Thanatephorus cucumeris

Rhizoctonia solani

General

• Globally fungus widespread soil .
• Considered as a parasitic fungus of the soil in temperate and Mediterranean zones, having carried sensitive crops on several occasions, in particular vegetables, it is particularly responsible for foliar attacks in very humid equatorial or tropical climates.
• Responsible for damping off seedlings in the nursery, and various symptoms in the field on underground or aerial organs (Leaf Rhizoctonia - web-blight) of vegetables.
• Several reported strains differ in their cultural characteristics, pathogenicity, and in particular their host specificity.
• Extremely polyphagous, capable of infecting several hundred different hosts, including a large number of vegetable crops.
• Known affected vegetables: eggplant, tomato, chili, various salads, melon, cucumber, watermelon, zucchini, squash, beans, carrot, cabbage, celery, radish, turnip, sweet potato, etc.
• particular on eggplant with Colletotricum coccodes , Interacting in Pyrenochaeta lycopersici , or even root-knot nematodes. Its parasitic effects seem rather limited on adult plants, occasionally damaging on seedlings and seedlings in nurseries.
• Organs attacked  : roots, crown and bottom of stem on young seedlings. Sometimes leaves and fruits in tropical areas.
• Symptoms :
  • Damping-off, occurring both pre- and post-emergence of seedlings.
  • Brownish alteration more or less surrounding the part of the stem located at or near the ground level, which can lead to the collapse and death of the seedlings.
  • Cankers at the neck of young or more developed plants, reddish-brown in color and varying in consistency with soil moisture (Figures 1, 3 to 10).
  • More or less extensive reddish-brown root lesions, rather moist, becoming superficially corky.

• >>> More pictures
• Signs  : several fungal structures make it possible to confirm the presence of R. solani on or near the altered tissues:
  • often inconspicuous whitish to brown filaments  running over germinated seeds, roots, stems and fruits;
  • more rarely poorly defined, brown masses visible locally on the damaged tissues (pseudo-sclerotia).

• Possible confusion :

Biology

• Storage : persists for several years in the soil as a saprophyte in the absence of susceptible hosts, in the form of mycelium (figures 1 to 5) and pseudo-sclerotia (figures 5 and 6) present in organic matter and plant debris. Sometimes present in certain substrates and composts.
• Sources of inoculum : contamination through mycelium already present in the soil or from sclerotia. They can also take place via basidiospores resulting from its sexual reproduction (figure 7).
• Infection : penetration of tissues directly through the cuticle or through various injuries. Rapid inter- and intracellular tissue invasion by lytic enzymes.
• Development, sporulation : formation of mycelium that travels on the tissues and on the ground, and reaches other healthy organs. Sclerotia, mixed with soil particles contaminating different materials, also contribute to its spread. Production of basidiospores formed on basidia present on the surface of the hymenium.
• Dissemination : by basidiospores carried by wind and air currents over fairly large distances. Transmission also ensured by soil contaminated by mycelium and sclerotia, tillage, soiled tools and tillage machinery, plants produced in infested nurseries.
• Favorable conditions : appreciates as well in humid and heavy soils as in lighter and drier soils, at acidic or basic pH and at temperatures between 5 and 36 ° C. Particularly damaging in the presence of humidity and when the temperatures are rather mild, of the order of 23-27 ° C or when they are unfavorable to its hosts.

Protection

• Carry out fairly long crop rotations in virgin land, they are no longer very effective in contaminated soil.
• Use a healthy substrate and quality plants.
• Possible soil disinfection: fumigant, solarization, biofungicides, etc.
• 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.
• Stake certain productions and aerate the vegetation as much as possible.
• Take care of irrigation: optimal quantity, localized supply, etc. Avoid sprinkler irrigation in the presence of Leaf Rhizoctonia. We will also avoid situations of shaded plots.
• 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.
• If necessary, spray fungicides taking into account the authorized uses.