First international workshop on grapevine trunk disease

Wines & Vines, Jan, 2000 by Ian Pascoe

The First International Workshop on Grapevine Trunk Diseases was held in Siena, Italy in the first week of October, 1999. The meeting was organized by the Mediterranean Phytopathological Union, and the International Council of Grapevine Trunk Diseases. Plant pathologists and viticulturalists from 20 countries were represented at the meeting, and almost everyone in the world who is involved in working with trunk diseases was there. This means that we were presented with the latest and best available information about trunk diseases. The proceedings of the workshop are being published in the journal "Phytopathologia Mediterranea". The references in quotes below are to presenters at the workshop and refer mostly to statements made in the workshop abstracts, or to comments made during the workshop.

So, what did we learn?

The first thing we learned is that there is still an awful lot to learn! More questions were posed than answers given. Very little is known about how these diseases spread, how they infect, how they affect the vine, and most importantly, how to manage them so as to return reasonable yields from affected vineyards. Much of the discussion during the conference (outside of the formal presentations) centered on the unanswered questions and concerned opinions more than facts. Presentations covered a wide range of topics including the taxonomy and molecular biology of the fungi involved, the biochemistry of toxins, the spatial distribution of disease in vineyards, the epidemiology of infection and spread, and the efficacy of fungicides and cultural control practices in management of the diseases.

Most significant were European papers on esca disease (black measles). It was clear that this disease is the major limiting factor in grape production and vineyard longevity in Europe, and especially in Tuscany. It became very obvious during the post-conference tour that most vineyards have a significant proportion of vines infected. This was corroborated by statistics on infection levels presented in several papers during the workshop. Cortesi et al. (Italy) pointed out that in Tuscany all vineyards have esca, and most have 20-40% of vines showing symptoms. However, because of the erratic distribution of symptoms from year to year, most of these vineyards actually have much higher levels of infection, and Surico (Italy) showed that over a three-year period up to 85% of vines showed symptoms in some vineyards. Faretra (Italy) was able to pull out all of the vines in an old vineyard, and show that almost all vines had internal rot symptoms. Several papers presented data to show that the spatial distribution of esca symptoms was random in vineyards, suggesting that the source of infection is primarily wind-borne spores blown in from outside the vineyard, rather than spread between vines within the vineyard.

It was generally accepted that esca is a complex disease caused by two main fungi, Phaeoacremonium chlamydosporum and Fomitiporia punctata. However, there was speculation about the role of some other fungi, particularly other species of Phaeoacremonium, in the disease and a number of speakers presented data suggesting that either Phaeoacremonium chlamydosporum or Fomitiporia punctata could induce the formation of esca leaf symptoms alone.

The importance of esca in Tuscany was a revelation to those of us from Southern Hemisphere countries, where esca either is not known to occur, or has only recently been diagnosed. It is unclear whether the lack of disease in our countries is due to very different environmental conditions or whether we just haven't had enough mature vines for long enough for the disease to become important. In Tuscany most vineyards are surrounded by oak forests and it is possible that these are a major source of spores from infected alternate hosts of Fomitiporia punctata. Esca-like diseases are increasing in importance in Argentina and Chile.

Southern Hemisphere pathologists from Chile (Auger), Argentina (Gatica) and Australia (Pascoe) showed that esca-like diseases could be caused by fungi other than Fomitiporia punctata growing in association with Phaeoacremonium chlamydosporum. The Chilean "chlorotic leaf curl" and the Argentinean "Hoja de Malvon" diseases are possibly synonymous, but not enough is known about the Australian situation yet, except that a Phellinus species is involved. What is clear is that fungi other than Fomitiporia punctata can cause esca-like white heart rots in grapevine trunks, and that they also cause leaf symptoms and decline of mature grapevines. It seems likely that these fungi infect and rot grapevine wood in the same way as Fomitiporia punctata, but that the different leaf symptoms are caused by production of different fungal toxins.

Unfortunately, since the banning of sodium arsenite by most countries, little progress has been made in esca control. Attempts to control the disease using chemicals such as fosetyl-Al in symptomatic vines have been unsuccessful, although there was some success with the same chemicals on young or newly-infected vines.