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Flowering crab tree death indicates possible scab

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Know It and Grow It
Lead Summary
By
George Bonnema, Luverne Horticulturalist

My trees are dying!!!!
I’ve had a few people question the leaf loss of their trees, and most often the trees they are alarmed about are flowering crab trees.
This year is exceptional for infection of a fungal disease called scab. The infection starts mid spring, and the first symptom is a black spot on the leaf which most people don’t notice.  Later the leaf turns yellow and drops, and that is when the evidence becomes very noticeable.
Cool wet weather invites the disease, and then it just advances with the season and the leaf loss can become severe. Good news is that the tree is not dying and that this will not necessarily kill the tree.
True, if the infection continues year after year, it will weaken the tree, and that invites all sorts of other problems that could lead to the death of that tree.
Once the tree becomes infected with scab, it is difficult to stop it. Remember that spray doesn’t make the fungus go away, rather it protects the new leaves from becoming infected. But at the point when most people notice the symptoms, the infection is pretty significant throughout the canopy and there are not many new leaves developing.
If you want to prevent scab, you have to spray before you see the evidence. Some years weather favors the development of this fungus, and other years not as much. Cleaning up infected leaves helps a little bit in that you remove that source of carryover for the next year; however, the fungus spores can also winter in crevices of the bark and twigs.  
Chances of a repeat infection next year are high because of the extreme humidity we have in this part of the country, so the best way to prevent the disease is to plant varieties that have resistance.
And while we are talking selecting resistant varieties, be sure the tree you are purchasing is resistant to fire blight. Infection of this virus is most often fatal to flowering crabs.
Fire blight is a virus, and evidence of infection is the sudden wilting and drying of leaves on a small or large branch. The leaves do not fall off. What has happened is the virus has totally blocked the flow of sap under the bark, thus effectively strangling the branch.
If you look closely at the bark, you will notice discoloration and a sunken area where this blockage has occurred. The disease will progress through the canopy sometimes slowly and in other varieties very rapidly.
Cutting out the infected branch sometimes will eliminate the problem, but be sure you are cutting back to healthy wood at least 4 inches before that evidence on the bark. Also, we recommend sterilizing any pruning tool with a 10-percent bleach solution between cuts to prevent spreading the virus from one area or tree to another.
Planting a resistant variety is an absolute must to prevent both of these diseases in our geographical area.
 
 

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