Athelia rolfsii  (Curzi) C. C. Tu & Kimbr.

Sclerotium  rolfsii  Sacc .  

Sclerotium rot

General  

• Soil fungus particularly prevalent in warm tropical and subtropical regions . It is particularly frequent and feared in many market garden soils of the French overseas departments and territories.
• Extremely polyphagous, affecting a large number of plants, whether cultivated or not: at least five hundred species belonging to botanical families representing both mono and dicotyledons.
• Known vegetables affected: eggplant, tomato, chili, various salads, melon, cucumber, watermelon, bean, artichoke, beet, carrot, cauliflower, celery, garlic, onion, radish, turnip, sweet potato, etc.
• Observed in the open field as well as under shelters.
• Organs attacked  : roots, crown, Sclerotium rolfsii attacks all Solanaceae organs located in or near the soil, whether the plants are very young (in nurseries), or adults in the field.
• Symptoms :
  • Wet and dark lesion on the collar (particularly vulnerable) evolving into rotting, and gradually girdling it over several centimeters. The affected cortical tissues turn brown more or less intensely and gradually decompose (Figures 1 and 2). The underlying vascular tissue may turn brown (Figure 3).
  • Root browning and rotting (Figures 4-6).
  • More or less reversible secondary leaf wilting, the plants regain their usual turgor during the night (figure 7). During hot weather, withering is sudden and complete, the plants eventually dry out (figure 8).
  • Large, moist, soft lesions on leaves and fruits in contact with the soil (Figures 9 and 10).
  • Distribution of diseased plants in focus.

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• Signs  : dense whitish mycelium covering the damaged organs (figures 11 and 12). Smooth, rather spherical structures (1 to 3 mm in diameter), first white in color, then tawny to reddish brown, gradually visible within the mycelium: sclerotia resembling mustard seeds. (figure 13)

• Confusions possibles  :  Rhizoctonia solani ,  Oomycètes  ( Pythium  et  Phytophthora  spp.) 

Biology

• Conservation : persists for several years rather on the soil surface on plant debris, in the form of aggregated mycelium ( figures 1 and 2 ), but especially sclerotia (figures 3 to 6) free or associated with plant debris. This fungus is also able to survive on different organic substrates.
• Sources of inoculum :  contamination takes place via the mycelium already present in the soil (Figures 4 and 5) or from sclerotia.
• Infection : directly penetrates the tissues and invades them thanks to lytic enzymes.
• Development, sporulation : Rapidly forms mycelium and sclerotia on damaged tissue.
• Dissemination : ensured by soil contaminated by sclerotia, tillage, soiled tools and tillage machinery, water and plants produced in infested nurseries.
• Favorable conditions : appreciates hot climates and proliferates following humid periods and / or irrigation. Favored by acidic, asphyxiating soils, and temperatures between 25-35 ° C.

 Protection

• Carry out fairly long crop rotations in virgin land, they are no longer very effective in contaminated soil.
• Possible soil disinfection: fumigant, solarization, biofungicides, etc.
• Liming the soil, ensuring a fertilization rich in calcium.
• Avoid planting too deeply, burying the crown of the plants.
• Put in place a plastic mulch to create a mechanical barrier between the soil and the plant organs.
• Eliminate healthy or diseased plant debris during and at the end of cultivation, as well as potential host weeds liable to harbor or promote the development and conservation of this fungus in the soil.
• Take care of irrigation: optimal quantity, localized supply, etc.