Pathogen Detection And Characterization
The session began with a controversial presentation by French scientists suggesting that bacterial rather than fungal pathogens might be the primary cause of grapevine trunk disease. The work using both molecular and microbiological experimental methods based its premise on the observation that similar fungal diversity is equally present in healthy and diseased vines. In contrast, they observed a large diversity of species of bacterial populations between healthy and symptomatic vines. Their conclusion that bacteria, rather than fungi, cause trunk diseases goes against the well-accepted etiology (cause of disease) of many known and characterized trunk diseases such as young vine decline (aka Petri Disease), Esca, and Bot Canker in which the fungal pathogens are endophytic organisms that live and colonize a vine without causing any disease until the vine is stressed. In other words, the traditional point of view is that the same fungal populations are present in healthy and diseased vines.
Related to stress and fungal pathogen association, another presentation given by Eurofins STA addressed the new syndrome, Red Blotch, reported in California vineyards in the 2011 fall season (for more, see page 10). The laboratory tested many samples from a variety of vineyards exhibiting Red Blotch foliar symptoms for the presence of viruses and fungi. No evidence of known virus infection in the Red Blotch syndrome plants was found. However, pathogenic fungi were routinely found in the symptomatic vines. In all instances of Red Blotch, the plant material showed typical fungal cankers and streaking in the wood. Additionally, most samples had constrictions caused by grafting tape or had twisted and/or “J” shaped roots.
Recently (17th International Grapevine Virology Meeting held in October 2012 in Davis, CA), a new Gemini-like virus was reported to be associated with the Red Blotch syndrome. There is no information on the biology of the virus and further studies are required to show that the virus causes this disease. It is possible that the presence of this virus makes the vines more susceptible to fungal pathogens or vice-versa. More research and the testing of many symptomatic vines will allow us to correlate symptoms with the presence of virus and fungal pathogens in diseased grapevines exhibiting red “blotchy” leaves.
The focus of the session then moved to pure fungal work, starting with a study on the genetic diversity of the fungus, Eutypa lata, in Californian vineyards, orchards, and riparian areas. This study showed that the fungal population is more genetically diverse in vineyards and orchards (planted to Prunus species) as compared to the riparian areas populated with willow. This suggests that Eutypa spp. fungal spores are dispersed between the orchards and vineyards, while the infections in the riparian areas most likely originate from the orchards and/or vineyards.
Phomopsis fungal species associated with wood cankers in North America were the subject of two papers. One presented the association of foliar symptoms with wood cankers. Another discussed the historical perspective of Phomopsis spp. as it is associated with trunk disease. According to the author, Urbez-Torrez, phomopsis has been overlooked as the cause of grapevine trunk disease for more than 40 years. Urbez-Torrez proposed the name “Phomopsis dieback” for the disease caused by P. viticola.
The session culminated with a presentation by Pedro Crous pleading with the audience to standardize the nomenclature of fungal pathogens. If all researchers agree with this proposal coined as “one fungus, one name,” the future nomenclature of fungal pathogens will be much simpler. The new proposal would drop the dual scientific names of pathogens with sexual and asexual stages in their life cycle. For example, the proposal would drop the name Diaporthe as it is less used than Phomopsis in scientific literature (Phomopsis is the asexual stage of Diaporthe).
For more on epidemiology, click on the additional page.