University of Illinois Extension
Troubleshooting Cultural Disease and Insect Problems - Successful Container Gardens - University of Illinois Extension

Troubleshooting Cultural Disease and Insect Problems

Aster Yellows (Phytoplasma)

Symptoms & Life Cycle: Causes stunting, yellowish-green foliage and flowers followed by browning and death of the plant. Symptoms can vary from plant to plant and area to area due to different strains of aster yellow phytoplasmas. Phytoplasmas are bacterial-like but lack rigid walls. The organism overwinters in insects such as leaf hoppers. Disease passes into the plant during insect feeding.

Control: Destroy infected plants immediately to reduce the risk of additional insects picking up and transmitting the disease to other plants. Minimize insect wintering sites by mowing tall grass and reducing tall brushy areas.

Alternaria (Fungus)

Symptoms & Life Cycle: On calendulas, Alternaria causes small reddish to purplish spots. On other flowers the most common symptom is brown or dark brown spots on older leaves. Spots are circular to angular in shape. Spots can enlarge to ¼ to ½ inch in diameter and develop dark, concentric rings or ridges, causing a target-like appearance. Angular spots are caused by the veins stopping the expansion of the disease spots. Spotted leaves soon turn yellow, wither, and drop. Spots frequently covered with dark velvety conidia. Disease over-winters in plant residue left on or below soil surface as well as in the seeds. When weather is suitable in the spring, the fungus produces spores that are carried by wind and splashing rain. Repeat infections occur as long as the weather remains favorable (heavy dews, rainy conditions and temperatures between 66 and 73F.) for the disease.

Control: Rotate susceptible flowers with more resistant ones. Practice good sanitation. Since this fungus is a weak pathogen, keep plants healthy (proper fertility, watering, as well as limiting insect damage)

Anthracnose (Fungus)

Symptoms & Life Cycle: Anthracnose usually causes brown spots of various sizes depending on host plant and the anthracnose fungus causing the infection. Spots may remain small and limited in number or infect and kill the entire leaf. Anthracnose can also infect and kill plant stems. Symptoms will vary from plant to plant and will even vary on the same plant depending on pathogen, host, time of infection, weather and so on. The disease over-winters on dead plant material and spores can blow in the wind for many miles. Wet conditions promote the infection and spread of the disease.

Control: Grow plants in sun if possible. Provide plenty of good air circulation around individual plants. Fungicides and sanitation may help.

Gray Mold or Botrytis (Fungal)

Symptoms & Life Cycle: On tulips this disease is known as Tulip Fire. Gray mold appears as brown dead areas but with proper moisture, a gray fuzzy appearance occurs producing "spores." The pathogen can attack both flower and leaf buds, flowers and leaves, and stems and fruit. It can eventual kill many herbaceous plants. Several fungi cause Gray mold/botrytis. The organisms overwinter on dead and live plant material. It grows at temperatures between 32 and 84°F. It releases spores on rising as well as lowering humidity. However, it needs free standing water to infect. It is an air-borne disease and can blow for many miles.

Control: Sanitation and fungicides may help.

Powdery Mildew (Fungal)

Symptoms & Life Cycle: White powdery growth develops on upper and often lower leaf surfaces, young stems buds flowers, and even the fruit of some plants. Entire leaves and plant can be infected. Depending on the powdery mildew pathogen, the white growth may become mealy or felt-like and turn gray to tan. Leaves may stunt, curl, become chlorotic and drop early. Flowers may distort. Some powdery mildew pathogens are very host specific while others have a broader range of host plants to infect. Mildew fungi can overwinter on dead plant material and flower buds. Spores are released in damp spring weather and are blown to uninfected plant tissue. Disease needs three or more consecutive days and nights where the days are warm and dry and the nights are cool and humid but plant tissue is NOT wet.

Control: Place plants in the proper growing site with good air circulation. Avoid shady humid areas. Follow good sanitation practices. This does not stop infection but can delay infection and reduce initial infection severity. Avoid over fertilization and watering. Grow resistant varieties. Fungus can mutate and attack what were resistant varieties whenever possible. If infection occurs early in growing season, consider a fungicide but if infection occurs late in the growing season, no fungicide treatment is recommended.

Root Rots (Fungal Water Molds)

Symptoms & Life Cycle: Pythium and Phytophthora are both water mold fungi that attach plant roots as well as the base (collar) of the plant stems when soil is kept too wet. Plants roots and stems rot and die. Top growth stunts, wilts, turns brown and dies. Infected tissue is brown to chocolate brown to black in color. Frequently tissue is "mushy." These pathogens spread by the movement of contaminated soil, tools, infected plants or splashing rain and wind.

