When fruit trees are stressed, so are the growers. Getting to the root of the issues, investigating what went wrong and taking measures to correct the situation can be a lot like solving a crime. There are the usual suspects, but things can get complicated when further insult is added to initial injury. Figuring out the best way to eliminate causative factors and stop further damage, as well as to save the investment in your trees and the season’s crop, is definitely a stressful situation for tree and human alike.
At Donaldson Farms, in Warren County, NJ, farmer Greg Donaldson has been working with Rutgers Cooperative Extension Agent Norm Lalancette PhD, specialist in tree fruit plant pathology, to combat serious concerns in one group of high density apple trees. The trees in questions are three and four-year-old specimens on M.9 rootstocks, and are clustered along the ends of several 500 foot rows in the high-density orchard. Older and newer trees of the same types are fine, as are the trees on other rootstocks.
“We have a major problem in the apples,” Donaldson said, and its cause doesn’t seem to be just one issue. Instead, a series of events have led to trees in decline, and a whole lot of stress.
In response to a rabbit problem, Donaldson added tree guards to these trees shortly after planting. He was also spraying herbicide in these rows and didn’t want the chemicals to touch the trunks. Unsure of when to remove the guards, they remained in place for several years. He is confident that there was no herbicide injury to the trunks.
“It wasn’t just the tree guards,” Donaldson said, as symptoms were not evenly dispersed, with some trees with tree guards doing just fine.
Of the trees with guards on them, the worst symptoms were appearing on those at the farthest end of the row. The 500 foot rows are irrigated with drip tubing, and the these would be the last trees to get water. And they weren’t getting enough. Donaldson relocated the irrigation system’s manifold from the end of the row to the center, and the distribution of water is now equitable from one end of the row to another.
To further complicate matters, last season Donaldson did not water the trees post-harvest. This contributed to drought stress that had occurred over the course of the trees’ lifetime due to the limited irrigation, as well as to the drought conditions that were experienced in the region in 2015.
According to Lalancette, all of this stress amounted to an “invitation for bot rot.” Also known as white rot, this fungal disease is caused by the fungus Botryosphaeria dothidea, and is most common in drought-stressed trees. The disease causes cankers on the trees, and also can cause fruit rot.
The fungus can get into the lenticels and will “grow longitudinally up and down” the trunk, Lalancette said. “You have to spray for white rot anyway. Use the lower nozzles to hit the trunks to prevent fungus from getting into the lenticels.”
And those tree guards added to the problems, too. The tree guards set up “the perfect environment” for the disease.
The guards, Lalancette said, decrease aeration, allowing moisture to increase around the trunk. Large cankers, caused by phytophthora collar rot, developed.
“It is on the scion, not the rootstock,” Lalancette said of these cankers.
The cankers, which are orange in the center, form a callus as the disease progresses, preventing nutrient transport to the tree. The disease requires cool temperatures and a moist environment, and is most active at blossom and the onset of dormancy. Spores will accumulate in the infected tree, and also survive for long periods in the soil, where wet conditions allow infections to spread via splashing onto fruit and trunks nearby, or via irrigation water.
Lalancette cautioned that phytophthora is a concern for all orchard growers.
“You have to assume the innoculant will be there.”
The infected trees in Donaldson’s orchard are expected to make a full recovery now that the tree guards are removed. They are receiving the proper amount of water and are being treated. The callus edges are beginning to heal over, indicating that the infection is an old one. Most of the infected trees are at least 50 percent in good condition, and able to recover.
M.9 rootstocks are considered highly susceptible to fire blight. In regions where rainy weather is occurring at open blossom, growers need to be concerned about fire blight, Lalancette said. In New Jersey, “we’ve met our requirements for the buildup of innoculants on the trees,” he said.
Growers are facing a high risk of wet weather continuing. Wind-blown inoculum during open bloom allows the causative agent, Erwinia amylovora, to thrive. The organism grows rapidly between 75 – 90 degrees Fahrenheit, so weather conditions are now ripe for infections to occur.
Streptomycin spray, 24 hours prior to likely infection periods — or 24 hours after infection if needed — is the best protection. There is a newer antibiotic, Kasugamycin, which has the “same efficacy as streptomycin,” and the two are “perfect partners for rotation,” Lalancette said. Oxytetracycline is an option, but it does not kill the organism, but rather prevents it from multiplying. Organic growers can apply Serenade Optimum, or copper sprays, to gain control in high-risk situations.
For all growers, cultivation practices need to include removal of all fire blight inoculum from the orchard. This will also protect against bitter rot and white rot.
“Get any deadwood out of your orchard,” Lalancette advised.
There are biological sprays that can help prevent fire blight, but they must be used earlier in the season, before full bloom, when the risk of fire blight is not fully known. These products contain organisms which directly compete with
Erwinia amylovora, but are not harmful to the trees.
If blight occurs, cut out any cankers immediately. Thin blossoms if risk of fire blight is high, as fire blight primarily infects via the blossoms. Bloom infections can spread to shoots prior to symptoms becoming noticeable. Shoots which are injured can also be a primary route of infection.
Even resistant trees can become infected when conditions favor fire blight development and inoculum is plentiful. Controlling aphids and other insects which cause injuries which serve as routes of entry for pathogens is important during the summer. Aphids and other insects can also transfer the fire blight pathogen directly into tissues.
There are numerous models to predict fire blight risk. Growers can consult these models to guide decision-making during high risk periods.