By Chuck Ingels, Paul Verdegaal, Doug Gubler, and Ria DeBiase: UCANR

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”.

Symptoms

Like Eutypa dieback, typical symptoms caused by bot canker on grapevines in California are the wedge-shaped canker in cross-cut cordons and dead spur positions. Eutypa dieback causes stunted shoots and leaves that are chlorotic, tattered, and cupped, but bot canker produces no foliar symptoms – i.e., the spur dies before spring push. Both diseases can be found on vines about seven to eight years of age and older or are common in vineyards older than 10 years. In susceptible varieties infection may occur after only four to five years when large cuts may be made during pruning.

Spore release

Eutypa lata overwinters in diseased wood and produces fruiting bodies called perithecia under conditions of high moisture (areas with rainfall exceeding 16 inches). Sexual spores (ascopores) are discharged from perithecia soon after rainfall. Infection occurs through pruning wounds, which remain susceptible much longer in December than in February.

Pruning wounds can be susceptible to infection by E. lata for seven weeks or more in late fall, but this varies with the time of pruning, size of the wound, and age of the wood pruned. With Botryosphaeria, asexual spores (conidia) are produced from black fruiting bodies called pycnidia during the entire season, including and perhaps especially in spring when temperatures are more conducive for sporulation. Another important source of pycnidia may be the shredded prunings or portions of arms and spurs left in the vineyards.

Varietal susceptibility

Wine grape varieties differ in their susceptibility to these diseases. In a 2003-2004 California survey, Botryosphaeria was isolated most often from Sauvignon Blanc (64 percent recovery from cankers tested) followed by Chardonnay (55 percent). Cabernet Sauvignon had the most cankers with Eutypa (58 percent).

Petite Sirah is extremely sensitive to infections as is Chenin Blanc, while Zinfandel and Syrah are moderately susceptible.

Control strategies

Training and pruning

Once a vine is infected, the canker should be completely removed in order to reduce spore production, and the wood should be removed from the vineyard. In most cases, this means removing a portion of the cordon and retraining a cane to recreate the cordon. A cut of this size should be made no earlier than March, and preferably around bud break. On older vines, doubling of spurs to replace lost spur positions and extensive cordon retraining or use of “kicker canes” may be necessary to maintain production.

There are a few key strategies for preventing canker diseases. One is to use a vine training method that reduces or eliminates the amount of cuts made during winter, such as minimal pruning or mechanical pruning in the late dormant period.

Research conducted in Sacramento County in the late 1990s showed that minimal pruning and mechanical pruning resulted in far less Eutypa dieback than spur pruning, while maintaining yield and fruit quality. Cane pruning also significantly reduced incidence of dieback. However, these training methods have not been widely adopted. In the case of machine, box or minimal (hedge) pruning, winemakers have been reluctant to accept these as standard practice, while cane pruning requires skilled labor and an extra operation of cane wrapping.

Another method is double pruning – mechanically pre-pruning to about 12-14 inches in fall or early winter followed by hand pruning before bud break. By removing most of the vine brush, the double pruning can speed up the final selective pruning, thus allowing growers to prune large acreages more quickly.

Research in the North Coast showed that neither E. lata nor species of Botryosphaeria could be recovered from farther than 1.5 inches below the pruning cut. When pre-pruning occurred in winter months, E. lata was recovered from 40 percent to 65 percent of canes, compared to only 7 percent to 10 percent when prepruning took place in February. But the hand follow-up pruning removes these infections.

When possible, prune in dry weather, and preferably when rain is not predicted for a week or more. The susceptibility of pruning cuts to infection declines over time, so a week of dry weather after pruning should result in less infection than when rain occurs the following day. Of course, this may not be practical on large acreage, where pruning must be done through most of the winter. Pruning less susceptible varieties first may be one strategy. Also, results from an unpublished study in the 1980s suggested that late pruning and shoot thinning in the establishment of young vines can significantly reduce later onset of severe dieback.

Late pruning reduces exposure of wounds to rain events. It provides a good deal of control in a currently infected vineyard since spores are depleted over the course of the winter. It is a wise IPM strategy to prune as late as possible.

Chemical fungicides

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.

Biofungicides

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.

Conclusions

The best strategy is still to prune as late as possible and minimize wounds greater than 5/8“ diameter, or the size of a dime. When the wound diameter is doubled, susceptible surface area is increased 4X. Also, shoot-thin young vines during the early years to reduce as much as possible the number of wounds at pruning time. Coupled with late pruning is the tractor application of Rally and Topsin M within 24 hours of pruning. This is a rapid treatment that can he applied quickly if a storm is forecast after late pruning.

References

• Gu, S., Cochran, C., Du, G., Hakim, A., Fugelsang, K., Ledbetter, J., Ingels, C., Verdegaal, P. 2005. Effect of training-pruning regimes on Eutypa dieback and performance of 'Cabernet Sauvignon' grapevines. J. Hort. Sci. Biotech. 80(3): 313-318. http://www.lodiwine.com/Gu_et_al_Pruning_Training_and_Eutypa.pdf

• Gubler, W.D., Rolshausen, P.E., Trouillas, F.P., Urbez, J.R., Voegel, T., Leavitt, G.M., and Weber, E.A. 2005. Grapevine trunk diseases in California. Practical Winery & Vineyard, Jan./Feb. 2005. http://www.practicalwinery.com/janfeb05/janfeb05p6.htm

• Kotze, C. , Fourie, P. H., and Van Niekerk, J. M. 2008. Biological control of the grapevine trunk disease pathogens: Pruning wound protection. Master’ Thesis. https://scholar.sun.ac.za/handle/10019.1/2117

• Rolshausen, P. E., Urbez-Torres, J. R., Rooney-Latham, S., Eskalen, A., Smith, R. J., and Gubler, W. D. 2010. Evaluation of Pruning Wound Susceptibility and Protection Against Fungi Associated with Grapevine Trunk Diseases. Am. J. Enol. Vitic. 61:1, 113-119 (2010) http://www.cdfa.ca.gov/plant/ppd/PDF/RolshausenEtAl2010.pdf

• Weber, E. A.; Trouillas, F. P.; Gubler, W. D. 2007. Double pruning of grapevines: a cultural practice to reduce infections by Eutypa lata. Am. J. Enol. Vitic. 58:1, 61-66. http://www.ajevonline.org/cgi/content/abstract/58/1/61