VT Apple IPM: Cold weather worries, scab model out of whack

I’ve been getting a lot of questions- interestingly, none from growers- about the effect of this weather on fruit buds in the state. My answer has been consistent: I’m not concerned about what we’ve seen so far, but I am concerned about what we may see ahead. Dr. Rob Crassweller at Penn State wrote a good summary earlier this month on Frost, Critical Temperatures, and Frost Protection. The general gist is that, once apple buds are at tight cluster bud stage, we should be concerned when temps go below 28°F, and expect near-complete crop loss below 21. That’s a gross generalization, as duration of cold, bud nutrient status, relative humidity, and other factors do come into play. Checking around the state, I see that where buds are likely most advanced at tight cluster or even very early pink (Chittenden, SW Windham, Bennington counties), it looks like the low dipped around 27-28 this morning. That could be enough to do a touch of damage, but likely not take out a whole crop. Where buds were more advanced (and therefore less cold-hardy), or in low-lying parts of the orchard, there could be more damage. The best thing to do is wait a few days, then assess some flower buds visually for signs of damage. Thanks fully, it looks like last night was the coldest we’re expecting for as far out as I trust the forecast, but there’s a lot of potential cold weather season ahead.

Dr. Crassweller does discuss some methods for protecting from frost damage in his article, Unfortunately, none of the methods is easy or can be found in a jug. To date, there remains no good research that has shown consistently (or even any) improved frost protection from something you can buy in a jug. The #1, 2, and 3 best methods for preventing frost damage are selecting a good site. Other practices that promote general tree health and good groundcover management are the next ones to consider. Active frost protection methods include various ways to either heat or move the air. The former includes heaters, and I know of no orchard that has them in sufficient quantity. Some growers have been known or light round hay / straw bales on fire in the orchard, but that has never been shown effective to my knowledge. The latter include either fixed wind machines, mobile orchard fans, or helicopters. Of all those, only the last is of any use if you’re not set up for them yet. Let’s just keep an eye on the weather and hope that we can ride things out for a couple more weeks.

I do want to cast a word of warning that there seems to be a glitch in the NEWA apple scab ascospore maturity model that is showing lower maturity in many orchards than we would expect. Several years ago, the model had a ‘dry switch’ included that paused the model’s spore development prediction in extremely dry weather, which we saw earlier this month. We suspect that’s what we’re seeing, but I’ve conferred with Cornell plant pathologist Dr. Kerik Cox and he agrees that the model seems a bit off. His prediction is that we’re closer to 15% mature ascospores (as seen in Shoreham, with 13% estimated) as opposed to the 4% that is shown for South Burlington. The point of this is to remember that a model is just that- a tool that helps us to organize information, in this case weather and disease life cycle parameters, to make decisions. I know, in the old days, we estimated ascospore maturity by collecting infected overwintered leaves and analyzing spores under a microscope. The time and facilities to do that no longer exist, and ascospore maturity models were developed because of issues with timeliness (you could only assess conditions for the day you did the squash mount) and site-specificity of the old system. My point is, assessing spore maturity, whether by NEWA, RIMpro, AgEye models or direct observation under a microscope is always an estimation. At the end of the day, we need to take the information that we know about the orchard-disease system and make a decision. With apples at tight cluster or later in the main production regions of Vermont, we’re entering the period where I will always recommend preventative, protective coverage prior to anticipated rain events, even if the latest model says that only 4% of overwintering spores are mature enough to cause infection. As Kerik reassured me, the NEWA models tend to behave better in due time when we sink into a more ‘normal’ weather pattern.

It looks like the next chance of rain is Saturday or Sunday, so plan accordingly.

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