Marin Master Gardeners
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Get garden pests while they’re dormant

January 9, 2015
Dave Phelps

Life cycles of some pests are complex, with multiple steps between egg and adult or spore and fruiting body and perhaps periods of dormancy.

The damage pests do to our gardens is usually associated with a certain stage, but can be best managed at an earlier stage, before most of the damage occurs. Because of this complexity and a desire to protect the environment, and especially beneficial organisms, it's important to follow the first two tenets of Integrated Pest Management (IPM) before applying a dormant spray:

  • Know the host plant, and
  • Know the pest.

Here are some questions to help you understand the pest problem and choose the best treatment:

  • Are the pests fungal in nature or are they insects?
  • If insects, are they homopterous: scale, mealy bug, leafhoppers, cicadas or aphids?
  • Could the pests be mites or are they boring beetles?
  • Or are they in the Lepidoptera class, such as butterflies, moths or caterpillars?
  • If fungal, do the spores overwinter on the leaves, under the bark, or on old flowers or fruit? Do they overwinter on alternate host plants nearby?
  • Could the disease be bacterial or viral?
  • Where does the mycelia, or body of the fungus, grow — and do they produce protective dormant structures?
  • Where on the plant does the damage occur?
  • How do the pests get there in the spring and summer?

One of the best methods to control pests during the winter is through a combination of good sanitation and the use of a dormant spray, another IPM tenet — using multiple controls synergistically.

Good sanitation involves removing organisms that would infest the host plants the following season. This can be as simple as removing infected leaves, old flowers or fruit, or removing wounds from the plants that may harbor pests or diseases. Covering the ground with fresh mulch can also help reduce the spread of pathogens by both interrupting spore transmission as well as creating competitive soil ecology where many pests become food.

Dormant spraying can be as simple as blasting the plants with water or an oil- or copper-based product, or an insecticide or fungicide. Sometimes an adjuvant, or "spreader-sticker," can be added for increased efficacy.

Knowing the pest and understanding the potential population and damage the following season can help you use two more IPM tenets:

  • Use the least toxic pesticides necessary
  • Keep the pest population under a damage threshold, typically aesthetic in the home garden

Sometimes it is best to remove alternate host plants or even replacing the infected plant itself with a more resistant plant or one better suited for the site. Peach leaf curl (taphrina deformans), fire blight (erwinia amylovora), or other blights (anthracnose, phytophthora) may require more control effort than is desired to maintain an aesthetic threshold. It may also be possible to adjust the environment to inhibit the pest population: Sunlight, air circulation and moisture can impact pest populations. Physical protection, as in the use of row covers, is another option.

In an extensive monoculture such as a commercial orchard, dormant sprays are usually necessary. In a home garden, they may not be warranted and can even cause more harm than good, if applied incorrectly. Monitoring pest populations throughout the growing season is the best way to detrmine whether a dormant spray makes sense. If so, it's important to know which spray to use and how to maximize its efficacy.

In Marin, this is usually a copper-based spray to control fungal diseases, but could be a light oil spray. Spraying to control bacterial or viral diseases is best done by a professional applicator. Prune the plant first.

Do due diligence to remove obvious contagions.Before using a spray, consult the "Pests and Other Problems" link on the home page of the Marin Master Gardener website (www. marinmg.org). This important resource will help you identify the pest and suggest the best control measures.

With the right information and the use of synergistic cultural and physical controls, a dormant spray may not be necessary. The University of California Marin Master Gardeners are sponsored by UC Cooperative Extension. For questions about gardening, plant pests or diseases, call 473-4204 from 9 a.m. to noon, and 1 to 4 p.m. weekdays, or bring in samples or pictures to 1682 Novato Blvd., Suite 150B, Novato.

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