What is in this article?:
- Grapevine canker disease a leading vineyard pest problem
- Control strategies
- Chemical fungicides
- For many years, Eutypa dieback, caused by the fungus Eutypa lata was thought to be the main dieback disease of grapevines, causing death of spurs and cordons.
- The disease resulted in a gradual but severe decline in yields. Although Eutypa is still present, its effects are complicated by other aggressive pathogens, mainly species in the Botryosphaeriaceae family, some of which are able to colonize wood tissue three times faster than E. lata. Nine Botryosphaeria species have been isolated from grapevine cankers from California. The disease they cause is referred to as “bot canker”.
Benlate (benomyl) was registered for 30 years as a pruning wound protectant for the control of E. lata. It required painting cuts to prevent infection. However, it was removed from the market in 2001, leaving growers with no alternative treatments.
Research conducted in northern California tested registered products, applied as paste, for use in preventing infection and dieback. Results showed the difficulty of using chemical treatments to control a broad spectrum of taxonomically unrelated fungi. Biopaste (5 percent boric acid) and Topsin M WSB were shown to provide excellent control of E. lata.
However, Biopaste did not perform as well against Botryosphaeria species. Cabrio EG was an effective fungicide against the Botryosphaeriaceae group, but was the least effective fungicide against the other species. The best overall product was Topsin M, which has the same mode of action as Benlate, and both are systemic fungicides.
Tractor-applied fungicides were the aim of a study in Napa County in 2008-09. Chardonnay vines were sprayed within 12 hours after pruning to the point of drip with single applications of Enable 2F, Rally 40W, Topsin M, and a combination of all three. Pentra-Bark was used on all treatments at a high label rate to ensure maximum penetration of the cork cambium. Pruning wounds were separately inoculated two days after treatment with several canker-producing pathogens.
Results from both years showed that Enable + Rally + Topsin M was the most effective treatment for all pathogens, although Rally alone was as effective against E. lata as the combination.
A limitation of fungicide formulations is that they do not offer full protection for the entire period of susceptibility of pruning wounds. These formulations may be easily washed off with rainfall, or simply degrade before significant rainfall ends and require reapplications (increased cost for little benefit).
One thing to consider is that applications of Rally made to protect pruning wounds must be counted as part of the seasonal limit of 24 oz/ ac.
Biocontrol agents have been tested as an alternative method for control of E. lata. and some other organisms. Bacillus subtilis, Fusarium lateritium, and Cladosporium herbarum all showed some potential activity in limiting the establishment of the pathogen.
However, unlike chemical applications, which have an immediate protective effect, maximum protection from biocontrol agents requires colonization of the surface of the wound. So there is a window of susceptibility after treatment, until the biocontrol agent is established well enough to prevent development of E. lata in the wounded tissue. Biocontrols tested as alternatives to fungicides showed mixed success, but both F. lateritium and C. herbarum worked well when they were applied two to three weeks before infection occurred.
In research in South Africa, fresh pruning wounds were treated with benomyl, two Trichoderma-based commercial products, Bacillus subtilis, and Trichoderma isolates, USPP-T1 and -T2. Seven days after treatment the pruning wounds were spray inoculated with four Bot. species, E. lata, and other pathogens. After 8 months, Trichoderma-based products and isolates in most cases showed equal or better efficacy than benomyl, especially USPP-T1 and -T2. The isolates demonstrated a very good ability to colonize the wound tissue. In California studies, Trichoderma resulted in only 58 percent control in two years of testing.