BOULDER COUNTY – Gardeners who plant apple trees are hopeful souls, putting in saplings as they dream of future pies, sauce, and tarts. With each season of growth, we croon encouragement at the plant. To have an apple tree is to have an Elysian dream, one that doesn’t include oozing bacteria, scab-spotted leaves, or wormy apples.
Occasionally we get those perfect seasons. But not this year. Instead, it’s a banner season for fire blight, apple scab, and codling moth.
Fire blight is a bacterial disease that is especially destructive to apple, pear, quince and crabapple. It attacks in spring, when temperatures reach 65 degrees and frequent rain occurs. Bacteria overwintered in cankers on the tree resume activity, multiplying rapidly.
Our wet early summer weather created good conditions for this damaging disease, and masses of bacteria forced up through cracks and bark pores to the bark surface, forming a sweet, gummy exudate called bacterial ooze. Insects picked up the bacteria on their bodies and carried it to opening blossoms where it infected the trees.
Girdling cankers – areas of disease on the wood – eventually develop from branch or blossom infections. Leaves wilt, darken and curl to form a shepherd’s crook. This gives the tree a fire-scorched appearance, thus the name “fire blight.”
There is no cure for this disease, so prevention is the best solution. Remove and destroy newly infected young twigs as soon as possible, so that your tree doesn’t become the mother ship for disease in the neighborhood. Do this when no rain is predicted for at least two weeks. It may be best to leave pruning until winter when the bacteria are not active. In young twigs, make cuts at least 12 inches below the dark, visible edge of infection to avoid slicing into the bacteria. Remove all blighted twigs and cankered branches. Prune larger limbs about six to 12 inches below the edge of visible infection.
After each cut, surface sterilize all tools used in pruning. Spray tools with Lysol or dip tools in 70-percent ethyl alcohol, or a 10-percent bleach solution. Bleach can rust tools, so if you use this to sterilize your pruners, wash them after you’re done and apply a light tool oil to keep them rust-free.
Apple scab, a fungus that attacks leaves and fruit, also was favored by the cool, wet weather. Gardeners are seeing the rapid spread of this disease across their apples and crabapples. At first, leaves get yellow or dark olive-colored spots, then turn yellow and fall off. Fruit develops dark, greasy-looking spots that then become sunken.
The disease overwinters on fallen leaves, so clean the area during fall. Avoid overhead watering that can splash spores around.
Coddling moth adults have come and gone, laying eggs on fruit so the larvae can hatch and burrow inside. A tell-tale sign of the wormy apple is a light brown, crumbly frass being pushed from a hole in the side of the apple. Larvae leave the apple when fully grown to pupae under the bark or in debris on the ground.
There can be several generations of codling moth each year, so the best prevention is spraying with a pesticide to control the moth when the adults are flying and picking off and destroying infested fruit. Using trunk bands to trap the larvae as they climb down the tree also helps.
For now, picking off and destroying the fruit is an option if you want unblemished fruit, but if you don’t mind sharing your treat with a legless, segmented intruder, you can wait until the apple ripens and simply cut off the worm-eaten part.
For more information on fire blight, apple scab, or codling moth, please see the CSU Extension website at extension.colostate.edu/topic-areas/yard-garden/?target=
Colorado State University Extension, together with Boulder County Parks and Open Space, provides unbiased, research-based information about consumer and family issues, horticulture, natural resources, agriculture and 4-H youth development. For more information contact Extension at the Boulder County Fairgrounds, 9595 Nelson Rd., Box B, Longmont, 303.678.6238, or visit the CSU Extension web site at