Verticillium wilt (Verticillium albo-atrum, Verticillium dahliae)

Verticillium wilt: biology

Verticillium wilt is mainly soil-borne, but can also be carried via seed tubers. The disease is favoured by crop stress induced by heat, drought, nutrient deficiencies and insect damage. Pathogenic fungi build up in the soil with repeated potato production and can survive there for long periods. Fungus is transported by anything that moves soil such as farm machinery, footwear, animals, water and wind. It can also be harbored by other crop species without visible disease symptoms.

The fungus penetrates plants through the roots and spreads upwards in the vascular tissue restricting water uptake and infecting stems, petioles and leaves.

Disease symptoms are seen on plants in patches throughout fields. The first symptoms usually appear immediately after flowering starting with the lower leaves. The area between leaf veins begins to yellow. Symptoms then move upward to the younger leaves. Initially, one side of the leaves turns yellow and wilts. Later, the entire leaf turns yellow. Leaf yellowing is followed by browning and necrosis.

The vascular area of infected stems also turns brown. It is easy to spot infected and dead plants as they stand up higher than other plants in the field, a condition called flagging.

In tubers, the vascular ring turns brown. This brown vascular discolouration starts at the stem end of the tuber and it does not usually extend more than halfway through the tuber.

This damage leads to the early crop senescence, approximately three to four weeks before reaching maturity. As a result, tubers do not size and serious yield losses occur.