Biology, epidemiology

• Conservation, sources d'inoculum
  

Although a poor competitor in the soil, Colletotrichum coccodes is easily preserved there, on plant debris or not, thanks to its sclerotia which allow it to persist there for several months, even several years. It is also capable of colonizing at least 58 plant species belonging to at least 19 botanical families, first of all the vegetables belonging to the nightshade families (pepper, eggplant, potato ) and cucurbitaceae (watermelon) which can play the role of role of alternate hosts or contribute to increasing soil inoculum. It would have been successfully inoculated with strawberry and onion. Several weeds perform the same functions: Solanum capsicastrum, S. dulcamara, S. nigrum, Abutilon theophrasti, Amaranthus retroflexus, Chenopodium album, Convolvulus arvensis, Capsella bursa-pastoris ). It has also been isolated from the roots of various plants showing no symptoms: cabbage, lettuce, watercress, white mustard, chrysanthemum We have isolated it several times from the water used for soilless cultures, water coming from open-air storage ponds.

• Penetration and invasion

Sclerotia are likely to produce mycelium or acervuli, in which are formed conidia . The latter germinate on the surface of green or mature fruits and adhere to them thanks to appressoria . From these structures, penetration takes place directly through the cuticle. It can also be achieved through various wounds present when ripe on tomatoes.

The methods of penetration into the root are quite comparable since appressoria form on contact with it. Note that the colonization of the roots increases with their age. In addition, the sclerotia and subsequently the mycelium coming into contact with the fruits and roots can also lead to infections. Once in place, the fungus colonizes the subcuticular tissues or the cortex through its mycelium, and exerts its parasitism through an extracellular protease. The infection can also get stuck. In the latter case, we will speak of “latent infection”. This type of situation can occur on green fruits, roots and even the stem as well. Green fruits see this latency lifted at maturity or if they are subjected to low temperatures.
 

• Sporulation and dissemination

Whether it is on the fruit, and to a lesser extent the roots, C. coccodes produces acervuli intra- or subepidermal, materializing its asexual multiplication (Figures 1 to 3). These structures, 200 to 300 µm in diameter, produce cylindrical, hyaline and non-septate conidia (16-24 x 3-7.5 µm) which are immersed in a gelatinous matrix that protects them from desiccation. These conidia partly ensure the dissemination of this fungus, which is carried out by splashing associated with rains and sprinkling irrigations, workers, animals and equipment moving through the plots. The flowing water, the soil particles transported by the agricultural 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. Of Microsclerotia , often smaller than one millimeter, also form in the tissues; they help both to conserve and to disseminate C. coccodes .

• Conditions favorable to its development

This fungus is able to develop over a fairly wide temperature range, although the germination of its conidia is optimal at 22 ° C. At this temperature, appressoria form after 6 hours. It no longer takes place at 7 ° C and is greatly slowed down at 10 ° C and above 35 ° C.

Once its conidia have germinated, the development of anthracnos e is greatly influenced by climatic conditions. Attacks can take place at temperatures between 15 and 30 ° C ; they would be much more limited to 10 ° C, and no lesion would appear at 38 ° C. Sporulation on fruits is optimal at 28 ° C. High humidity and the presence of liquid water are necessary for the germination of the spores: rainy periods and irrigation by sprinkling favor anthracnose. Free water on fruits for several hours, linked to the duration and frequency of rains and sprinkler irrigation, greatly increases its impact on the crop.

Monoculture of tomato or KNVF-type rootstocks on the same soil leads to an increase in the rate of inoculum therein, and therefore worsening of root symptoms. Wet soils, excess salinity predispose plants to attacks by C. coccodes .