|
Plant Nutrition: Zinc
Apple zinc (Zn) deficiency symptoms are often seen in orchards planted in sandy soils and where no Zn-containing fungicides were applied. This year, we have seen these symptoms at the UVM Horticulture Research Center in a block where a study is taking place and no Zn-containing fungicides were used. It has been observed that fruit trees are more subject to Zn deficiencies than most annual crops. Cherry appears to be more susceptible to Zn deficiencies than apple and apple is more susceptible than other deciduous fruits.
The total soil Zn content usually ranges from 10 to 300 ppm depending on the parent soil material. Soils originating from basic igneous rocks have a high Zn content whereas siliceous parent materials have a low Zn content.
In addition, the availability to soil Zn to the plant depends on the form that Zn is found and the pH of the soil. Zn solubility is low in soils of high pH (>7.0) and especially when calcium carbonate is present. Those orchards where the soil have a high calcium (Ca) content need to be especially careful with the balance between Ca and Zn. Cultivar differences in Zn requirements and response to Zn applications appear to be inversely related with the leaf phosphorous levels. For example, Zn deficiency symptoms are not as pronounced in 'McIntosh' as they are in 'Golden Delicious' which generally has a higher leaf phosphorous content than McIntosh. Studies have shown that Zn availability in most soils is influenced by soil moisture. Under conditions of similar vigor, a higher leaf Zn content was found in trees that were irrigated on a daily basis when compared to trees that were irrigated twice a week.
In summary, the following is a list of factors that reduce the availability of soil Zn to the plants:
Heavy phosphorous applications
High soil pH
Low soil Zn
Low soil organic matter
Cool soil conditions
High iron or manganese levels
Restricted root zones
Liberal nitrogen applications
Function of Zn in plants
Enzyme cofactor
Needed for protein synthesis
Needed for auxin synthesis
Essential for uniform fruit maturity
Essential for seed formation
Aids in chloroplast formation
Deficiency symptoms
The most important symptom of Zn deficiency is known as "little leaf" which is associated with reduced auxin content of the shoot apices. The characteristics of this symptom is the rosetting of the leaves at the shoot tip. The leaves are small, mottled, abnormally shaped, narrow, and bunched together at the tip of the shoot. Death of the terminals may occur during the following season with a number of laterals producing nonvigorous shoots. This effect is usually not uniform throughout the tree and may be more severe on individual limbs than others.
Toxicity symptoms
Zn toxicity often induces copper (Cu) deficiencies. Cu deficiencies symptoms include:
Younger leaves are affected first and they appear misshapen or stunted with irregular margins
Whitish mottled chlorosis between veins
Fruit is small with poor color
Stunted shoot growth
Flowering is reduced
Zn applications
Apple trees are generally unresponsive to ground applications of Zn. Foliar application of Zn is the most effective method for supplying Zn to established orchards. There are various methods and times to supply zinc to apples such as the application in the late dormant season of zinc sulfate, or summer applications of zinc chelates, or postharvest applications of zinc-containing products. Zn-containing fungicides have been partially effective in established orchards but generally, they do not meet the Zn requirements for apples.
· The application of Zn sulfate (20 to 36% Zn) from late dormant to silver-tip at 3.5 to 5 pounds of actual Zn per 100 gallons as dilute sprays alone or with hydrated lime as a softener is effective in supplying part of the total Zn requirement.
CAUTION: Oil sprays following Zn sulfate dormant sprays increase the penetration of the Zn sulfate into buds and spur tissues and have resulted in extensive damage.
Freezing weather occurring within 2-4 days before or after dormant spray increases uptake of Zn sulfate and may result in spur death.
NOTE: Stover et al., 1999 determined that following winter injury to buds, the application of a combination of boron and EDTA-Zn chelates at ½ inch green followed by boron plus urea at tight cluster to pink was helpful in improving bud survival and fruit set.
· Summer applications of ETDA-Zn chelates provide a higher degree of safety. Two or more sprays of EDTA-Zn chelates at 10 to 14 day intervals beginning at 1-2 weeks after petal fall is recommended. A Prebloom application may be needed to stimulate early bud, leaf and shoot development.
· Postharvest sprays of Zn materials have shown variable results and are generally not recommended .
Literature Cited
Faust, M. Physiology of Temperate Zone Fruit Trees.
Neilsen G. H. and D. Neilsen. Tree fruit zinc nutrition. In Tree Fruit Nutrition. Eds. A. B. P. Peterson and R. G. Stevens. Good Fruit Grower , publisher.
Stiles, W. C. 1999. Effects of nutritional factors on regular cropping of apple. Hortechnology. 9(3) : 328-331
Stiles, W. C. and W. S. Reid. Orchard nutrition management. Information Bulletin 219. Cornell Cooperative Extension.
Stover, E. W., M. J. Fargione, W. A. Stiles, and K. Inguerman. 1999. Effects of prebloom boron, zinc, and urea applications on spur development, fruit set, and cropping of 'Empire' and 'McIntosh' apple. Hort Sci. 34 (2) : 210-214.
|
|
