Native to the U.S., pecan trees (Carya illinoinensis) become large shade trees that are also appreciated for tasty crops of nuts. Many fungal diseases and insect pests can inflict harm on a pecan tree, but the deficiency of the micro-nutrient zinc negatively affects trees' growth. Zinc deficiency is often dubbed rosetta disease even though it's not caused by a fungus, bacteria or virus.
According to "Your Florida Landscape" contributing authors, deficiencies of zinc in soils are most likely in four agricultural situations, and vary across the United States. Highly leached acidic sand, muck, alkaline or over-fertilized soils often are devoid of sufficient levels of zinc, especially for pecan trees. Agricultural fields that have been extensively cropped for decades and maintained with heavy applications of nitrogen, phosphorus and potassium most often develop low zinc availability.
Significance of Zinc
Zinc is essential for plant growth because it controls production of indoleacetic acid (IAA), a plant hormone, according to Texas A&M University. Zinc ions also play active roles in enzymatic reactions within living plant cells. Pecan trees suffering from insufficient zinc levels develop smaller-than-usual leaves that are sickly and weak with yellow mottling. Leaflets look narrow and become crinkled, especially on foliage on lower branches. Newest growth on stem tips is highly branched and irregular, giving rise to the term rosette or rosetta disease, according to North Carolina State University.
Texas A&M horticulturists mention application of zinc to the soil to correct zinc deficiencies is futile, while those from North Carolina State University advise soil fertilization. Regional soils and climate play a factor. Both universities do recommend spraying foliage of pecan trees with liquid zinc solutions. Zinc sulfate or zinc nitrate are the two forms of zinc applied to pecan trees during spring and summer growth, especially during the first seven years of growth.
Texas A&M University advises these formulations of solution: Liquid zinc nitrate is used 2 to 4 tsp. per gallon of water or 1 to 2 quarts per 100 gallons of water. Powder-form zinc sulfate is mixed 2 tsp. per gallon of water or 2 lbs. per 100 gallons of water. In contrast, North Carolina State University recommends use of powder-form zinc sulfate at a rate of only 1 tsp. per gallon but at 2 lbs. per 100 gallons of water. Contact your local cooperative extension office for region-specific recommendations.
Zinc is needed by the fast-growing and expanding new leaves on pecan trees. Douse trees aged 7 years or younger with zinc spray every two weeks from the emergence of buds in spring through midsummer. In the southern United States, foliar spraying winds down in August. Larger, mature pecan trees need spraying of zinc three times annually as advised by Texas A&M anytime between leaf break in mid-spring to no later than the summer solstice.
While solutions of both zinc sulfate and zinc nitrate in water are not harmful to humans, some plants are damaged from spray drift. Texas A&M University still recommends people and animals are not exposed to zinc solution sprays during application. Other orchard trees like peaches, cherries, plums and apricots must not be exposed to the zinc spray, as it will burn their leaves and immature fruits. Generally speaking, zinc solution can be mixed with other fertilizers or pesticides for spray application, but consult product labeling to confirm compatibility.