Dormant Sprays

By Yvonne Rasmussen, U. C. Master Gardener

What are dormant sprays and what are they used for? In Napa Valley, most deciduous fruit trees benefit from these protective treatments, applied in winter after leaves have fallen and before new growth has fully emerged. 

Dormant sprays help control many insects and fungal diseases. Applied in winter, when some insects are in their most vulnerable stage (crawlers or eggs) and many fungi are at their most active, these sprays can be highly effective. Spraying trees when they are bare helps you get good coverage of branches and insects, key to good control. Sprays can damage tender young growth, another reason to apply them when trees are dormant.

Some dormant-season sprays are fungicides. Others are horticultural oils derived from highly refined petroleum. When applied in the dormant season or early spring, before bud break, horticultural oils help control scale, spider mites and leaf-chewing caterpillar eggs. They can also help eliminate sucking insects such as plant bugs and aphids when they are immature and present in high numbers.  Later in the summer, you can use these oils to control spider mite eggs and adults on leaves.  

Most experts agree that horticultural oils kill insects by smothering them. The fine droplets coat tender-bodied insect and egg surfaces, preventing oxygen uptake. The spray must fully coat branches, including those little nooks and crannies where small insects and eggs can hide.  If you don’t get good coverage, or if the insects or eggs are in a protected stage of growth, such as a hard-shell stage or cocoon, you will not get good control.  Note that oil residue will not harm beneficial insects that may fly into the area after you spray.

Look for horticultural oils at nurseries and garden centers, where they are often labeled as “superior oil” or “summer spray oil.”  They generally flow easily and have low sulfur content so you can use them on growing plants in winter or summer, but not on tender new growth.

Fungicides protect fruit trees by inhibiting fungus growth that might infect branches. These products are generally composed of copper or sulfur or some combination of the two. Again, good coverage and timing are critical. You need to apply fungicides before fungi are actively growing. And if it rains, you must reapply them to keep plants protected. As a result, you may need to treat your trees several times in early spring to protect new growth. Once the fungus is actively growing, it is too late for effective control or treatment.

Our wet weather and mild temperatures encourage fungi growth. Many of our fruit trees (such as pear, apple, peach, prune and apricot) are especially susceptible. If properly applied, copper and sulfur sprays can help protect trees from such problems as leaf curl, scab, powdery mildew, anthracnose, cankers and bacterial blights. To be most effective, fungicides should be applied in the fall, around November, and again in the spring before buds swell, usually early February.  In a dry year, you can forego the February treatment. In wet years, you may need a third spray near bloom time.

Good tree care throughout the year is also important in keeping fungal problems at bay. Thinning the canopy in midsummer will improve air flow. Removing diseased wood and plant parts during winter pruning will also help control disease.

Keep pruners clean and sharp. When using any chemical or pesticide, follow label directions exactly, including instructions for clean-up and disposal. Read the label before you start.

For more information on dormant spraying and other fruit-tree topics, consult the Master Gardeners’  Healthy Garden Tips (http://ucanr.org/sites/ucmgnapa/Gardening_Books/Healthy_Garden_Tips/) and Pest Notes (: http://www.ipm.ucdavis.edu/PDF/PESTNOTES/index.html).  For more information about home fruit trees, consult the University of California’s backyard-orchard web site (http://homeorchard.ucdavis.edu/).

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