Sclerotinia sclerotiorum  (Lib.) de  Bary

General

• Soil fungus widely distributed throughout the world; although rather feared in temperate regions, it is also occasionally observed in warmer regions but its incidence has nothing to do with that of Sclerotium rolfsii .
• Rather polyphagous, it is capable of infecting many vegetables belonging to various botanical families (Solanaceae, Cucurbitaceae, Compositae, etc.).
• Observed in the open field, more damaging under shelters, in soil but also above ground.
• Organs attacked  : stems, inflorescences and fruits, sometimes leaves.
• Symptoms :
  • Damping-off, collar lesions on seedlings and collapse of these.
  • Elongated lesions on stems  initiating from senescent or injured tissues (aborted fruits, floral parts, senescent leaves, pruning wounds and various ...). These can be localized near the collar or distributed along the length of the stems. They appear damp and dark at first, then necrotize and take on a beigeish and brown tint, and eventually surround them by several centimeters (Figures 1 to 3).
  • The distal part of the twigs girdled, the leaves may turn yellow, wither and wither (Figure 4).
  • Browning and necrosis of vascular tissue (figure 5), recess of the marrow invaded by mycelium and black sclerotia (figure 6)
  • A moist, soft, dark rot developing on fruits, at the stylar scar or in contact with the soil. It spreads rapidly and gradually spreads; the fruits end up shrinking and / or collapsing more or less completely (figure 9).
  • Diseased plants divided into foci (figure 8)

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• Signs  : mycelium  more or less cottony and white on all affected tissues (figure 9), as well as large black irregular structures, sclerotia which are  rather elongated, measuring 2-20 mm x 3-7 (figures 9 and 10). Small "trumpets", apothecia, forming on the larger sclerotia (Figure 11).
• Confusions possibles  :  Sclerotium rolfsii ,  Rhizoctonia solani  aérien, 

Biology

• Storage : has significant saprophytic potential. It can be maintained in the soil for 8 to 10 years thanks to its sclerotia (figures 9 and 10) that it produces on the affected organs and / or on its mycelium (figure 3) present in the plant debris abandoned on the plots. Reported on more than 400 different plant species, cultivated or weeds, in particular many vegetable crops (salads, beans, cabbages, peppers, eggplant, many cucurbits, celery, peas, carrot, rutabaga, potato, etc.) and a certain number of weeds unnoticed.
• Sources of inoculum : contamination occurs through the mycelium from sclerotia found near organs in contact with the soil. Possible airborne contamination via ascospores produced by apothecia (figure 11) ensuring its sexual reproduction. these germinate on plant tissues only in the presence of water from rain, sprinkler irrigation or condensation or dew.
• Infection : easily penetrates and invades living, injured, senescent or dead organs whether or not in contact with the ground.
• Development, sporulation : produces mycelium and sclerotia in and on tissues.
• Dissemination : through the soil present on tillage tools or on plants soiled by sclerotia, and by ascospores carried by wind and air currents over significant distances.
• Favorable conditions  : thermal optimum located slightly below 20 ° C, but capable of developing at temperatures between 4 and 30 ° C. Development facilitated by humid and rainy periods, by tissues having reached an advanced stage, light soils rich in humus.

Protection

• Carry out fairly long crop rotations in virgin land, they are no longer very effective in contaminated soil.
• Possible soil disinfection: steam, fumigant, solarization, biofungicides, etc.
• Well worked and drained the soil in order to avoid the formation of puddles of water favorable in particular to the formation of apothecia responsible for air contamination
• Labor deeply in order to bury the sclerotia in depth which will be destroyed more quickly. Immersion of infested plots would reduce the number of viable sclerotia present in the soil. Orient the rows of planting in the direction of the prevailing winds so that the crown of the plants and the vegetation cover are well ventilated.
• Control nitrogenous manure, which should neither be too strong (at the origin of very receptive succulent tissues), nor too weak (sources of chlorotic leaves constituting nutrient bases).
• Reduce the ambient humidity of the crops and avoid the presence of free water on the plants: ventilate the shelters as much as possible, irrigate preferably in the morning and early afternoon - never in the evening. Prefer localized irrigation to sprinkler irrigation.
• Put in place a  plastic mulch in order to create a mechanical barrier between the soil and the plant organs., Even in order to trap the ascospores released from the apothecia.
• 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.
• If necessary, use a biopesticide based on Coniothyrium minutans , and / or spray fungicides taking into account the authorized uses.