Didymella lycopersici Kleb. (1921)

Chancres à Didymella

( Didymella stem canker and fruit rot)

General

• Fungus reported in many countries , especially in Europe (Great Britain, Germany, France, Netherlands, Italy, Spain, Norway, etc.). Also described in New Zealand, India, Africa, and in particular Morocco, several American countries (Mexico, Canada, United States), Asia.
• Sevit on Solanaceae crops rather in the open field than in shelters, in soil as well as in soil.
• A fungus rather specific to the aerial organs of the tomato, which can also be observed much more occasionally on the plant cover of eggplant, or even on peppers and potatoes .
• Its attacks are sporadic and rather rare on aubergines in France.
• Once present in a farm, it remains there for the long term.
• Organs attacked  : leaves, fruits, stems.
• Symptoms :
  
  • Canker lesions on stems, slightly concave, located at different levels of the stem: sometimes on the surface of the soil or just below (black foot), high at the level of leaf stripping and disbudding wounds. Rather moist and dark brown, the epidermis and cortex gradually break down and the xylem tissues turn brown.
  • These cankers spread and gradually surround the stem and / or the petioles, thus disrupting the sap currents.
  • Eventually, yellowing, wilting and drying out of the leaves located downstream of the lesions. When a canker has surrounded the crown, the entire plant may die.
  • Spots on leaves initially brown and damp, gradually necrotic. Brown patterns in more or less concentric arabesques are sometimes visible on the injured tissues. These spots are often chlorotic on the periphery.
  • Alterations also develop on the fruits, often at the level of the stalk scar; these are wet, black, and evolve quite quickly. Eventually, large spots with concentric rings cover the fruits. These can gradually mummify or fall. Symptoms on fruits also appear during their conservation and marketing.
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• Possible confusion  : Sigatoka.
• Signs : tiny globular structures (distinguishable with the naked eye but more easily with a magnifying glass), more or less brown to black, dot the altered tissues. Two types of conceptacles, brownish to black, are formed in the tissues:
  • Of pycnidia subepidermal ostiolées diameter of between 180 and 250 .mu.m on B and between 140 and 200 .mu.m on culture medium ( anamorph form of the fungus); they are the source of conidia colorless , mostly unicellular but also bicellular (2-3 x 6-10 µm).
  • Of perithecia , embodying the teleomorph are rarely formed in nature (diameter between 120 and 210 .mu.m). These give rise to ascospores also hyaline and bicellular (5.5-6.5 x 16-18 µm).

Biology  

• Conservation : in the soil on plant debris and organic matter. Its survival is increased in the presence of humidity, organic matter and fairly low temperatures. Can be found on seeds, in the form of mycelium and pycnidia. It could be preserved on various nightshades (eggplant, potato, tomato, pepper, black nightshade).
• Sources of inoculum : present in the environment of greenhouse plants and on equipment that has been in contact with diseased plants (stakes, stakes, pottery, etc.).
• Infections : The pycniospores are mainly at the origin of the primary contaminations; the role of perithecia and ascospores appears to be much more limited. The first contaminations take place following a wet period, directly through the cuticle, via the stomata or via wounds. This fungus then quickly invades the tissues that it alters.
• Dissemination : ensured mainly by pycniospores following splashing occurring during rain or sprinkler irrigation; to a lesser degree the tools, hands and clothes of workers during cultivation operations. Ascospores, when perithecia form, are dispersed by the wind. Seeds can also contribute to the sustainability and dissemination of this fungus.
• Conditions favorables : L'humidité est le facteur qui influence le plus le développement de D. lycopersici. Les pluies, les rosées matinales prolongées, les aspersions favorisent la maladie. L'eau stagnante sur les feuilles stimule la germination des spores, la pénétration des filaments germinatifs, et ultérieurement la formation des fructifications. Ce champignon se développe à des températures comprises entre 13 et 29°C, l'optimum se situant aux alentours de 20°C. Il affectionne plutôt les vieilles plantes et celles ayant reçu des fumures azotées ou potassiques réduites. Sa croissance est sensiblement réduite à partir de 30°C. 

Protection

• Carry out crop rotations of several years. It will of course be advisable to avoid alternating the eggplant with other nightshades or leaving weeds of this botanical family in the plot or its environment, in particular the black nightshade which must be destroyed fairly systematically.
• Disinfect particularly contaminated soils. The method used will vary depending on the country and the type of crop: steam, fumigant, fungicides, solarization. The latter method, used in particular in Sicily and Morocco, seems particularly advantageous for greatly reducing the quantity of inoculum on the surface of the soil.
• The greenhouse environment and the equipment that has been in contact with diseased plants must be carefully disinfected (bleach, fumigants, various products). The stakes can be solarized. They will be covered with a transparent polyethylene film and exposed as they are to solar radiation in summer. The high temperatures obtained under the film will destroy a large proportion of the resident inoculum, this fungus being eliminated at more than 50 ° C.
• Thoroughly disinfect affected nurseries.
• Use unharmed seeds.
• Do not plant in hydromorphic soils.
• Choose a planting density ensuring good aeration of the vegetation, good wiping after rains or sprinkling irrigations.
• Avoid any stress to the plants, and ensure them a balanced manure, in particular in nitrogen.
• Mulch the soil to form a mechanical barrier reducing contamination
• Prefer drip irrigation rather than sprinkling. If this is not possible, the latter will be carried out at times of the day allowing the rapid drying of the vegetation.
• Note that the rootstocks resulting from interspecific crosses with Lycopersicon hirsutum are resistant to D. lycopersici . Grafting will be of interest if the attacks are mainly localized at the neck.
• Eliminate plant debris, remove them from the plots and destroy them. They must in no case be buried in the ground.
• When the first spots or the first cankers observed affect a limited number of plants, it may be wise to remove them quickly and carefully by placing them in a plastic bag before removing them from the crop.
• If necessary, spray fungicides taking into account the authorized uses. Applications will be made every 7 to 10 days, on dry plants, and should be repeated following heavy rains exceeding 20 mm. It should be remembered that strains tolerant to certain fungicides have been reported. Distrust, alternate families of different fungicides.
• It should be noted that some attempts at biological control of D. lycopersici by Trichoderma harzianum varying have been carried out with degrees of success.