Causal agent(s) and transmission
Two fungi, of the genus Verticillium (Verticillium dahliae and V. alboatrum), are responsible for Verticillium wilt in potatoes.
The primary inoculum usually comes from the soil, where the fungus can survive for several years in the form of microsclerotia on crop residues or weeds. It can also be borne by seed potato tubers.
The infection may start through the sprout, the root system or the wounds. In time, the fungus gets into the vascular system and then spread systematically leading to complete wilt.
Verticillium wilt can be the cause of significant yield losses under high disease pressure on susceptible cultivars. Intensification of potato production with very short or no rotation may lead to high incidence of soil-borne diseases and occasionally be expanded by co-infection with root lesion nematodes (Pratylenchus penetrans) or with the potato cyst nematodes, Globodera spp.
Symptoms on foliage
The disease causes wilting, dwarfism, premature senescence and early death of the plants (photo 1).
Symptoms in growing crops usually start on one side of the plant or the leaf (photo 2) with chlorosis beginning between the leaf veins. This then develops into necrosis and finally foliage dehydration or wilting.
The withered leaves turn brown, fall or remain attached to the stem, which remains green. At the base of the diseased stems, a brownish discolouration of the vessels can usually be observed (photo 3) when sectioning the stems.
On the stems and roots of the withering plants, small black sclerotia or mycelium may be observed, depending on the species of fungus (photo 4).
Symptoms on tubers
The infected tubers display brown spots on the vascular ring (photos 5 and 6) and may be of a smaller size. Pinkish-brown necrosis of the eyes may be observed.
Control
The disease is favoured by high temperatures (optimum development temperatures of 22 to 27°C with V. dahliae and 16 to 25°C with V. alboatrum), stressful growing conditions (alternating dry and wet periods), high water salinity and, above all, short rotations.
Penetration by the fungus into the roots may be favoured by the presence of free-living nematodes of the genus Pratylenchus in the soil or co-infection by Pectobacterium bacteria in the seed tuber.
Verticillium wilt remains marginal in temperate climates. Overall control measures for soil- borne pests and pathogens are effective in preventing verticillium wilt. They involve:
- a minimum rotation period of three years, without Solanaceae crops;
- the control of groundkeepers;
- the use of certified healthy seed tubers;
- the application of adequate fertilisation and irrigation;
- when authorised, a fungicide treatment can be applied to the soil prior to planting.
In specific environments with a high disease pressure due to favourable warm climatic conditions, soil chemical control and the choice of less susceptible cultivars (when available) might be necessary to limit yield losses due to verticillium wilt.