Fig. 3.12
Zinc deficiency in an avocado tree showing stunted leaves with marginal burn (chlorosis) and smaller, more rounded fruits.
deficiency progresses, the yellow areas get larger and the new leaves produced are smaller. In advanced stages of deficiency, a marginal burn develops on these stunted leaves, twig die-back occurs and the distance between the leaves on the stem is shortened giving a crowded ‘feather duster’ appearance. Yield is reduced and some of the fruit may be smaller and rounder than is normal for the variety.
The use of copper-based fungicides on avocado trees can exacerbate zinc deficiency.
Small rounded fruits are often the main symptom but can also occur combined with small leaves which also have interveinal chlorosis. These symptoms are easily confused with those of iron and manganese deficiencies and, in the case of small leaves, with ‘root rot’ disease.
Treatment: 3.4 kg (7.5 lbs) of ZnSO4 / tree, either as an annual soil application or in quarterly irrigation.
Problems have been experienced with foliar applications because the Zn remains on the leaf surface and does not appear to be absorbed (25). However, foliar spray recommendations which have been used (in 1978) were:
for young trees 1364-1818 L ha-1 (300-400 gallons/acre) of either 0.45 kg ZnSO4 (36% Zn) in 455 L water (1 lb in 100 gallons) or 0.91 kg ZnO in 455 L of water. For mature groves of large trees 2728-3637 L ha-1 (600-800 gallons/
acre) (26).
3.5.4.2 Citrus Trees (Citrus limon L. lemon; Citrus sinensis L. orange; Citrus paradisi MacF., grapefruit;
Citrus reticulata, mandarin)
(Indicative Annual Yield = 25-60 t ha-1)
Citrus trees are particularly susceptible to zinc deficiency, which appears in the early stages as small blotches of yellow between green veins on the leaf (sometimes called 'mottle-leaf'). With severe deficiency, leaves may become increasingly yellow except for the green veinal areas.
The leaves will also be small with narrow pointed tips on terminal growth. Narrow yellow leaves and delayed maturity are often considered characteristic of this deficiency in citrus.
Since zinc deficiency can be caused by high soil pH, which can also induce deficiencies of manganese and iron, it is relatively common for the symptoms of deficiencies of all three micronutrients to occur together. These are: (a) for manganese deficiency: dark green bands along the midrib
and the main veins surrounded by light green areas giving a mottled appearance; with increasing severity light green changes to yellow-bronze; incipient Mn deficiency can disappear as the season progresses; (b) for iron deficiency: leaf veins are darker green than interveinal areas appearing on the first foliage; with increasing severity, the interveinal area becomes increasingly yellow with the entire area eventually becoming ivory in colour;
trees may become partially defoliated with eventual twig die-back.
Treatment: Foliar fertiliser application of zinc. It is important to note that trees with the pathogenic disease
"citrus blight" will also show similar leaf symptoms to those caused by zinc deficiency (27). Citrus trees obviously need careful investigation before treatment owing to the similarity in symptoms with manganese and iron deficiency and 'citrus blight'.
3.5.4.3 Pome Fruits (Apple-Mallus domestica;
Pear-Pyrus communis L.)
Zinc deficiency in both types of tree is manifested, in the spring, by small, stiff and sometimes mottled leaves developing in whorls (rosettes) near the tips of the previous season’s growth. Except for these rosettes, the twigs are bare for some time. Later branches may arise below the twigs and produce leaves that at first are almost normal but later become mottled and misshapen.
The formation of fruits is reduced and many of the apples that develop are small and malformed. Twigs may die back after the first year ('blind wood') (4).
Fig. 3.13
International Plant Nutrition Institute Zinc deficient citrus tree showing patches of chlorosis between the veins.
International Plant Nutrition Institute
Fig. 3.14
Zinc deficient apple tree showing rosetting due to reduced internodal growth and interveinal chlorosis.
Treatment: Foliar sprays of chelated forms of zinc at manufacturer’s recommended rates and timing are preferred. Zinc sulphate may be used with equal amounts of hydrated lime in the first two cover sprays. Soil applications of zinc in high pH soils give inconsistent responses. Zinc-containing fungicides (Dithane M-45 and Zineb) are also effective sources of zinc if applied at times and rates specified on the labels (28).
3.5.4.4 Apricot, Peach and Cherry
(Apricot, Prunus armenica L.; Peach, Amygdalus persica Stokes, Cherry, Prunus avium L.)
These stone fruits can all be affected by zinc deficiency, which is most common on sandy and calcareous soils of neutral or alkaline reaction. In regions such as South Australia, apricot trees suffer from zinc deficiency, especially on sandy soils. Peaches and nectarines can also be affected by zinc deficiency, but tend not to need a spray application of zinc during the dormant period except at sites with a history of recurrent zinc deficiency problems. Normally, an adequate amount of zinc can be supplied by adding zinc oxide to two of the foliar sprays applied from late spring to early summer.
