Deciduous Tree Fruits and Nuts

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Almond Defoliation

The price of almonds was in the basement in 2001 and 2002. This forced many growers to cut corners in production cost. Fall zinc sprays was considered by many almond growers unnecessary since zinc as a nutrient can be included with any other sprays during the growing season.

In addition to the nutritional value, a fall zinc spray has other important benefits. It accelerates the abscission of the leaves which, at this time, are ready to fall from the trees. If you are planning to prune your trees early, it allows you to prune them early.

A more important benefit is the elimination of the leaves from the trees. These leaves can carry disease inoculum from one season to the next. Shot hole and rust are diseases that can overwinter in leaves.

Shot hole disease is a problem when prolonged wet conditions persist in the spring. This disease can cause defoliation, yield losses and tree weakness.

The shot hole fungus survives on infected twigs, and as spores in healthy buds. However leaf lesions in the fall are important sources of inoculum for spring infection. For this reason, it is important to eliminate all the leaves in the fall.

Rust, Tranzschelia discolor is a foliar disease that survives from one season to the next in infected leaves and possibly in infected twigs. This disease was very common in many of our orchards this year. The reason may be due to lack of defoliation at wintertime. Warm winters in previous years have allowed some old leaves to stay on the trees until new leaves appeared in the spring. This situation has made possible the continuation of the disease cycle from old to new leaves.

Rust appears as a small yellow spot on the upper surface of the leaves. On the lower surface of the leaf, one finds rust-colored spores. This disease can cause premature defoliation in early summer. Premature defoliation can lead to weak trees and yield losses.

Rust, like many other diseases, is driven by high humidity in the spring and summer. It is difficult for us to control weather conditions. We can however break the disease cycle by defoliating the trees in the fall.

In addition to disease management, the fall defoliation may allow the tree to go into rest early. This will permit the tree to begin the process of chilling hours accumulation.

Fall zinc sulfate spray cannot only provide a mineral nutrient to the tree, it can also eliminate inoculum of rust and shot hole diseases. It will also allow the tree to accumulate chilling hours to break the rest period. Furthermore, the dormant spray will have better coverage on trees that have been defoliated than a tree that still maintains the old leaves.

One of the best defoliant for almonds is zinc sulfate (36% zinc) which should be applied during the last week in November when the leaves begin to fall off. The rate should be 10 to 15 lbs. of zinc sulfate (36% zinc) per 100 gallons of water. It should be applied in the diluted form which means 300 to 500 gallons of water per acre.

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