Verticillium spp.

(Verticillium  wilt)

General

• Soil fungi reported from many countries in temperate and subtropical zones.
• Several species * of Verticillium are likely to attack vegetables, and in particular eggplant, causing similar symptoms on this species.

* Knowledge about the nature of these Verticillium species is still incomplete; in addition to V. dahliae and V. albo-atrum , other species such as, for example, V. isaacii , etc. seem to appear in France contributing to the complexity of understanding the interactions between eggplant, the various rootstocks (and their resistance) and verticillium spp. (even possible races). Note that other pests of eggplant and its rootstocks could influence the parasitism of Verticillium spp. ( Colletotrichum coccodes , Rhizoctonia solani , Meloidogyne spp., Etc.)

• Very polyphagous, they affect more or less wide ranges of hosts depending on the species.
• Known vegetables affected: eggplant in particular, but also tomato, pepper, potato, Cucurbitaceae, artichoke, lettuce, cabbage, strawberry, rose bush, aster, chrysanthemum, tobacco, cotton, various weeds (black nightshade, amaranth), etc.
• Observed in the open field as well as under shelters.
• Organs attacked: leaves, vascular system.  
• Symptoms :
  • Partial wilting of some lower leaves at the hottest times of the day, reversible during the night.
  • Progressive softening and yellowing of areas of leaf blade, often interveinal and "V" shaped. The latter end up turning brown and necrosis.
  • Premature death of the leaves, leaving the fruits exposed to the sun.
  • More or less marked browning of the vessels of the lower part of the stem extending over time and reaching the twigs.
  • Weakening of plants or even premature death, reduction in yields.

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• Possible confusion : other vascular diseases of Solanaceae ( bacterial wilt wilt , Fusarium ).
• Signs  : no particular signs present on the affected oganes of the plants.

 Biology

• Conservation : poor competitors in the soil, their conservation is ensured by mycelial fragments still in place in the plant debris, but also by its microsclerotia (figure 1) which perpetuate them for more than fifteen years. Their polyphagia allows them to attack and persist over more or less wide ranges of hosts.
• Infection : Direct penetration of the mycelium into the roots, either through various injuries, and / or attacks by root-knot nematodes and Pratylenchus spp. Invasion of the vascular system of plants that they gradually colonize. These react by forming gum or tyloses which prevent their progression. These defense mechanisms, associated with colonization and clogging of vessels by the mycelium, contribute to the wilting of plants.
• Sporulation : production in the vessels of fragile whorled conidiophores forming ovoid conidia (figure 2).
• Dissemination : possible through soil, agricultural equipment soiled by contaminated soil and plant debris. Soil dust containing microsclerotia and / or conidia is easily disseminated by air currents, as well as by splashing water and soil insects.
• Favorable conditions : appreciate variable climatic conditions depending on the species and strains. Their thermal optima would be between 20 and 32 ° C. Short photoperiods and low illumination sensitize plants to verticillium wilt, which is believed to 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.

 Protection

• Use resistant rootstocks if possible.
• Carry out fairly long crop rotations, at least 4 years. Cereals do not appear to be affected by these vascular fungi. Peas, beans and cabbage could be used in the rotations as they would not favor the maintenance of the inoculum in the soil.
• Possible soil disinfection: fumigant, solarization, biofungicides, etc. Let us add that it will be advisable not to work too deeply the disinfected grounds before the setting in culture, under penalty of causing microsclerotia to rise in the treated stratum.
• Destroy host weeds.
• Ensure a balanced manuring in order to avoid obtaining young plants with too succulent tissues.
• Take care of irrigation: optimal quantity, localized supply, etc.
• Irrigate optimally during hot periods to limit wilting.
• Soak the plants at the hottest times of the day to reduce leaf wilting.
• Clean tools and agricultural machinery used in contaminated plots before using them in other still healthy plots. Thorough water rinsing of this material will often be sufficient to rid it of contaminated soil.
• Eliminate diseased plant debris during and at the end of cultivation, as well as potential host weeds likely to harbor or promote the development and conservation of these fungi in the soil.
• If necessary, add a fungicide taking into account the authorized uses.