Late blight
Phytophtora infestans
Causative agent and transmission
Potato late blight is caused by a Phytophthora infestans fungus belonging to the genus Zygomycotina of the family Pythium.
Factors particularly favourable for late blight include temperatures in the range of 17-20°C, a high level of humidity and dense vegetation.
Description of symptoms on crops
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The appearance of small discoloured blemishes on the foliage which turn brown and are surrounded by a yellow ring on the upper side of the leaves. On the underside, the fruiting bodies of the fungus (conidiophores and conidia) appear under damp conditions on the rim of the blemishes and give a characteristic white felting.
An increase in the number of blemishes, their spread and subsequent desiccation can rapidly lead to destruction of the foliage.
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Brown blemishes, sometimes necrotic, are often observed on the stems and the terminal bouquets; these also carry fruiting bodies in damp weather.
Description of symptoms on tubers
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Externally, the affected tubers show blemishes with poorly defined outlines which are coloured brown or blue-grey and which can be slightly depressed.
The cut surface of the tuber shows rust-coloured mottled areas on the surface which may extend towards the centre of the tuber.
Other pathogens can then develop and cause damp rot if the crop harvested harvest is not well-dried.
Control
In all cases, control must be preventive: the use of healthy seed potatoes, destruction of waste heaps, good covering and fungicide protection ensured until haulm defoliation.
There are a large number of products against effective late blight: products with a copper base (risk of phytotoxicity), contact fungicides (maneb, mancozeb, fluazinam, etc.), penetrating fungicides (cymoxanil), spreading (dimetomorph, propamocarb) or systematic/systemic (metalaxyl, oxadyxil).
Applying products must be well thought out according to the type of epidemic risk (warnings), the risk of resistance to certain fungicides and varietal sensitivity.
