George Sundin, Plant Pathology, and Amy Irish-Brown, MSU Extension Educator

Orchards with existing apple scab infections on leaves will carry over significant inoculum into next season. In addition, much of this inoculum will likely be resistant to strobilurin fungicides. Thus, any methods that would be useful in reducing this inoculum load are important to consider.

The apple scab fungus overwinters in fallen leaves. During the following spring, the fungus undergoes a sexual cycle and produces a fruiting body called the pseudothecium that contains the ascospores. These ascospores are the spores that represent the primary inoculum. Development of these spores is timed with the development of the tree, and the spores can begin to be released around green tip.

Inoculum-reduction methods serve to reduce the primary ascospore load by eliminating some of the apple scab-infected leaves. Any reduction of scab-infected leaves directly correlates with a reduction in primary inoculum. Now, it is impossible to completely eliminate this inoculum, but spore-reduction strategies have been effective in reducing spore loads by 50-80 percent.
The two main methods for spore reduction are:

1) Application of urea to fallen leaves in fall or spring.
2) Shredding of leaf litter with a flail mower.

A 5 percent solution of urea (spray urea or greenhouse grade) (40 lb urea in 100 gallons of water) is used to increase the breakdown of leaves. Urea will stimulate indigenous microbial breakdown of leaves; urea can also soften leaves which are then more easily ingested by earthworms. These native organisms will work better as the temperatures rise; thus, urea applied in November may not be as effective if followed relatively quickly by freezing temperatures and snow cover. An application in spring can be highly effective in spore reduction and the urea may also directly inhibit ascospore formation. Another possibility is the direct application of urea to leaves on trees; this method usually is less effective because if the leaves do not drop within seven days after application, the nitrogen present will be taken up into the tree and not be available for leaf degradation. Finally, urea sprayed on the ground beneath the tree canopy will also add to the nitrogen fertilization of trees and subsequent N fertilization rates should be adjusted accordingly.

Shredding leaf litter in the spring increases microbial breakdown of leaves by providing more pieces that can be invaded and consumed. In addition, mowing tends to re-orient most of the leaf pieces on the orchard floor. When the scab fungus is developing pseudothecia, the structures are all oriented in a vertical direction with the opening facing up. The spores are forcibly ejected out of the top of the pseudothecium. If a leaf piece containing a pseudothecium is inverted, the spores are ejected into the soil and not into the air. Thus, re-orientation also decreases inoculum load by preventing the fungus from effectively discharging spores.

A few other points: 1) the mower must be set low enough to reach leaves low to the floor; 2) the mower must be offset to reach leaves beneath the trees; 3) if the mowing is done later in the spring, the re-orientation of leaves is also effective. Prior to spring, the pseudothecia structures have not developed yet.