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Verticillium wilt - Verticillium dahliae
Verticillium spp. Nees. are considered to be one of the most widely distributed and destructive plant pathogens in agriculture. They cause wilt of a wide range of broad-leaved hosts belonging to over 40 families. They have not yet been documented as major soil borne plant pathogens in Vietnam, but the symptoms have been recognised in the field. Verticillium dahliae is a common pathogen in many countries but has not been reported in Vietnam.
The severity of the wilt disease depends on factors such as the strain of the fungus, the level of inoculum in the soil, the cultivar and the temperature. Wilt symptoms are generally more severe on younger plants than older plants. Early symptoms consist of interveinal chlorosis of leaflets, loss of leaf turgidity and curling of leaves. Later symptoms consist of general yellowing, vein browning, leaf necrosis, wilting, leaf defoliation, stunting and eventual collapse of infected plants. Symptoms intensify in sumnmer, when plants are under moisture stress and temperatures are over 26°C. With adequate moisture, infected plants may live to maturity, with only moderate symptoms. These plants may mature earlier. Internal symptoms include brown vascular decolouration in the crown, main stem, branches and petioles, best observed with a long oblique section of the stem just above ground level.
In stone fruit, the bark is peeled away to show the brown cambium layer. Young trees may be killed, but older trees should recover with new leaves the following year. Verticillium wilt of cotton reduces the yield of the lint and seed and also lowers the quality of the fibre. Infection in the cotton field is not usually evident until 35-40 days after plants emerge. Tubers of infected potato plants may develop a light brown discolouration in the vascular ring. Severe vascular discolouration may extend over half way through the tuber. Cavities may develop inside infected tubers. Pinkish or tan discolouration may be seen around the eyes or as irregular blotches on the surface of infected tubers. Both V. dahliae and V. albo-atrum may be present in wilted potato plants. In cucurbits, symptoms will progress from the crown region and along the runner. Fruit quality will decline, vascular discolouration will be evident in the node tissue and the plant will die.
When grown on potato dextrose agar, V. dahliae produces white, fluffy aerial mycelium with slender, often branched conidiophores in whorls. Conidia are hyaline, unicellular, oval and borne singly or in clusters apically. Microsclerotia may form in one week at 20°C.
Distribution within Vietnam
Not yet documented in Vietnam.
The host range of V. dahliae includes peanut, brassicas, capsicum, cinnamon, cucurbits, cotton, tomato, potato, mango, tobbacco, pistachio and sesame.
V. dahliae survives in the soil as microsclerotia for several years. When the infected plant debris decomposes, the microsclerotia that were produce abundantly in the infected tissue are released in the soil. They remain dormant until germination is stimulated by plant root exudates. The infection hyphae grow through the root cortex into the xylem. In the xylem, mycelia continue to colonise the vessels and produce conidia that move rapidly through the transpiration stream to the aboveground parts of the plant. Weed plants are also important to the persistence of the pathogen between susceptible crops.
Infection occurs in spring and symptoms are visible by midsummer. The symptoms become more severe in late summer, but may not appear until the following year or years after infection.
The pathogen is disseminated throughout the field soil by farm equipment, wind and water movement or by infected seed.