Sooty molds are Ascomycete fungi which grow on plant exudates and the sugary honeydew secreted by insects such as aphids, scales, the whitefly, and other insects which suck sap from their host plants. The name itself is descriptive, as sooty mold is a black, powdery coating adhering to the leaves of ornamental plants such as azaleas, gardenias, camellias,crepe myrtles, and laurels.
Pathogen-caused leaf spot diseases, particularly those of stone fruit trees and such vegetables as tomatoes, peppers, and lettuce are of two types, those caused by bacteria and those caused by fungus. Leaf spotting of either kind is generally similar in appearance and effect. Prevention and treatment of both kinds often involve the same practices.
A serious problem in home gardens, club root can be managed using these organic methods.
Affecting members of the cabbage family, club root is a serious plant disease in North America. It is caused by the soil-borne fungus, Plasmodiophora brassicae, which infects susceptible plants through root hairs. Diseased roots become misshapen and deformed (clubbed), often cracking and rotting. As a result, plants have difficulty absorbing water and nutrients properly.
A fungal disease, downy mildew (Plasmopara viticola) affects many plants and appears as yellow to white patches on the upper surfaces of older leaves. On the undersides, these areas are covered with white to grayish, cotton-like fungi. These “downy” masses are most often noticed after rain or heavy dew and disappear soon after sunny weather resumes. As the disease progresses leaves may eventually turn crisp and brown and fall off even though the plant has ample water.
Found on a wide range of plants (too many to mention), gray mold (Botrytis cinerea) is a fungal disease that travels quickly through gardens, especially during damp, cool to mild weather. It can be identified as grayish colored soft, mushy spots on leaves, stems, flowers and on produce. Spots may become covered with a coating of gray fungus spores, especially if humidity is high. Fruit or plants shrivel and rot and often develop black, stone-like sclerotia under rotted parts.
A fungal disease that affects peaches and nectarines, leaf curl (Taphrina deformans) is one of the most common disease problems found in backyard orchards. It appears in spring as reddish areas on developing leaves. These areas become thickened and puckered, causing leaves to curl and distort. When severe, leaf curl can substantially reduce fruit production.
Affecting a wide variety of plants, including roses, beans, tobacco, tomatoes, potatoes and peppers, mosaic is a viral diseases found throughout the United States.
Plant viruses can be difficult to detect as symptoms look similar to several nutrient deficiencies. Look for:
- Yellow stripes or spots on foliage
- Wrinkled or curled leaves
- Stunted growth and reduced yields
- Infected fruit appears mottled and develops raised “warty” areas
Symptoms. All aboveground parts of the plant can have symptoms of this disease. The fungus can kill seedlings before or after they break the soil surface. It can also cause collar rot in young plants, in which a dark, slightly sunken spot develops on one side of the stem. This spot will either grow completely around the stem or weaken the stem, causing the plant to fall over and die.
Septoria Leaf Spot
Septoria Leaf Spot
Symptoms. Symptoms of this disease can be found on the leaves and other green parts of the tomato plant. Small, round, yellowish spots appear on the leaves, usually after the first fruit sets. Symptoms generally start on the lower leaves first. The spots later become brown or grayish and may have a yellowish or dark border. On susceptible tomato varieties, leaf spots may be up to 1/8″ in diameter; but they are smaller on resistant varieties. Dark specks or dots appear in the center of the spots on susceptible varieties. These are usually fewer or absent on resistant varieties. Heavily infected leaves fall off, normally starting at the bottom of the plant and progressing upward. Fruit infection is rare.
Symptoms of leaf rusts include bright yellow, orange or red leaf spots. These leaf spots often produce an abundance of yellow, orange, red, or brown powdery spores that can be easily rubbed off and seen on a white tissue or paper towel. Spores may be produced in little blisters within leaf spots or may emerge from tiny cups or tubes on the lower surface of the leaf. In some hosts, leaf rust fungi also infect petioles, young green stems, and fruit.
Snails and Slugs
Snails and Slugs
Snails and slugs are among the most bothersome pests in many gardens and landscapes. The brown garden snail, Cornu aspersum (formerly Helix aspersa), is the most common snail causing problems in California gardens. It was introduced from France during the 1850s for use as food. Another troublesome snail is the white garden snail, Theba pisana. It currently is established only in San Diego County but has been found in Los Angeles and Orange counties as well.
Aphids are small, soft-bodied insects with long slender mouthparts that they use to pierce stems, leaves, and other tender plant parts and suck out fluids. Almost every plant has one or more aphid species that occasionally feed on it. Many aphid species are difficult to distinguish from one another; however, management of most aphid species is similar.
Thrips, order Thysanoptera, are tiny, slender insects with fringed wings. They feed by puncturing the epidermal (outer) layer of host tissue and sucking out the cell contents, which results in stippling, discolored flecking, or silvering of the leaf surface. Thrips feeding is usually accompanied by black varnishlike flecks of frass (excrement).
Earwigs are among the most readily recognized insect pests in home gardens. Although earwigs can devastate seedling vegetables or annual flowers and often seriously damage maturing soft fruit or corn silks, they also have a beneficial role in the landscape and have been shown to be important predators of aphids.