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Southern California Gardening: Insects and Diseases

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Glossy foliage and colorful flowers are the hallmarks of a healthy garden. With a little care, you can prevent diseases and insects from ruining the effect.

brown snail

By Evelyn Alemanni

sooty mold on sweet potato foliage

We all strive to maintain healthy gardens. But sometimes, despite our best efforts, culprits, such as diseases and insects, sneak into the flower and vegetable beds, raising havoc. Early detection and control are essential to keeping your plants healthy.

Diseases that affect our plants can cause leaves to yellow, discolor and drop off, resulting in stress or even death to the plant. Rust, black spot, sooty mold and mildew are common plant diseases that have these effects. It’s important to remember that once these diseases occur, it is necessary to remove the leaves and prevent recurrence. The leaves do not heal and return to a healthy condition.

black spot on a rose leaf

Black spot is a common problem on rose foliage. The best prevention is to research plant varieties that are disease resistant. Otherwise, you may need to spray fungicides weekly.


Rust is a fungal disease that appears on the underside of foliage as dusty orange pustules. You can treat this disease with fungicide sprays.

Good to Know: Be sure to spray the underside of the leaf, as well as the top.

To discourage leaf diseases, prune plants to allow good air circulation and keep beds clear of dead leaves. It’s also important to remove diseased leaves from plants and rake them from the ground to prevent reinfection. To prevent the spread of disease, put the leaves in the trash rather than the compost. Spraying with horticultural oil in late winter/early spring, before trees and shrubs leaf out, also helps.

brown snail

One challenge in Southern California is that winters are not sufficiently cold to kill insects. Some common insects affecting our gardens are snails and slugs (in the mollusk family), mealybugs, ants and aphids.

Brown snails cause the most problems in my garden. Snails are usually present in wet months, and they thrive in piles of damp leaves. They come out at night and feast on foliage, fruits, vegetables and flower petals. Effective deterrents include handpicking them at night or early morning, attaching a copper band to pots or tree trunks, and spreading a border of horticultural diatomaceous earth around affected plants. Snails don’t like to cross those two barriers, but if they are already on a plant, you need to handpick them.

Decollate snails dine on brown snails, so they can be a helpful control method. However, if you’re relying on decollate snails, you won’t be able to use snail bait because it kills all snails, good and bad. Speaking of snail bait, use a type that does not harm pets or wildlife, and reapply it every two weeks so it controls newly emerged snails.


Although ants don’t harm our plants directly, they are little farmers that work hard to keep honeydew-producing insects such as aphids, scale and mealybugs on plants. Ants “farm” these insects, eating the honeydew they release. If the ants don’t eat the honeydew, it can lead to sooty mold, which further damages leaves.

Good to Know: You can control ants with bait or sticky traps, and remove aphids and mealybugs with a strong jet of water, insecticidal soap, or neem oil.

Good to Know: Before using insecticides (chemical or organic), be sure to read the label to determine whether they affect fish, pollinators, pets, birds, wildlife, and waterways.

Integrated pest management (IPM) is a broad-based approach that focuses on long-term prevention of pests. It does this by creating inhospitable conditions that cause pests to die or move elsewhere. IPM consists of a combination of cultural, mechanical/physical, biological and chemical controls, using the least toxic remedies.

Thousands of pests and diseases can attack our plants. If you have a problem you can’t identify, check with your local extension office or the University of California statewide integrated pest management program. The best treatment for pests and diseases is prevention.

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