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Diseases
Chlorosis: Chlorosis causes the leaves, particularly vein areas, to turn yellow. The plant turns yellow because the soil has too much iron. Too much iron can be caused by a higher pH than what the plant needs or by waterlogged soil. Chlorosis also occurs when plants are placed close to concrete surfaces.
Leaf Spots: Bacteria or fungi cause brown or black spots on leaves. The leaf spots have no particular shape, but the edges look yellow. People, rain and insects spread leaf spots. When the plant is dry, remove the infected leaves. Be sure to keep the plant pruned properly, and clean up any dropped leaves under the plant. Water plants from below, taking care to keep the foliage dry. Leaf spots can also be controlled with fungicide.
Powdery Mildew: Powdery mildew is a fungus that usually affects plants that have low air circulation and inadequate light. It is a bigger problem in the early spring and fall months, when the temperatures swing from high during the day to low at night, especially in zones with a lot of humidity. Powdery mildew forms a white or gray coating on the top of the leaves, making them turn yellow, curl up and drop off. It also causes stunted fruit that drops early. To control powdery mildew, decrease watering, use a fertilizer that is low in nitrogen and keep water off the foliage. Apply fungicides as directed.
Pythium and Phytophtora Root Rot: Pythium and phytophtora root rot attack a plant if there are fungal spores in the soil and the plant is overwatered, creating a high moisture level. Roots turn black and break. Leaves yellow and fall off, usually from the bottom up. Contaminated water or soil mix is the usual cause of root rot. To control this disease, remove infected plants, make sure the soil is well-drained and do not overwater or overfertilize plants. There is no chemical treatment for root rot.
Rusts: Rusts generally overwinter on plant foliage and spent flowers. They are host-specific and usually appear as small, yellow or bright-orange pustules under the leaves. Sometimes the pustules can be brown. Other fungi and splashing water causes most rust problems. Rust tends to worsen when the weather gets moist. To control rust problems, water the plant from the base, keeping the foliage dry. Make sure the plants have enough air flow to all leaves by thinning and pruning. Water only during the day, giving plants a chance to dry before nightfall. Rusts can also be controlled with a fungicide.
Southern Blight: Southern blight is a fungus that leaves lesions on the stem of a plant. The lesions can usually be found near the soil line. The lesions cause the plant to suddenly wilt. This fungus likes temperatures above 85 degrees Fahrenheit. It can be controlled with a fungicide.
Pests
Aphids: The aphid is a small insect that forms clusters on seedlings and new plant growth. It's pear-shaped body ranges from 1/16- to 3/8-inch long. Aphids spread quickly, feeding on plant juices, which weakens the plant. Signs of infection include honeydew and sooty mold on plants, curling leaves, stunted growth and yellowing of foliage. Remove infected plant matter and keep the garden weed-free. Do not use high-nitrogen fertilizer, as this promotes aphids. If you have a light infestation, you can spray the aphids off the plant with water, then apply insecticidal soaps or pesticides. Aphids will develop a resistance to pesticides, so use them sparingly and as a last resort. If you do not wish to use insecticides or pesticides, you can introduce natural enemies of the aphid, such as lacewings, parasitic wasps or pirate bugs.
Caterpillars: Caterpillars are heavy feeders and can be quite destructive. They are the immature forms of butterflies and moths. Control caterpillars by removing all weeds around the area and introducing parasitic wasps. Certain insecticidal soaps and oils will kill caterpillars.
Lacebugs: Lacebugs are rectangular, short bugs with wings that look like lace, hence their name. They suck the sap out of plants. Though they have wings, they do not fly. They leave spots on plants that look like bleach was splattered on them, as well as hard, black excrement. Most of the damage by lacebugs is done underneath the leaves of a plant. They can be washed away with soap and water and insecticides or, in a mild infestation, by simply removing infected leaves.
Scale Insects: The 1/8-inch-long scale insect attaches itself to the leaves and stems of plants. While the males have wings, the females do not, and spend their lives attached to the plant, sucking out juices with their piercing mouth parts. The scale insect excretes honeydew, which causes sooty mold that attracts ants. Cut any infected leaves off the plants. If the stems are also infected, you may have to dispose of the entire plant. You may be able to control them with insecticidal soaps or pesticides. You can also introduce parasitic wasps to your garden, as they are the scale insect's natural enemies. Lady beetles are also natural enemies.
Whiteflies: Whiteflies are less than 1/8 of an inch long, with white wings and yellow bodies. Their eggs hatch in 8 to 11 days. They have piercing mouthparts and suck the juices from plants, which causes stunted growth and yellow leaves. They can be controlled with pesticides, but in order for them to be effective, the underside of the leaves must be treated. Parasitic wasps and lady beetles can be used for outdoor control.
