Several forces at work suggest that orchardists should look more closely at how they manage the floors of their orchards and vineyards.
For one thing, there is strong evidence now that some fungicides, when used near and during bloom, contribute to the declining health and vigor of honeybee colonies.
These fungicides increase the toxicity of insecticides, in a process called synergism, that boosts their killing power by 2,000 times or more. Orchard floor management practices can reduce levels of disease inoculum, especially apple scab, and thus reduce the need to use fungicides early in the growing season, when control of primary scab is essential for season-long scab control.
“We should do what we can do to scab while it’s sitting there on the orchard floor.”
—Dr. Matt Grieshop
Third, organic growers aren’t making progress toward finding effective chemical weed-control products. After trying alternative practices, like flaming, with poor results, they’d like to find good tillage methods that would make in-row weed control less labor-intensive, faster, and more effective.
And fourth, it is generally believed that vegetation on the orchard floor interacts in complicated ways with tree and vine nutrition, water use, nitrogen release, fruit and grape quality, beneficial and pest insects, and diseases, and soil quality measures such as compaction, microbial populations and activity, and organic matter content.
At Michigan State University, Dr. Matt Grieshop has embarked on orchard floor management research projects to address both weed and disease control. He’s head of MSU’s Organic Pest Management Laboratory. An entomologist, he’s working with plant pathologists and horticulturists as well. He, his graduate students, and colleagues are looking at practices that can be used by either organic or conventional growers—probably both.
Scab inoculum reduction
“I think raking and flail mowing are good options for organic growers wanting to reduce apple scab inoculum levels in their orchards,” Grieshop said. “For conventional growers, I think they should use urea treatments, ideally in combination with flail mowing.”
MSU plant pathologist Dr. George Sundin recommends urea treatment on apple leaves, in the fall or spring or both. The nitrogen speeds up the breakdown of leaves, where the scab organism overwinters, ready to release spores the following spring.
The treatment—which is also widely recommended by land-grant university plant pathologists across the eastern United States—uses feed-grade urea in a 5-percent solution sprayed on the leaves.
If applied in fall, it can be sprayed on the trees if leaves haven’t fallen, or on the leaves as they lay on the ground. The recommended treatment is 42 pounds of urea in 100 gallons of water. Applying 100 gallons per acre adds 20 pounds of nitrogen to the orchard. This treatment reduces spore inoculum by two-thirds or more.
In other studies, flail mowing to shred the leaves in November or April reduced scab inoculum by 85 percent. Flailing in the spring, after the scab fungus has started to grow, will reduce inoculum by 50 percent just by turning the leaves over so spores release downward and thus can’t reach new tissue to infect, Sundin said.
“Earthworms seem to prefer eating urea-treated leaves,” he said. Spraying the leaves, then chopping them, provides a salad that brings earthworms into the scab control program.
Sundin said that it only takes 1 percent scab-infected leaves to cause infection in the orchard. Still, reducing overwintering levels of inoculum greatly reduces the -number of spores released in the spring and makes fungicide applications at green tip more effective. Reducing the number of fungicide applications each year would not only save money, it would reduce the chance for -disease organisms to build resistance to fungicides.
Sundin said there are no new classes of fungicides in the pipeline, so growers need to undertake resistance-management strategies to preserve the -effectiveness of the fungicides they have now.
Apple scab’s overwintering inoculum is in the primary (sexual) stage, a phase in which genes mix into new combinations, some of which may be resistant to fungicides. Later, secondary spores are asexual clones, so new resistance is not likely to occur. Control of primary scab is essential, he said, and the fewer spores growers have to contend with, the better their fungicides will perform.
“The fewer sexual spores there are, the fewer combinants there will be for developing resistance,” Grieshop added. “We should do what we can do to scab while it’s sitting there on the orchard floor.”
For organic growers, Grieshop is investigating using a simple machine called the Kuhn Haybob, a hay rake that reaches under the tree canopy and sweeps out leaves into a windrow so they can be chopped in the alley center using a flail mower.
He thinks the machine could also be useful in cherry orchards to reduce cherry leaf spot inoculum.
During a webinar for Michigan growers in early March, a listener asked, would it be possible to vacuum up all the leaves and remove them from the orchard completely? Possible, yes. But feasible? Not likely for a large orchard operation, Grieshop said.
For the last two years, Grieshop’s graduate student Brad Baughman has been studying the effectiveness of tillage compared to herbicides in two vineyards and an apple orchard in Michigan.
The apple experiments were conducted at Steve Tennes’s Old Country Mill in central Michigan and at Cornel Oliver’s 2 Lads winery and Paul Dalese’s Chateau Chantal winery near Traverse City.
While all orchards had grassed alleys, trees were in strips that were either treated with herbicides or tilled with one of two tools.
One tool is the Wonder Weeder, from Harris Manufacturing in Burbank, Washington, and the other the Clemens Radius Hoe, a German machine used in grape vineyards.
The Wonder Weeder consists of a gang of rolling spider wheels and is equipped with a shear bar that sweeps under the canopy close to the tree or vine row.
The Clemens Hoe consists of a blade that runs under the soil surface, uprooting vegetation. It has a “feeler” bar that keeps the blade away from trees or vines, allowing the blade to sweep back and forth into the row and between plants.
Both machines have the advantage of being front mounted on a tractor, providing good visibility for the operator and faster operation, Grieshop said.
Are there other such machines, listeners asked. Tillage tools like these are used in Australia and New Zealand, and the once-popular Lilliston rolling cultivator is still used by some growers when they can find the units.
Grieshop said the rolling and slicing tools are much better than rototiller-type powered-rotation tools that break up and damage soil structure. •
So how can downy mildew be managed in the vineyard?