Macrophomina phaseolina (Tassi) Goid., (1947)

Root rot

 

General  

  • Very polyphagous telluric fungus (more than 500 known hosts), with rather limited saprophytic potential, would be an optional parasite appreciating high temperatures.
  • Present in many countries on various cultivated plants, especially active in hot, tropical and temperate production areas.
  • Occasionally found on eggplants grown in soil in France, especially in the South-West. Its damage can occasionally be very spectacular on roots.
  • Can act in complex with other telluric pests ( Colletotrichum coccodes , Pyrenochaeta lycopersici , Pythium spp., Rhizoctonia solani , Meloidogyne spp.).
  • Synonymy: Rhizoctonia bataticola (Taubenh.) EJ Butler, (1925)
  • Organs attacked  : mainly the roots which can be completely destroyed.
  • Symptoms :
    • Extensive, moist, brown to black root lesions, sometimes superficially suberized. Eventually, many roots take on a black tint and rot.
    • Reduced plant growth, or even more or less reversible secondary leaf wilting is observed, as well as the decline of certain plants.
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  • Signs  : dense brown to black superficial mycelium reminiscent of Rhizoctonia solani ; multitude of black microsclerotia (50 to 200 µm); Globular and black pycnidia (whose diameter can fluctuate between 100 and 250 µm) forming hyaline, ellipsoid to oval conidia, measuring 16-29 x 6-9 µm.
  • Confusions possibles  :  Rhizoctonia solani ,  Oomycètes  ( Pythium  et  Phytophthora  spp.).

Biology

  • Conservation : M. phaseolina is able to be preserved in the soil for several years in the absence of susceptible hosts, in particular thanks to its sclerotia  (figures 1, 2, 3, 6). These are very resistant, they withstand various difficult conditions, including temperatures above 55 ° C. Affected roots "harbor" sclerotia and are invaded by its mycelium (Figures 4 and 5), also contributing to its conservation. Alternative hosts, cultivated plants (tomato, pepper, various Cucurbitaceae, tobacco, potato, bean, chickpea, strawberry, okra, sunflower, corn, sorghum, soybean, alfalfa, white clover ... ??) or weeds also ensure its multiplication and maintenance in the soil.
  • Sources of inoculum : the sclerotia, vore the mycelium are at the origin of the majority of telluric contaminations.
  • Infection : The mycelium present in the soil or from the sclerotia comes into contact with the roots and penetrates them.
  • Development, sporulation : it gradually invades them and reaches the vessels. Once firmly in place and having more or less degraded the tissues, the fungus produces more or less sclerotia depending on its location on the plant and the surrounding conditions.
  • Dissemination : the sclerotia ensure the dissemination of this fungus, which is carried out by splashing, workers, running water; soil particles transported by tillage machinery also contribute to its dispersion. It is also easily disseminated by the nutrient solution in soilless crops, even more so if it is recycled. The role of conidia in the dispersal of this fungus seems poorly understood.
  • Favorable conditions : the survival and activity of this fungus in the soil is influenced by mineral fertilization and organic amendments. Hot temperatures, around 28 to 35 ° C, and water stress favor its attacks. Its growth would be possible between 10 and 40 ° C.

Protection

  • Choose plots free from this fungus, or not having grown crops of very sensitive plants such as sunflowers.
  • Carry out preventive crop rotations, before the soil is heavily contaminated. They should last at least 3 to 5 years. Of course, the other plants entering the rotation will not be sensitive (certain cereals such as wheat or barley for example). In addition, weeds likely to harbor the fungus will be eliminated from the plots. with will be carried out
  • Soil fumigation and solarization do not seem to effectively control the attacks of this fungus.
  • Disinfect or replace the bags, pots, gutters of infested soilless crops.
  • Choose a draining, well-worked soil: a good subsoil will allow the roots to access new layers.
  • Use healthy substrates (disinfected) and avoid placing the clods on the ground. Indeed, they often become contaminated in contact with the latter, in particular if it has not been disinfected.
  • Avoid high plant densities and ensure optimal fertilization, especially nitrogen and phosphorus;
  • Avoid water stress on plants; avoid harvesting injured and / or overripe fruits.
  • In the presence of wilting during cultivation, try to keep the plants alive as long as possible; for this, it is advisable to butter them in order to promote the emission of adventitious roots which can replace the old damaged roots; to bathe them during the hottest periods of the day to avoid excessive and uncompensated evaporation, leading to wilting, drying out and death of the plants; carefully monitor irrigation and avoid excess which amplifies root lesions by asphyxiation.
  • eliminate and carefully destroy diseased plants and their root system during and at the end of cultivation; bury them deeply so that the fungus can be subjected to the action of potential soil antagonists.
  • Grafting on KNVF type rootstocks should not solve this problem.
Last change : 10/12/21
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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Figure 5