When to prune out fire blight: To prune or not to prune
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
The question of pruning out fire blight strikes during the growing season, or leaving them until the winter is debated whenever we have wide spread fire blight. The answer to this question depends upon the number of strikes and the weather conditions suitable for infection. Generally, pruning out fire blight strikes only makes sense in young orchards and when there are a relatively small number of strikes that are easy to get to.
If you decide to prune out strikes in a young orchard, the strikes should be pruned out as soon as they appear. Failure to do so increases the likelihood that blight will continue to spread both to adjacent trees and into the rootstocks of affected trees. Pruning out infections in mature trees may not be practical, but mature trees with a full crop will set terminal shoot buds earlier than young trees. When trees set terminal buds, blight stops spreading both between trees and within the affected trees.
Remove strikes before the cankers extend into the tree. Trees must be examined at least two or three times weekly until the epidemic slows as tree growth slows. In sections where trees are severely affected, it may be more cost-effective to immediately remove entire trees, especially if trees are a susceptible cultivar like Gala. Pulling out badly affected trees will allow blight removal crews to focus their efforts on trees that can be salvaged.
Blight removal crews should be trained to recognize the early symptoms of blight on terminal shoots. The first or second fully expanded leaf will droop and closer examination will show blackening along the mid-vein at the base of the leaf blade. The entire shoot tip may appear to be slightly yellowed. Remove such shoots by cutting back into two-year-old wood at least 8-12 inches below the last visible symptoms. If a spur or shoot on the central leader shows signs of blight, immediately remove the central leader down to 8-12 inches below the last visible symptom. Immediate and aggressive removals reduce the need for repeated pruning in the same tree and may result in fewer trees lost to root stock blight.
Makecuts into at least 2-year-old wood where bacteria will be less able to multiply. Also, leave "ugly stubs" by cutting branches between nodes and several inches away from the central leader or other branch union. Small cankers that form on these stubs can then be removed during winter pruning whereas a canker that forms at a flush cut on the central leader will be missed during winter pruning.
Prune during dry weather. An extension specialist in California reported that he failed to transmit fire blight with pruning tools when he purposely made cuts through active cankers in dry weather. However, he succeeded in transmitting blight on pruning tools when pruning was done in wet weather. Blight removal operations should be suspended in wet weather. If wet weather persists, the risk of spreading the disease during pruning is too great and it may be better to leave the fire blight strikes in the orchard until dry weather arrives.
Should prunings be removed from the orchard? My recommendation is to toss prunings in the row middles and allow them to thoroughly dry before mowing them. Dry, dead prunings on the orchard floor do not present a danger to spreading the disease. Dry means that the bark no longer slips on the cut branches, and the cambium is brown. With today’s tightly spaced orchards, I am concerned that carrying prunings out of the orchard may spread more blight than occurs when prunings are left to dry in the row middles. I normally do not recommend disinfecting the pruning tolls because you need to soak them for several minutes to do a good job and most will not take the time.
Avoid hand thinning, bud pinching and other manipulation activities until after terminal bud set. Delaying hand thinning may result in some loss of fruit size, but risks of spreading blight out-weigh the benefits of early hand thinning. You can spread blight on your fingers while pinching buds (or hand-thinning).
Under dry conditions when only a few strikes occur, immediately pruning down to non-infected 3-year-old wood reduces the potential of the disease spreading. This strategy works where infections are located in only an isolated area of the orchard. This strategy works best with old trees.