May 2007

 

Q:  Should I try to prune out fire blight when it appears in young trees?

 A:  Absolutely, unless blight is so severe that the orchard is beyond hope.  Strikes should be pruned out as soon as possible after 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 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.
Q:  Is it necessary to disinfect pruning tools between cuts?
A:  Dr. Paul Steiner has shown that disinfecting pruning tools is a waste of time because minute cankers often form on the ends of cuts even when pruners are disinfected.  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 usually be suspended in wet weather, but that is not always possible. Instead of wasting time disinfecting pruning tools, Paul recommends making all cuts 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 at least several inches away from the central leader.  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. 
Q:  Should prunings be removed from the orchard?
A:  My personal recommendation is to place prunings in the row middles and allow them to thoroughly dry before running a mower over them.  "Thoroughly dry" means that the bark no longer slips on the branches that have been removed, and the out bark and cambium have turned 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. 
Q:  Can I do hand thinning or bud pinching while blight is active in the orchard?
A:  Avoid these 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.   Pinching is done to succulent shoot tips that are highly susceptible to blight whereas cuts made to remove blight are made in wood that is at least two years old. 
Q:  What can be done to stop the spread of blight to new terminal shoots?
A:  No good answers here.  Anything that helps to shut down tree growth will help to limit the spread of blight since the epidemic stops when terminal buds are set.  Apogee  is useful for arresting blight epidemics.  Apogee causes trees to set terminal buds beginning about two weeks after it is applied.  To control vegetative growth in overly vigorous blocks, Apogee application will be recommended at late bloom or at petal fall. If Apogee is applied after the first symptoms of blight appear in an orchard, two applications will probably be needed to rein in the growth process and blight will continue to spread for at least two weeks after the first Apogee application.  Copper sprays applied in summer theoretically should inactivate blight bacteria on plant surfaces and thereby help to reduce inoculum and slow the epidemic.  Proponents of using copper during summer admit that benefits of copper will be limited because copper is not systemic and therefore will not affect bacteria inside plant tissue.  Furthermore, actively growing terminal shoots "outgrow" the copper residue, thereby leaving the blight-susceptible shoot tips unprotected within several days after an application.  Copper applied in summer is also phytotoxic to fruit, with injury appearing as necrotic black spots at fruit lenticels.  Thus, copper sprays are not acceptable where the crop is destined for fresh market.  In young orchards, salvaging the crop may be less important than salvaging the trees.  The bottom line:  If I was managing a young Gala block with fire blight, I would be applying a low rate (about 4 oz/100 gallons dilute spray) of a fixed copper on a 7-10 day schedule until terminal buds are set.  Copper sprays should be applied under good drying conditions.  The alkaline nature of copper sprays means that they probably cannot be combined with other pesticides that are subject to alkaline hydrolysis.

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