Biology, epidemiology

• Conservation, sources d'inoculum

Phaemoniella chlamydospora is very frequently isolated from young or old vines, whether or not expressing esca symptoms. It is also produced from rootstocks, roots and soil. It is preserved on the vine by the intermediary of its mycelium but especially of these pycnidia which form in particular near old pruning wounds, in protected areas of the bark (cracks, cracks) or under the latter. Phaeoacremonium aleophilum fruits and is also preserved in crevices in the bark.

These two fungi are also associated with host plants other than the vine that can multiply and preserve them, and act as sources of inoculum when located near vine plots. P. aleophilum is described in particular on alder, willow, prunus, kiwi and olive. Fomitiporia mediterranea is present on many hosts including oak, ash, birch, rowan, poplar, kiwi and apple (Figures 1 and 2) which also ensure its sustainability from one year to the next.

• Penetration, invasion

The conidia produced by the pycnidia of P. chlamydospora (figure 3) pollute pruning wounds, then enter them during the winter, during mild and rainy climatic periods. Note that infections can also occur through any other injury, throughout the year. P. aleophilum (figure 4) is also capable of contaminating pruning wounds, most frequently during the vegetative period of the vine (suckering, disbudding, etc.); peaks of conidial emission were observed throughout this time.

Once in the strain, the mycelium of P. chlamydospora gradually colonizes the various tissues present. Infection by P. aleophilum of the woody tissues of the xylem of the vine leads to a pink-brown necrosis, always starting from the pith. In association with P. chlamydospora , it forms the `` pre-necrosis '' of esca. F. mediterranea , considered secondary, degrades the cell walls of plants thanks to a battery of enzymes emitted in the tissues.

The incubation period is usually quite long, approximately 5 to 8 years.

• Dissemination

P. chlamydospora produces pycnidia on the trunk surface in bark crevices, in which numerous conidia are formed. These are disseminated by water and wind. P. aleophilum also fruits in on the same plant supports and its conidia are dispersed in the same way. The carpophores (or basidiomas, figure 1) of F. mediterranea are sometimes observed on the surface of affected vines. The basidiospores produced, resulting from sexual reproduction, mainly ensure the dissemination of this fungus by air.
It should be noted that P. chlamydospora and P. aleophilum are present in an epiphytic or endophytic state on and in one-year-old vines. They are therefore likely to be propagated by the wood used to make the scions and rootstocks, and to contaminate the plants via various wounds present on them.

Thus, in the nursery, contamination of the plant material by P. chlamydospora is possible during the different stages of the plant development process, in particular during stratification with water or sawdust. It should be noted that the development of this fungus in young vine wood and its ability to form necrosis can considerably depend on the growing conditions.
During the production of the plants, the transmission of the fungi present on the one-year-old wood can be done via the rehydration baths and the grafting tools, or during the stratification in pot. Note that the epidemic impact of contaminated plants in the vineyard is not known with precision. The presence of fungi associated with esca in a young vine does not mean that the vine will necessarily express the disease quickly.

• Favorable factors

The esca is influenced by many more or less well-known factors such as climatic conditions. Mild, rainy summers favor the expression of the slow form, while high summer temperatures lead to the manifestation of the apoplectic form of the disease. The disease rate also varies enormously from one plot to another depending on the age of the vines, the grape variety and the nature of the rootstock, and pedo-climatic conditions.

The driving system strongly influences the incidence of this disease. In fact, pruning, in addition to generating wounds allowing infections, is at the origin of the establishment in the vines of numerous scarring cones of dry wood, the size and location of which in relation to each other generate difficulties in sap circulation. This phenomenon can be problematic for certain pruning methods and constitute an aggravating factor of esca syndrome, in particular with regard to the apoplectic form.

Differences in behavior in Esca (and BDA) between grape varieties are evident in the field. Sauvignon, Cabernet Sauvignon, Chenin, Ugni Blanc and Auxerrois are the most sensitive. Pinot noir, grenache, and muscat appear less sick. The characteristics of the rootstock have an impact on the manifestation of this disease. Rootstocks conferring strong vigor or those poorly adapted to a soil limiting factor appear to show higher mortality rates.

The cultural context also influences the esca. It was shown that in plots with a high useful reserve, characterized by a non-limiting water supply, the disease rate on the sensitive cabernet sauvignon grape was greater than in plots where the vines were under water constraint.