Humid, Wet Weather Brings Tree Diseases

The humid and wet weather of spring and early summer has been ideal for the development of several woody plant diseases.



Fire blight, a bacterial disease, is rampant this year. It is commonly found on apples, crabapples, mountain ash and pear. Look for dark brown or blackened branch tips bent back into a “shepherd’s crook”. You might also find droplets of a creamy- colored liquid called bacterial ooze on infected stems.

This disease causes branch dieback and flower and fruit blight. At this time of year, cut out the affected branches about a foot below the visible signs of infection. Disinfect your pruning tool between each cut using a 10% bleach solution, disinfectant spray or rubbing alcohol. Fire blight can be spread on the blades of pruning tools and easily enters the plant through pruning cuts.
Fire blight is found on pome-forming (core forming) fruits of the apple family that include the plants mentioned earlier as well as roses, pyracantha and hawthorn. Similar shepherd crook damage is often observed on plums, cherries and apricots. Even though they are members of the rose family, they are stone fruits and in our area do not succumb to the disease. Look for other causes of this type of injury.



Sycamore and oak anthracnose fungal diseases are also common this summer. Infected leaves develop irregular brown leaf blotches. Leaves drop prematurely when the disease is severe. These anthracnoses may be confused with frost damage, scorch or leafminer injury depending on when the problem is first observed. If you have a hand lens of 30 power or more, you may be able to find telltale fruiting structures embedded in the darkened areas of the leaf undersides. If you’re not sure, contact the Jefferson County Plant Diagnostic Clinic at 303-271-6620 for assistance. See this fact sheet about anthracnose for more information.

Rake up leaves and dispose of them in the fall. Generally anthracnose diseases stress trees in situations where defoliation and re-leafing occur every year. Protectant fungicides should be applied in the spring at leaf emergence if this is the case. In drier years, anthracnose diseases are less of a problem.
Marssonina leaf spot of aspen has been brought into the clinic already. The fungal disease causes dark brown spots often surrounded by yellow “halos”. Spots may range in size from pinhead to larger blotches, where several smaller spots have merged together. Leaves may drop prematurely. Usually the problem is more of an aesthetic one than a plant health problem, although yearly premature defoliation stresses aspen.

Rake up leaves this fall. Trees may also be sprayed with a protectant fungicide next spring beginning at bud break. The disease is more prevalent and is found a bit earlier this year than normal due to the spring and early summer weather. See this fact sheet about foliage diseases for more information.



Anthracnose in Cucumber