Chapter authors - Ron Godin, Grant Cardon
Of the various nutrients that can be limiting in Colorado orchards, usually zinc, iron, copper, manganese, calcium, magnesium, nitrogen, and boron can be successfully corrected temporarily with sprays. Zinc deficiencies are most successfully corrected through dormant sprays, while the most effective responses to the other nutrients are through soil applications. Iron sprays are not effective on peaches nor, possibly, pears. Fruit trees showing severe deficiency symptoms may respond temporarily to some of these other nutrients applied as sprays. However, these sprays should only be used in conjunction with soil applications of the same nutrient. The sprays will provide temporary relief until the soil applied nutrient can be translocated throughout the tree. Researchers in Utah have reported a summary of their studies on the effect of foliar nutrient sprays on sour cherries and apples. Further information may be obtained by contacting Dr. Ron Godin, Colorado State University Extension, (970) 874-2195.
Chronic micro-nutrient deficiencies, typical on peaches on alkaline soils, can most times be corrected by reducing the soil pH through acidification of irrigation water and/or application of elemental sulfur to the soil. Soil application of chelated micro-nutrients can correct these deficiencies from 1 – 3 years depending on soil pH. Western Colorado soils are typically deficient in boron regardless of soil pH.
CAUTION: Nutrient sprays can cause phytotoxic injury to foliage and tree if not applied correctly in the correct amounts and at the right time. To avoid potential injury, verify the nutrient deficiency through tissue analyses or visual observations. Use caution when using a concentrate sprayer because of potential injury. Some (like zinc sulfate) can cause tree injury if applied within 3-5 days of an application of oil. Others (like Leffingwell products) may be generally compatible with most fungicides and insecticides if the pH is adjusted so that it remains close to neutral (pH 6-7). Pyrethroid insecticides are rapidly deactivated at low pH values, especially those around 3.5-4.0.
Leaf analysis results show some boron deficiencies in peaches and apples. However, pears are the fruit crop that most often shows boron deficiency. Where pear trees are affected by "blossom blast" or wilting of the flower buds in early spring due to boron deficiency, a spray should be applied before bloom. A single maintenance spray, applied each year at a low rate, should supply enough boron to prevent the development of a deficiency. While the spray may be applied at any time, late fall applications when leaves are still green or spring pre-bloom applications are recommended.
CAUTION: Do not confuse "blossom blast" with such diseases as fire blight.
Spray applications of calcium can reduce the incidence of bitter pit and cork spot in apples by 35 to 50 percent. Under average conditions, three sprays are suggested. The first should be applied about mid June. It should be followed by a second spray in mid July and a third in mid August. With young and very vigorous trees or trees with large fruit which have a history of serious bitter pit, more sprays may be necessary. Applications should begin at the same time (mid June) and should be carried on through to mid August. The more severe the history of bitter-pit, the more frequently should calcium be applied.
Manganese deficiencies are especially common in peach orchards located on highly alkaline soils. This deficiency is often masked by zinc and iron deficiencies. While it may not be visually detectable, a tissue analysis will identify the deficiency. The deficiency also can be induced by applying excessive amounts of iron chelate. Usually one foliar application of manganese sulfate, applied when the first leaves are fully expanded, is sufficient to maintain an adequate level of manganese in the leaves.
Iron sprays with iron salts or chelates usually give temporary correction of chlorosis although peach trees are less likely to respond than other fruits. Soil applications of chelates are much more effective than foliar sprays, but need to be protected from breakdown by the sunlight. For soil applications, apply 2-4 oz Sequestrene 138 Fe or Ferriplus 138 per inch of trunk diameter shortly before the first or second irrigation. Distribute the material evenly along the tree in the nearest furrow on each side of the tree and cover lightly. The irrigation water will dissolve the chelate and move it into the root zone.
Zinc deficiency symptoms are common in Colorado. Soil applications of zinc have not proven effective. Where zinc levels are known to be low, annual spray applications should be made to avoid deficiency symptoms. Once symptoms are detected, they should be treated as soon as possible to avoid further injury.
1. Verify need by tissue analysis or visual deficiency symptoms. Zinc sprays can cause severe injury to shoots, buds, fruit, and leaves. Adjust the rate, formulation, and time of application according to the kind of fruit, the season of the year, and the amount of zinc required.
2. Applications made within 3 days before or after an application of oil can cause injury. Longer periods may be required during cool weather. Application of zinc sulfate spray within five days of any oil-containing spray may damage apples and should be avoided during that time.
3. Because of the problem of multiple applications of oil to pears in the spring, it may be necessary to apply zinc in the fall instead.
4. Do not use fall applications on apricot because of potential injury.
5. When using zinc sulfate crystals, be sure all crystals are dissolved before spraying because of potential injury.
6. Zinc sulfate is highly corrosive. The spray tank, pump, lines, and nozzles should be thoroughly rinsed and flushed after using.
7. Foliar application during or followed by damp weather may result in spray injury on some varieties of stone fruits.
Dormant Application: Higher rates of zinc can be applied in the spring before the buds are open than during the growing season. Sprays are more effective and appear to cause less injury when delayed as late in the spring as possible, but before buds scales open.
Fall Application: Zinc can be applied after the trees have begun to go dormant (usually after October 10), but while the leaves still remain green and active. Fall applications are usually less effective than spring dormant applications, but the former may be needed in cases of severe deficiency. With sweet cherry, both a fall and a dormant application may be necessary.