"Green" Approaches to Scab Control
(Continued from page 7)
will not provide adequate scab control when used alone, but it can reduce losses that might otherwise be incurred with sulfur programs or even with protectant fungicide applied in high-inoculum orchards.
The objective of inoculum reduction is to eliminate some of the ascospores that overwinter in fallen leaves. Reducing the number of ascospores makes it easier to prevent leaf infections with fungicides applied in spring, and it decreases the likelihood that scab infections will be initiated at green tip or half-inch green when only a small proportion of ascospores are ready to discharge. By avoiding early-season infections, the risk of developing fruit scab is significantly reduced.
Three effective approaches for inoculum reduction have been documented in the scientific literature. None of these approaches will eliminate 100% of the ascospores, but any one of them can reduce inoculum production by at least 50 to 80%.
1) Urea sprays (40 lb urea/A) applied to fallen leaves in
autumn or spring
2) Shredding leaf litter with a flail mower
3) Application of dolomitic lime (2.5 ton/A) over fallen
leaves in autumn
Urea works by stimulating microbial breakdown of overwintering leaves. It may also inhibit ascospore formation in the surviving leaf litter. Urea should be applied at 40 lb/A to fallen leaves using a sprayer that provides coverage of the entire orchard floor. Spraying trees with urea before leaf drop in autumn is less effective than spraying leaves on the ground, because leaves that remain on the tree for 7 days after the urea application will translocate the nitrogen into the twigs, thereby making it unavailable to assist in decay of the fallen leaves. Surprisingly, even when ground sprays of urea are applied as late as green tip, they have been shown to reduce ascospore release by 40-86%.
Shredding leaf litter with a flail mower can reduce inoculum in several ways. First, it provides more "edges" in the leaf litter for invasion by the microflora that cause the leaves to decay. Second, if flail mowing is done in spring, the chopping action will result in the reorientation of most leaf pieces on the orchard floor, and many ascospores will discharge into the soil rather than into the air. In New Hampshire, leaf shredding was least effective when it was done in December, presumably because shredding at that time did not allow for leaf decomposition before winter and also failed to cause disorientation of ascospore release (the pseudothecia in overwintering leaves have not yet formed in December and therefore cannot become disoriented). Effective leaf shredding can be accomplished only with a flail mower that is set so low that it nearly scalps the sod in the row middles. Effectiveness is also dependent on having a very level orchard floor and on being able to shred most of the leaves beneath the tree canopy. If the flail mower cannot be offset to reach beneath trees, then it may be necessary to blow leaves from beneath trees into the sodded row middle or to remove leaf litter from beneath trees using a brush rake ahead of the flail mower.
Dolomitic lime has been less widely tested as an inoculum reduction technique, but it was very effective when tested in Oregon, where it was applied at 2.5 ton/A after leaf drop in autumn. Lime presumably works by raising the pH of fallen leaves to a level where they are more subject to breakdown by