Wet Weather Breeds Phytophthora in Young Apple Plantings
August 15, 2017 
Roots and lower stem of a tree infected with Phytophthora crown and root rot. The rootstock has died with a red-brown discoloration. In this case, tissue in the scion is still green, though it will soon die as well. (Photo: Dan Cooley)
This growing season has been a tricky one for Eastern apple growers – with a plethora of fungal and bacterial diseases flourishing under the higher than normal rainfall, researchers are also seeing diseases that haven’t been a major problem for many, many years.
In fact, when a couple of growers contacted Dan Cooley, Professor of Plant Pathology with the University of Massachusetts with new trees dying back, the original culprit was suspected to be fire blight.
“We went out to visit and when we saw it the symptoms, [they] didn’t match up,” he says.
The trees lacked the tell-tale fire blight symptoms. These trees in question were either completely dead with brown leaves to some that were wilting to some that had chlorotic, yellow leaves that would eventually turn brown.
And, in a very wet part of the orchard, the incidents were concentrated.
The trees were dug up to be examined further. When cuts were made into the trees, wood was turning brown, beginning at the roots and extending up to the crown area, depending on how symptomatic the tree was. Cooley also said the trees that were beginning to wilt still had a lot of green tissue in the stem.
“‘It looks a lot like, to me, Phytophthora crown and root rot,’” he told the grower.
Phytophthora is a water fungi, so this year’s wet weather has been a breeding ground.
“It needs a lot of water to do its thing,” he says. “In fact, it literally produces swimming spores.”
Cooley says soils saturated for more than 24 hours, but more like 48 hours, are breeding grounds for Phytophthora.
“We’ve really had at worst normal levels of rain, at least in the Northeast for the last 5 or 10 years and the other extreme has been dry weather, not wet weather,” he says. “We really haven’t had those sorts of years for quite a while.”
Phytophthora Prevention
While there are rootstocks that provide some resistance to Phytophthora and crown rot, such as M.9 and B.9, there are limitations to this resistance.
“It’s not absolute. It doesn’t mean the tree is immune, but under normal conditions, even under some semi-wet conditions, those rootstocks are not going to get Phytophthora,” he says. “The resistance is conditional on wet weather or wet sites and on how stressed that tree is.”
The plantings that Cooley diagnosed as having Phytophthora were on B.9.
“It was surprising to see that they were getting the disease,” he says.
Cooley suggests growers ensure that wet sites within an orchard have proper drainage. Growers should also consider berming, especially on sites with heavy soils and wet spots.
“They’ve only got to get the soil up maybe 6 or 8 inches. It doesn’t have to be up very high to get them a lot of benefit,” he says. “If they were able to build a raised bed 6 or 8 inches high and maybe 3 feet wide, for their trellised plantings that would help quite a bit particularly with heavy soils.”
Treatments
Cooley says one of the best things a grower can do is take the mindset of prevention when it comes to Phytophthora.
“Assume it is there and the best thing a grower can do is to try to keep it in check,” he says.
Cooley says growers can also use some chemical aids as soil drenches to help new plantings get off on the right foot. He suggests labels be checked, but some fungicides could be used as root dips for replant sites as well.
Fungicides such as Ridomil Gold SL (Syngenta) or MetaStar (Arysta LifeScience) could be used as a soil drench and phosphite fungicides could be applied as foliar sprays or as a root dip. Phosphites, Cooley says, can help stimulate the tree to be more resistant. Copper fungicides could also be applied as soil drenches. Phosphites shouldn’t be applied within one to three weeks of copper sprays, though, as they could damage the young tree’s leaves.
But, if a tree is showing serious symptoms, Cooley says the best course of action is to pull out trees with Phytophthora and replant. The site will be more productive than nursing along symptomatic trees.
“Phytophthora is most problematic on new plantings, trees in the first one to three years. That’s when most of serious problems pop up,” he says. “Once they make it to larger trees and the root systems have a chance to develop, they have a better chance of handling it OK.”
As long as the soil is still wet, the fungus is able to reproduce. This is important to understand, especially with young trees.
“One of the reasons we see this so often on these newly planted and young plantings, is the combination of transplanting shock and getting that root system established,” he says. “Anything you can do to minimize the transplanting shock — such as getting trees in early, making sure they’re not sitting out in the sun for a half a day before they actually get in the ground — that sort of thing is going to help you stay away from transplant stress.”
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