Bloom sprays: What and when to use to provide effective disease control.

Written by Brent Holtz, UCCE San Joaquin

Almond trees are susceptible to bloom and foliar diseases when it rains at bloom, and the decision of when to spray and what fungicides to use can be quite difficult. In the San Joaquin Valley we are usually in a low precipitation region and we cannot predict when and how much it is going to rain. We often receive rain during bloom which can result in favorable conditions for several plant pathogenic fungi to cause spring time diseases of almonds. The main diseases in almonds are Brown Rot Blossom Blight, Green Fruit Rot or Jacket Rot, and Shothole. Other less familiar diseases include Scab, Rust, Leaf Blight and Anthracnose. The fungi that cause these diseases are usually always present in almond orchards, sometimes in higher or lower amounts depending on the previous year’s disease levels and current environmental conditions.

Fungicide Control Programs
Generally, a good disease control program is based upon a wise choice of fungicides and good timing and coverage. Growers should assess the diseases present in their orchards and select materials carefully. Not all fungicides are equally effective on all diseases (fig. 1). It is a good idea to use more than one kind of fungicide for a broader spectrum of activity. This will be especially important with using Strobilurin fungicides (Abound, Cabrio, Flint, Sovran, and Pristine). Pristine is a Strobilurin fungicide that is combined with Carboxyanilide to slow resistance development. Resistance to these fungicides can develop over time and repeated use, thus try to rotate the fungicides you use. Two successive applications of the same mode of action should be avoided. Information on effective fungicide rotations can be found in figure 2.

A suggested bloom spray strategy:
Usually two sprays are made for brown rot control. The first is usually done at 5-20 % bloom using a systemic fungicide. The second spray, using a different material, should be done at about 80% to full bloom or two weeks after the first spray. This is the most effective brown rot spray (fig. 3). Depending on the weather, a third spray may be necessary if rains persist and two weeks of protection have gone by. Since we cannot predict the weather at bloom time, we must at least take some

Application techniques are also important. Usually ground application is better than air; but care must be taken that both are applied correctly. In general, use properly calibrated and directed nozzles and maintain a slow speed.

Figure 1: Efficacy of fungicides for use on almond foliar and bloom diseases.

Figure 2: Suggested FRAC group rotations for the various modes of action of fungicides.

Figure 3: Treatment timings of major foliar diseases of almonds.

More information on the timing of fungicide applications for orchard tree and vine crops can be found here, at the UC IPM website.