Reduction of overwintering inoculum in orchards with apple scab

Cultural controls reducing apple scab-infected leaf litter prepare the orchard for more effective control next year.

With all of the control difficulties we are facing due to fungicide-resistant isolates of the apple scab fungus in Michigan orchards, we need to increasingly utilize additional management options. Cultural control methods that reduce leaf litter that contains scab lesions are a great physical method of inoculum reduction. These are equal opportunity methods, i.e., both fungicide-resistant and fungicide-sensitive isolates are affected, which is excellent. While degradation of leaf litter in orchards is never 100 percent effective, use of urea or flail mowing should result in a significant reduction of the spore load for next year. Reduced spore loads mean more effective scab control next year.

Orchards with existing apple scab infections on leaves will again carry-over significant inoculum into next season. Same as last year, much of this inoculum will likely be resistant to strobilurin fungicides and to sterol inhibitor fungicides. Thus, any methods that would be useful in reducing this inoculum load are important to consideration.

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 tree’s development, 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 to 80 percent.

The two main methods for spore reduction are:

  • Application of urea to fallen leaves in fall or spring
  • Shredding of leaf litter with a flail mower

A 5 percent solution of urea (spray urea or greenhouse grade) (40 lbs. 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. Earthworms will work better as the temperatures rises; 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 is usually 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 be preventing the fungus from effectively discharging spores.

A few other points about mowing:

  • The mower must be set low enough to reach leaves low to the floor.
  • The mower must be offset to reach leaves beneath the trees.
  • 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.

Dr. Sundin’s work is funded in part by MSU‘s AgBioResearch.

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