Low levels of leaf zinc and associated zinc deficiency symptoms are common in eastern Washington. The visual symptom is small, thin leaves. With acute deficiencies, leaves also appear chlorotic (pale yellow), and new growth is limited to a short rossette. The first symptoms of a deficiency with spur-type Red Delicious apples may be a limited number of small leaves on spurs, poor fruit set, and small fruit size.
Soil applications of zinc have not been effective except with young trees where applications have been worked into the soil prior to tree planting. Effects can last for 3 to 5 years. Spray applications of zinc are required with established trees. Annual applications are more effective in preventing deficiencies than making applications at high rates every 2 or 3 years. Ground sprayer applications are more effective than those by aircraft because they give a better distribution of zinc to the lower and inside portions of trees where weak spurs, lower levels of zinc, and small fruit size are greater problems.
Where zinc levels are known to be low, make annual spray applications at low rates, either after harvest in the fall while leaves are still green and active, or as a dormant spray in the spring. Higher rates of zinc can be applied in the early spring than in the fall. With an acute deficiency, both a fall and spring application may be necessary.
Where zinc deficiency symptoms are observed during the growing season, avoid the use of zinc sulfate because of potential injury to fruit and foliage. Various zinc chelates and organic complexes are available which reduce the potential for injury. They differ in their compatibility with oil, and in their effectiveness, in correcting deficiencies. Follow manufacturer's directions carefully.
Caution: Excess zinc and high rates of application can cause severe injury to shoots, buds, leaves and fruit. When using zinc sulfate crystals be certain all crystals are dissolved before spraying or injury can occur. Zinc sulfate is highly corrosive. After use, thoroughly rinse spray tank, pump, lines, and nozzles.
Zinc Dormant Spray Application. The most effective time to apply zinc is in the spring before the buds open. Higher rates can be used at this stage than later in the season. To improve effectiveness and reduce potential injury, delay spray applications as late as possible, but spray at or before stage 2 (greentip).
Caution: Injury from spring applications has been associated with oil sprays and cool weather at the time of application. Some zinc formulations can be applied with oil as shown on the label. It is suggested that zinc sulfate and other formulations not be applied within three days before or after applying oil. Longer periods may be desired during cool weather. Where multiple applications of oil are required on pear, or where management problems occur, late fall applications of zinc are suggested.
Zinc Foliage Applications. Where deficiency symptoms occur during the growing season, spray applications should be made. If symptoms occur late in the season and fruit is present, delay applications until after harvest. To aid absorption, thoroughly wet foliage.
Caution: Where fruit is present, applications to apple and pear can cause fruit russeting. This is most likely to occur under cool, slow drying conditions in the spring. On bearing stone fruits, use lower rates or organic complexes.
Zinc Postharvest Applications. Zinc may be applied in the fall, but fall applications are usually less effective than those made in spring as a dormant application. Fall application may be needed where deficiencies are difficult to correct or where multiple applications of oil in the spring may cause injury. With some deficiencies, particularly on sweet cherry, both a fall and spring application may be necessary.
Make applications after harvest while leaves remain green and active but before the trees have begun to go dormant.
Caution: High rates of zinc sulfate create potential for injury, particularly to buds. Late-maturing varieties are more susceptible to such injury. Postharvest applications of zinc sulfate, especially concentrate sprays, frequently cause direct injury to leaves and possibly buds. Zinc chelates are less likely to cause direct injury. Do not make fall zinc applications to apricots.