Apart from the physiological disorders caused by heat (phenomenon of secondary growth), other symptoms may be observed:
Symptoms on foliage
Under the effect of continuous intense heat over a period of several hours, the entire plant starts to wilt. The tissues lose their turgor pressure, and the growth habit is less erect. Subsequently, leaves and/or leaflets dehydrate, at least partially (photos 1 to 3), others take on a coppery appearance on their undersides. This phenomenon may be associated with a lack of water in non-irrigated crops, and the foliage then turns dark green due to the unavailability of nitrogen.
Symptoms on tubers
When the crops are subjected to temperatures above 30°C, the skin of the tubers, especially those at the top of the ridge, may turn brown; this is called “skin browning” or “bronze spots” (photos 4 and 5). Intense heat can also cause colouration or necrosis of the vascular ring (usually limited to the first centimetre from the stolon) (photo 6) and, exceptionally, black heart.
Seed tuber leaking or liquefaction
During very hot spells (> 28-30°C) following cool, humid periods, some plants lose their turgor pressure and tend to wilt (photo 7). This phenomenon occurs on one or more stems. The mother tuber is then in an advanced state of liquefaction. It cannot satisfy the needs of the developing aerial part of the plant, which has still to attain its full “independence”.
The plant then appears especially susceptible to latent parasitism and may develop symptoms of bacterial rot, first on stems and then on tubers.