Photo by Tom Walker
SAD
Jesse MacDonald KTT specialist at AAFC Summerland, in his family orchard with a Salish tree that died quickly last summer.
Growers across the Okanagan are seeing productive apple trees quickly die only a few weeks after showing signs of stress. The tree does not seem to have one particular cause for this mortality, and roots appear to be healthy. This “Sudden Apple Decline,” or SAD, is affecting orchards across the Okanagan and has destroyed as much as 40 per cent of some blocks.
At the recent BC Tree Fruit Horticultural Symposium several experts spoke to this issue, including the role of fungal pathogens, or cankers, including Cytospora in SAD, and management methods and future research.
“This is not a new problem. Cytospora was first identified in 1818,” says Dr. Danielle Hirkala, a pathologist formerly with the BC Tree Fruits Coop until a recent restructuring of the company. “Nor is it limited to any one area. I see it across the Okanagan.”
“Things have changed so much in our growing practices,” Hirkala notes. “In the past we simply cut off the fungal affected branch and left the rest of the healthy tree to grow. Now we are growing “branches” on root stock, at very high densities and these trees are essentially weak and easily compromised.
“We may have more virulent strains of pathogens, but also by moving from 800 to 2000 trees to the acre, we are giving it way more hosts,” she adds.
Fungal pathogens are opportunistic, Hirkala stressed. The spores are active in the tree bark year-round and are able to infect the tree when it is weakened in any way. Wounds are the main source of infection, whether a leaf scar, a damaged bud, pruning or grafting wounds, but insect damage can also provide entry to the tree. The spores may be present in the tree for a number of years, but if it is affected by winter damage, or heat or drought stress during the summer, that may give the fungus an opportunity to infect the tree. An infected tree will show cankers, or splits in the bark. When the bark is pulled back, tissue damage is easily seen.
“You have to play the fine line with controlling vigour,” says Hirkala, “This is a lack of vigour disease. The tree needs to be strong enough to fight back.” It is important to flag trees that show signs of stress immediately and inspect them for damage.
“I encourage you to be proactive,” says Hirkala. “If you see a tree with canker damage, take it out now. It may survive, but the chance to grow 30 more apples does not out-weigh the risk of the spores spreading through your whole block.”
And while the tree may continue to survive, bullseye rot caused by canker spores on the apples could appear post-harvest and cause large amounts of product shrinkage. “It looks like you delivered bins of great fruit and then when you get your pack outs, you have 40 per cent culls due to rot,” says Hirkala.
“These infected trees are going to have delayed bloom, delayed fruit maturity and reduced fruit size and are not going to produce the way they should.” Hirkala says. The fungus remains alive in the bark of a dead tree up to six months, she adds. “Burn the dead tree if you can, or get it off your property.”
The fungal problem can remain under the radar for a number of years, says Hirkala. “It’s not like fire blight which is really obvious and can whip through your block very fast,” she says. “This can exist in your trees for several years until the trees are no longer productive and then when you walk your block and really take a look at the trunks, it’s not 10 per cent, it’s more like 75-80 per cent.”
Check your nursery trees carefully for canker signs, Hirkala advises. If the young trees are infected, don’t plant them.
“Your nursery trees are a significant investment,” Hirkala points out. If they are compromised from the start, the additional inputs over the years to a tree that may die, are simply a waste of money. “Take the time to paint the trunks of those young trees when they go in. That is the easiest time,” she adds. Growers who raise their own trees need to be sure that the nursery area is not close to a block with fungus infected trees.
Be mindful of your pruning Hirkala adds. “It may not be practical to clean your tools between each tree, but go back and prune infected trees last after the rest of the orchard.” Painting pruning wounds has been shown to help keep the fungus out, she adds. And be careful with summer pruning. Spores spread rapidly through moist warm air, so wait for dry weather.
“After a winter injury year, the next two years you will see more trees infected with canker,” Hirkala says. “Water or heat stress, insect and rodent damage, anything that causes the tree stress will make it more prone to canker disease.”
There are no fungicides registered in Canada for the control of cytospora on pome fruit Hirkala notes. However, products for apple scab or powdery mildew may also provide some preventative suppression of fungal cankers.
“The message I really want to send today is maintain your vigour. If you have healthy trees you are not going to have a problem,” Hirkala says. “And remove all affected wood at the first sign of disease. Canker is much easier to manage with fewer resources when you catch it at the beginning, rather than trying to play catch up.”
Is it Sudden Apple Decline?
Daniel O’Gorman, plant pathologist with AAFC Summerland, reviewed recent apple mortalities across the Okanagan at the BCTF Hort Forum. “We started to get calls from orchardists last summer who were very concerned. They were seeing trees in their blocks that were showing signs of stress and then two or three weeks later they were completely dead,” says O’Gorman. “The rapid death was atypical of anything we have seen around the Okanagan and up to 40 per cent of the trees in some blocks were dying. This was a big deal. We went and visited orchards or had people send samples to us.”
The leaves on the trees turned red or purplish, says O’Gorman. Most of the trees had cankers at the graft union and extending up the trunk. “You could see the reddish coloration indicating the canker and see fungal spore producing structures emerging through the bark,” he says. “There was insect damage in most, but not all trees. If you dug up the roots they were healthy.”
O’Gorman says similar problems have been identified in New York and Ontario orchards and given the label ‘Sudden Apple Decline’ or SAD. “They haven’t identified a single cause of this problem. They suspect that it is multiple actors that are involved,” he explains. “The symptoms Okanagan growers are seeing, the pests that are associated and the pathogens that are isolated are similar, but not identical, to those identified in the east.”
Cornell University has looked at possible factors that are contributing to SAD and have listed fire blight, herbicide damage, boring insects, winter damage, drought stress, fungal pathogens and viruses, says O’Gorman. “In BC we have one sample positive with fire blight and herbicide damage has been identified,” he says. “As far as boring insects, we do have a growing population of apple clear wing moth.” Winter damage, drought stress and cytospora and other fungal pathogens are certainly present although they are a different group than those back east he adds.
Many cultivars are susceptible. Honeycrisp in particular, as well as Gala, Sunrise, Silken, Fuji, Aurora, Ambrosia and Macintosh. O’Gorman says they plan to investigate whether there is an apparent genetic factor. “Researchers back east say trees less than eight years old are the problem, but in the Okanagan we have nine and 10-year-old trees and some up to 18 years old,” he says.
“We just took a quick look at this off the corner of our desks,” says O’Gorman. “We didn’t have a specific project. We tried to gather as much information as we could quickly, but obviously there are a lot of question marks here.”
