For a few weeks in spring, your ornamental apple tree becomes a diaphanous cloud of white blossoms, apparently floating in light. After the petals fall, little fruits called crabapples appear and persist through winter. Unfortunately, several fungal scabs or powdery mildew can cause yellowing or coated leaves that shrivel and drop all summer long. Timely application of fungicide can keep your tree and its leaves healthy and handsome all year long.
Flowering Crab Fungus Foes
Ornamental apple trees, crab apple trees and orchard apple trees are all members of the apple (Malus spp.) genus and share the group’s susceptibility to several specific fungal infections. Apple scab, caused by the fungus Venturia inaequalis and pear scab caused by Venturia pirina start as chlorotic spots on leaves as they follow spring blossoms, gradually overspreading the entire tree. They overwinter in ground litter and re-infect the tree by riding rain or irrigation splash back up to lower tree limbs. Cedar-apple and cedar-quince rust fungi (Gymnosporangium juniperi-virginianae) migrate from Eastern red cedar trees (Juniperus virginiana) and other junipers in late summer. Powdery mildew, (Podosphaera leucotricha), emerges in shaded areas with poor air circulation, during cool, rainy spells or near apple orchards filled with susceptible species.
Although good sanitation and pruning practices can help control fungi population, proximity to infected Malus or other plants will introduce new pathogens. Replacing your tree with disease-resistant variety might be an option. If you want to fight the fungus, however, fungicides provide a barrier -- but they do not actually kill the fungus. Bordeaux mixtures (copper sulfate and hydrated lime), copper soaps (copper octanoate), sulfur, are “organically acceptable” sprays, several of which require mixing just before use. Horticultural oils, including mineral, jojoba or neem oils are eradicants, meaning that they will kill fungi on plant surfaces. Chemical sprays containing myclobutanil and a biological control containing Bacillus subtilis for powdery mildew are protectants. The effectiveness of any fungicide treatment, however, is entirely dependent on the timing of its application.
Preventative fungicides must be applied before pathogens contact the plant; eradicants are applied as soon as symptoms appear. In spring as soon as green shoots appear, repeated applications of fungicides for apple and pear scab every 10 to 14 days protect against infection. Bordeaux sprays and other copper sprays must be suspended at bloom drop, but as fruit begins to set, the tree becomes more resistant and treatments may be suspended. Sprays for cedar-apple scab can begin somewhat later -- as blossoms begin to turn from green to pink and white -- and continue at 10 day intervals. Powdery mildew requires treatment every 14 days in cool, wet -- or foggy -- periods.
In a warm Mediterranean climate or a foggy coastal area, gardeners may practice winter spraying. After a final fall clean up and removal of all dead leaves, trees can be sprayed in November and February with a follow-up spray near bloom time if spring weather is wet or foggy.
- Kansas State University Agricultural Experiment Station and Cooperative Extension Service: Flowering Crabapples
- University of Kentucky Cooperative Extension Service: The Flowering Crabapple
- University of California Integrated Pest Management: Apple and Pear Scab
- University of California Integrated Pest Management: Powdery Mildew on Fruits and Berries
- University of California Cooperative Extension: Winter Fubgicide Tree Sprays
- Medioimages/Photodisc/Photodisc/Getty Images