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Healthy tomatoes grow in a garden. The leaves and stems of those infected with tomato russet mites typically turn bronze and then brown. Photo courtesy Getty Images.

Do you have a gardening question? The UC Master Gardeners of Santa Clara County can help. Our volunteers are part of the University of California’s Cooperative Extension program and we are trained to use research-based information to educate our fellow residents on sustainable gardening and landscaping practices specifically for our area.

Our monthly “Ask a Master Gardener” column answers your questions and offers tips. Have a question? Email lifestyle@embarcaderopublishing.com.

I’m noticing that the lower leaves and stems on my tomato plant are starting to turn a bronze color.  What is this,  and what can I do about it?

There are lots of tomato pests out there, and it can be hard to identify a problem. It sounds like your plant could have tomato russet mites.  These are tiny sucking insects that thrive in high temperatures and low humidity, exactly what summers on the Peninsula provide – and especially with the weather we’ve had lately.  They multiply rapidly under those conditions, so we start noticing the problem in mid-summer as the bottom leaves and stems turn bronze, then brown, moving upward through the plant. The fruit can be damaged by both the mites and by sunburn as many leaves die, leaving the fruits exposed. Spraying with sulfur can help control the mites – just remember to apply sulfur when temperatures are expected to stay below 90 degrees to avoid possible damage to the plant, and always carefully follow instructions for application.  

August gardening tips

Prune apricot and cherry trees 

By pruning at least six weeks before the first rainfall, you reduce the chance of a fungal disease called Eutypa dieback. Spores from this fungus enter fresh pruning wounds by splashing water or rain, so you want to be sure your pruned trees are healed by fall.  Symptoms take time to develop. In spring, you may observe faded leaves and wilted limbs. Later, cankers form, and limbs die back. The tree will die if not managed. Remove infected limbs, make cuts at least 1 foot below the site of any canker, and cut until no discolored diseased wood remains below the cut.

Keep spider mites away

Tiny spider mites are ubiquitous pests that attack a wide variety of ornamental and edible plants. To the naked eye, they may look like tiny moving dots, but you can see them with a magnifying glass. If you look closely, webbing may be visible. Stippling of the leaves is the first symptom. The color of the plant will start to fade and eventually leaves may drop. They suck out plant juices from leaves, flowers and the blossom end of the fruit. 

Dry hotter temperatures and dusty conditions encourage them, so water is an important management tool. Be sure to water the plants sufficiently, and wash off the leaves with water to knock off a lot of the dust and spider mites. If treatment is necessary, spider mites can be controlled with insecticidal soap, horticultural oil or neem oil. 

More information is available in the UC Quick Tips on Spider Mites.

Give shelter to succulents

While succulents may not require much water, they are not necessarily “desert” plants. Many succulents will look better if given a little shelter from hot afternoon sun in the summer. Some can even be grown indoors. Too much sun may lead to some plants looking parched and sunburned. But if given too little sun, other plants may grow lanky, stretching for the light. Try to provide each succulent with the lighting conditions it prefers.

Learn how to tell if fruits and vegetables are ready to pick

 The UC Davis Postharvest Technology website was designed for commercial growers, but the information on how to tell When Fruits and Vegetables are Mature is handy for home gardeners as well. 

Know your planting zone 
 Planting zones help you select plants that are right for your garden. There are two systems, USDA and Sunset Western Garden. USDA divides the U.S. into hardiness zones based on average high and low temperatures. While it was updated recently, Santa Clara County didn’t change much – most of Santa Clara County is in zones 9b and 10a. You can look up your area by zip code through the USDA Plant Hardiness Zone Map Lookup. Sunset makes many finer distinctions, taking into consideration rainfall patterns, ocean influence, and more. In the Sunset system, Santa Clara County is largely zones 15 or 16 with some zone 7 for areas with more extreme highs and lows. Sunset zone maps: Central and South County and North County

Take a gardening class

UC Master Gardeners offers several gardening-related classes in Santa Clara County. Here are some August events along the Peninsula in August.

Ask questions or listen in at our monthly Zoom Plant Clinic, which is held the second Tuesday of the month from 7– 8:30 p.m.  Dates for 2024 are Aug. 13, Sept. 10, and Oct. 8.You can also subscribe to our monthly Tips and Events newsletter. San Mateo residents can also find gardening advice on the UC Master Gardeners of San Mateo and San Francisco website.

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