Eggplant - Colletotrichum coccodes

Colletotrichum coccodes (Wallr.) S. Hughes, (1958)

Root rot

General

A very cosmopolitan and polyphagous fungus, it is especially reported in many tomato and potato-producing countries, on all continents.
Rather frequent and underestimated on eggplant, affecting in particular crops under cover, whether the plants are grafted, or even free-standing.
It should be noted that the "monoculture" of tomato and eggplant plants grafted on KNVF type rootstocks is at the origin of the strong extension in many soils of this parasitic fungus.
It is especially part of a root parasitic complex of several soil pests classified according to their decreasing incidence: Colletotrichum coccodes , Meloidogyne spp., Phytophthora nicotianae , Pyrenochaeta lycopersici , Rhizoctonia solani, Globodera tabacum .
Isolates of C. coccodes appear to exhibit quite variable pathogenic powers, growth rates, and sclerotia sizes.
Organs attacked : mainly the roots which can be completely colonized.
Symptoms :
- Lesions on extended roots, moist, brown to black, sometimes superficially suberized. They are present in low numbers on the main roots.
- The rootlets are few or non-existent because a brown cortical rot has destroyed them. The cortex of the main roots shows dark brown lesions, more or less suberized.
- Stem base rot has occasionally been reported. The decomposed cortex is detached in places from the central cylinder.
- In soilless culture, the attacked roots are poorly developed, discolored and partially decomposed.
- More or less reversible secondary leaf wilting is observed, as well as the dieback of certain plants.
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Signs : acervuli 200-300 µm in diameter, producing 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. Microsclerotia, often smaller than a millimeter, also form in the tissues.
Confusions possibles : Rhizoctonia solani , Oomycètes ( Pythium et Phytophthora spp.)

Biology

Conservation : a poor competitor in the soil, Colletotrichum coccodes is still easily preserved there, on plant debris or not, thanks to its sclerotia which allow it to persist there for several months, even several years.
- Capable of colonizing at least 58 plant species belonging to at least 19 botanical families, primarily vegetables belonging to the Solanaceae families (tomato, pepper, eggplant, potato ??) and cucurbits (watermelon) which may play the role alternate hosts or contribute to increased soil inoculum.
- It is said to 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.
Inoculum sources : the sclerotia are capable of producing mycelium or acervuli, in which are formed conidia.
Infection : appressoria from conidia form on contact with the roots. In addition, the sclerotia and subsequently the mycelium coming into contact with the 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.
- Note that the colonization of the roots increases with their age.
Development, sporulation : it rapidly colonizes the tissues of the cortex and the vessels that it causes to rot. In damaged tissues, it produces numerous chlamydospores. It also forms on the surface of the roots, along with a multitude of endoconidia.
it produces intra- or sub-epidermal acervules, materializing its asexual multiplication (figures 1 to 3), as well as numerous dark brown to black sclerotia.
Dissemination : conidia like 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.
Favorable conditions : it thrives in a fairly wide temperature range, even if the germination of its conidia is optimal at 22 ° C. The monoculture of eggplant 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 .

Protection

Carry out preventive crop rotations, before the soil is heavily contaminated. They should last at least 3 to 4 years. Of course, other plants entering the rotation will not be susceptible. In addition, weeds likely to harbor the fungus will be eliminated from the plots.
Disinfect heavily contaminated soils (solarization, biofumigation, fumigant, etc.).
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.
In the presence of wilting during cultivation, try to keep the plants alive as long as possible; for this it is appropriate:
- to butter them in order to promote the emission of adventitious roots which can replace the old damaged roots. In soil-less cultivation (on peat or on pozzolan + peat) and during serious attacks, peat can be added locally to the collar to allow additional rooting. Sawdust is sometimes used;
- 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. Indeed, if the plants wither, it is not necessarily linked to a lack of water, but rather to root alterations due to Colletotrichum coccodes . In some cases, growers tend to increase irrigation to respond to wilting, leading to amplification of root lesions through asphyxiation.
Eliminate and carefully destroy diseased plants and their root system during and at the end of cultivation.
To our knowledge, no resistance to root attacks by C. coccodes has been described. Grafting is not a possible alternative, because KNVF type rootstocks are sensitive to C. coccodes , and their intensive use leads to an inexorable increase in damage to the roots.