Inoculum reduction recommendations in 2024.

With the late season rains in 2023, there could be high levels of apple scab or apple blotch in the coming spring. Reduction of overwintering, or “primary ascospore inoculum”, will allow for easier season long management especially if there is considerable rain fall from green tip to petal fall. Any efforts to reduce initial inoculum will delay the epidemic, and in theory, if there is little rain early in the season, it could possibly delay the epidemic to a point in the season where it would be too dry and too hot for the apple scab fungus to cause high levels of disease. Since it’s hard to predict if there will be an early season drought, we should keep suppressing apple scab to keep orchard populations low early on. In addition to apple scab, reducing orchard floor leaf litter and fruit drops may greatly reduce the inoculum for other foliar diseases like apple blotch and numerous fruit rot diseases including bitter, black, and white rot. As soon as it is possible to safely get a tractor in the orchard, remove any remaining fruit drops and shoots left on the floor from winter pruning as they may contain bitter rot or black rot inoculum. If orchard floor management was practiced in the fall with flail mowing or urea sprays, it won’t be necessary to repeat the practices this spring. Research out of the University of New Hampshire has demonstrated that there are diminishing returns for practicing inoculum reduction in both the fall and spring. Even if the planting is in green tip, inoculum reduction may still provide considerable benefit by reducing inoculum pressure by tight cluster or pink, when tissues are at their greatest susceptibility to apple scab.

The two best options for inoculum reduction are to apply the urea to leaf litter or use a flail mower to shred the leaves. These practices hasten decomposition of the leaf litter. In the case of flail mowing, leaves should be first swept or raked from underneath the canopy into row middles as most of the apple scab inoculum is present on litter under the trees. Subsequently, go over the row middles with the flail mower set to scalp the sod. If urea is used, apply 40 lbs. of feed grade urea per acre in 100 gallons of water to the herbicide strip (5% solution). Dolomitic lime applied at a rate of 2.5 tons per acre can be used of in place of urea. Of the various options, applying urea is the simplest approach, but take care to flush the sprayer pumps with water afterwards since the urea is caustic and can corrode a pump over time. As suggested above, the use of orchard floor urea may also reduce inoculum of other diseases such as apple blotch, bitter rot, and black rot as it hastens decomposition of leaf litter, fruit drops and pruned shoots that harbor the pathogens causing foliar diseases, cankers, and summer fruit rots.

Delayed-Dormant copper for apple scab and fire blight inoculum reduction.

Excessive warm weather after petal fall in the last few years has led to some devastating fire blight epidemics in Western NY. Although fire blight was devastating statewide due to a cooler bloom in 2023, orchards with late season shoot blight could still have overwintering cankers. Presently, with the cool weather following budbreak last week, overwintering fire blight cankers may still be dormant in commercial orchards. As the weather begins to warm (> 60°F) in the coming day/weeks, fire blight cankers could begin to ooze. While we are having this cool spell, scout for oozing cankers, especially in the eastern part of the state and New England. It’s important to note that cold weather will not kill fire blight bacteria overwintering in cankers; the bacteria will remain inactive, but viable at low (< 32°F) temperatures. To mitigate the threat of oozing cankers and reduce both fire blight inoculum and early season apple scab inoculum, make one or two “delayed dormant” silver-tip applications of a high (>15%) metallic copper equivalent (MCE) copper fungicide (e.g. Badge, Kocide, Cuprofix). It may be hard to get into the orchards at silver tip due to wet fields, but the application can be delayed to ‘green tip’. Even at green tip, it is generally still safe to apply high MCE copper products but may be advisable to leave out oil at these late development stages. In the Geneva research orchards, we often make a second application of a high MCE copper fungicide at ¼” green with no consequences.