Fall urea applications can subdue apple scab
While apple scab pressure in the eastern United States was much lower this year than during the cool, wet summer of 2009, plant pathologists are sticking with their advice for growers who are troubled with scab-susceptible varieties (like McIntosh) or scab populations with resistance to fungicides.
For those growers, a fall application of urea fertilizer applied to fallen leaves can reduce overwintering scab inoculum by more than 80 percent, reduce primary scab pressure at green tip next spring, and maybe save a spray or two of fungicide after the primary scab period is over.
Cornell University Plant Pathologists David Rosenberger and Kerik Cox put that recommendation at the top of their list of ways to reduce inoculum in problem orchards: Apply sprays containing 40 pounds per acre of urea in 100 gallons of water to the fallen leaves in fall, or even early next spring after the snow is gone.
Shredding leaf litter with a flail mower will also help.
A question now being debated is whether the urea can be sprayed onto trees when perhaps only 10 percent of the leaves have fallen, Cox said. That would provide more thorough coverage, and leaf damage or excessive nitrogen uptake shouldn’t be issues that late in the season, he said.
“Urea works by stimulating microbial breakdown of over-wintering leaves,” they wrote in a paper published this spring. “Applications as late as green tip can still reduce the number of ascospores available during peak discharge periods between tight cluster and bloom.”
Urea contains 46 percent nitrogen, so nitrogen fertilizer rates should be adjusted accordingly. Urea applied in the tree rows will be available to the trees next spring, but that in the row middles will likely feed the ground cover, they said.
In orchards where additional nitrogen may be a problem, they recommend flail mowing as a way to provide more leaf edge area for better invasion by microflora and to reorient leaf pieces so more ascospores will discharge into the soil and less into the air.
Leaf removal by vacuuming is being used in Europe, they note. “Removing leaves from orchards may be the most effective option for organic growers because, if done carefully, it can reduce ascospore availability to almost zero.”
It doesn’t take many overwintering scabby leaves to start an infection, they said. Scab spores travel only about 300 to 500 yards, so most infections originate inside the infected orchard.