Biology, epidemiology

The few Alternaria growing on grape berries have biological characteristics comparable to other pathogens in the bunch. They often live as saprophytes on the vine and in its environment, becoming opportunistic pathogens when the physiological state of the berries is modified (presence of wounds, advanced maturity ... ).

• Storage and sources of inoculum

The Alternaria spp. belong to the superficial flora of many plant species, and in particular to that of the vine. Although very present on this plant, they cause damage in France only very occasionally in vineyard plots intended to produce table grapes in particular. Their status as opportunistic pathogens and their saprophytic potentials allow them to easily maintain themselves on vines, particularly at the level of their bark, senescent floral parts but also in their environment (various plant debris, on and in the soil, alternative hosts) .

• Host penetration and invasion

These fungi penetrate berries mainly when ripe, in at least two ways:
- directly through the cuticle in the presence of exudates;
- through the stomata, lenticels and microcracks, various injuries such as damage linked to pathogenic insects or fungi, physiological micro-bursts, bird pecks or solar burns.

Subsequently, their mycelium grows very extensively, growing in all directions inside the flesh which gradually degrades as the berries slowly shrivel.

• Sporulation and dissemination

The Alternaria spp. show a behavior comparable to that of many rotting agents. They sporulate quite easily on the surface of berries and plant debris and their conidia are dispersed by air currents and / or splashing water following rains.

• Factors influencing development

The growth of Alternaria spp. is favored by humid climatic conditions and optimal temperatures between 18 and 30 ° C. Alternaria alternata can thrive at temperatures between 2 ° C and 32 ° C, its thermal optimum being between 25 and 29 ° C. Berries that have lost their integrity (wounds, damage linked to a primary colonizer ) and / or have reached a more or less advanced stage of maturity are particularly vulnerable. This is particularly the case for grains damaged by solar burns. The rains of late summer and autumn favor contamination.