In the Garden:
Western Mountains and High Plains
September, 2009
Regional Report

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Look for telltale signs of fireblight infection in apples to plan for control measures.

Be on the Watch for Fireblight Disease on Apple Trees

Growing fruit trees can be a rewarding experience in our region when you plan ahead and plant varieties that are most adapted to your area. Apples are among the hardiest of fruit trees to grow. They tolerate a wide range of soils as long as there is good drainage. If you're cramped for space, you can plant semi-dwarf or dwarf varieties.

However, this year's weather conditions have been unlike those of any other, with the frequent rains and cooler temperatures. So it is not unusual to see diseases on some of the hardiest and most disease-resistant fruit trees.

Fireblight is a disease that is caused by a bacteria rather than a fungus, making it more difficult to control. The timing of control measures is of utmost importance. This bacteria can be spread by insect vectors including aphids, bees, psyllids, and other insects that visit the apple tree. The most vulnerable parts of the tree include the young tender shoots, twigs, injury points on the tree, and the flowers.

This year fireblight can be easily detected by examining the tree for the telltale signs. Infected branches wither rapidly and turn black or brownish-black, giving them a sooty, scorched appearance. The tips of the tender growth, once infected, will take on the shape of a shepherd's crook. You might also see blossom clusters scattered around the tree which have wilted rapidly and turned brown or black.

Fireblight can become a very serious disease and wipe out all susceptible trees in a home orchard in a short time. It is important to control the problem to avoid future outbreaks. Timing control measures correctly will help reduce the spread of fireblight.
The bacterial infection is at its highest in the spring when the apple trees are in bloom. Bacteria can easily enter the blossoms with activity of insects. Tender, lush growth can also be infected. Once inside the tree, the bacteria will multiply rapidly and move to the roots. Pustules may form on the tree bark which will exude an orangish-brown liquid and cankers will follow. These are the most severe cases. Bacteria in a canker can survive through the winter and be ready to infect more trees the following year.

For trees that are at most risk to fireblight, it might be wise to apply an antibiotic spray during bloom and shortly after to prevent the entry of the disease through the blossoms. This would be especially advisable if it is a wet, warm, and humid spring since these conditions favor the bacteria.

Control Insects
Keep insect vectors such as aphids from spreading the disease by making daily inspections and hose them off or spray with a soap spray. These insects are noted for producing a sticky material called honeydew on which black fungus grows. Yellow jackets and wasps are common around honeydew, indicating pest activity. Now is a good time to control insect pests on fruit trees rather than spring since you avoid harming many of the beneficial insects that are present in the trees in early spring. Horticultural oil sprays are safe and effective. Dormant oils can be applied in early and late winter as indicated on the label.

Pruning
Remove suckers that sprout around the base of the trunk at any season of the year to reduce succulent, soft growth attractive to bacteria. This increases air circulation to the bark as well. Other pruning activities should be limited to the dormant period of the tree in late fall and early winter. Use a sharp pruning shears and cut off all of the wilted or infected branch or twig at least six inches below the point of last visible wilt. After each pruning cut be sure dip the pruner or saw in a disinfecting solution made by mixing one part household bleach in nine parts of water. Caution: when you are finished pruning, wash your pruning equipment with soapy water, and then dry and oil the tools to prevent damage from the bleach. I like to use a spray disinfectant which I apply to the shears after each pruning cut.

By staying alert and checking your trees often, you can avoid a severe outbreak of fireblight in your home orchard. Maybe next year will be better year for apples.


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