Developing a threat-specific contingency plan for the exotic pest angular leaf scorch

Abstract

Angular leaf scorch (ALS), caused by the fungus Pseudopezicula tetraspora, is a high-priority disease threat to the $40 billion Australian wine industry. Research undertaken at the Cornell University New York State Agricultural Experiment Station in Geneva, NY, USA, has provided contingency plans for the Australian wine industry in the event of an ALS incursion. A diagnostic protocol has been developed to enable a rapid response. Cultivars widely grown in Australia are susceptible to P. tetraspora, highlighting the threat that an incursion of ALS presents to Australia, and provides information to underpin effective monitoring and surveillance. The fungicides; trifloxystrobin (Flint), pyraclostrobin (Cabrio) and tebuconazole (Folicur) were effective at controlling ALS and could be used for containment or eradication. A drastic pruning eradication strategy has been validated for ALS and, along with the diagnostic protocol, will be included in the Viticulture Industry Biosecurity Plan. This research has increased the biosecurity capability of the Australian viticulture industry and improved industry preparedness, with potential to save the wine industry many millions of dollars in lost production and vineyard re-establishment costs, whilst maintaining the competitive advantage conferred by freedom from exotic diseases.

Summary

Angular leaf scorch (ALS), listed as a high-priority threat to the Australian wine industry, is caused by the fungus Pseudopezicula tetraspora and can cause yield losses of up to 90%. Developing contingency plans for ALS will contribute to maintaining freedom from exotic pests and diseases in Australia, providing financial benefit to the $40 billion per annum industry, and a distinct advantage in a very competitive world market.

Research was undertaken at the Cornell University New York State Agricultural Experiment Station in Geneva, NY, USA, to develop a diagnostic protocol for ALS and methodologies for undertaking research. Attempts to isolate P. tetraspora and induce development of fruiting bodies and sporulation on symptomatic leaves were largely unsuccessful, most likely due to the intensive fungicide spray program applied in commercial vineyards of NY, which may have reduced the viability of the fungus in infected leaves. Due to the inability to produce spores as an inoculum source, a method was developed for inoculation of leaves with mycelium of P. tetraspora grown in culture. This proved to be very effective, and provided a reliable inoculum source for subsequent greenhouse experiments and field trials.

Identification of P. tetraspora was achieved by microscopic analysis of symptomatic leaves, and through culture morphology and DNA sequencing of P. tetraspora (ATCC 62299). Submission of the DNA sequence data to GenBank will provide a reference for future diagnosis. A diagnostic protocol for ALS has been drafted and submitted to the Subcommittee on Plant Health Diagnostics from the Commonwealth Department of Agriculture and Water Resources, and once endorsed will be included in the Viticulture Industry Biosecurity Plan. 

Grapevine cultivars vary greatly in their susceptibility to ALS but only a few V. vinifera cultivars have previously been evaluated. Greenhouse evaluations revealed that the seven cultivars most widely planted in Australia can be infected by P. tetraspora. Riesling was extremely susceptible, Sauvignon Blanc, Cabernet Sauvignon and Pinot Noir, moderately susceptible, and Chardonnay, Shiraz and Merlot, slightly susceptible. This highlights the threat that an incursion of ALS poses to Australia, and provides valuable information for effective monitoring and surveillance should an incursion occur.

Fungicides currently registered for use on grapes in Australia were evaluated in the laboratory and greenhouse for efficacy against P. tetraspora. Three fungicides; trifloxystrobin (Flint), pyraclostrobin (Cabrio) and tebuconazole (Folicur), belonging to three distinctive chemical groups that represent two different modes of action, were effective at controlling P. tetraspora and so can be used for control, containment or eradication of ALS in Australian vineyards, in the case of an incursion.
A drastic pruning strategy, previously validated for black rot disease, was evaluated in a simulated incursion of P. tetraspora in New York State. Over the three years following implementation, symptoms were recorded only on control vines, and not on treated vines, confirming the success of the eradication strategy for ALS. In the event of an incursion of P. tetraspora in Australia, the drastic pruning protocol has potential to save the wine industry many millions of dollars in lost production and vineyard re-establishment costs. The eradication protocol will be published and included in the Viticulture Industry Biosecurity Plan.

Australian researchers have gained expertise in the diagnosis, eradication and management of ALS and strengthened the existing collaborative partnership with Cornell University.