Verticillium species are non-specific plant pathogens. Many species are saprophytes, Verticillium albo-atrum and Verticillium dahliae are the most important plant pathogens. Verticillium dahlia causes verticillium wilt and early dying of potato.
Life cycle and appearance of Verticillium wilt
Verticillium dahliae survives for prolonged time periods (many years) in the soil as microsclerotia, small hard survival structures that can withstand high and low temperatures and dehydration. Verticillium dahliae also survives as hyphae (mycelium) in crop residues. The fungus infects the roots of plants, directly or through wounds caused by for example nematodes. The fungus then grows into the vessels of the plant and produces spores that are transported upwards with the transpiration stream. As a defence mechanism, the plant produces gum in the vessels. The presence of both the fungus and the gum causes the xylem vessels to get blocked, which in turn causes the plant to wilt. Spores may be wind-dispersed to new hosts. In the senescent tissue, the resting mycelium or the microsclerotia are formed. The spread of the fungus by mycelium growth is limited. Dispersal from plant to plant takes place via water, soil particles, tools, etc. Verticillium can also be seed-borne and tuber-borne.
The optimum temperature for Verticillium albo-atrum is 21 °C and growth stops above 30 °C, whereas Verticillium dahliae has an optimum temperature around 25-27 °C.
Verticillium species mostly cause wilting of plants. The wilting starts at the lower leaves. These leaves are pale-green, then orange-yellow. In the beginning the plant seems to recover at night. The symptoms may occur only in parts of the plant. Cutting the stalk horizontally reveals a slight discolouration of the vessels, brown or greenish, but much less distinct than the discolouration caused by Fusarium. The root system usually stays mostly intact. In rose, branches die off from the top and purple brown stripes occur on the stem. In tomato, sometimes V-shaped lesions are formed on the leaf, between the veins. 'Early dying' disease of potato only occurs as premature senescence, without any other typical symptoms. In many crops, the fungus can be present without clear symptoms or with only partial wilting.