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Disease Management

Management of Leucostoma canker is based on preventative measures designed to decrease winter injury and insect damage, promote optimum plant health, and facilitate healing of wounds and other injuries. As with any other disease, once it is established in an orchard, new infections are exceedingly difficult to control.

Cultural Management

Establishing new orchards: Nursery stock should be disease-free and not excessively large (greater than 11/16 caliper (0.6875 inches or 1.75 cm diameter)). Trees with small cankers on lateral branches may be planted if they are pruned so that at least 10 cm (4 inches) of healthy tissue below the canker is removed. Examine all trees closely. Plant trees immediately after receiving them from the nursery to avoid any additional stress. Protect trees from peach tree borer with an appropriate insecticide. Newly planted trees should be pruned when their buds begin to break and trees should be pruned back to about 102 – 114 cm (40 - 45 inches) to promote wide-angled branching. Small trees can be pruned to whips, but four to six side branches on larger trees should be pruned to two or three nodes since trunk buds may not develop. Trees should be inspected after growth begins, and any dead branches should be removed.

Site selection: Proper site selection for new peach plantings is essential if young trees are to enter their productive years free of disease. Sites should have deep, well-drained soil and good air circulation to minimize the chances for winter injury. Grade the site for proper surface drainage. Tile drainage systems should be installed where feasible and whenever natural drainage is impeded. New plantings should be reasonably isolated from sources of inoculum. Young trees should not be planted adjacent to older, heavily infected peach blocks and the down-wind side of older blocks should be avoided. Interplanting young trees with older, diseased trees may appear economical; however, young trees planted in this way are at a much greater risk for developing cankers and have a shorter productive life than young trees planted in solid blocks.

Orchard care: Sustainable management of Leucostoma canker results from the integration of several management strategies. Optimum disease management is achieved when all the practices listed below are followed.

Insect and disease control. Control oriental fruit moth and peach tree borer even in the first few non-bearing years. These insects can cause serious damage, and their feeding activity creates infection sites for Leucostoma spp. It is also important to control brown rot because twig infections caused by the brown rot fungus are often subsequently invaded and then enlarged by Leucostoma spp.

Mound soil at the bases of trees. This practice drains water away from the trunk and may prevent direct cold temperature injury to the crown. It also may prevent the formation of ice collars which could cause physical injury. Do not use gravel to fill depressions around the bases of trees. Where high wind and resulting tree movement causes open areas in the soil at the ground line, these may be most effectively filled in with sand.

Orchard floor management. In clean cultivation management systems, cease cultivation and sow a cover crop within 3 weeks of early fruit drop. Sod management with trickle irrigation, in addition to maintaining tree growth and fruit size, has the added benefit of making trees more resistant to Leucostoma spp. by improving tree health.

Train trees properly. Trees must be trained during the first season so that tree branches develop the wide crotch angles that are necessary for long orchard life. Where narrow crotch angles form, the tissue in the crotch is susceptible to winter injury, invasion by borers, and the trapping of moisture that favors spore germination. Also, portions of bark become included in narrow crotches where normally there should be solid wood, thus making the branch more likely to split when bearing a heavy crop. Wire spreaders or wooden spreaders with nails should be avoided because they injure the bark which may then become infected by Leucostoma spp.

Deter rodent injury. Rodent damage should be prevented with wire or plastic guards. Plastic wrap-around guards should be removed each summer because they may delay hardening of the wood in late fall; they may harbor boring insects and interfere with trunk sprays for borer control. Latex paint with thiram (fungicide) also discourages rodent feeding.

Optimize cold tolerance. Low temperature injury is always a potential problem with stone fruits. This injury occurs to buds, twigs, branches and branch crotches, and trunks. Cold temperatures can injure peach trees early in the winter before the trees are completely acclimated to the cold. Practices to avoid include excessive or late fertilization with nitrogen and late season cultivation.

Southwest-injury or sun scald is caused by the warming of the bark by direct sunshine on the south and west exposures of the trunk and scaffold limbs and may occur even during relatively mild winters. This injury may be the most damaging because it occurs on trunks, scaffolds, and crotches. These sites are commonly infected by Leucostoma spp. To avoid southwest injury, trunks and scaffolds should be covered with white latex paint which can reduce bark temperatures on sunny winter days.

