Eutypa dieback is a degenerative disease of mature vines which threatens the sustainability of premium Australian vineyards. Vines become infected when ascospores of the fungus Eutypa lata land on wounds made in the wood. The fungus grows slowly in the wood and foliar symptoms are not evident until several years after infection. Therefore, the fungus is well-established in the vine before disease symptoms are visible. There are currently no fungicides or biological agents registered for the control of Eutypa dieback in Australia, and remedial surgery is often undertaken to restore vines to productivity. There is a need to develop improved strategies for the management Eutypa dieback, based on a sound understanding of the epidemiology of the disease. Also, rapid diagnostic techniques will facilitate studies of epidemiology and control, and improve the prospects for management of this intractable disease.
Methods for studying the release of ascospores from infected, dead wood in the laboratory were developed. Spores were ejected up to 10 mm above the surface of wood bearing perithecia of the fungus. Release of ascospores from wood of Grenache was greater than that from wood of Shiraz. Ascospore release increased with increasing volume of simulated rainfall applied, from 1 to 20 ml, but was not affected by increasing the temperature of incubation from 4 to 22°C.
Foliar symptoms in naturally infected, mature Shiraz vines varied from year to year, and there was a constant relationship between the severity of foliar symptoms and yield. The absence of foliar symptoms was not a reliable indicator that a vine was healthy, as shown by the presence of discoloured wood in vines examined during re-working trials. Discoloured wood, which contained the pathogen, was observed in the below-ground parts of the trunk. Such vines would not be candidates for reworking. Reworking trials, using the ‘cut and train’ method on mature, naturally infected Shiraz and Malbec in two commercial vineyards, suggested that production of watershoots was influenced by the cultivar and the location of the vineyard. Due to the long latent period for Eutypa dieback, mature vines of nine cultivars inoculated in the field have not yet shown foliar symptoms, and monitoring for the effects of cultivar on disease progress will continue. A survey of the Hunter Valley and Mudgee regions of New South Wales showed that Eutypa dieback was not responsible for the dead arms prevalent on mature vines, and isolations yielded Botryosphaeria spp. which cause the disease known as Bot canker or Black dead arm.
The benzimidazole products Benlate (benomyl) and Bavistan (carbendazim) were the most effective for protecting pruning wounds from infection following artificial inoculation. Although Benlate has been withdrawn from the market, benomyl is still available in Marvel. Application of large amounts of Benlate to pruning wounds did not result in residues in the fruit. However, it is important that alternatives to the single-site activity benzimidazoles are sought, to delay the development of fungicide resistance in this naturally variable pathogen. Of the biological products tested, Trichoseal (based on the fungus Trichoderma harzianum) afforded some protection when applied to pruning wounds 14 days in advance of the pathogen. Further testing is underway to assess reliability. Physical barriers such as acrylic paints and mastics were also effective, especially when supplemented with Benlate. Materials that prevented infection of pruning wounds on 1-year-old wood were also effective on larger wounds made on cordons. Comparison of the efficacy of protecting pruning wounds by spraying rather than by labour-intensive painting gave variable results and further trials are planned.
PCR (polymerase chain reaction) and Southern hybridisation assays developed here are capable of identifying E. lata in culture and detecting the fungus in infected grapevine wood. Only the Bio-101 soil DNA extraction kit yielded DNA of a quality suitable for PCR amplification without inhibition by the phenolic compounds which occur in discoloured, diseased wood. However, this kit costs approximately A$1,000 for 50 DNA extractions, and is not cost-effective for use in routine detection of E. lata DNA. Southern hybridisation, a technique much less susceptible to inhibition by phenolic compounds, has the potential to allow reliable detection of E. lata DNA in artificially inoculated and naturally infected grapevine wood. Detection of E. lata DNA using Southern hybridisation appeared to be more sensitive than re-isolation into culture and, therefore, could provide an accurate and efficient means of assessing both the extent of infection in diseased vines and the efficacy of potential control agents.
The production of potentially phytotoxic acetylenic secondary metabolites by E. lata was most consistent in culture media containing grapevine wood and sucrose. Eight of the nine isolates tested produced eutypine and an unidentified compound when grown in media derived from grapevine wood. Extracts from cultures of E. lata were toxic to grapevine leaf discs. Results suggested that compounds other than acetylenic phenols may contribute to the foliar symptoms characteristic of Eutypa dieback. Detection of eutypine in the foliage of diseased vines using Raman spectroscopy was not possible due to autofluorescence of the chlorophyll. Research on the identification and detection of a suitable biochemical marker for infection continues.
For more information please contact Mark Sosnowski