Dr. Kari Peter, Penn State tree fruit pathologist, has seen trees in apple orchards declining over the past 10 years. These declines have resulted in fruit rots, a serious issue for packers.
In 2023, Peter and her colleagues saw issues primarily with G 11 and Bud 10 rootstock as well as with pear trees.
“We were seeing a lot of weird fungus right underneath the bark of the roots,” said Peter. “What we’re pulling out is a lot of different things. In some cases it was all neopestalotiopsis, which is a big problem in strawberries. It was first talked about in Florida just a few years ago but we’re finding out it’s in the soil and is pretty much everywhere. We’re finding it on apple trees, especially apple trees that are declining.”
Fusarium is also the cause of root rot and crown rots.
Why are fruit rots becoming more common? Peter’s philosophy is that everything in the growing system is connected. “We’re seeing different root rots pop up in storage and in the field,” she said. “We’re seeing all this tree decline and I believe it’s our lack of winter, which would have a ‘sterilizing’ effect when the ground freezes. It kills off the bad guys in the soil.”
Pest species that haven’t been able to withstand winter in the past are now surviving because temperatures are warmer. “It’s Pandora’s box at the moment,” said Peter. “A lot of rots are secondary and the fruit is in the way. The origin of the rot is associated with the tree itself and not the fruit.”
Until fruit pathologists solve this issue, the key is creating healthier orchards with healthier trees. This will in turn limit tree decline and fruit rots.
Saleable fruit starts with proper soil preparation. “Growers can lose a lot of money as a result of not preparing soil adequately,” said Peter. “I see people pushing out orchards, not waiting, and replanting. There’s good reason to let the soil rest, grow cover crops or fumigate before planting. Healthy soil will create healthy trees.”
An important aspect of soil preparation is soil fumigation, which requires a fumigation license. “When you chemically fumigate, you have two years before problems such as weeds, nematodes, pathogens creep back in,” said Peter. “But that’s two years of nothing there to compromise the trees.
“Another method is to grow cover crops or biofumigants, which helps kill nematodes. Crops such as rapeseed and sudangrass release natural gases from the plant material when plants are chopped. This also adds biomass.”
Since the goal is to grow stronger, healthier trees that can withstand stress, it’s critical to provide nutrients according to soil tests, prune trees properly and ensure there isn’t too much vegetative growth.
“It’s a delicate balance, and it’s part of nutrient management,” said Peter. “If you go too far in one or the other direction, you’re going to have a tree stress issue, which can be related to tree decline, abiotic injury, herbicide injury or some other stress.”
Black rot (Botryosphaeria obtusa) symptoms. Photo courtesy of University of Georgia Plant Pathology, Bugwood.org
Peter outlined some of the most common stressors for trees: Too much water can result in crown rot. Old cankers can be problematic. Winter injuries. Too many weeds close to lower limbs can choke out trees. Trees planted close together, as they are in high-density systems, is a stressor.
“We’re expecting a lot from trees with fewer resources, and having them produce in a shorter time,” said Peter. “That is a stressor. Wood-boring beetles, such as ambrosia beetle, come in when trees are stressed. Trees produce ethanol when they’re stressed, and ambrosia beetles are attracted to ethanol.”
Other wood borers, such as dogwood borer and black stem borer, can do significant damage if not controlled.
Water is a critical aspect of tree growth. Irrigation is usually defined by water availability – it’s important to keep irrigation consistent throughout the season.
“It’s good to have an even level of water throughout the season,” said Peter. “Avoid ‘burst’ irrigation to avoid crown rot issues.”
She added that if trees are already compromised with fungi present, flooding them with water creates a hospitable environment for fungi and the result is a domino effect.
Invest in large caliper trees and plant them at appropriate depths according to the graft union. “The graft union shouldn’t be too high or low,” said Peter. “Avoid any potential infection near the graft union – that’s the most vulnerable part of the tree.” (Dogwood borers are often found on the lower trunk near the graft union.)
It’s important to manage fire blight, especially shoot blight. Pruning out dead shoots during winter is the key to ensure no dead wood remains on tree to provide a desirable environment for rot pathogens. A variety of products, including but not limited to Actigard®, Regalia® and Fosphite® can help manage fungal infections.
“Choose hardier rootstock to help avoid fungal issues,” said Peter. “We’ve seen issues with M9 rootstock, which is vulnerable to winter damage. Inspect your trees before planting. Look for trunk abnormalities, crown gall and check the roots. Crown gall might not kill the tree but will slow it down and make it weaker. The tree is now more susceptible to other pathogens that can issues with the tree, which can then lead to a source of inoculum for fruit rot.”
Prevent winter injury and herbicide injury. Tree guards and painting trunks help to reflect sunlight and prevent herbicides from getting on trees.
“The graft is a very vulnerable part of the tree,” she said. “Herbicides can be absorbed and interfere with the tree’s defense system, making it weaker.”
Peter said growers should be mindful about weed control. Unmanaged weeds create more competition for trees that are already compromised, especially dwarfing trees. Weeds can be a reservoir for tomato ringspot and tobacco ringspot viruses, which won’t kill a tree but will weaken it. These viruses enter the tree by the dagger nematode.
Rodent control is also important to protect the graft union.
“Everything is connected in the orchard – horticultural practices, tree health and rots,” said Peter. “If you have rots, chances are the rots are there as a byproduct of something else. Creating a healthy orchard has a ripple effect.”
by Sally Colby
