Frost or cold damage

■  On foliage

• Frost causes burns and leaf withering. The worst affected leaves are usually the top leaves and the ones around the edge of the plant (photos 1 and 2).

In the case of young plants (5-10 cm), a severe frost can cause the green parts to wither completely. Growth recommences with the unaffected parts.

• Prolonged cold can cause a purplish discolouration of the edges of the leaflets, although this formation of anthocyanins may be due to other stress factors (deficiency or virus infection).

■  On tubers

The symptoms appear when the plants are getting warm again after frost. The frozen part of the tuber becomes soft. The inside becomes liquid and blackens. In the case of a partial frost, irregular grey to black spots (photo 3) may be observed in the flesh but more commonly in the outer part of the tuber.

Damage can also occur during storage at low temperatures (close to 0°C) for a relatively long period. In this case, sprouting may be inhibited and marks may appear on the tuber: necrosis and sometimes depressed areas on the skin (photo 4) possibly with long, hairy sprouts (seed potatoes). Skin necrosis may appear when fine skinned cultivars are suddenly put into storage at low temperatures (photo 5). Severe symptoms of black and glossy tubers, prone to subsequent rotting, may be observed after storage at low temperature, frost or oxygen deficiency (photos 6 et 7).

The phenomenon of premature tuber formation is partly due to the effect of low temperature on incubated plants (see Sprouting disorders due to physiological age).