PESTS: California Pests and Pathogens

The pests and pathogens listed below damage several of California’s most economically valuable crops every year. They have physiological and behavioral characteristics that give them an advantage over their competitors, and they require vigilant management attention to keep populations under control. Tri-Tech Pest Control Advisors attend meetings and complete continuing education units to stay up to date on the latest, and most environmentally safe pesticides and management practices to thwart pest outbreaks before they can cause serious crop damage.

Two spotted spider mite, Tetranychus urticae, adults and eggs.

Two-Spotted Spider Mite (Tetranychus urticae)

Commonly found on the underside of strawberry and raspberry leaves, Two-Spotted Spider mites range from 1/100 to 1/50 of an inch (0.3-0.5mm) in diameter. Mites feed on plant tissue where they spin fine webs and lay tiny, clear eggs. Adult spider mites are greenish yellow and have two black spots on their backs. They have four sets of legs and two body segments, the head, or gnathosoma and the abdomen, or idiosoma. Plant damage symptoms include leaf flecking and stippling, and dull bronzed tissue. Fine webbing often covers the undersides and sometimes the tops of leaves. Severe mite infestations can damage fruit and cause significant yield loss. Several beneficial predatory insects and mites prey upon two-spotted spider mites. One common predatory mite, Phytoseiulus persimilis, can be differentiated from the herbivorous mites by their red-orange color, larger size, and highly active behavior.

Pea aphids extracting sap from the stem and leaves of garden peas.

Aphids (Chaetosiphon fragaefolii, Aphis gossypii, Myzus persicae)

Sap sucking aphids damage many crops including strawberries, peppers, tomatoes, brassica, and more. Aphids are generally light green, small, pear-shaped, and sometimes have wings. They transmit many viral plant diseases and their sticky secretions often attract pestilent fungi and bacteria. Many insects, including the syrphid fly and green lacewing larvae, feed on aphids and can help keep populations low.

Lygus pratensis

Lygus Bug (Lygus hesperus, Lygus lineolaris)

Lygus bugs attack off-season (summer) strawberries in Ventura County, as well as cotton, alfalfa, beans and other crops. Lygus bugs puncture seeds, causing fruit malformation and poor pollination. They are generally brown or sometimes green, with a small yellow-green triangle on the back, and have a reddish-brown pattern on the wings. Adults are about a quarter inch long and have a slender, oval shape. Nymphs are light green and are easily mistaken for aphids, though lygus nymphs are more active and fast moving.

Bagrada Bug (Bagrada hilaris)

Bagrada Bug (Bagrada hilaris)

Bagrada Bugs are an invasive species from Africa, that first appeared in southern California in 2008. Bagrada bugs mainly infest brassica crops, although they have also been found on other plants, including strawberries in Ventura County. In the winter, Bagrada bugs colonize wild mustard weeds on foothills adjacent to agricultural fields. The weeds provide a breeding ground while temperatures rise, and by late summer when the weeds dry up, enlarged bagrada bug populations move downhill to infest cole crops. Bagarda bugs are oval shaped and about 1/4 inch long and 1/8 inch wide. They are shiny black with white and orange marks on their backs, and may be easily confused with Harlequin bugs, another type of stink bug with a similar color pattern. Bagrada bugs damage crops by injecting enzymes into plant tissue and sucking out the partially digested liquid. Their feeding activity causes lesions, wilting, stippling, and in some cases, plant death.

Spodoptera exigua larva feeding on Nicotiana attenuata.

Beet Armyworm (Spodoptera exigua)

Beet armyworms commonly attack tomatoes, cole crops, and other plants during their larval stage as caterpillars. Although their color varies, caterpillars are usually light green with pale stripes down the back and dark spots above the second true legs. Armyworms feed on plant tissue, creating holes in leaves and shallow depressions in fruit. Plant wounds caused by feeding provide entry sites for pathogenic fungi and bacteria causing secondary disease. Caterpillars eventually transition into the pupal stage, then re-emerge as gray and brown moths with a 1 inch wide wing span. Eggs are laid on leaves in clusters of up to 100. When the armyworms hatch they start feeding on the leaf and disperse outward as they grow.

Raspberry plants showing symptoms of Phytophthora root and crown rot.

Phytophthora Root Rot (Phytophthora megasperma, Phytophthora citrophthroa, Phytophthora parasitica, etc )

The soil fungus, Phytophthora, attacks cole crops, citrus, caneberries, and other crops. Phytophthora thrives in poorly drained soils that stay moist for long periods of time. The fungus decays the plant roots, causing leaves and stems to wilt and die. Plant crowns turn dark brown and soften as the fungus infects the tissue.

Powdery mildew, species Podosphaera fusca

Powdery Mildew (Sphaerotheca macularis)

Powdery mildew is a fungus commonly found on strawberry plants. It often appears as a white powdery substance on the undersides of leaves. Infected flowers may fail to produce fruit or yield deformed berries. When powdery mildew infects mature fruit, it appears powdery, white, and filamentous. Powdery mildews found on peppers and other crops display similar characteristics, but are caused by different fungal species.

Late Blight

Late Blight (Septoria apiicola)

Late blight is a fungal disease often appearing on celery. Early in the infection, late blight forms small, round, yellow spots on leaves. The lesions turn tan or dark brown and become papery as they dry out. Late blight forms reproductive structures, called pycnidia, in the center of lesions. Pycnidia are triangularly shaped, black flecks that resemble grains of soil splashed onto the leaf. Late blight thrives in warm, humid environments and spreads through contaminated seeds.

Sclerotinia (Sclerotinia sclerotiorum, Sclerotinia minor)

Two main species of the fungi Scerotinia, S. sclerotiorum and S. minor, infect a wide range of vegetable crops, including celery, lettuce, brassicas, and more. Pale brown and gray-brown lesions form on leaf tissue early in the infection and as the disease progresses, Sclerotinia produces a fluffy, thick white mat of mycelium. In celery, Sclerotinia is sometimes called “pink rot” because of the pink and red discoloration that appears on infected tissue. Sclerotinia minor usually infects the plant at the roots or the base of the stem near the soil, destroying vascular tissue and the crown. Sclerotinia sclerotiorum can also start near the soil surface, but often appears first on the leaves. Crops are most susceptible to Sclerotinia under warm, moist conditions.

Botrytis Fruit Rot, or Gray Mold (Botrytis cinerea)

Botrytis fruit rot, a fungal infection, attacks strawberries under cool, moist conditions. Botrytis spores often land on strawberry flowers and remain dormant in the plant tissue as the berry matures. Once the sugar content in the fruit increases, Botrytis will activate, producing brown lesions near the calyx. As the lesions spread, the fruit tissue becomes gray-brown and soft. The fungus produces billions of spores on the fruit that can spread by wind or water. Removing infected berries and plant tissue from the field can help prevent disease spread.

The University of California Integrated Pest Management Program
Ontario Ministry of Agriculture, Food and Rural Affairs Integrated Pest Management

For links to more information see the Sources page.