Micronutrients

The following elements are classified as micronutrients because they are required by plants in smaller quantities. With the exception of manganese, toxicities are very difficult to diagnose visually. Deficiency symptoms tend to be characteristic of the lack of a particular element. Pictures of many of these deficiencies and toxicities can be seen under “Nutrient Deficiency and Toxicity Symptoms in Tree Fruit.”

Iron
Deficiency: Iron deficiencies are very common in plants. Initial symptoms are a loss of green color in the very young leaves. While the interveinal tissue becomes pale green, yellow, or even white, the veins remain dark green. New leaves may unfold completely devoid of color, but the veins usually turn green later.

Toxicity: Although rare in the field, an excess of iron usually produces symptoms similar to those of manganese deficiency.

Manganese
Deficiency: Symptoms begin as chlorosis between the main veins starting near the margin of the leaf and extending toward the midrib. Symptoms can often be confused with those of iron and magnesium deficiencies. But unlike magnesium deficiency, manganese deficiency symptoms seldom develop so far as to produce interveinal chlorosis, the chlorosis normally being confined to leaf margins. The other distinguishing characteristic is that manganese deficiencies appear on the youngest leaves first, and the finest leaf veins do not remain green as they do with iron deficiencies.

Toxicity: “Measles” is a disorder of apples, especially Delicious and Jonathan. It is caused in part by an excess of manganese accompanied by low calcium levels.

Boron
Deficiency: In most fruit crops, boron deficiencies show up in the fruit before appearing in the leaves. Symptoms in apples and pears are similar: gnarled, misshapen fruit caused by depressions usually underlaid by hard corky tissue. This symptom is often confused with bitter pit or corking caused by calcium deficiency. Researchers disagree whether the two symptoms can be told apart visually. Boron deficiency might be distinguished from bitter pit by the presence of pitting from the peel to the core, whereas in bitter pit the pitting usually occurs only at the calyx end and only very close to the skin. In some instances of boron deficiency, the entire surface is covered with cracks that have callused over, producing a russeted appearance.

In plums, the symptoms appear as brown sunken areas in the fruit flesh, ranging in size from small spots to almost the whole fruit. The fruit usually colors earlier than normal, and falls. Gum pockets may also form in the flesh. In peaches, the flesh adjacent to the pit develops brown, dry, corky areas, and some fruits may crack along the suture. The most typical vegetative symptom is the death of the terminal growing points, resulting in a “witch’s broom” appearance.

Toxicity: Symptoms in apples include dieback of twigs, greatly enlarged nodes on 1- and 2-year-old twigs, early fruit maturity, internal breakdown, and dropping of fruit. Foliar symptoms occur first on the older leaves and include a yellowing along the midrib and the large lateral veins. In peaches vegetative symptoms include necrotic lesions on leaves, crinkling of margins and tips of leaves, reduced flower bud formation and set, and pit splitting.

Copper
Deficiency: Younger leaves appear stunted or misshapen, narrow, and slightly elongated with wavy margins. There may be some terminal dieback. Copper and zinc deficiencies often occur together and are aggravated in soils that have a high pH.

Toxicity: Symptoms are almost nonexistent under orchard conditions, but when present they may resemble those of zinc deficiency.

Zinc
Deficiency: Symptoms have often been described as a “rosetting” of leaves or “little leaf.” Newly developing leaves are smaller than normal. Reduced shoot elongation keeps them close together, resulting in the rosette appearance. In severe cases, older leaves may drop, resulting in a more pronounced rosetting. In the early spring, the observer might notice a delayed foliation of lateral leaves on last year’s shoots. This symptom has been confused with that of winter injury, but the distinguishing characteristic is that winter injury will also produce browning of the cambium.

Toxicity: Symptoms are rare and most likely are masked by secondary symptoms resembling those of other micronutrient toxicities.