Mar 17, 2016Orchard sanitation a key tool to manage apple scab
The New England apple industry is not without its regular bouts with scab, a major disease that impacts the region’s six states, which have apple harvests amounting to about 25 percent of that in Michigan.
Daniel Cooley, professor of plant pathology, Stockbridge School of Agriculture at the University of Massachusetts, spoke at the recent Southwest Michigan Horticulture Days in Benton Harbor, Michigan, on the topic of apple scab management.
“A major annual disease problem is scab,” Cooley said. “Fire blightis increasingly damaging. Powdery mildew, sooty blotch/flyspeck and summer rots are also becoming common problems.”
Cooley said every year growers need an overall plan to effectively manage scab.
“In the long run, we need to preserve fungicide efficacy and limit resistance development,” he said.
Scab epidemics start on the orchard floor, Cooley said. Infected leaves carry scab through the winter. At about green tip, inoculum just starts to mature, and gets ready for release in rains. Ascospores continue maturing and releasing until fruit are set and begin to grow.
“I think one of the most important things that growers might not do is orchard sanitation. There are a lot of reasons people don’t do it. They don’t see that they have scab in their orchard, that the fruit at harvest looks great, clean.”
He said a survey of New England growers from 2012 — and repeated in 2014 — showed most apple growers use some form of orchard sanitation for scab control. The most common reason for not doing it was lack of time when it needed to be done. The majority of growers use flail chopping, though the use of urea is increasing.
“In fact, we are seeing buildup of inoculum that overwinters as more and more of a problem,” Cooley said. “And I don’t think that that correlation with what your fruit look like and what is going on out there in the orchard is very good. We are recommending that people do orchard sanitation of one sort or another every year.”
Cooley said those sanitation approaches include either flail chopping or applying urea.
“Of those things, probably the least expensive is doing the flail chopping, but with urea you can do it relatively easily,” he said. “We’ve been thinking of that as a ground application for leaves that have already fallen off the tree, but a fall application when leaves are on the tree just before they’re ready to fall is going to be more effective and is going to be just as easy to do for growers, so we think that is definitely something to plan for as part of an annual management plan for scab and other diseases.”
The benefits of such management approaches include reduced chances of resistance development.
“It makes it so that mistakes or long wetting periods or things that might otherwise start some scab going in an orchard are much less likely to happen because there’s not enough inoculum there to make it go. We highly recommend sanitation.”
Orchard sanitation destroys overwintering scab inoculum in orchards. Less scab inoculum reduces risk of primary infection, particularly early in the season. It leads to less chance of mistakes having serious consequences and less chance of resistance development.
“Sanitation costs about as much as a scab spray,” he said.
Leaf chopping can be done in the fall or spring. Spring chopping flips leaves over, eliminating on average 50 percent of inoculum immediately.
“Get as much of the leaf cover as you can – rake to the row middles,” Cooley said. “Allow for time for microbes, earthworms to break down leaves.”
For urea treatment, mix a 40-pound feed-grade urea at a rate of 100 gallons per acre. Apply it in the fall, just before leaf fall.
“You can apply in the fall or early spring to leaves on the ground, but it’s not as effective. It allows less time for microbes, earthworms to degrade leaves.”
Cooley also advocates an annual application of a copper spray, but it shouldn’t be applied past green tip.
“I would like to see that go on the entire orchard, first and foremost, to knock down fire blight inoculum, but also because it can act as your first fungicide of the year going on at about silver tip to green tip,” he said. “That is a spray that gives you multiple benefits and should go out there.
“Another thing I think people really need to pay attention to is putting in a broad-spectrum protectant in every spray. By that I mean Captan or mancozeb, and that includes even if you’ve got a premix like a Luna or a Merivon, I still think you have to put the broad-spectrum in there in order to do resistance management.”
Cooley said growers “have to be aware that formulations for all pesticides are changing, and we need to be aware that there are things in insecticides that may work like oil to interact with Captan to cause fruit and tree damage. You get phytotoxicity when Captan gets into the tissue, and if you have oils and spreader stickers then these will tend to carry Captan in, and we’re seeing more and more of these sometimes serious problems when you get a tank mix of four, five, six, seven materials — one or two or three of which may help Captan get into a plant.”
Cooley said growers “have a choice. One thing would be to just avoid Captan, say from about pink on through first or second cover, and just stay away from it because those are the most complicated sprays, or otherwise pay close attention to whether there might be an interaction problem with Captan.”
He said an early season mistake caused by skipping early sprays “can cause worse disease than a late-season mistake. Savings on even three fungicide applications may not justify the risk of a scab outbreak. It may take several seasons to reduce inoculum levels. Scab outbreaks breed pathogen resistance.”
Model gone awry
Cooley’s second presentation focused on the fact that the ascospore maturity model used to estimate the amount of available inoculum does not perform well in dry conditions.
“We’ve been seeing over the past four or five years that a tried and true aspect of our scab management models – the things that forecast what the risk of scab is going to be – we’ve seen problems and they’re not working as well as they used to,” Cooley said.
“One of the issues that really pops out is the model that talks about ascospore maturity. This is the model that says, ‘OK, if we consider that ascospores are mature at green tip, and we keep track of the temperature, we can keep track of how that inoculum is going and do a good estimate of when the end of primary scab season is.’”
He said the model was developed in New Hampshire in the early 1980s and it worked pretty well, “and it’s worked in a lot of places where the climate is similar to New Hampshire, most of the Eastern United States for example, to give a good estimate of when the end of scab season will come.”
“Unfortunately, about 2012 we saw that there was an exceptionally dry year during primary scab season and the whole period of scab development was drawn out, so that instead of ending around first cover or second cover as it usually does, it went well into third or fourth cover – so about one, two, three weeks longer. And we saw that scenario repeat in two of the last three years, as well.
“It’s apparent that dry weather periods are not accounted for well enough in our ascospore models, and that we need to do something to change that, or at least be aware that when there’s a dry year, they may not be functioning that well. I think that an attempt was made by a researcher in Norway to put an accounting of these dry periods into the model, but it still has not worked very well.
“In the future, we’re going to have to make adjustments, or not pay as much attention to it,” Cooley said. “As they say, ‘Don’t drive your car into the water just because Google Maps tells you to do it.’”
— Gary Pullano, associate editor