Causal agent(s) and transmission
Grey mould caused by the fungus Botrytis cinerea is a very common disease on many plants. In general, it is not a serious problem for potatoes except in cold (average of 10 ° C) and humid (especially foggy) climates. The foliage of maturing crops may be partially or totally destroyed by this fungus, but usually without a significant impact on production.
Botrytis cinerea is a necrotrophic polyphagous fungus belonging to the Ascomycota division and the Sclerotiniaceae family. It is highly polyphagous and it is able to cause the grey mould disease on several hundreds of plant species. The damage is usually very limited on potato but on other crops, it may cause significant damage, both in open field or under glasshouses, causing stems cankers as well as losses associated with rotting tissue, especially on fruits.The resulting losses on fruit are important worldwide notably on economically important crops such as tomato, strawberry or grapevine.
Grey mould caused by Botrytis cinerea is present all over the world on the potatoes, but the damage is usually not significant on this crop. However, in areas with a long growing season, low temperatures, long periods of wetting the leaves and a lack of light, grey mould can be a very harmful disease. Such conditions exist in regions where potatoes are grown at high altitude and in tropical and subtropical areas, when potatoes are grown at low altitude in very short days with a formation of thick fog.
Chemical treatment usually controls this problem, whose economic impact is quite rare in potato crops.
Symptoms on foliage
Symptoms on foliage due to Botrytis cinerea may be confused with late blight symptoms, caused by Phytophthora infestans. Cool and wet conditions clearly favor Botritys cinerea, the causal agent of grey mould (Photos 1 to 3). However, due to the saprophytic nature of the fungus, it tends to infect vulnerable and senescent plant parts.
Symptoms on foliage
Botrytis cinerea cause damping-offs or seed decays on various crops.
Most of the plant organs can be affected but this saprophytic fungus usually infects the weak and senescent plant tissues.
Potato crops are mainly affected on the foliage. Affected leaflets often present brown to light grey spots, becoming darker when they are wet, and initially with a rather circular shape. The lesions are frequently starting from the tip of the leaflets and the spots develop later along the central vein (figures 1-3). The yellow to light brown lesions have a thin aspect and may present concentric forms. Depending on the climatic conditions, the foliage symptoms can either dry or macerate with a rot extending to the whole leaflet, which necrotizes gradually. A grey to grey-brown mycelium can be present or develop under humid conditions. Lesions progressing quickly along the main nervures is a typical feature of Botrytis cinerea, as well as the presence of grey mycelium on the lesion and the deformation of affected tissues.
Rotting tissues can progress to the petioles and then to the stems, causing brown stem rots which first are wet and later dry. Stem rots have well bounded outlines and rot can progress on several centimetres long. Afterward, distal leaves may yellow, dry out and then die.
Leaf and stem lesions due to Botrytis are often confused with late blight lesions caused by Phytophthora infestans. However, when sporulating naturally or after having being kept in a wet room during one or two days, the mycelium of potato late blight is white and silky whereas Botrytis cinerea forms a grey dark mycelium.
Senescent petals are particularly vulnerable to B. cinerea which can later develop on the inflorescences and make them rot, but also may initiate secondary infections on leaflets and fruits.
Regardless of the affected organs, the infected tissues are covered with a dense grey mould, which is very characteristic, consisting of conidiophores and conidia of the fungus. B. cinerea can produce black sclerotia from 2 to 5 mm in diameter, which are rarely visible on lesions.
Symptoms on tubers
Botrytis affects more easily tuber tissues which have been frozen or affected by other diseases or secondary invaders. Damage can be associated to a rotting of affected tissues and to the presence of a typical grey mould.
Botrytis cinerea is a very polyphagous fungus which can colonize hundreds of cultivated plants or weeds that contribute to its conservation and are potential sources of inoculum. Due to its saprophytic ability, the fungus can survive on the organic matter in the soil as mycelium or sclerotia, these latter surviving several years in the soil and on plant debris. On all its hosts as on plant debris, the fungus produces a grey mould formed by a mycelium and many long and branched conidiophores bearing at their ends spherical to ovoid conidia which ensure the dissemination of B. cinerea. Sporulation can begin 3 days after the first contamination. The dissemination is carried out mainly through the wind and to a lesser degree through the rain and splashing water. The spores germinate on the leaves within a few hours in the presence of high humidity.
Botrytis cinerea is particularly favored by wet environments and the optimal conditions for its development are a relative humidity around 95% and temperatures between 17 and 23 ° C. These conditions can be found in the field during rainy periods or as a result of irrigation by aspersion. The weakened or or etiolated plants are particularly vulnerable. Sometimes, the withered flowers serve as starting point for the development of the disease. Low light intensity promotes the development of grey mould and botrytis is especially a problem when the light intensity is low.
Some potato varieties are more sensitive than others to grey mould.
Control of the grey mould disease is possible but it is difficult and is rarely justified on potato.
In senescent cultures, which are very prone to Botrytis, treatments are not useful as they do not give any yield increase.
For high altitude crops, where the disease can be problematic, yield can be increased by the application of fungicides. However, as the growing season is relatively long (7 to 9 months), the crop must be protected for long periods and, as a result, plant protection is generally too expensive to be profitable.
Furthermore, prophylactic measures limiting the disease are to promote aeration of the foliage, to avoid the presence of water on the plants, to opt for the same reasons for an drip-irrigation system, avoiding to wet the foliage (unlike irrigation by aspersion), to use an adapted nitrogen fertilisation in order to avoid stress to plants leading to disruption in plant growth and to remove plant debris at the end of culture.