Dormant Spray for the Control of San Jose Scale in Almonds

Mario Viveros

UCCE Farm Advisor, Kern County

Deciduous Tree Fruits and Nuts

December 22, 1999

Dormant Spray for the Control of San Jose Scale in Almonds

A dormant spray is made up of a dormant oil plus an organophosphate or synthetic pyrethroid. This spray traditionally has been effective in controlling San Jose scale, peach twig borer and European red mite eggs. However, in the last few years, it has been under the scrutiny of regulatory agencies and growers. Regulatory agencies are looking at the dormant spray because the organophosphates are causing environmental problems. Growers are thinking twice about using dormant sprays because it may not effectively control peach twig borers and San Jose scale.

San Jose scale, three years ago, showed us what it can do to almond trees. This insect infested and killed big tree limbs in some orchards. The death limbs were located not on the top but on the bottom part of the trees. This indicates that dormant spray didn't control the scale. The reason is not known at this time. However, if a grower has San Jose scale in his orchard, we recommend a dormant spray with oil and an organophosphate insecticide.

The best time to apply a dormant spray is in the winter, during the first week of January. If by this time, there has been no rain, irrigate your orchard. Oil sprays can be phytotoxic to water-stress trees. Before a dormant spray is applied, irrigate your orchard using two- to three- acre inches of water. This will allow you to wet the soil profile 24 to 36 inches. This water is not wasted water since root growth and development starts between Christmas and the New Year. Furthermore, if the water that is not used by the roots, it will be stored in the soil profile for use in the early spring.

The application of the dormant spray should be properly done. The sprayer should be well calibrated. It should deliver two-thirds of the spray to the upper one-half of the tree. Speed should be kept below two mph. The faster you travel, the greater the bending effect on spray droplets. Concentrate applications are particularly susceptible to the effect of speed due to the fine particle size of these sprays.

The dormant spray should be diluted because San Jose scale occurs in dense colonies. A diluted spray consists of 300 to 500 gallons of water per acre. This is not a popular spray because it takes a long time to do one orchard. However, this is the only spray that is going to cover all the parts of the tree.

The amount of oil should be 1.5 to 2 gallons in each 100 gallons of water. For example, if you are applying 300 gallons of water per acre, you will be using 4.5 gallons of oil. If you are applying 500 gallons of water per acre, you will be using 10 gallons of oil. The oil alone will control low to moderate infestations of San Jose scale. However, if the infestation of scale is high, add an organophosphate insecticide such as Diazinon® to the oil mixture. If you have resistance to Diazinon® or Lorsland®, Seven® is a good alternative. Please read and follow directions of all insecticide labels.

Copper in the dormant spray has been controversial. Some researchers believe that copper aids in the control of shothole disease and "blast" in almonds. Others believe that copper has no value in a fungicide program for almonds. Dr. Beth Teviotdale and I have studied dormant copper sprays for seven years in an almond orchard in Kern County.

Based on seven years of data, I will make the following statements are on the use of copper in the dormant spray.
· Dormant copper alone does not control shothole disease, therefore, it is not a substitute for a good fungicide program in the spring;
· In years with heavy shothole infestation, dormant copper has increased the effectiveness of the fungicide program in the spring;
· Even though copper in the dormant spray decreases the amount of shothole disease, it did not show an increase in yields.
In conclusion, if a grower wants to have a clean orchard, (especially during a heavy disease pressure year), he needs to include copper in his/her dormant spray. The grower needs to keep in mind that copper in the dormant spray is not a substitute for a good fungicide program in the spring.