Zinc deficiency is probably the most common micro-nutrient affecting prune trees.
Micro-nutrients are chemical elements used in relatively small quantities by plants and are essential for growth and development. Zinc deficiency can be diagnosed using visual and/or leaf tissue analysis.
The University of California has established critical leaf tissue levels at 18ppm for spur levels sampled in July. Sample leaves are selected from fruit spurs reachable from the ground and picked at random around the tree at different heights. One or two leaves can be taken from each of about 50-60 spurs for total of about 100 leaves. Zinc is considered deficient below 18ppm.
Visually the beginning and less severe stages of zinc deficiency are often characterized by interveinal chlorosis of older leaves at the lower shoot positions. Leaves slightly zinc deficient are only slightly reduced in size but show many small chlorotic areas between their lateral veins. If zinc deficiency is moderate to severe, symptoms are typically seen as trees leaf out.
The first evidence is delayed opening of vegetative and flower buds. A zinc sufficient tree could be in full leaf while a severely zinc deficient tree or shoot is just beginning to leaf. When vegetative buds do open, the leaves are small, chlorotic and appear in tufts, often described as “little leaf”. In severely deficient cases, terminal dieback may occur. As the season progresses, normal leaf growth tends to mask early season zinc deficiency symptoms making visual evaluation more difficult. Fruits on zinc deficient shoots or trees are markedly smaller in size than are normal fruits.
Soil applications to correct zinc deficiency produce variable results and are not normally recommended. Soil type and texture, severity of the deficiency, tree age and zinc source all complicate getting zinc into trees. Many prune orchards are planted on heavier clay soils which tie up or fix zinc making it unavailable for plant use.
Typically zinc correction strategies involve foliar and dormant sprays. Spray application in early spring, before leaves reach full size, are effective. Zinc should not be applied after mid-May due to the risk of phytotoxicity with certain zinc materials. A fall spray of zinc sulfate applied at the beginning of normal leaf drop can also correct zinc deficiency.
Leaf burn and defoliation, depending on material rate, usually occur but are not considered detrimental to tree performance and may help with other orchard management objectives.
• Reasons to consider fall zinc applications:
1. Effective to correct zinc deficiency.
2. If defoliation occurs after a fall zinc spray, the danger of trees blowing over is reduced.
3. Loss of foliage can facilitate early tree pruning.
4. Zinc sprays applied mid- October to November, reduce aphid habitat if defoliation occurs.
5. Zinc can be tank mixed with a fall aphid spray to “kill two birds with one stone”.
6. Fall sprays have the advantage of being easier to plan. Orchard floors are dry and weather is more spray friendly.