Biology, epidemiology

Phomopsis viticola performs only one biological cycle per year.

• Storage and sources of inoculum

Phomopsis viticola is easily preserved from one year to the next on the vine, mainly thanks to its mycelium and its pycnidia. The first is very present in the buds infected last year; in the spring, in addition to causing their death, it can initiate superficial lesions on the young grow. It is also found on excoriated branches but also in certain parts of the wood since it is easily isolated there.

Mature pycnids are very numerous on bleached wood (Figure 1) and old lesions, and mummified berries (Figure 2). During wet and rainy spring periods, masses of conidia (spores α and β) (Figure 3) escape from pycnids in the form of cirrhes (Figures 4 and 5) and many conidia pollute the young shoots as a result. of water splashes.

P. viticola appears to be able to survive on a few other Vitis spp. secondary: Vitis rupestris , Vitis labrusca and Parthenocissus subquinquefolia . These hosts can ensure its conservation and serve as sources of contamination when they are present near vineyard plots.

• Penetration and invasion

Only alpha spores are able to germinate easily, at temperatures between 1 and 37 ° C. Once the germ tube is initiated, it enters the tissues through stomata, lenticels and wounds. Infections take place in a few hours in the presence of humidity and temperatures around 23 ° C. They are possible at least between 5 and 35 ° C, and they would be optimal between 16 and 23 ° C. The first symptoms appear between 2 and 4 weeks after the first infections. Note that P. viticola is found in wood galleries made by insect larvae; it is also considered to be endophyte in the vine.

• Sporulation and dissemination

Pycnidia form in greater or lesser quantities on the lesions rather at the end of the season, but they do not mature until the end of winter, as bud break approaches. In the spring, the disease is spread by conidia as described above, and this over short distances near sources of inoculum and during rains and splashing water. It has been shown that infected tissues can continue to produce spores for at least 3 seasons. Let us add that P. viticola can also be disseminated via vine wood, cuttings, certain insects, by pruning equipment and agricultural machinery. Finally, it should be noted that it has been found, in a latent state, on many apparently uncut grapevine wood used in nurseries in Poland.

• Conditions favorable to its development

P. viticola appreciates mild to cool temperatures, its thermal optimum is around 23 ° C; its development would be reduced or even zero during very hot summer periods. The period of maximum sensitivity of the vine is between bud break and the 2-3 leaf stage. The continuous spring rains, repeated showers, high humidity, the presence of water on the various organs promote the development of the excoriose agent.

Finally, remember that only young tissues are sensitive and that the grape varieties show differences in sensitivity to excoriose. For example, cabernet franc, cinsault, carignan and merlot would not be very sensitive, while cabernet sauvignon and grenache would be much more.

• Synoptic of the development of P. viticola   (figure 6)