Gardening Articles: Care :: Pests & Problems
Growing Organic Apples (page 2 of 4)
by Michael Phillips
Three diseases and three insect pests most often ruin home apple crops. The diseases are apple scab, powdery mildew, and cedar-apple rust. The insect pests are codling moth, apple maggot, and curculio. Scab, powdery mildew, and codling moth are nearly universal; others are more regional.
Scab is generally considered the most serious apple disease. Initially the fungus causes olive green spots, which darken and harden over time. Scab develops in the mild temperatures and high humidity of spring. During particularly wet summers, subsequent scab infections, called "secondary scab," will cause problems through the summer if you were unsuccessful preventing infection in spring.
Scab is the most common and most potentially damaging apple disease. It occurs throughout the apple regions of the North, the Northeast, northern California, and the Northwest. It starts in spring about the time buds show their pink color, and that's the time for you to spray.
Scab spores overwinter in infected leaves, so if the disease is common where you live, rake fallen leaves every fall and destroy them. Alternatively, scatter ground limestone over the fallen leaves under the tree after harvest, and follow up with an earthworm-friendly layer of compost. This hinders spore reproduction that normally occurs at that time.
Powdery mildew is common to many kinds of plants besides apples. Infected leaves are covered with a thin layer of white fungus. The disease develops fastest when days are warm and nights cool. It does not require high humidity. Spores overwinter in already infected buds.
Even though scab and powdery mildew are very different diseases, when you spray for scab, you also prevent powdery mildew. Control both diseases with sprays of sulfur. Many forms of sulfur are available; which one you choose depends on how you plan to apply it. In my orchard, I use liquid sulfur applied with the spray adhesive NuFilm 17, a sticker-spreader made of pine resin that helps hold the sulfur to the leaves. Also, even though sulfur is an ancient and safe pesticide, it is a potential irritant, so protect your eyes, ears, and nose.
Cedar-apple rust. This pest is aptly named because quarter-inch pustules on leaf undersides look just like spots of rust. Primarily a pest in the East, it can also occur in the Northwest. Two sulfur sprays beginning at pink bud control it, along with scab and powdery mildew. The spores spread to apple trees from red cedars up to 2 miles away. For spores to germinate, apple leaves must remain wet for four to six hours. Control it with the same sulfur spray you use for scab and powdery mildew, being sure to apply it at pink-bud stage and again three weeks after petal fall.
Codling moth: This is the most famous insect pest of apples and it occurs virtually everywhere apples grow. The time to take action is petal fall and early summer. Use sprays of the insecticide Bacillus thuringiensis (Bt), possibly traps.
Many kinds of fruit moths and their caterpillars strike in different regions of the country, leaving behind entrance holes marked by sawdust-like frass, winding feeding depressions in the skin, deep sunken areas of light corky tissue, or clusters of tiny circular excavations on the fruit beneath a touching leaf. But codling moth is by far the most prevalent nationwide. Adult moths lay eggs on leaves and twigs about the time petal fall begins. Within days, larvae find their way to fruits and tunnel inside them, often beginning with the tiny openings left by the flower.
Home gardeners might sufficiently protect their trees with codling moth traps, which contain a hormone attractant that lures males and traps them, provided that wild or unmanaged fruit trees are not within 100 yards. Make sure traps are in place when bloom begins. Use two per standard-sized tree, one per dwarf tree, and replace them after eight weeks.
If codling moths are a persistent problem, spray with Bt. Add fish oil to the spray mix to help stick the bacterium to leaves and slow its breakdown. At least three sprayings are needed, and timing is critical: spray initially 15 days after petal fall begins, and subsequently at 5-day intervals. Note when petal fall begins and mark the spray days on a calendar.
Apple maggot earned the name railroad worm long ago for its meandering tunnels beneath the apple skin and eventually throughout the flesh. A practiced eye can pick out the puncture where the fly laid its egg by catching the slightly depressed sting mark in the right light.
Apple maggots overwinter in the soil after emerging from fallen fruits. That's why the first step to reduce their numbers is picking up and disposing of fruit when it drops from the tree.
Control apple maggots during the growing season by trapping them on fake apples that are coated with sticky insect trap coating. Called red sphere traps, these hang on the tree like Christmas ornaments. You can buy disposable plastic or long-lasting wood spheres. You need six traps for a standard-size tree, and they must be in place three weeks after petal fall.
Curculio is a tiny beetle with a long, anteater-like snout or nose. It lives east of the Rocky Mountains and often drives apple growers there to distraction. Beetles overwinter under fallen leaves, in brush piles, and along fence lines--wherever conditions provide some safety. They become active just about the time apple trees bloom in spring. The adult beetles then crawl or fly to the trees, snacking on tender new growth and flower petals, until curculio night: the first evening at or above 70° F after fruits have set. Furious mating takes place, and serious egg laying begins shortly thereafter in the tiny fruits.
For each egg, female curculios eat a tiny hole in the skin. After depositing the egg, she cuts a crescent-shaped slit beneath the egg so that the growing fruit won't crush the egg. Usually within a week, the eggs hatch and the young larvae begin feeding on the apple. Shortly, the tree responds to the feeding larvae by dropping the fruit. The larvae crawl out and burrow into the soil to begin the transformation into adults.
This is the most maddening apple pest of all. Luckily for Westerners, it is a problem throughout apple regions in the eastern U.S. Try to repel them with special sprays described in the text, and collect fallen fruit in June.
There is no trap or botanical spray that will control curculio. But the insect does have the curious habit of playing dead when surprised, a habit you can take advantage of: turn-of-the-century advice was to spread sheets under your tree each morning (for three weeks after petal fall), then shake or jar it, causing the beetles to fall onto the sheets. Then roll up the sheets and dispose of them. This technique still works, and for most home gardeners it is sufficient.
Repellent sprays help, provided the curculios have a flowering crab apple nearby on which to turn their attention. To repel them, I recommend combining garlic extract spray (2 tablespoons) with Safer's Bioneem (1 tablespoon), fish oil (1 tablespoon), and liquid seaweed (2 tablespoons) per gallon of water. Apply this mixture at petal fall and a week later, preferably early in the morning or at twilight when leaves won't dry off as quickly.
Some commercial growers have resorted to an organophosphate insecticide, phosmet (Imidan) to control curculio. While seemingly safe and fast to decompose, it is toxic to honeybees. I mention it here because you're likely to read or be told about it for curculio control. Some otherwise organic growers use it, but I don't.