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Marin IJ Articles

Keeping blight off of fruit trees

  • Nanette Londeree
  • You don't need a calendar to know that spring is right around the corner. Signs of the season abound as bulbs pierce the ground with vibrant bouquets and trees and shrubs begin to strut their floral finery.

    New foliage erupts from the bare branches and slender stems of your prized fruit trees, creating visions of bountiful crops, fresh peach pie, baked pears or fruit plucked warm off the tree.

    But your daydreams might be squelched when you instead see blistered, bumpy, puckered, twisted leaves on your peach tree or pear tree branches that look like they've been torched. What's up with that?

    As spring unfolds, so do a couple of common fruit tree maladies. Peach leaf curl is a fungal disease that affects only peach and nectarine trees, potentially weakening them and reducing yield. Left untreated, it can result in the demise of the tree. Fire blight is a nasty infection by a destructive bacterium. It most frequently affects apples, pears and related plants — crabapple, ornamental pear, Pyracantha and quince.

    Peach leaf curl produces malformed, thickened and often pink to reddish green leaves that punctuate healthy foliage and in severe cases, affect twigs and branches of the entire tree. The sadly deformed and discolored leaves turn brown and die. During cool rainy weather, the infecting agent increases in number, eventually forming a film on the tree's surface. The good news is that once the distorted leaves drop, if the tree is otherwise healthy, it will produce another set of normal leaves when weather turns dry and warm (79 to 87 degrees).

    While it's admittedly not pretty to look at, peach leaf curl is generally not difficult to control. Selecting varieties resistant to the disease is the easiest preventive measure. "Frost," "Indian Free," "Muir" and "Q-1-8" are some resistant peach varieties; "Kreibich" is one of the few resistant nectarine varieties.

    Keeping trees healthy and vigorous, providing adequate water to reduce drought stress, and thinning fruit help to reduce demands on peach and nectarine trees. Along with good garden sanitation — removing diseased leaves, twigs and branches from the garden — these measures go a long way in fending off the disease.

    An annual application of a fungicide is usually sufficient to protect the tree from peach leaf curl, and timing is everything. Apply after leaves fall, usually in late November. If it's a very wet winter and early spring, a second application just before buds swell may be beneficial. Treatment after symptoms appear won't have any effect in controlling the disease.

    A fungicide containing copper ammonium is the easiest to use, and you can improve its effectiveness by adding a 1 percent horticultural spray oil to the application mix. Bordeaux mixture is another option — it is a copper sulfate and lime mixture that should be prepared just prior to use. The synthetic fungicide chlorothalonil is currently the only other noncopper fungicide available to the home gardener for managing this fungal disease.

    The telltale "scorched" look of fire blight may appear scattered throughout the crown of the plant and is usually seen first on blossoms and stems, turning them brown or black. It can spread from the point of entry (blooms, stems and branches, limbs, trunk or roots) and may severely disfigure or kill individual limbs or the entire plant. Mild temperatures (65 to 85 degrees) with some intermittent rain provide perfect conditions for the pathogen to develop. As trees resume growth in the spring, the bacteria become active and are spread by insects — aphids, flies, leafhoppers and honeybees, pruning tools, wind and splashing rains. As long as the warm, wet conditions exist, the disease can continue to spread and infect new sites.

    The most effective means of managing the disease is to plant varieties that are not susceptible, and as most infections originate in the flowers, avoid varieties that bloom late or throughout the season. Most pear tree varieties are very susceptible as are "Fuji," "Gala," "Gravenstein" and "Jonathan" apples.

    Keep plants vigorous and healthy through good cultural care and promptly remove and destroy any portions that appear infected. Prune diseased branches, cutting at least 8 to 12 inches away from the visible injury or canker, or until you see healthy tissue. Sanitize tools between each cut, to prevent spreading the pathogen.

    Treatment with a very weak (about 0.5 percent) Bordeaux mixture or other copper product applied several times as blossoms open might reduce new infections but won't eliminate them or those already existing in wood. Apply spray to open blossoms; repeat applications every four to five days during the bloom period.

    Don't let those forlorn looking leaves deter you; the scrumptious orbs of summer and fall are worth the little extra effort that may be needed to keep your fruit trees healthy and productive.