Zinc deficiency causes waving, crinkling and chlorosis in the leaves at the terminals of twigs and shoots. In severe cases, considerable defoliation occurs and rosettes of small leaves form at the terminals. The leaves in these rosettes sometimes do not have leaf stalks and are small and rigid; defoliation progresses upward. Formation of fruit buds is drastically curtailed and the few peaches
formed are misshapen and worthless. Symptoms of zinc deficiency in cherries resemble those of peaches.
Treatment: Foliar sprays are applied during the winter dormant period (July in Australia).
3.5.4.5 Grapes (Vitis vinifera L. wine grape; Vitis rotundifolia Michx. Muscadine grape)
(Indicative Annual Yield = 15-30 t ha-1 in sub-tropics) Zinc deficiency is a common problem in grapes in some areas and may be more widespread than recognized due to the occurrence of hidden deficiency in some locations.
Zinc deficiency can cause poor fruit set, stunted shoots and small misshapen leaves with interveinal chlorosis.
Generally, deficiencies tend to occur in localised areas and not uniformly over a whole vineyard. The deficiencies are generally associated with sandy soils and soils with high pH, or high phosphorus levels. Clay soils with a high magnesium content can also have low concentrations of available zinc (29).
In Iran, multi-micronutrient treatments including zinc, copper, iron and manganese were found to be effective in improving yields and quality of grapes.
Grape petiole analysis is widely used for the diagnosis of zinc and other nutrient problems. A satisfactory range of zinc in petioles is 35-50 mg Zn kg-1 dry matter. If the zinc concentration is less than 35 mg Zn kg-1 then treatment will be necessary (30).
Treatment: Foliar application of zinc is the most effective treatment. Neutral zinc products (50-52% Zn) or zinc oxide (75-80% Zn) are both suitable. Foliar sprays are most effective in improving fruit set when applied two weeks prior to bloom, up to full bloom. If the foliar deficiency symptoms persist, a second foliar treatment may be required (29).
3.5.4.6 Bananas (Musa x paradisiaca L.) (Indicative Annual Yield = 40-60 t ha-1)
Bananas have an optimum pH of 6.5 but tolerate pH 5.5- 7.5. Zinc deficiency has been associated with high levels of available phosphate, alkalinity (pH > 6.0) and sandy or gravelly soils. Zinc deficiency symptoms are a ‘bunchy’
top effect.
3.5.4.7 Pineapple (Ananas comosus L.)
Zinc deficiency is common in pineapple plantations in Hawaii, French Guiana and Queensland, Australia.
The disease often occurs simultaneously with copper deficiency which causes a characteristic symptom of 'crookneck' (pale green leaves which curve downwards and grow along the ground).
The first symptoms of zinc deficiency in pineapples appear at the end of the dry season. Transparent spots appear on the young leaves. These spots eventually turn yellow, spread and unite in the central region of the leaf blade. In advanced stages, larger areas become yellow and brown specks appear at the centre and form shallow cavities.
Treatment: Where zinc and copper deficiencies occur together, as in Queensland, the disorders are treated by incorporating 10 kg (12 lb) of zinc sulphate and 10 kg copper sulphate into each tonne of fertiliser used on the crop.
In Hawaii, zinc sulphate or chelates are used in fertiliser mixtures or nutritional sprays at 22.4 kg Zn ha-1 (20 lb/
acre). In French Guiana, sprays containing 1% zinc are applied at the end of the rainy season and these should supply 20.2 kg Zn ha-1 (18 lb/acre) (4).
3.5.4.8 Guava (Psidium guajava L.)
This is an important crop in South Asian countries, Hawaii, Cuba, Brazil, India and Pakistan. In Pakistan alone, it is grown on an area of 58,500 ha. It is a crop which is able to adapt to a wide range of climate and soil conditions.
It can grow in soils with pH values ranging from 4.5-9.4 and it is moderately resistant to salinity. Guava trees can suffer from zinc deficiency (and also iron deficiency) especially under high pH conditions.
Treatment: Foliar sprays with 7g ZnSO4 L-1 are used in Pakistan (31).
3.5.4.9 Strawberry (Fragaria ananassia) Deficiencies have been observed in strawberries in Oregon, Louisiana and Washington, USA. The young leaves are chlorotic, except for the longer veins and narrow border at the margins, which remain greener.
The leaf blade is relatively narrow and concave. Both leaf
blades and petioles grow slowly, or not at all and many stunted leaves are visible on older plants (4).
Treatment: Either foliar sprays or soil applications of zinc compounds can be used. In Louisiana, strawberry fields high in phosphate are top-dressed with a zinc chelate.
3.5.4.10 Berry Crops (General)
Zinc deficiency symptoms include short internodes (stunting), small, narrow leaves, and interveinal chlorosis with shoot and branch die-back. In advanced stages, small, narrow terminal leaves are arranged in whorls or ‘rosettes’ (32).
3.5.5 Nut, Seed and