Biology, epidemiology

• Conservation, sources d'inoculum

Cladosporium cucumerinum is preserved on fruits and plant debris present on and in the soil, but also on seeds. It is able to maintain itself on the walls of shelters and in the environment of young plants in the nursery.

Note that this fungus would be endowed with important saprophytic abilities allowing it to easily perpetuate itself in plots in the absence of cucurbits, for example on organic matter from various plants. Under these conditions, its survival could be greater than 3 years. As soon as the climatic conditions allow it, it produces numerous conidia which ensure the primary contaminations.

• Penetration, invasion

In the presence of free water on the plant, the conidia germinate , emit a germ tube and mycelial hyphae directly penetrate the cuticle in less than 9 hours, and gradually invade the tissues. Its progression in the leaf tissues would be intercellular and its pathogenic power would be exerted thanks to enzymes (cellulases, pectinases ) and toxins. The first symptoms are sometimes visible in less than 3 days.

• Sporulation and dissemination

Quite quickly, this fungus sporulates on weathered tissue as a velvety dark gray to black. The numerous conidia formed (Figures 1 to 2), carried by conidiophores branched (Figure 1), are disseminated by wind and drafts in the shelters, as well as by the clothing and tools of workers and gatherers. These spores form when climatic conditions are humid. They are dispersed more easily when the climate becomes drier. Note that the conidia are relatively resistant and withstand transport over long distances. During heavy rains and sprinkler irrigation, a redistribution of conidia on the vegetation also occurs as a result of splashing. Let us add that the seeds, and even insects, contribute to their dispersion.

• Conditions favorable to its development

C. cucumerinum "appreciates" very much climatic conditions cold and humid and is severely affected in poorly drained plots.

Its cardinal temperatures of development are 5 and 30 ° C; the optimum for spore germination and mycelial penetration is around 17 ° C to 20 ° C. Nights at 15 ° C and days at 25 ° C are also very favorable. Penetration can take place after a period of saturated humidity at night of 6 hours or three times 2 hours. The disease progresses rapidly thanks to 30 hours of saturated humidity. It decreases as soon as the temperature rises above 22 ° C, and hardly manifests itself at 30 ° C. Following heavy rains, for example, symptoms on leaves and fruits appear in 3 to 5 days and sporulation occurs a day later.

Periods of fog, abundant and frequent dews, and light rains are also very conducive to leaf spot.

Remember also that young tissues (seedlings, tips, young fruits) are particularly sensitive. The epidemic cycle of leaf blight is relatively short, less than 7 days. Thus, many cycles can take place during plant growth, as long as the climatic conditions are favorable. Note also that cladosporiosis can develop during storage of fruits at low temperatures, for example between 2 and 8 ° C.

Many artificial culture media are favorable for the growth and sporulation of C. cucumerinum . On these media, mycelial colonies are often greenish black to pale brown in color (Figures 3 and 4).