VT Apple IPM: Scab season has started, NEWA orientation

At UVM Catamount Farm orchards, we were at late silver tip bud stage last Monday, April 18, when I applied a copper spray. Bud stage tracking is a critical IPM and general management tool to help track orchard activities that correlate to tree phenology. My observations that support these comments are often made from a single site at 200 feet elevation in South Burlington, or from one of the 2-3 trees at the edge of my yard at 1500 feet in Calais.

To better inform these notes, please consider reporting your bud stages. It’s really simple to bookmark this link on your phone and tap in the bud stage as you go about your orchard business: https://go.uvm.edu/22applebudstage

When I left the farm Friday, I noticed Zestar, an early-developing cultivar, were approaching the half-inch green stage, but the bulk of varieties were at a solid green tip. This is the time we start to really think about apple scab management, after that first copper application which is really more targeted at fire blight. Conventional wisdom has always said to maintain full protective fungicide coverage from green tip until all ascospores, the inoculum on last-year’s leaves on the orchard floor that cause primary infections, is expended around 900 degree days (base 32°F) from the green tip date. That’s one example of why knowing the dates of bud stage development allows us to fine-tune our pest management programs.

NEWA is the expert system of web-connected weather stations that we use in much of the northeast to help with IPM implementation. Every grower should have a nearby station or consider getting one. The data is as accurate as the weather is from your site to wherever the station is located. If you are interested in getting a station, please reach out to me. They run about $1500-2000, and last five years with minimal maintenance, maybe 7-10 years with some parts replacements. So for $200-300 per year, you can accurately implement IPM on your farm in a way you couldn’t without this critical information. The UVM Fruit Program covers the NEWA network subscription for the whole state. NEWA is a product of the New York State IPM Program, and is run by our colleagues at Cornell University.

After selecting a station from the dropdown list, various tools can be selected at the bottom of the page. If you set up a free account (you don’t need a station to do so), you can select a favorite station and preferred models that will show up at the bottom of the page. NEWA supports many crops besides just apples, so you may want to narrow down to the important ones to make it easier to check. Clicking on ‘Apple Scab’ brings us to a new page. Enter your green tip date- NEWA will guess based on accumulated degree days, but for our site, that was about six days early based on my observations. That will generate two tables- Ascospore Maturity Summary and Infection Events Summary. The first one predicts (that’s important- these are based on models and not observations in the field, but the models were developed based on extensive prior observations) the maturity of the spores that develop on overwintering leaves, from 0-100%. As spores mature and are wetted, they release and may cause infection if conditions are appropriate.

In ‘old IPM’, we would cover the orchard with prophylactic fungicides to prevent infection from the moment there was susceptible tissue, green tip, until a few weeks after petal fall, for a total of 8-10 or more sprays. Fast forward to ‘new IPM’ (skipping over a brief period in the 1990s when fungicides that had activity to be used after infection were used to arrest infection after it occurred but which also led to substantial fungicide resistance in the population of the scab fungus) where we can use the NEWA tools to fine-tune our apple scab programs.

I still recommend prophylactic coverage with protective fungicides- sulfur for organic growers; mancozeb early season or captan after bloom for non-organic growers, but tailored to your disease and orchard conditions to apply them with intentionality, rather than spray on a 7-day schedule. We also still have fungicide tools that have that retroactive activity of managing disease after infection occurred, but we want to minimize the potential for resistance development, so do not use them alone in that ‘backward only’ use. This allows multiple tools and conditions to be considered when developing our IPM tactics. For example, at our site, NEWA predicts 2-5% ascospore maturity, with some spore discharge later this week and likely conditions for infection (extended wetting). While best done by conducting a thorough assessment of disease in the fall, I feel comfortable in our orchard with not being covered for this relatively minimal disease event. However, in an orchard that had any substantial scab, and it takes very little to be substantial, 2-5% of a large overwintering scab population can cause substantial headaches moving forward.

This is a good thing, as I also discovered Friday that my spray tractor has a flat tire, and my spare spray tractor has a frozen clutch plate, which happens when it sits in winter and I wasn’t able to free up before I left. Our tire service company is short-staffed and can’t get to it repair until early this week, and my retired-since January staff mechanic won’t be popping inn to show me the trick to free the clutch until tomorrow morning. That highlights a key component of IPM- readiness to take action should a condition that requires treatment be found. I am lucky that, under my assessment of the IPM situation based on NEWA, that I have a week or so to get things in order, but it’s critical at this time of year to make sure your equipment and personnel are in top shape to get the job done. Using risk management tools like NEWA to make nuanced decisions means living a bit on the edge- being ready to hop on the sprayer at short notice, and constant checking of the actual orchard situation though scouting and the modeled environment through daily NEWA checks.

Other orchard activities to think about: clear up any remaining brush and consider a close to the trunks, low to the ground flail mowing to reduce debris and overwintering scab inoculum; hang white sticky traps for tarnished plant bug, if that pest is of concern (pick-your own growers typically don’t need to worry about this cosmetic pest), get traps ready to hang at tight cluster-early pink for European apple sawfly; apply herbicides if you are using them; get irrigation up and running; order fertilizer.