Prune correctly and at the proper time. Infection at pruning cuts (Figure 12) is less frequent when pruning is delayed until late in the spring. The more rapidly that a wound heals, the less risk there is for infection. Wound healing is temperature-dependent; therefore pruning should be delayed until the first forecasts of warm, dry weather. Approximately 390 accumulated degree-days from the time of wound initiation (base = 0 °C) are required for complete wound healing to occur. In general, any practice that promotes tree health promotes more rapid healing. Pruning should be well planned each year so that large cuts, which heal more slowly, are minimized. Avoid leaving susceptible stubs. When pruning side branches from larger limbs, the cut should be made just beyond the ridge of thickened bark where the smaller branch joins the larger limb (Figures 13 – 15). The branch bark ridge should not be removed because it is in this region where the most rapid wound healing occurs. On one-year-old wood, the ridge of thickened bark is slightly inset, and it is difficult to make the proper cut. In this situation, cut as close as possible to the larger branch without injuring it or leaving a noticeable stub. Prune to open the center of trees to light penetration because shaded branches are weakened and more susceptible to winter injury and Leucostoma infection (Figure 16). Remove all dead and weakened wood and burn it immediately. In irrigated regions avoid summer pruning during irrigation events.

Figure 12	Figure 13
Figure 14	Figure 15
Figure 16

Surgically remove cankers. Cankers should be removed, and (if possible) burned, buried, or moved out of the orchard (Figure 17). Cankers on trunks and large limbs can be removed surgically in mid-summer when trees heal most rapidly. Surgery should be performed in dry weather with a forecast of dry conditions for at least three days. During surgery, remove all diseased bark around the canker and about 4-5 cm (1.5-2 inches) of healthy tissue from the sides and ends, respectively. Disinfest cutting tools between cuts with an alcohol or bleach solution. The resulting wound when finished should have a smooth margin and be slightly rounded above and below to favor rapid wound closure. The practice of covering pruning cuts in spring with a thiram-latex paint mixture provides some degree of protection against fungal infection. Sites of surgery heal best if left uncovered.

Figure 17

Tree fertilization. Trees under stress are more susceptible to Leucostoma canker. Practices that promote tree health minimize the damage resulting from infection by these pathogens. Nitrogen fertilizer should be applied in early spring to avoid inducing late, cold-susceptible growth in the fall. Use leaf analysis to determine proper fertilization amounts so that fertility is balanced and not excessive. Foliage should show a healthy green color and terminal growth should be about 30 cm (12 inches) on bearing trees and 46-61 cm (18 - 24 inches) for non-bearing trees. Trees with pale, nitrogen deficient leaves are more susceptible to infection by Leucostoma spp. Balance nitrogen fertilizer application with an adequate supply of potassium.

Irrigation management. In irrigated orchards with impact sprinklers, ensure that water streams do not impact directly on canker surfaces. Also keep sprinkler anglers as low as possible. In severely infested orchards consider changing to drip or microsprinkler irrigation. Avoid summer pruning during and immediately following irrigation sets. Avoid under-tree impact sprinkler irrigation systems - they are extremely efficient at disseminating the canker pathogens.

Genetic resistance

Cultivar selection is important and only cultivars tolerant of cold temperatures should be planted. No commercial cultivars are immune or highly resistant to the Leucostoma canker pathogens. Although there is no known immunity to the fungi that cause Leucostoma canker, cultivars that are tolerant of cold conditions may vary in their ability to tolerate or resist infections. Up-to-date listings of cultivars are often available from local extension offices.

Biological control

There has been much interest in biocontrol agents for Leucostoma canker because the disease is so difficult to control with standard cultural and chemical treatments. Some organisms are antagonistic to Leucostoma, but none has been developed to the point where it provides a practical level of disease control.

Chemical Management

There are no fungicides registered specifically for control of Leucostoma spp., and efficacy trials have produced conflicting results. The combination of captan and thiophanate-methyl, when used for brown rot blossom blight control, may provide recent pruning cuts some degree of protection against infection by L. persoonii. Demethylation-inhibiting (DMI) fungicides have very little activity against L. persoonii. The application of canker paints during moist weather should be avoided.

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