"My crabapple tree is dropping leaves like crazy! Almost half the foliage is gone and the remaining leaves have orangish-spots. "
Cedar-apple rust is a disease that causes substantial injury to apple, crabapple and hawthorn trees in Nebraska. Trees severely defoliated by the disease are a common site in late summer. Infection of trees, by the fungal spores that cause the disease, begins when the flower blossoms are pink but before they have opened, usually May and June. The fungal spores are released at this time by their alternate host, juniper. The spores are blown by wind to susceptible apple trees and soon leaf spots develop.
Leaf spots caused by cedar-apple rust are bright yellowish-orange in color and vary in size. The spots may have a band of red or yellow around them depending on the tree's susceptibility to the fungus. Fruit symptoms are similar to leaf symptoms. Infection of the fruit occurs near the underside, blossom end of the fruit and the spots are yellowish-orange in color.
The most effective method control for this disease is to plant resistant varieties of apple, crabapple and hawthorn. Unfortunately some very popular older varieties of apple and crabapple, which are still commonly found in our landscapes, are susceptible to cedar-apple rust and must be sprayed annually to control the disease. Injury to the apple trees occurs when extensive infection leads to premature leaf loss. Repeated infection for several growing seasons will weaken the tree and may result in decreased fruit production and death.
Susceptible trees need to be sprayed regularly during the period just before the blossoms open, or the early pink bud stage. Several additional fungicide applications should be made at 7-10 day intervals to provide good protection. Apple trees can be treated with several fungicides during this time, although Funginex (triforine) is the preferred product. As always, follow the label directions carefully, especially if this is an apple tree from which you plan to harvest fruit.
Resistant cultivars of apple include Granny Smith, Red Delicious, Early MacIntosh, Freedon, Liberty, Northwestern Greening, Nova Easygrow, Priscilla, Redfree, Sharon and Sir Prize. Resistant cultivars of crabapple include Bob White, Donald Wyman, Indian Magic, Prairifire, Red Jade, Sugar Tyme, and Zumi Calocarpa. Resistant hawthornes include Crataegus crus-galli, C. intricata, C. laevigata 'Autumn Glory', C. phaenopyrum, C. pruinosa and C. virdis.
Cedar-apple and Related Rusts of Apple and Ornamentals, University of Nebraska- Lincoln Extension