Biology, epidemiology

• Storage and source of inoculum

Eutypa lata is a very polyphagous ubiquitous fungus, living both saprophytically and parasitically. It is preserved mainly thanks to its mycelium, but especially to its sexual reproduction organs, the perithecia present on the surface of dead wood (figures 1 to 3). Thus, all the vines, the arms, or the horns which remain in place in the plots, at the end of the row or placed in piles near the buildings of exploitation for various uses, are as many inexhaustible sources of inoculum to the origin of the contamination of neighboring vines and vineyards.

It has also been isolated from a large number of angiosperm perennial plants. At least 89 very different host plants have been identified to date. Among them we can cite many forest (willow ), fruit or ornamental species, in particular rosaceae ( Prunus, Malus, Pyrus , ...) and more recently Actinidia . It is not excluded that some of these species, cultivated near vineyard plots, may help to conserve and multiply E. lata .

• Sporulation, dissemination

Moistening the wood promotes the formation and maturation of perithecia (Figures 1 to 3) which can remain fertile for at least 3 years. The emission of ascospores (Figure 4) takes place two hours after the onset of a rainfall of at least 0.5 mm. It can be intensive for about 2 to 3 days. The wind ensures their dissemination sometimes over long distances. The ascospores, which each year constitute the primary inoculum, are projected throughout the year in the climatic conditions of the vineyards on the Atlantic coast. Note that in vineyards where the climate is drier (South of France, California), we do not observe aerial spores during the summer. In addition, in climatic zones where the annual rainfall does not exceed 250 to 300 mm, the presence of perithecia is rare and the impact of eutypia is generally low.
Note that this fungus also forms pycnidia and pycniospores whose germination capacity is very low. Their epidemic role remains to be clarified.

• Plant penetration and host invasion

The ascospores are deposited randomly on the vines, especially on pruning wounds. They germinate at temperatures between 1 ° C and 45 ° C. The mycelium penetrates the wounds of size, gains the vessels of the wood, then the adjacent tissues by the punctures approximately 4 to 14 days after the contaminations. Its progression in wood in the vineyard can reach 5 cm / year.
Wounds of recent size show a significant receptivity which gradually decreases with age under the combined effect of tissue healing and competition from other pathogenic microorganisms or saprophytes occupying the same ecological niche. As spring approaches, crying (flow of sap from pruning wounds) hinders the installation of the fungus, reducing the duration of receptivity of pruning wounds.

E. lata , the aggressiveness of which can vary between isolates, is capable of colonizing all wounds including those produced in the spring during suckering or disbudding. But winter pruning wounds remain the preferred site of penetration. Let us add that this fungus seems specific to woody tissues because it has never been found naturally in herbaceous organs; it is therefore very rare to isolate it from young plants. E. lata is not a fungus transmitted by young plants.
Subsequently, it spreads slowly in the wood and causes a characteristic V-shaped sectoral necrosis. The mode of degradation of the wood observed in transmission electron microscopy, shows that the three main constituents of the woody cell walls, namely the lignin, celluloses and hemicelluloses are attacked simultaneously in sensitive grape varieties, with formation of cavities in the walls, thus giving the wood its brittle character. Several toxic metabolites involved in leaf symptoms, including eutypine, eulatinol, etc., are produced.

The life cycle of eutypia is very long; it can last from 4 to 8 years.

• Favorable conditions

Many cultural or environmental factors can influence the development of eutypia.
The presence of dead wood (horns, stumps) in or near the plots is a major factor in the development of this decline of the vines.
The vigor of the vines was first cited as a factor favorable to eutypiosis in the 1980s, because it obliges the cutters to carry out wounds of more numerous and larger sizes. Later, vigor was considered more of an asset because vigorous vines emit vines allowing them to be coppiced and restored when they are sick. In addition, vigorous vines support the disease longer than others.

Among the other factors also favorable to eutypiosis, let us quote the forms or the modes of size generating more or less numerous and large wounds. The latter are particularly conducive to pollution by ascospores and then to easy colonization of vessels by the mycelium. In this regard, the transformation of the vines in the 1980s, from a low Guyot form to a high form allowing the implementation of mechanical harvesting, led to the formation of significant wounds and thus contributed to the extension of this disease. Forms with long arms (cords) die back more slowly than those with short arms, but may express leaf symptoms longer.
The availability of water, an essential factor for the growth of the vine and of all the pests associated with it, seems to be an essential factor for the development of E. lata . It is generally considered that the expression of eutypia is regular in a region as soon as the rainfall reaches 500 mm / year. Water (rain, dew) is essential for the sporulation of E. lata , but also for the expression of symptoms. Late winter and rainy springs were often reported as favorable to strong expressions of leaf symptoms. In addition, plots submerged each spring for several years showed a progressive worsening of eutypiosis.

The exact influence of the nature of the soil (structure or texture) on the development of eutypia is not yet clearly specified. But it is very likely that the water reserve or the richness in nutrients play an important role. The influence of fertilization is under study. "Chlorosing" soils and hydromorphic soils have already been cited as favorable to this disease.