Verticillium spp. (Cotton)
|Scientific Name||Verticillium albo-atrum Reinke & Berthold, Verticillium dahliae Kleb.|
|Synonyms||Verticillium ovatum G.H. Berk. & A.B. Jacks., Verticillium albo-atrum var. medium Wollenw., Verticillium trachiephilum Curzi For a long time V. dahliae has been considered to be synonymous with V. albo-atrum. The two species, however, can be distinguished based on morphological characteristics.|
|Common Names||English: Cotton wilt; German: Verticillium-Welke; French: Verticillioses; Spanish: Verticilosis|
|Description||Verticillium spp. are members of the Ascomycota, class Sordariomycetes, order Hypocreales (family incertae sedis).|
Although referred to as a wilt disease, wilting may not be the main symptom seen or may be absent in the Verticillium disease. Symptoms depend on cultivar resistance and environmental conditions. The severest symptom is an irreversible wilting of the whole plant after several weeks of vegetative growth followed by plant death. Typically, wilting may affect only some shoots or leaves; Verticillium infection may result in defoliation, but sometimes, sectorial chlorosis and necrosis of leaf tissue are the only visible symptoms. Vascular staining may be present, but stunting may be the only effect of the disease. Wilt diseases caused by V. albo-atrum are indistinguishable from those caused by V. dahliae, which is likely to predominate in cotton. It is often necessary to isolate the pathogen before the cause can be verified.
Colonies of V. dahliae produce moderately fast-growing white mycelium which turns black from the centre when microsclerotia are formed. Aerial mycelium is often limited. Hyaline, ellipsoidal, one-celled conidia (2.5 - 6 x 1.5 - 3.0 µm) are produced at the tips of narrow, pointed, conidiogenous cells subtended in whorls (2 to 3 per node) on hyaline conidiophores. Conidia produced in succession form moist spore aggregations at their tips. The dark brown to black microsclerotia have irregular shape and size (50 - 200 x 15 - 90 µm). The dark, thick-walled resting mycelium of V. albo-atrum has less longevity than the microsclerotia of V. dahliae, and rapidly loses viability.
Wilt diseases caused by Verticillium spp. are monocyclic. V. dahliae may survive for many years in soil as microsclerotia - free or embedded in plant debris; their germination is stimulated by root exudates. The pathogen can be disseminated with cotton seed and as microsclerotia associated with the lint. Hyphae produced from germinating microsclerotia or conidia infect roots at or just behind the tips and progress into the cortex and towards the developing vascular tissues. Once in the xylem vessels, the fungus spreads by mycelial growth and the production of conidia which are transported into the shoot with the transpiration stream. Symptoms (and damage) result from the occlusion of vessels and toxin production. Microsclerotia are formed in senescing diseased tissues.
Additional Crop Information
Verticillium species have a worldwide distribution and attacks more than 200 plant species including important field crops such as cotton and numerous solanaceaous vegetables.
Verticillium wilt is the most important disease in the major cotton-producing regions. Most severe losses occur in the CIS with annual losses of up to 20 % in many farms. Losses in cotton quality - fibres from infected plants are shorter and weaker than from healthy plants - have to be added to production losses.
Strains of V. dahliae attacking cotton are present in most cotton growing areas. In many regions, cotton wilt is caused predominantly by a pathotype which defoliates susceptible cultivars, but has limited virulence to other host plants. These isolates belong to one vegetative compatibility group. Resistance of some cotton cultivars have been overcome in several areas.
Cotton wilt disease is favored by moderate to high temperatures, although temperatures > 30 °C are inhibitory. The disease is often more severe in irrigated crops as irrigation reduces soil temperature after irrigation. Salinity of irrigation water may exacerbate cotton wilt. V. albo-atrum is limited by high temperatures and is the main cause in cooler production areas.
Integrated Crop Management
For effective management, an integrated approach is necessary involving cultural practices which minimize disease - crop rotation, fertilization, irrigation, pathogen-free seeds, use of resistant cultivars - and pre-plant soil treatments like soil fumigation or solarization that reduce the inoculum in the soil.
The development of resistant cultivars has been very important, particularly where defoliating strains have become established. The usefulness of some resistant types has been restricted by the evolution of strains able to overcome the resistance.
Cotton wilt has been reduced by rotations with cereals, legumes and crucifers and with lucerne. However, there are reports that rotation systems have not been effective in providing control.
There are no (cost-) effective chemical controls available to reduce the soilborne Verticillium wilt disease in field crops. In cotton, crop rotation with grains such as corn or wheat or with dicotyledonous crops such as sorghum, safflower or alfalfa seems to be (beside the use of tolerant varieties) the only way to reduce significantly the incidence of this disease.
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