ON THE FOLIAGE
Under the direct action of the sun (and sometimes in the presence of dew, which has a magnifying glass effect), the green tissues start to dehydrate and turn black. Necrosis of the tissues is complete after a few hours. It affects the leaflets or parts of the leaf blade. Some cultivars are more susceptible than others.
ON THE TUBERS
Tubers exposed to the sun and high temperatures for several hours, in the field or later, suffer from “sunburn” and take on a metallic appearance with browning of the underlying tissues (“golden blotch” or sunscald). The surface tissues dry out (photo 1) and the tubers are unsuitable for storage.
Damage due to excess water
Too much water damages the potato plant and injuries can take different forms:
If the potato plants are waterlogged (by flood) for a period greater than 24 to 36 hours, the tubers are asphyxiated and become prone to attacks from various latent micro-organisms in the soil (Pectobacterium, Pythium, and Phytophtora erythroseptica). Potato tubers in the bottom of the mound are more exposed than the others (photos 1, 2 and 3).
In certain rare cases, black heart may be observed.
ENLARGED LENTICELS (lenticellosis)
A high soil moisture content lasting for several days favours lenticel enlargement on the tubers. This alters their appearance at harvest time and favours penetration by rotting agents.
Rain damage to plants or newly-lifted tubers
Seed tubers that hace been dampened by the rain before being planted (e.g. in the planter hopper or skip) are more susceptible to rotting pathogens. This may cause emergence losses. Ware tubers rained on before storing may also be more susceptible to rotting.
The effect on the yield and on the quality of the harvested tubers produced also varies greatly, with potential significant loss of yield due to late growth and/or fewer daughter tubers.
Areas of necrosis (burns, dehydration) in the foliage of all, or part, of the plant (photos 1 and 2) appear in a usually circular area after it has been struck by lightning. This could be confused with late blight symptoms (close observation of the foliage and an absence of mycelium will provide an accurate diagnosis). The presence of a metallic element may sometimes be observed on the soil in the centre of the area.
The extent of the damage depends on the force of the hailstorm. In extreme cases, the surface of the foliage is completely destroyed (photos 1 and 2) with significant damage to the stems which are “lacerated” (photos 3 and 4). In some cases, the hailstones damage the tubers directly (sides or tops of the ridges). An appropriate late blight protection is then necessary.
The effect on the yield and the quality of the tubers produced are also highly variable: significant loss of yield due to a late crop and/or fewer daughter tubers. In certain cases, re- growth and symptoms of glassiness may be observed
The wind acts mechanically on the aerial parts of the plant. It can destroy leaflets, entire leaves and stems, depending on its strength. Leaflets worn down by rubbing may sometimes be observed (photos 1 to 3).
The resistance of the plants can vary according to their stage of development; cultivar susceptibility is also a factor. Crops are more susceptible with incomplete leaf cover and when the foliage is not naturally crumpled. There is usually only a limited effect on the yield as new tissues or organs form afterwards.
In some cases, a recurrent wind will limit the subsequent development of the crop and will have a negative effect on the yield and the quality (grade) of the tubers produced.