In wet weather the berries swell and the beneficial action of the noble rot is lost.

Biology

The mycelium appears as small cylindrical, bulging, brown or olive coloured filaments. It lives in plant tissues, both within and outside the cells. Botrytis cinerea will overwinter mostly as mycelium either in the bark or on plant debris on the ground.

Asexual reproduction is by the production of conidiophores which appear on both sides of infected leaves and on the outside of bunches, often in dense tufts. They are around 1 - 3 mm in length, branching, and firstly colourless, then brown in colour. On their ramifications they carry many bunches of conidia or spores that are unicellular, oval and grey. The conidia are dispersed by wind and rain and can survive for one month after dissemination. They germinate producing a tube that enters the plant generally through a wound or weak point in the epidermis.

Botrytis cinerea can also overwinter by sexual reproduction as masses of agglomerated mycelium (sclerotia) on poorly ripened canes and rotted berries. These appear at the end of the growing season and are first white in colour, then turn black and hard and crusty with a length of 2 - 3 mm and variable shape. They resist winter temperatures well.

The Spread of Botrytis cinerea

Botrytis cinerea should be considered as an organism that is almost always present, but requires certain conditions for its development. However, it is important to bear in mind that it can evolve brutally towards grape maturity from centres of infection formed early on in the year.

The germination of conidia requires adequate temperatures, persistent humidity, and is stimulated by the presence of substances found on the plant's surface. Germination is very low at temperatures below 8ºC and above 30ºC, the optimum temperature being around 18ºC. Spores are killed in 48 hours at 35ºC.
Rainfall is not necessary, conidia will germinate with persistent mists or dews providing there is at least 90% humidity on the plant surface for long enough.

The most common path of invasion of the fungus is through disruption of the berry skin by:-

• Physical shock - e.g. hail, frost, friction
• Pest attack - especially Oidium, Peronospora, moths, insect and bird damage
• Spray or sun scorching
• Bunch bursting due to over-compact bunches or heavy rainfall after a drought period leading to rapid swelling

It can also invade micro-fissures in the berry skin (due to over-compact bunches) and flower ovaries can become infected during fertilisation via the stigmatal pore leading to a 'quiescent' contamination that will evolve at a later date. Once an organ has been infected, mycelial spread is achieved by the production of a large number of enzymes that break down the plant cells and cause them to brown.

Noble rot will only occur if the berries are brought to full maturity in a healthy state. In particular, the berry pellicles should remain intact. Generally speaking, the sugar content of the berries should be around 250 g/l and the nitrogenous content low. Spread is slow and encouraged by alternate periods of humidity and sun.

Control

Preventative measures:

• Select thick-skinned cultivars with loose bunches
• Reduce leaf-bunching; control vine vigour, and select appropriate trellis and pruning systems, summer pruning
• Protect the vine from other pests and diseases
• Cultivate the ground to bury infected debris
• Leaf removal to expose grape bunches

Chemical control

Botrytis is an extremely difficult pest to control, as it is omnipresent and capable of explosive growth given the right conditions. Treatment must be preventative, once the disease has appeared; it can only be contained, and not eradicated.Effective pesticides can be grouped in the following way:

• Broad-spectrum fungicides, often used for powdery mildew, e.g. dichlofluanid (ELVARON)
• Specific preventive and curative fungicides (Cyclic amides or dicarboximides) e.g. iprodione (ROVRAL), pyrimethanil (SCALA), effective, but should be used sparingly, as there are resistance problems.

The five critical periods for treatment are:

• Beginning of flowering
• Start of berry set
• Bunch closure
• Start of veraison
• Three weeks before harvest (least effective)