Biology, epidemiology

• Conservation, sources d'inoculum
  

Verticillium dahliae is a poor competitor in the soil, lacking saprophytic capacities allowing it to maintain itself easily in the latter. Despite this, its conservation is ensured by mycelial fragments still in place in the plant debris, but above all by its microsclerotia (figure 1) which perpetuate it for more than fifteen years. Its large polyphagia allows it to attack many cultivated host plants (tomato, pepper, potato, cucurbitaceae, artichoke, lettuce, cabbage, strawberry, rose bush, aster, chrysanthemum, tobacco, cotton ) or weeds (black nightshade, amaranth ) which promote its multiplication and conservation. In fact, we find in this fungus many strains with variable parasitic potentials allowing them to attack more or less wide ranges of hosts.

• Penetration and invasion

Contaminations occur either by direct penetration of the mycelium from microsclerotia into the root, or through various root injuries . In addition, they can be facilitated by attacks by root-knot nematodes and Pratylenchus spp. Once introduced, the fungus reaches the vascular system of plants, which it gradually colonizes. These react to this vascular invasion by forming gum or tyloses which prevent its progression. These defense mechanisms, associated with colonization and clogging of vessels by the mycelium, contribute to the wilting of plants. Note that V. dahliae produces microsclerotia (at temperatures between 10 and 20 ° C) in the tissues and fragile whorled conidiophores forming ovoid conidia.
 

• Sporulation and dissemination

It is possible by soil, by agricultural equipment soiled by contaminated soil and by plant debris. Soil dust containing soil microsclerotia (figure 1) and / or conidia (figure 2) is easily disseminated by air currents, as well as by splashing water and insects.

• Conditions favorable to its development

V. dahliae is rather known to appreciate cold climatic conditions. In fact, there appear to be a number of strains with varying thermal requirements. Their thermal optimum would range between 20 and 32 ° C . Short photoperiods and low light sensitize plants to the disease. The latter would be more severe in neutral to alkaline soils. The monoculture of sensitive plants or too short or poorly chosen rotations contribute to increasing its incidence in certain plots.