This is a bad year for fireblight which is a bacterial disease which affects mostly apple, pear, crabapple, hawthorn and Mt. Ash. It is spread in the presence of moisture and enters the trees through wounds and open blossoms. Colorado is the worst state in the US for fireblight.
Because of our moist spring and extra wounding caused by hail, there is a lot of infection. Symptoms include black, hooked twig ends, brown or blackened leaves, dried up fruits and sunken cankers that can be a dull orange or black. Fireblight is a difficult disease to manage because there is no cure. Spraying chemicals is not recommended by CSU because they are not very effective, timing is critical and spraying must be repeated.
Pruning out infected branches is the main active solution that can prevent the spread of the disease, but it is tricky because pruning can spread or worsen the disease. There are many views of the “right” way to do it.
Mikl Brawner dealt with fireblighted trees as an arborist for over 35 years. Here is a simplified version of his approach:
Do not prune in spring because the moist conditions and pruning cuts create opportunities for the fireblight to enter the trees. Once conditions are drier, prune fireblighted twigs and branches back 6″-8″ from the infection, and sterilize tools after every cut with Lyson, alcohol or a bleach/water solution.
If there are hundreds of blackened leaves and twigs, wait until the leaves are falling to prune, because when the tree is dormant, pruning can be done without sterilizing every cut. In general make more small cuts than big cuts which take longer to heal and cause more stress to the tree. The infection can be superficial and not as bad as it looks, so pruning less can be better. Also radical pruning stresses the tree and creates more sucker growth, both which increase infection by fireblight.
Prevention is important when possible. Plant fireblight-resistant varieties. (there are conflicting views, unfortunately) Irrigation that sprays the leaves and keeps the soil saturated encourage fireblight. Strong chemical fertilizers cause fast soft growth that is especially susceptible. Do water when soil is really dry, because stress makes fireblight worse.
There is evidence that organic fertilizers and mineral supplements (like rock dust) applied in September can prevent or reduce fireblight infection by increasing nutrient availability and vitality. Inoculating new tree roots with mycorrhizae can also be helpful.
Don’t ignore fireblight or it will spread, but don’t be afraid to take 2 or 3 years to prune out diseased wood if that allows for smaller cuts. A susceptible variety like Jonathan apple or Bechtel crab may die or have to be removed.
This year may also be bad for fungal diseases like Powdery Mildew which appears as a white covering of leaves and stems with curling of leaves and stems. This can be managed and cured with Green Cure, Neem, Compost Tea and Fungus Pharm—all non-toxic products.
Cultural Prevention: Water deeply and only once or twice (3x in July?). It is best not to spray the leaves with water, and best to water in the morning so the soil can dry out by evening when our cool nights can condense moisture onto the leaves. A mulch reduces fungus spores from splashing up onto leaves.