Symptoms

•  Symptoms on foliage

On the upper side of the leaves, small discoloured spots may be observed; these turn brown and are surrounded by a light-green to yellow halo (photo 1).

On the underside of leaves and in wet conditions, the asexual fructifications of P. infestans (sporangiophores and sporangia) appear around the spots giving a characteristic white felting. An increase in the number of spots, their expansion and subsequent desiccation can rapidly destroy the foliage (photo 2).

Brown lesions, sometimes necrotic, can be observed on the stems (photo 3) and on the apical parts of the canopy (photo 4). Under moist conditions, all the plant organs can be infected including the berries.

Grey mould due to Botrytis cinerea

Symptoms on foliage due to B. cinerea may be confused with late blight symptoms, caused by P. infestans. Cool and wet conditions greatly enhance grey mould infection due to B. cinerea (photo 5). However due to the saprophytic nature of the fungus, it tends to infect weak and senescent plant parts.

Chemical treatment usually controls the problem, which rarely has an economic impact on potato crops.

• Symptoms on tubers

Externally, the tubers display unclearly delineated, brown or purplish-grey spots (mottling), which may be slightly depressed (photos 6 and 7).

When infected tubers are cut open, they contain areas of rust-coloured mottled flesh just under the epidermis (photo 8), although the decay may extend to the centre of the tuber.

Other pathogens can then develop and cause damp rot if the harvested tubers are not well- ventilated.