We have noticed many growers using mulch in their groves. Unfortunately, many people appear to be applying it too thickly up against the tree trunks, and covering the entire grove floor. We are already beginning to see leaf wilting and drop in avocado groves. It is highly likely other fruit crops will eventually show symptoms of improper mulch applications.

Several points must be made:
1. Applying mulch too thickly may lead to aeration problems for the tree roots; this may be from the extremely thick mat of mulch (especially wet mulch) impeding gas exchange between the tree roots and atmosphere. This leads to anaerobic conditions in the soil. Tree symptoms of anaerobic conditions include leaf chlorosis (yellowing), desiccation, leaf spotting (brown), leaf drop, stem death, and possibly even tree death.

2. Wet mulch up against tree trunks may lead to rotting of the bark and infection of the wood with pathogens. Symptoms of rotting bark include soft wet bark, peeling bark, sap oozing, leaf chlorosis, stem dieback, and tree death.

3. Covering the entire grove floor with a thick layer of mulch will make it more difficult for fertilizer and water to penetrate through to the soil and reach plant roots. In effect, you must apply enough water and fertilizer to satisfy the water requirements and fertilizer needs of the micro-organisms which live in the mulch (decomposing the mulch) before any will be available to the tree. Due to this, the tree may suffer nutritional deficiencies. Symptoms again include leaf chlorosis and drop.

We recommend:
1. Mulch be applied at no thicker than 3-6 inches. This will decrease the chance of aeration problems.

2. Keep all mulch at least 12-24 inches from the trunk of all trees. This will decrease the chance of trunk rotting and disease problems.

3. That mulch be applied mostly around the trees in-row, leaving some bare ground between trees in row and between rows. This will leave sufficient open space for fertilizer applications directly to the soil profile for use by the tree roots.

4. In those groves showing any of the above symptoms, we recommend that:

* Use of a trade name does not constitute endorsement of that brand of the active ingredient.

Dr Carlos Balerdi and Dr Jonathan Crane
RFCI Tropical Fruit News, July 1993, Vol.27, No.7

DATE: November 1993

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