Biology, epidemiology

• Conservation, sources d'inoculum

Verticillium dahliae , soil fungus, has no capacity saprophytic s allowing him to easily keep in the soil. Despite this, its conservation is ensured exclusively by mycelial fragments still in place in the plant debris, and by its microsclerotia (figure 1) which perpetuates it for more than fifteen years.

Its large polyphagia  allows it to attack many 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 note for example Amaranthus viridis , Xanthium strumarium, Datura sp., Erigeron sp. and black nightshade. Note that this fungus does not induce systematically 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 produced in tissues of microsclerotia (Figure 1) at temperatures between 10 and 20 ° C and fragile conidiophores verticillate (Figure 2) forming conidia ovoid (Figure 3). The brown to black, thick-walled, variable-shaped microsclerotia (measuring 15 to 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 microsclerotia soil (figure 1) and / or conidia  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.

• 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 varying thermal requirements. 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).

Short photoperiods and low light sensitize plants to the 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.