Symptoms

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.