Biology, epidemiology

• Conservation, sources d'inoculum
  

Verticillium dahliae and V. albo-atrum , telluric fungi that can easily coexist in the soil, do have not saprophytic capacities allowing them to easily maintain themselves in the latter. Despite this, their conservation is ensured exclusively by mycelial fragments still in place in the plant debris for V. albo-atrum and by microsclerotia (figures 1 and 2), sometimes also by mycelial fragments, for V. dahliae which makes it more durable. about fifteen years. This is the reason why verticillium wilt is more observed in plants grown in soil such as greenhouse cucumber or field melon, although the disease can also develop in artificial substrates if they are contaminated with these agents. pathogens.

The large polyphagia of V. dahliae allows it to attack numerous cultivated host plants or weeds (weeds), which promote its multiplication and conservation. Among the sensitive plants, we can cite: cotton, strawberry, stone fruit trees, nightshades (tomato, tobacco, chili pepper, and especially potato and eggplant which are particularly sensitive). It is also mentioned on lettuce, radish, turnip, olive, apricot, sunflower, artichoke, rapeseed, cabbage, cauliflower, mint, pistachio, pepper, mango, horseradish, alfalfa, peanuts, cowpeas, ornamental trees and tropical trees. ... Among the weeds let us mention by way of example Amaranthus viridis , Xanthium strumarium, Datura sp., Erigeron sp. and black nightshade. Note that this fungus does not systematically induce symptoms on these hosts.

• Penetration and invasion 

Contamination occurs either by direct penetration of the mycelium from microsclerotia through the root and especially the root hairs, or through various root injuries . In addition, they can be facilitated by attacks by root-knot nematodes and Pratylenchus spp. Once introduced, the hyphae of the fungus enter the vascular system of plants. The xylem vessels are gradually colonized causing blockages and interference with the flow of water in the plant. This reacts to this vascular invasion by forming gum or tyloses which prevent the progression of the fungus. These defense mechanisms, associated with colonization and clogging of vessels by the mycelium, contribute to the wilting of plants.

• Sporulation and dissemination 

V. dahliae produces in the tissues microsclerotia (figures 1 and 2) at temperatures between 10 and 20 ° C) and fragile conidiophores whorled forming ovoid conidia (figures 3 to 5). The brown to black, thick-walled, variable-shaped microsclerotia (measuring 15-100 µm) are found in the wastes of infected crops and are eventually released into the soil. Dissemination is possible by soil, by agricultural equipment soiled by contaminated soil, by plant debris or directly from one plant to another by root contact. Soil dust containing soil microsclerotia (figures 1 and 2) and / or conidia (figure 5) is disseminated by air currents, as well as by splashing water and insects.
This fungus was found outside the seeds of a squash ( Cucurbita pepo ), allowing its transmission to seedlings, and therefore its dissemination.

V. albo-atrum , unlike V. dahliae , does not produce microsclerotia but a dormant mycelium , partitioned and dark on stems and culture medium. In addition, this Verticillium does not seem to be able to grow at 30 ° C.

• Conditions favorable to its development 

The Verticillium spp. are rather known to appreciate cold climatic conditions. In fact, there appear to be a number of strains with thermal requirements varying . Their thermal optima would be between 20 and 32 ° C. However, note that the growth of V. dahliae is stronger in soils with high temperatures (22 to 27 ° C) while V. albo-atrum adapts to a wider temperature range (16 to 27 ° C). ).

The photoperiod short and low illumination educate plants to disease. The latter would be more severe in neutral to alkaline soils, but also during periods of cold stressing the plants. The monoculture of sensitive plants or too short or poorly chosen rotations contribute to increasing its incidence in certain plots.