I assume many of you are still pruning or getting started with it by now for sure. Relative management of diseases associated with pruning cuts, there are a multitude of questions and much still to be learned. I contacted Dr. Kendra Baumgartner, a USDA expert on pruning wounds of grape, to ask a few pertinent questions relative our environment and diseases associated with pruning wounds. See below for the questions and abbreviated answers.
1. You mention winter temperatures and infection of the pruning cuts. My growers often prune in January and February, and days are highly variable relative temperatures. Do you know the cardinal temperatures for any of these pruning wound pathogens relative infection? If it is too cold for infection to occur, then application of fungicides would be wasteful. On the other hand, trunk temperatures in sun are different than air temperatures, so are the pathogens actively sporulating even on some rather colder days?
Answer. We do not know the cardinal temperatures for any of the trunk pathogens for infection. All we know is that they need rain to produce spores. Beyond that, there are no details! Some of the spore trap studies suggest they make spores over a huge range of temperatures, but I think we are all in agreement that the conditions for infection are likely to be much more narrow, in terms of temperature, relative humidity, etc…. I would guess that “yes” they are sporulating on cold days, but those spores might not germinate until it warms up. By the time it warms up, the spores may not have made their way to pruning wounds in the first place, so too bad for them! We know that rain-splash is the only mode of dispersal for many of the trunk pathogens (Botryosphaeria-dieback pathogens, Phomopsis-dieback pathogens, and some of the Esca pathogens). As such, I would say that pruning in the rain is a bad idea, no matter the temperature, because you can be sure spores are being splashed around.
2. Vines bleed as we get into the later pruning periods. Would you spray a fungicide on bleeding vines and would that do any good at all?
Answer: We don’t spray our field trials when there are a ton of bleeding wounds. Also, we don’t inoculate the bleeding wounds afterwards, to see if the fungicide works to protect the wound. As such, I don’t know if a fungicide on bleeding vines will help.
3. Relative Pristine, I have recommended it for pruning wounds on blackberry and blueberry – essentially since it was introduced on the market (based on limited field studies I conducted at the time). Newer studies have confirmed the activity, so I got lucky on that one. Blueberries are now often pruned and sprayed simultaneously with specific equipment designed for that purpose. Can we recommend Pristine for activity on pruning cuts?
Answer: You can do Pristine applications in the dormant season for grape (NOT some of the V. labrusca cultivars, though….there is a phytotoxicity problem) against Phomopsis. I would definitely recommend Topsin, as it seems to work against the broadest range of trunk pathogens.
Note that Dr. Baumgartner indicated that Rally is not particularly effective, and Dr. Mizuho Nita (see the link below for his presentation) says basically the same thing. Based on this newest information, if using Rally, I would probably tank mix with the Topsin M as a potential resistance management tool. However, unless we have better research over time to support use of Rally for pruning wounds, we may hold off on this one. I have recommended Rally in the past based on the initial information that we had available, but we learn over time. On the other hand, I had not really thought about Pristine, and based on the label, I think it would be a legal application for pruning cuts, as it does have Phomopsis on the label and nothing in the label language seems to preclude its use at that timeframe.
Though the pruning cut protection saga continues, I think the following is clear:
- Wet environments such as ours are ideal for pruning cut diseases, especially those caused by Botryosphaeria species.
- Once the pathogen is in the cut, you can’t take it out, and we see plenty of these diseases on Georgia grapes. Obviously, infections are occurring when we are pruning. I would apply Topsin M on the day of pruning, and based on Kendra’s recommendation, I would consider application of Pristine 10-14 days later. You want to allow the pruning wound to heal without sealing in pathogens that will later kill the cordon or the whole vine. As indicated, we don’t know enough about cardinal temperatures for fungal infection or spraying of bleeding wounds to make educated opinions. However, I would currently err on the more conservative side. If it is above 50 F and wet, you will likely have infections, and some fungi are moderately active at lower temperatures. Based on a consensus of opinion from those conducting research on these pathogens, Rally no longer should be a product of choice for these diseases as a solo product, but you might tank mix it for resistance management.
The following link for a Penn State training has presentations concerning pruning, as well as information on disease management associated with pruning cuts.
https://extension.psu.edu/dormant-grapevine-pruning-workshop-and-commercial-grower-panel
