Biology, epidemiology

• Conservation, sources d'inoculum

Colletotrichum orbiculare can be preserved during the winter in the ground, in particular on plant debris, or from straw manure persisting for up to 5 years on the latter. The stromatic masses (figure 1), which it forms on the fruits, represent a certain form of preservation.

In addition, it is a so- called seminicolous fungus because it can also be maintained on the seeds which become contaminated during their extraction from infected fruits.

Let us add that it is probably preserved on various cultivated or wild cucurbits in regions where climatic conditions allow their development all year round. It has been reported on: calabash, pumpkin, cucumber, squash, spaghetti squash, zucchini, christophine, giraumon, bitter gourd, melon, watermelon, patisson, pumpkin, Luffa spp. , Momordica charantia ...

• Penetration and invasion

The conidia of C. orbiculare germinate on the surface polluted plant organs, and form a appressorium , at the end of short germ tube, which allows it to bind and penetrate tissues directly. Note that the production of melanin by this fungus would be essential for its penetration into the host plant. Subsequently, the mycelium, mainly intracellular, invades the tissues. The invasion takes approximately 72 hours after the spores arrive on the plant organs, and the first symptoms appear within a week after infection.

• Sporulation and dissemination

As previously stated, C. orbiculare produces in about five days, weather permitting, acervuli (Figures 2 and 3) on leaf lesions or on those localized on stems and fruits. These structures, materializing its asexual reproduction, generate abundant conidia (13-19 x 4-6 µm) (figure 4) grouped together in the form of a mucus which are disseminated in different ways:
- by water and splashing water occurring as a result of rains, runoff, condensation, or sprinkler irrigation;
- by the wind carrying in particular the fine droplets of water. Thus, the disease often spreads rapidly during heavy windy thunderstorms;
- by workers during cultivation operations and agricultural tools;
- by certain insects .

Note that seeds contaminated also contribute to the spread of this disease, and sometimes over long distances.

The perfect or teleomorphic form of this fungus is rarely seen in nature.

• Conditions favorable to its development

The humidity especially , temperature to a lesser degree, influence anthracnose epidemics on cucurbits.

Contamination often takes place following wet periods, thanks to rains and sprinkler irrigation. They are carried out for example in 24 hours at 100% humidity and for temperatures between 19 and 24 ° C. Note that the spores germinate at temperatures between 5 and 30 ° C, they have poor resistance to desiccation once disseminated. On appressorium the contrary, is very resistant. In addition, the frequency of infections increases proportionally with the duration of leaf humidity, whatever the temperature. The optimum temperature for infection would be between 21 and 24 ° C.

A few studies carried out in various countries supplement these data, in particular:
- in China, anthracnose appears mainly between 20 and 30 ° C, and its extension is reduced at an air temperature below 20 ° C. Optimal conditions are temperatures between 25-30 ° C, plus 50 mm of precipitation and relative humidity above 80%;
- In India, during the monsoon period, infection rates are reported to be highest at temperatures between 25 and 32 ° C, and relative humidity above 90% in the morning and 80% in the evening, associated with rainfall.