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Predictions for this season include a high incidence of both fire blight and cedar apple rust. The signs of fire blight (scorched-looking foliage and stem tips) are rather boring compared to the freaky looking appendages that “ripen” with cedar-apple rust. Get ready to start seeing large, orange gelatinous spore horns hanging from cedars like they are decorated with Japanese lanterns. They strike awe and dread from me because they are fabulous looking creations of nature but they also forewarn a rust problem in the orchard for next year.
Dr. John Hartman of University of Kentucky explains the disease cycle: many rust diseases must have two specific hosts for the disease to complete the various stages of its life cycle. For example, the fungus that causes cedar-hawthorn rust needs a Juniperus species, like the eastern red cedar or an ornamental juniper to spend its parasitic stage of life.
During the parasitic stage of life, the fungal spores appear in the springtime as bright orange teliospores on the diseased cedar. As these teliospores develop they are carried through the air to the next host plant. These other hosts must be roseaceous, like apples, hawthorns, crabapples or serviceberries.
While on the second host plant, the rust disease continues to develop into the fruiting stage that then travels back to re-infect nearby cedars or junipers. Apparently after eighteen months in this stage the spores return in the second spring to start all over if the conditions are favorable. They were very favorable for cedar-apple rust in the last few weeks.
All of the various rust diseases are caused by specific species of the fungus Gymnosporangium. The fungus must travel between a Juniperus species and some roseaceous plant; and back again. However, there are different types of rusts. Cedar-apple, caused by G. juniperus-virginianae, rust primarily infects the foliage of apples and crabapples. Yellow spots appear on the foliage in the spring, turning orange as the season progresses. Whitish, tubular spores will develop around these spots by summer. This seems to be the most distinguishable characteristic for diagnosis.
Cedar-hawthorn rust is caused by G. globosum and appears on hawthorns, apples, crabapples, pear, quince, service berry, and mountain ash. The appearance of this rust disease is similar to cedar-apple rust but the white, tubular spores are not present, just the leaf spots.
Cedar-quince, caused by G. clavipes, rust proves to be the most damaging to plant material. The list of susceptible plants include those that have been mentioned above plus, quince, photinia, medlar, chokeberry and cotoneaster. The foliar spots are not apparent here rather the infection is manifested in the twigs and fruiting structures of the plant. Young twigs that become infected can die and repeated infection can cause the decline of the tree. According to University of Kentucky plant pathologists we will see few cases of cedar-hawthorn and cedar-quince because the telia (the orange, gelatinous horns) where finished producing once the rains hit; not so with cedar-apple, though.
These diseases are very difficult to control using fungicides because timing is critical. Dr. Hartman suggests that we grow disease resistant varieties of trees and that we remove any old, abandoned or worthless host trees.