Stalk break, White mould

Causal agent(s) and transmission

White mould disease, also named stalk break, is caused mainly by the fungus Sclerotinia sclerotiorum and, more rarely, by Sclerotinia minor, both of which can survive for several years in the soil in the form of resting sclerotia. These fungi have a wide host range, including cultivated crops such as beans, peas, sunflowers, colza, other solanaceous species, and weeds.
Such a large host range contributes to the survival of the soil inoculum from season to season.

In moist soil conditions, the sclerotia form fruiting bodies that release numerous ascospores; these spores are spread by the wind and may be transported to potato stems or leaves.

After germination of the spores in humid conditions, the fungal mycelium penetrates the plant before colonising the vascular system. The infection of stem and leaves is more severe if the plants are damaged by the wind, hail or heavy rains.

In case of severe soil infestation, sclerotia may germinate directly and produce infectious mycelium which typically induces stem lesions at soil level.

The disease occurs sporadically in Northern Europe and in temperate climates. It may be significant on dense foliage and in humid conditions related, for example, to the use of sprinkler irrigation and intensive fertilisation.

Symptoms on foliage

In growing crops, the symptoms appear after long periods of rain and are characterised by a whitish mould on the lower part of the stems and at the leaf axils. The tissues are gradually destroyed and the diseased stems fold, wilt and fade (photos 1 to 5), and finally, in the case of early and severe soil infestation, the plants may die.

In favourable conditions, e.g. when humidity is high, large black sclerotia can be observed on the diseased tissues or inside the stems (photos 6 to 8).

Symptoms on tubers

In wounded tubers, the symptoms of rot at the heel-end which are quite rare, are caused mainly by S. minor. Typical sclerotia are visible in the affected tissues (photo 9).

Control

The severity of the disease is greater in cool, humid and windy climates.

Excessive haulm development, such as intensive nitrogen fertilisation or high planting density increases the risk of damage by maintaining a high moisture content at the base of the stems.

Wounds at the stem base can provide a point of entry for progression of the mycelium.

The most effective control measure is cultural and consists of limiting the presence of host plants in the rotation (potato, rape, leguminous plants, etc.) and reducing favourable microclimates (dampness at soil level and basal foliage). In the case of high risk of disease and intensive potato production, care should be taken to limit nitrogen fertilisation.

Potato cultivars susceptible to wind are more easily attacked.

Chemical control is another available control measure though with little effect on the primary soil inoculum, the resting sclerotia.

Preventative biological control can be used to treat the soil so as to reduce its infective potential prior to planting any susceptible crop.