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Peach Leaf Curl

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December is the Time to Manage Peach Leaf Curl

Most gardeners in the Klamath-Trinity have encountered Peach Leaf Curl – the curled, deformed leaves that appear on peaches and nectarines in the early spring.  Problem is, by the time it’s visible it is already too late to treat it; in fact, the ideal window for controlling peach leaf curl is coming up in early to mid-December.  Peach leaf curl is caused by a fungus, Taphrinia deformans, and can be either a nuisance, or lethal to the tree, depending on its severity and the age of the tree.  The fungus is found just about anywhere peaches are grown, and is particularly damaging following warm, wet winter-Spring weather.  The options for control are selecting immune varieties such as Frost, Q-1-8, Oregon Curl Free, resistant varieties (such as Red Haven, Rio Oso Gem) that contract the disease but bounce back quickly; and/ or applying a sufur-based dormant spray before the leaf buds swell, which occurs in our area in mid-December.   Many exasperated gardeners miss this critical window and find that even after spraying they still have the curl.  A brief look at the life cycle of peach leaf curl will reveal that timing of this spraying is essential: a dormant spray must be applied before the buds crack and swell, or it will not have any effect on controlling the disease.  The best material to use, lime sulfur, is also the least toxic.   

 Life Cycle - By the time a gardener sees peach leaf curl in the spring, the fungus has already penetrated the cellular tissue of the foliage and is doing its damage.  At this point the fungus is immune to treatment.  Though foliar feeding with compost tea or concentrated kelp solution will help to fortify the tree and bring it through the infection, no amount of fungicide will help the situation.  Leaves affected by the curl eventually shrivel completely and fall off and the tree pushes new leaves to replace the damaged ones.  The peach leaf curl will continue to attack the new foliage until temperatures rise and humidity decreases; it takes several days in a row above 85 degrees F for the fungus to go dormant, so only after a stretch of warm, dry weather will the disease symptoms will slow down and eventually disappear.   The fungus forms tough spores that are resistant to heat and desiccation in order to survive the summer.  These spores germinate with the first fall rains and remain active during the winter as long as temperatures are over 45 degrees F. 

 Look closely at a dormant peach bud in late November/ early December and you’ll see a tightly closed bud protected by a waxy scale.  Sometime in late December or January, depending on location, peach buds will swell slightly and crack the protective scale, leaving them susceptible to the fungus.  In our mild winters this bud swell happens 1 to 2 months before the peach blooms, much earlier than most gardeners realize.  Applying a dormant spray before this bud swell is the key to controlling leaf curl.  Timing depends on weather, but in our low elevation areas it can occur as early as mid-December in a sunny winter. In higher elevation colder areas it will be much later. For these low, warmer areas, apply the first spray in early December, and then another in mid to late January.  The first spray is the most critical, and some gardeners forgo the second application and accept some peach leaf curl.  Researchers at Washington State University found that lime sulfur (calcium polysulfide), a low-toxic spray that is approved for organic production, is the best material for controlling peach leaf curl.  A dormant oil will help the mix stick to the branches and also smother soft body insect pests and eggs.   Copper sulfate is also used, but copper can build up in the soil and is toxic to earthworms and beneficial soil fungus, whereas the lime sulfur breaks down rapidly.   Lime sulfur used to be readily available in any gardening store but is now hard to find.  Peaceful Valley Farm Supply sells it in a 2.5 gallon container, which is far more than most gardeners or homesteaders need.  It does not store for more than a few months, so if you buy this much then be sure to share it with friends.  

 Cultural Control – A healthy soil and a healthy tree is the first step in disease control.  Fertilize your trees each year with compost and or aged manures.  It is good to add a source of calcium and phosphorous every 3-5 years, use oyster shell flour for calcium and soft rock phosphate for phosphorous.  Foliar feeding with concentrated kelp solution, will help to fortify the tree and bring it through the infection.  Turning in the fallen leaves in autumn or raking them up and adding them to your compost pile will reduce the amount of fungal inoculant.