Urban Gardener: Hot summer days can create garden problems, foster mites

Summer presents a challenge for vegetable growers, as many plants struggle with the heat. Lettuce may bolt, and other leafy greens will struggle as well. Tomatoes may experience blossom drop as nighttime temperatures exceed 70 degrees.

Hot days may cause fruit to be deformed or ripen unevenly. Chili peppers and eggplant may also experience similar problems, though my University of Hawaii Hawaiian chili pepper and long eggplant varieties are producing nicely through the heat. Crops that may grow well through the summer include corn, beans, squash, taro and sweet potato. Regular irrigation in the morning and midday can help your plants deal with the heat.

Our summer season can also lead to other problems. Dry, hot conditions are favorable for spider mites. Their populations can increase rapidly and the damage they cause can destroy your prized plants within days. Mites are very tiny and difficult to see, so scout regularly for their signs.

Mites are closely related to spiders, ticks and other arachnids. There are a number of mite species that are pests on our ornamental plants, fruit trees and vegetable crops. Hibiscus erineum mites cause the bumpy galls on hibiscus leaves. Lychee erinose mites cause leaf curling and the growth of velvety hairs that look like fungus on lychee leaves, and occasionally on flowers and fruits. Broad mites can cause distorted “shoestring” leaves on peppers, papaya and other plants. Spider mites on the other hand, cause a characteristic discoloration called stippling. This consists of many tiny white or yellow spots on leaves, which together can create a silvering or bronzing discoloration. This is very typical on tomato and eggplant, but can affect plants ranging from orchids to taro. On fruit trees such as papaya and citrus, spider mites commonly cause rough scarring on the fruits. When large populations build up, you may see webbing on leaves and stems.

How to control spider mites?

Beneficial insects such as predatory mites and ladybugs feed on mites, but may not control them before their outbreaks kill your plants. Insecticidal soap or neem oil are effective against mites and other insect pests, while posing less risk to people and beneficial insects. Apply them in the early morning or late in the day when temperatures are cooler, as they may stress plants in the hot sun. Products with sulfur can also be very effective against mites.

Avoid broad-spectrum pesticides with ingredients such as carbaryl or pyrethrin. Even though they may be considered lower in risk to people and animals, they are highly toxic to all insects, even good ones. They often encourage outbreaks of mites, thrips or other pests after application.

My favorite control for mites is to wash them off with a strong spray from the water hose, making sure to get the tops and bottoms of leaves. This is just as effective as using pesticides, and easy to do as I water the plants. Make sure to do this in the morning or early enough in the day so the plants can dry off. Leaves that stay wet overnight or during prolonged rainy periods can encourage fungal diseases.

Learn more about plant pests and how to contact your local master gardeners with questions at ctahr.hawaii.edu/UHMG.

Kalani Matsumura is a junior extension agent with the University of Hawaii’s College of Tropical Agriculture and Human Resources and coordinates the UH Master Gardener Program on Oahu.