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Controlling Garden Pests and Diseases of Spring

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The alternating cool and warm weather of spring, plus intermittent rains, create an ideal environment for some garden pests and diseases that can make for a bad start to the gardening season. But there are preventive measures you can take now and throughout spring to prevent or minimize damage. Using synthetic chemicals should always be a last resort. Here are some safe, effective ways to get a jump on garden pest and disease control.

Horticultural Oils

Using horticultural oils on fruit trees during their dormant season to control overwintering insects has been a common integrated pest management practice for years. But these sprays are also effective in the ornamental garden both before (dormant oils) and during the growing season (summer oils). If applied at the right time and in the right way (always read the label), horticultural oils can be one important tool in controlling many common soft-bodied garden pests such as mites, aphids, white flies and mealy bugs.

They work primarily by suffocating insects— and sometimes the eggs —that are on the plant at the time you spray. But unlike broad-spectrum pesticides, horticultural oils are much safer because they do not leave a long-term toxic residue that can harm beneficial insects. “Horticultural oils…degrade rapidly through evaporation, and have very low toxicity or almost no toxicity to humans or wildlife at the rates used to control pests,” says UC Davis Integrated Pest Management (IPM) Associate Director Mary Louise Flint in Pests of the Garden and Small Farm: A Grower’s Guide to Using Less Pesticide (University of California Press, 1998).

Like most pest controls, the sooner you spot an infestation, the more effective horticultural oils will be. Spray when populations are relatively low; you will have a better chance of minimizing damage to your plants.

Mechanical and Cultural Controls

Soil and wind-borne pathogens such as molds and mildews that prefer cooler, wetter weather can be difficult to control, but there are things you can do to minimize damage.
For fruit trees and bushes, organic mineral-based products such as fixed copper can help prevent or control diseases like peach leaf curl. While these products are organic, they can affect beneficial insects, so read the labels carefully for instructions on when and how to use them.

If you don’t already have several inches of coarse mulch protecting plants, add some now. Soil-borne diseases can be carried onto stems and leaves when hard rains splash dirt back up onto the plant. A good mulch can prevent the problem.

Make time at least once a week to look for early signs of pests and/or disease and remove them by hand. Being diligent about pulling weeds among ornamental and crop plants also helps, as weeds host pests that can spread quickly in the garden.

Insecticidal Soaps and Home Remedies

These days, there are safer pesticides (for humans and the environment) available, which can, when used early and wisely, help you stop infestations before they can do widespread damage to your garden. Insecticidal soaps and neem oil are examples of such products. To learn more about the more environmentally friendly controls, visit, the UC Davis Integrated Pest Management website.

Some gardeners swear by homemade concoctions such as diluted dish soap (best to use milder kinds, such as baby shampoo), sprays made from a water and vegetable oil base with added ingredients like cayenne pepper (for chewing insects) or garlic (for deer). While the latter methods are not proven scientifically, they are inexpensive, harmless tactics that just might be a good addition to your gardening tool box.

The first day of spring this year is Monday, March 21st. This is the time to start planning, planting and cleaning up your garden.

Rachel Oppedahl is a University of California Cooperative Extension Master Gardener of Tuolumne County.

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