Every year hundreds of questions come to the Help Desk, a UC Master Gardener project designed to bring information about sustainable gardening to Contra Costa residents. At the Help Desk, a team of Master Gardeners answers questions about insect problems, plant diseases and how to manage the ever-pesky critters that love a perfectly ripe tomato.
Recently we gathered the most frequently asked questions and published them on our website. Here are the top five, along with abbreviated answers.
1. Why are the leaves on my citrus tree turning yellow?
Yellow leaves on citrus trees can be caused by a variety of factors throughout the growing season. The good news is that almost all causes are treatable by the home gardener. The most common reasons that citrus leaves turn yellow (this is also called chlorosis) are:
• Watering: too much water can cause the tree’s roots to drown and rot (https://ipm.ucanr.edu/PMG/GARDEN/FRUIT/CULTURAL/citruswatering.html)
• Fertilizing: too much or too little can be a problem https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=56049)
• Nutrient deficiency in the soil: these include iron, zinc and magnesium deficiencies
• Seasonal factors: Cold and rainy conditions
• Fungal diseases: like Phytophthora or root rot https://ipm.ucanr.edu/home-and-landscape/phytophthora-root-and-crown-rot/#gsc.tab=0)
• Pests: Mites can cause yellow leaves especially in hot and dry conditions https://ipm.ucanr.edu/PMG/GARDEN/FRUIT/PESTS/citrustmite.html)
2. I get lots of flowers but almost NO tomatoes. What’s wrong?
There could be several reasons for lack of fruiting in tomatoes:
• Night temperatures too low, below 55°F (12.8°C) or daytime temperatures too high, above 90°F (32.2°C). The plants will produce when temperatures become favorable.
• Excess nitrogen fertilizer. Lots of healthy green leaves and stem growth, but few flowers or fruit.
• Too much shade. They need a minimum of six hours of sun to produce fruit.
• Lack of pollination. While tomatoes are self-fertile, the conditions mentioned above will inhibit pollination.
• Plants set out too early in spring. We recommend planting tomato seedlings the first of May.
3. How can I get my soil tested?
Although the UC Master Gardeners of Contra Costa cannot test your soil for you, many commercial laboratories provide a variety of soil testing services to the public. Most have informative web sites that describe what tests and other services are available, how to submit soil samples for testing, etc.
• For more information: https://ucanr.edu/sites/default/files/2025-08/405a%20Soil%20and%20Water%20Testing%20Laboratories%20for%20Home%20%20Gardeners%202025_02_28%20Final.pdf
4. There are small insects all over my roses and sticky stuff on the nearby walkway. What is it?
The small insects you are seeing are most likely aphids. Aphids can be identified by their soft pear-shaped bodies and a pair of tubelike structures (called cornicles) projecting out of the hind end of their body. Aphids feed by sucking plant sap from leaves which they then excrete onto the plants. This excretion (called honeydew) is the source of the stickiness on your walkway. Aphid populations are generally not damaging to garden plants and are kept under control by beneficial insects. However, if aphid populations are too high and are causing problems you can prune off damaged areas or use a strong spray of water to knock the aphids off the infected plants. Make sure you also check for ants on your roses. Ants love honeydew and will protect aphids for their honeydew. Ant bait stations can be used to keep the ant population under control.
5. There is a powdery growth on the leaves of some of my plants. What is it?
White powdery growth on leaves and shoots can be a sign of powdery mildew. This disease affects many plants, and one of several fungi can cause it. Manage powdery mildew by growing resistant plant varieties and altering the growing environment. In some situations, fungicide treatments might be required for susceptible plant species.
For the full list of FAQs with responses, visit us at https://ucanr.edu/site/uc-master-gardener-program-contra-costa-county/faq.
And if you have a question of your own, you can email or call us or make an appointment to visit us at our Concord office.
• Email us at ccmg@ucanr.edu, especially if you want to submit photos along with your question(s).
• Call us at (925) 608-6683. If you call outside Help Desk hours, please leave both your phone number and email address so we may respond to you in a timely manner.
• Visit us for a consultation at 2380 Bisso Lane, Concord, by appointment only. To set up an appointment, please call or email us.
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Help Desk of UC Master Gardeners of Contra Costa County (RDH)