Spray Spring Snow Crabapple Trees for Fireblight this spring 2016 in Denver Metro Area?

Asked April 6, 2016, 4:32 PM EDT

Is it a good idea to do this every year or at least this year? Our HOA in Littleton CO has a lineup of a dozen large Spring Snow Crabapple trees that have been sprayed in most but not all 20 of their years. Thoughts or recommendation on the effectiveness of the sprays commonly used? Cheap insurance policy, or don't bother? Thanks in advance.

Arapahoe County Colorado trees and shrubs

1 Response

Fire Blight is a bacterial disease that can occur i many varieties of crabapple. 'Spring Snow' is one of those varieties that is susceptible to Fire Blight. The bacterial disease can be spread from one tree to another
and can cause considerable damage to the trees.
Disease incidence varies from year to year and severity is influenced by cultivar susceptibility, tree age, succulence of tissues and spring meteorological conditions. The disease is most serious when spring temperatures during pre-bloom and bloom are warmer than average. Warm rainy springs are particularly conducive to rapid spread of the pathogen, resulting in blossom blight. Blight of twig terminals can occur in late May through June during wind driven rain events. Hail and wind damage provide wounds that allow the pathogen to enter at other times. Hot summer weather generally slows or stops the disease.
The following information is from CSU Fact Sheet: Fire Blight:
There are some cultural practices that will help with prevention:

Cultural practices: Minimizing rapid growth and succulent tissue will reduce the risk of fire blight developing on the susceptible young, succulent tissue. Annual pruning with avoidance of major cuts will help minimize tree vigor. Similarly, limiting the amount of nitrogen fertilizer will reduce twig terminal growth. Fertilization should be based on the results of foliar and/or soil nutrient analysis and should not be applied in excess.

Regarding the use of chemicals:

Chemical sprays: Chemical sprays are preventive treatments that must be applied prior to the onset of fire blight symptoms; sprays have little effect after the onset of symptoms. Expect blossom infections and plan to apply chemical sprays if: temperatures remain between 65 F and 86 F for a day or more during flower bloom, there is at least a trace of rainfall, the relative humidity remains above 60 percent for 24 hours, there is abundant succulent shoot growth, or there are fruit injuries from hail or other agents. For specific instruction on sprays and timing please use the Midwest Tree Fruit Spray Guide. The chemicals may be sold on various trade names.

Streptomycin is an antibiotic that is acceptable for use to protect trees but may be difficult to obtain. Do not use streptomycin after symptom development since it may lead to antibiotic resistance in the bacterial population.

Aluminum tris is a bactericide used prior to and during bloom.

Copper sprays are toxic to many species of bacteria. Copper sprays are best used during dormancy and prior to bud break because they may damage leaves and young fruit. Do not apply sprays within 50 days of apple harvest or within 30 days of pear harvest. Do not mix with oils or phytotoxicity issues can occur. Copper is available in several forms and sold under various trade names, including Bordeaux mixture.

Prohexadione-calcium is a plant growth regulator that reduces longitudinal shoot growth by inhibiting gibberellin biosynthesis. Prohexadione-calcium does not possess antibacterial activity but alters host biochemistry and tissues in ways that are not favorable for infection by E. amylovora. The length of time that shoot growth is inhibited depends on the application rate and tree vigor. Prohexadione-calcium is ineffective for control of the blossom blight phase of fire blight.

A tree professional may be the best way to go with a spraying program. The timing of the spray is very important and a good tree professional will be able to help your HOA. If the HOA decides to do this job then read the label of any chemical carefully and follow the directions as specified on the label. Preventative spraying may be a "good insurance policy" in your case, you would hate to have to replace these trees.