Tree fruit scientists spoke about recent research on bitter rot and apple replant disease on Tuesday during the Mid-Atlantic Fruit and Vegetable Convention.
Bitter rot causes sunken lesions in apples and produces orange spores. At least eight species of fungus cause bitter rot, but they are closely related, produce identical symptoms and can be managed essentially the same way, said Phillip Martin, a Penn State doctoral student.
Bitter rot has been increasing in recent years, thanks in large part to the expanding acreage of Honeycrisp. Popular with consumers, the variety is also highly susceptible to bitter rot fungi.
Bitter rot grows best in long periods of wetness between 70 and 80 degrees. The fungus often infects the fruit and stays dormant until late in the season or even after harvest.
“If it’s warm and it’s wet, you’re going to have infection pressure,” Martin said.
Martin believes that bitter rot spores initially blow into an orchard from woodlands during strong storms. Once in the orchard, the pathogen completes its life cycle within a small area of the orchard. Successive generations might even infect the same tree.
The fungus can live in leaves without causing disease, so it would be impossible to eliminate.
But growers can reduce the spore load, Martin said, by removing mummy fruits from the orchard, using good horticultural practices that keep the trees healthy, and pruning to allow good air flow and quick drying.
Fungicides effective against bitter rot include Captan, Merivon, Pristine and Luna Sensation. Omega should be used at the full rate, Martin said.
Fungicides should be applied before a wet period to prevent the fungus from starting an infection.
“Once it’s underneath the apple’s skin, it’s very hard to kill, even with systemic fungicides,” Martin said.
Tianna DuPont, a tree fruit Extension specialist at Washington State University, has been researching alternatives to fumigation to prevent apple replant disease.
The disorder involves trees failing to thrive and producing poorly in areas that have been repeatedly planted to fruit trees.
It’s caused by a complex of pathogens, including rhizoctonia, phytophthora and pythium species as well as certain nematodes. Compaction and herbicide residuals can add to the problems.
In Washington state, growers typically fumigate with 1,3-dichloropropene plus chloropicrin to control nematodes and fungi before planting new trees, but it would be good to have other options.
One promising tool is brassica seed meal, which releases noxious mustard gases into the soil and kills the pathogens.
DuPont and her team rototilled into the soil a finely ground amendment of equal parts white and brown mustard. They laid a totally impermeable film within 30 minutes to trap the gases.
This process works best above 65 or 70 degrees. “These are volatile compounds that need to move through the soil,” DuPont said.
DuPont also tested anaerobic disinfestation, a practice that starves the pathogens of oxygen and allows more desirable microbes to take over.
DuPont grew triticale or timothy, about 3.5 tons per acre, and windrowed it into the tree rows so those strips had 8 to 10 tons of biomass per acre. The grasses were then chopped very finely and incorporated into the soil in several passes.
The team then laid a double line of orchard drip tube covered by a totally impermeable film.
Both the brassica and disinfestation treatments seemed to work. They shifted the microbial communities in positive ways, and tree growth was better than in the untreated control, though it didn’t always keep up with the trees in the fumigated rows, DuPont said.
A few steps are needed to propel the new strategies from academia to commercial production.
Brassica seed meal is still considered experimental. It’s treated as a fertilizer, though a manufacturer is pursuing a pesticide label in Washington state.
DuPont also plans to test the mustard in reduced doses because her experimental rates would be too pricey for commercial orchards.
To determine whether a disinfestation treatment had reached an anaerobic state, DuPont used specialized probes that farmers aren’t likely to have.
Her data could be expressed as a percentage of the field’s soil moisture capacity, a number that farmers could work with.
“I’m hoping that that’s going to be a good enough measurement that it would be pretty simple to monitor,” she said.
Both the brassica and disinfestation programs require about three weeks to work, and growers should wait another couple weeks after the treatments before planting the new trees to allow any harmful compounds to dissipate.