Diseases of Ornamental Crabapples
Ornamental crabapples are by far the most popular flowering tree in our region. They provide us with a wide array of bloom and leaf colors, fruits and tree shapes. Unfortunately, these plants are also subject to several serious diseases that can threaten their health. The three most serious diseases are scab, cedar-apple rust and fireblight. While there are cultural and chemical methods that can help in prevention and treatment of these diseases, the best defense is to carefully choose your varieties.
Disease Resistant Varieties
Choosing a disease resistant variety is the most important step you can take to insure your crabapple will be healthy. Always buy trees from a reputable nursery that can guarantee the variety you are getting and tell you about disease resistance. Here are a few of the many beautiful varieties of flowering crabapples that have a high resistance to disease:
Prairie Fire The truest red flowers of all crabs with blood-red fruit.
Donald Wyman Bears white flowers and produces small, persistent red fruit.
Louisa Flowers start out bright red and fade to white and produce small yellow fruit.
Madonna Upright habit makes it great for a boulevard and produce white flowers.
Tina Dwarf variety; single pink buds open to white flowers.
Red Jewel White flowers followed by long-lasting red fruit.
Sugar Tyme Pink buds that open into white flowers and produce dark red fruit.
Adams Deep pink buds open to clear pink flowers and produce small red fruits.
Candy Mint Bright rose/pink flowers followed by small red fruit.
Pink Spires Pink buds open to white flowers.
Cedar-Apple Rust
Prognosis: Cedar-apple rust can seriously affect the fruit quality and weaken the tree.
Cause: Several fungi in the species Gymnosporangium.
Spread: Requires two years to complete its life cycle. Brown galls that over wintered on certain cedars and junipers emit spores that are carried by wind and rain to nearby crabapples. In late summer, infected leaves give off spores that are carried back to its alternate host, where the galls develop the following year.
Symptoms: The initial symptoms are small, yellow spots on the leaves in early summer that gradually get bigger and turn orange. These rust-like lesions can also develop on the fruit, as well as being unsightly. Severe infection can result in little or no fruit.
Cultural Prevention: While it is often impossible to remove all potential cedar/juniper hosts in the area, avoid planting ornamental crabs in combination with susceptible junipers. Plant resistant varieties when possible, and reduce any additional stresses.
Chemical Prevention and Treatment: Repeated applications of a broad spectrum fungicide from bloom time through mid-summer are relatively effective in preventing cedar-apple rust.
Apple Scab
Prognosis: If left untreated, apple scab can weaken the tree.
Cause: A fungus named Venturia inaequalis.
Spread: Apple scab is spread from tree to tree by spores that develop on infected leaves. These spores can winter on leaves left on lawn, to be carried in spring to new young foliage by the wind and rain.
When the conditions are favorable for scab (wet and warm), infected leaves give off spores that affect the fruit on the tree.
Symptoms: When a leaf is infected, the spores start to develop small, rough patches on the leaf, usually on the bottom surface. These patches or scabs start out a light green, turning dark brown or black. It is common for affected leaves to fall early in the season. The fruit on the crabapple may develop similar scabby areas and, when severe, the fruit will be misshapen and won't fully develop.
Cultural Prevention: Plant resistant varieties when possible. Remove the fallen leaves from the ground, especially in fall. Dispose of infected leaves; do not put them in the compost or use them as mulch on other plants. Prune to open up the branching for better air circulation. Avoid getting water on the leaves whenever possible. Minimize stresses to the health of the tree by keeping it well watered and fertilized.
Chemical Prevention and Treatment: Several fungicides have proven to be effective in minimizing and/or preventing apple scab. Fungicides require several applications. Read the label and follow directions carefully.
Nothing has been found to be very effective in treating diseased trees. Since it is not immediately fatal, preventive measures can be taken for the following season.
Fireblight
Prognosis: A potentially devastating disease in ornamental crabapples. Left untreated, it is almost sure death for the tree.
Cause: A bacterium called Erwinia amylovara.
Spread: Fireblight is readily spread between trees by insects and rain, targeting the blooms in spring.
Symptoms: The infection is usually first noticed as a sudden wilting at the tip of a branch. After wilting, it will rapidly turn dark brown like it was burned. Fireblight then rapidly moves down the branches toward the trunk, forming cankers and blistered areas on the bark.
Cultural Prevention: As soon as you identify fireblight on a tree, prune it out. Do this by cutting at least 12 inches beyond affected areas into healthy tissue, sterilizing your pruners between each cut with a strong bleach solution. Remove the prunings from the area. For new plantings, always choose disease resistant varieties.
Chemical Prevention and Treatment: Since fireblight is a bacteria disease, the chemical treatment available is an agricultural grade of the antibiotic streptomycin. Timing on chemical treatment is crucial and effectiveness is questionable.
Recommended Products
Bachman's-Grown Crabapple Trees
Quality Pruning Shears
Compression and Hose-End Sprayers
Bonide Fungonil™ Fungicide
Fruit Tree Spray
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