Various physiological disorders, which are not of parasitical origin, can be caused by environmental and nutritional conditions. If conditions are favourable, these disorders are sometimes combined with the subsequent appearance of pathogenic agents.
Premature tuber formation
Formation of daughter tubers from a mother tuber without foliage development. Premature tuber formation is the result of poor storage or storage over an excessively long period. Planting at low temperatures or repeated desprouting favour this phenomenon, which is also associated with varietal sensitivity during incubation.
When planting substantially sprouted tubers in cold ground, which is particularly the case for early potatoes, there is sometimes a delayed emergence - or no emergence at all - due to abnormal sprout growth (particularly in rings). These conditions favour attacks of black scurf on the sprouts.
Second growth, tuber cracks, glassy tubers
The term "second growth" refers to crop conditions associated with alternating hot and dry periods following cool and damp weather (rain or irrigation) favouring rapid plant growth. Tuber growth ceases during second growth.
Tuber deformation then occurs when growth resumes : deformed and cracked tubers, lumps, growth cracks or splits, etc.
In the most severe cases, the starch in the older tissues can be converted into reducing sugar giving a translucid and glassy aspect, sometimes becoming liquid and making the tuber unfit for consumption.
Freezing or chilling injury
• On the crop: symptoms of heat necrosis, with withering of the leaves. Generally, the most affected leaves are on the upper part and around the edge. The plant starts growing again from unaffected parts.
• On the tuber: symptoms appear on warming after frost. The frozen part of the tuber becomes soft. The inside becomes liquid and blackens. In the case of a partial frost, grey to black irregular blemishes may be observed in the flesh but more commonly around the edge of the tuber.
Damage can also occur during storage at low temperatures (close to 0°C) for a relatively long period. In this case, germination may cease and/or staining appears on the tuber (necrosis and sometimes depression of the skin).
Skin necroses on fine skinned varieties can appear when seed potatoes are suddenly put into storage at low temperature.
Tubers exposed to the sun at high temperatures for several hours, in the field or later, suffer from "sunburn" and the tubers take on a metallic appearance, with browning of the underlying tissues. The superficial tissues dry out and the tubers do not keep well.
Strong heat can also cause a black centre and sprouting of spindles.
Water damages (lenticellosis), enlarged lenticels
Conditions of substantial soil humidity for several days favour enlarged lenticles on the tubers, which minimise the appearance of the harvest and encourage penetration of rotting agents.
Internal rust spot
On the cut surface, the tuber flesh is punctuated with blemishes or "rusty brown" spots. The disease can be mistaken for late blight or Tobacco Rattle virus.
Unlike late blight, the outer part of the tuber displays no symptoms, and blemishes do not affect the skin.
The phenomenon is linked to varietal sensitivity, combined with weather conditions.
Brown center and Hollow heart
These two internal tuber physiological problems seem well correlated because hollow heart is preceded by brown centre in some varieties, such as Russet burbank or BF.15. The factors involved are low temperatures after the start of tuberisation (<15°C for 5 to 7 days) as well as substantial humidity (80-85%) which destroy the pith cells. Then, slow and regular growth favours brown centre while rapid growth is favourable to hollow heart.
Brown centre starts with a brown necrotic blemish at the tuber centre which can grow and also crack.
In the case of hollow heart, a relatively large longitudinal cavity forms in the centre of the biggest tubers, sometimes in the shape of a cross. The walls of this cavity turn brown with formation of a cork scar. Hollow heart is a varietal phenomenon linked to weather conditions favourable for the rapid growth of tubers (abundant supply of water and nitrogen, low density, etc.).
Blackheart occurs in cases where there is a shortage of oxygen required for the respiration of tubers. It is linked with poor storage or a long period without oxygen (refrigerator or poorly ventilated container). This may or may not be combined with storage at high or, alternatively, at very low temperatures.
It is sometimes associated with the presence of small brown sunken blemishes on the tuber surface, caused by asphyxiation of the tissues, in the case of insufficient air renewal.
Black spot and bruise damage
Blackspot and bruise damage is characterised by the presence of grey-blue blemishes (ash spots), just beneath the tuber skin. This colouration, due to the conversion of tyrosine into melanin, is caused by damage to the tuber (the spots appear 1 to 3 days after the lesion) and is often combined with compression of the underlying tissues.
Blackspot and bruise damage is favoured by varietal sensitivity, dry matter content (harvesting after a dry period or drying out during storage), handling the tubers in low temperatures (less than 10°C) or unbalanced fertilisation.
Pinck discoloration of the flesh
A pink and purplish colouration of the flesh, forming mottles and generally localised in the tuber pith, appears in some varieties such as Roseval but also in yellow-skinned varieties (Record). This non-toxic pigmentation is due to anthocyanic pigments, and is favoured by various factors :
- light: the tubers close to the surface of the soil are the most affected;
- alternating cool nights and hot days;
- dry or nitrogen-rich soil.
Greening of the tuber skin is caused by photosynthesisis of the chlorophyll and also the presence of toxic alkaloid substances such as solanine.