Pruning and Trunk Diseases

Gabriel Torres, UCCE Viticulture Advisor for Tulare and Kings Counties and UC IPM Affiliate

Grapevine trunk disease (GTD) is a general term that includes at least four distinct diseases including: Esca, Eutypa, Phomopsis, and Botryosphaeria dieback. Going a step further Esca, Eutypa and Botryosphaeria are all caused by multiple species of fungi. Overall, more than 150 species of fungi are involved in the GTD complex, and it is not difficult to find mixed infections in a vineyard. Some of the pathogens that cause trunk diseases are endophytes. This mean that they can grow inside the vines without causing outward symptoms for years. Most of the fungi that cause GTD are fungi that evolved to feed on decaying wood and are abundant in nature, but under the right conditions can infect living plants, especially those with lignified tissue (hard wood).

Most of the GTD pathogens infect grapevines through injuries such as pruning wounds. Cankers on > 95% of infected vines can be traced back to an infection that was initiated at a pruning wound. The following factors determine whether or not our vines will get infected after pruning:

Type of pruning: The type of pruning will affect both the size, number, and location of pruning wounds. The size of the wound is very important to determine the risk of infection. The larger the wound, the higher the chances that a pathogenic spore from one of the GTD will land on it. Larger wounds also give the chance for multiple spores, possibly from different GTD pathogens, creating a greater opportunity for not just infection but mixed GTD infection.

Another factor to consider is the proximity to the trunk. The closer a wound is to the trunk the more dangerous it is. This is because cankers that are initiated close to the trunk can quickly compromise the entire vine and are more difficult to prune off than cankers that originate on the periphery of a vine. Vines that are head-trained and cane pruned will have all their pruning wounds closer to the trunk than cordon-trained vines (Figure 1 A).

Cordon-trained spur pruned vines may have a higher number of pruning wounds than head-trained cane-pruned vines (Figure 1 B); however, the cross-sectional area of each individual pruning wound may be smaller and further from the trunk than those created with cane pruning. Cane pruning put the lesions next to the trunk while in spur prune the distance is bigger (green marker on Figure 1) and permit replace infected spurs more easily when they get infected.

Cut angle: Cut angle depends on the viticulturist preference. Some people prefer to give some angle to the cuts to let rain if occurring to be removed from the lesion by gravity. One downside of this cutting type is that the lesion size increase as the cutting angle increases. Some viticulturists, especially in table grapes do not like to give an angle to the cuts since pointy spurs can puncture the clusters and thereby cause rot problems. Flat cuts can result in a surface for a droplet of water to pool on providing ideal conditions for GTD spore germination if present.

Pruning image 1.png — **Image 1**. Pruning damage on mechanical pruned vineyard. Red circles show the expected pruning lesion size if manual pruning was used.

Pruning Tools: Fortunately, dispersion of GTD by pruning tools is minimal and does not represent a major risk in infected vineyards. That means that disinfestation is not required. However, sharpness of the tools is critical to have clean cuts. Dull tools can be problematic leading to uneven cutting zone and ripping the bark leading to larger pruning lesions the same applies for the tool size; using an undersized tool will requires several cuts to remove a cane. Selection and maintenance of a proper tool makes pruning easier and reduces damage to the vines.

In recent years, the use of mechanical pruning has increased as an alternative to manual pruning. This method does leave larger than expected pruning lesions and can increase the risk for GTD development (Image 1). Future research on the effect on mechanical pruning and trunk diseases is needed.

Pruning time: For California pruning is done during the dormant season between December and March. This also coincides with the rainy season in California. There is a strong correlation between rain events and spore releases for Botriosphariaceae, Eutypa, Esca and Petri disease (Urdez-Torres et al. 2010). Pruning wounds themselves also become less susceptible to GTD pathogens when pruning is done later in the dormant season (Urdez-Torres and Gubler 2011).

Based on the relationship between the principal GTD pathogens, California rain, and wound susceptibility, one of the most effective strategies is to postpone pruning until as close to budbreak as possible. Labor shortage in California, makes it impossible for all growers and companies to postpone pruning until late-winter. Forcing many vineyard managers to need to prune as soon as they have access to the farm labor to do it. In addition, recent changes in wage laws take affect January 1 in California; again, giving vineyard managers another reason to take care of pruning before the higher wages take effect. If pruning is done during the rainy months fungicide protection of the wounds is advised.

Another strategy is for growers to use a double pruning system. At the beginning of the winter the vines are pre-pruned leaving an extra 12-28 inches past the final desired cane or spur length. At the end of the winter the canes or spurs are cut to the desired length and number of shots. This normally helps to remove any infected tissue from the pe-pruning phase. However even in this system it is advised to protect the wounds.

Wound protection: Grape trunk disease infection are not curable with fungicides. However, to apply a fungicide or to puts a protective paste after pruning can help to reduce the risk of get GTD infections. Currently, Topsin-M, Rally and Mettle 125 are registered as pruning-wound protectants. In addition, pastes that include 5% boric acid and acrylic paint have been tested effectively to control Esca and Eutypa dispersion.

There is not a single protocol that growers need to follow to manage trunk diseases. Preventive measures can prolong vineyard lifespan. Integrating pruning time, with tool selection and wound protection help reduce the number of vines affected by GTD. If you see an increase of trunk disease pressure, consult with your PCA or your local Viticulture Advisor about the best strategies to manage the disease.

Grafting: Another major wound that can allow for GTD to infest a vineyard happens during grafting and top working. Spores landing at fresh grafting points (Figure 2 arrows) or selection of infected scions or rootstocks can result in field infections. To reduce the risk of infection from nursery, always acquire your plant material from a reliable nursery, and inspect suspicious plants for internal damage. In addition to plants grafted at nursery, field grafting or regrafting always have a risk to serve as point of entrance for trunk diseases. Special attention is needed when regrafting (Figure 2) since the cut is large and the inoculum from old decaying wood is readily at hand. In all cases, it is important to protect the exposed tissue until the lesions heal.

Pruning image 3.png — **Figure 3.** J root planting. A. Correct, straight root distribution; B. J root at planting; C. Cracking points at J rooted plants bend points (red arrows).

Natural Infection: In addition to injuries associated with cultural practices, GTDs can also occur via naturally occurring wounds. A good example of a “natural injury” serving as entry port is the “J” rooted plants. J rooted plants occur when the roots are bent into the bottom of the planting hole. The geotropic nature of roots forces the tips to grow downward creating a crack that serve as point of entry for the black root or petri pathogens (Phaeomoniella and Paheoacremonium spp). Under this scenario the pathogen is obtained at planting and external symptoms of Petri disease are normally observed between the first 3 years after planting.