Causal agent(s) and transmission
Early blight is caused by a group of species of the fungal genus Alternaria. At least two species are responsible for early blight symptoms on potatoes, A. solani and A. alternata. These fungi overwinter in the soil, on crop debris and infected tubers. They are pathogenic to Solanaceous plants, potatoes and tomatoes, but can also be considered as saprophytes. The spores are spread by the wind and rain splashing.
Early blight occurs in most of the continents where potatoes are grown. Its real severity is probably disguised by efficient chemical control against late blight. In view of predicted climate change, the reduction of mancozeb- based chemicals and the use of susceptible cultivars, severe early blight epidemics may become problematic in the future.
Symptoms on foliage
There are clearly delimited necrotic spots, of variable size which are mainly located on the bottom leaves and concentric rings on the larger spots (photos 3, 4). Symptoms usually appear on the bottom part of the plant where the leaves are older and start senescing (photo 1, 2)
Early blight symptoms may vary greatly from one growing environment to another. The type of potato cultivar may also contribute to different expressions of the disease but, until now, no clear link between the Alternaria species and the type of symptoms has been demonstrated.
Symptoms on tubers
The affected tubers are very dry and depressed (photos 5 and 6) and show brown to black superficial rot.
Risk factors
The disease causes damage mainly in hot, dry climates and its effects are exacerbated in irrigated crops. Early blight develops at temperatures between 20 and 30°C and with alternating dry and humid periods.
High temperatures (20-25°C) on sunny days and dew at night are favourable conditions for the development of the disease.
Early blight is also considered as a sign of weakness in the plant, brought about by nutritional imbalance, drought, plant senescence, insect attacks or mechanical damage.
Control
Management of early blight consists of general measures limiting the agronomical stresses and the factors favouring the disease:
- balanced fertilisation and adequate irrigation reduce the growth stress and delay plant senescence;
- several foliar fungicides, such as dithiocarbamates or chlorothalonil, used to control late blight are also effective against early blight;
- use tolerant or less susceptible potato cultivars when available;
- limit the inoculum by the destruction of infected crop residues, groundkeepers, and weed management and avoid planting susceptible crops such as tomatoes in the rotation;
- harvest when tubers are sufficiently mature and limit the wounds at harvesting and handling to avoid tuber rots.