Control: Make sure soil drains well and do not over water. Do not let pots/containers sit in water filled saucers.

Sooty Mold (Fungal)

Symptoms & Life Cycle: Sooty mold fungi have a black mycelium and feed/grow on insect honeydew. The honeydew is excreted by certain types of scales as well as aphids. The insects may not be on the plants that are covered by the sooty mold. The insects could be on the trees the containers are under. The severity of infection varies with the individual sooty mold pathogen as well as whether or not multiple sooty mold fungi are growing together. Sooty mold fungi continue to grow as long as there is adequate honeydew being developed. Sooty mold kills the foliage by reducing light to the chlorophyll which eventually dies and in turn cause the leaf to die.

Control: Eliminate or reduce the insects producing the honeydew.

Verticillium and Fusarium Wilt

Symptoms & Life Cycle: These pathogens live in the soil and attack plants that are under stress such as over watering, drought, over fertilization, and so on. Plants may be infected for awhile before showing symptoms. Initially plants wilt. Wilting tissue soon begins to yellow then turn brown and die. Internal discoloration or streaking of the vascular tissue occurs in most plants. The wilt diseases enter through the roots. They can survive in the soil as saprophytic (live on dead plant tissue) for years. Infected dead plant roots improve their survival in the soil. These wilt fungi are often moved with infected soil or infected plants.

Control: Avoid stressing the plant roots. Avoid overwatering, drought, over-fertilizing, deep cultivation, or use of herbicides. Buy resistant plants when possible. Destroy infected plants, and use sterilized containers and soil.

Aphids (Insect)

Symptoms & Life Cycle: Aphids have piercing, sucking mouth parts. On tender shoots and leaves, the feeding can cause distortion and puckering of plant tissue. The insects suck sap from plants. As the sap moves through the insect, necessary nutrients are removed and the excrement is called honeydew. Aphids overwinter as eggs. The eggs hatch in the spring. All newly hatched aphids are females who can produce live young in just a few days. When the aphids become overcrowded, the adults give birth to winged aphids that fly to other plants or other plant areas where they give birth to wingless aphids. As temperatures fall and days shorten, the aphids are stimulated to give birth to live males and egg laying females. The males only mate with egg laying females.

Control: There are insect predators such as lady beetles, lacewings, syrphid flies and parasitic wasps. There are also fungi that attack aphids. Plain water from a hose can wash them off. There are also both organic and inorganic insecticides. Follow all label directions and precautions when using any pesticide.

Fungus Gnats (Insect)

Symptoms & Life Cycle: Fungus gnats emerge from soil as adults. They are small, thin and dark in color. They can run across soil, plant and pots and will fly readily. They fly around people when disturbed. They are mostly a nuisance insect. Eggs hatch in potting soil as well as garden soil especially after the pot/container is potted and watered frequently to establish the plants. The prolonged moisture allows the insect time to complete its life cycle and emerge as an adult.

Control: No insecticide is needed. Carefully dry out soil without allowing the plant to wilt. This allows eggs and larvae in the soil to dry out and die.

Meadow Spittlebug (Insect)

Symptoms & Life Cycle: The immature insect produces protective foam that looks like "spit" around itself. This foam helps protect the insect from ultraviolet light, predators and insecticides. Adults overwinter, and lay eggs in the spring. Spittle-covered green nymphs become obvious on various plants during the spring. Later in the summer, the brown leafhopper-like adults are common.

Control: Use a strong spray of water to wash the foam and the insects off the plant. This expose the insects to predators and many will die before they can find their way back to a plant. Many insecticides and insecticidal soaps are effective too.

Mealybugs (Insect)

Symptoms & Life Cycle: Mealybugs typically are coated with a white waxy substance that covers the insect. When plants are grown in a very porous soil as often used in pots and containers, there are often mealybugs feeding on the roots as well as the above-ground portions of the plants. Feeding damage can cause distortion of leaves and death of plants especially small herbaceous plants grown outdoors. These insects produce honeydew that is sticky. Feeding usually occurs on the underside of the leaf, where leaf and stem attach, or at the base of whorled leaves and on plant roots. When mealybugs congregate in masses, they cause the plant to look "cottony" in spots. Sometimes these "cottony masses" are egg clusters.

Control: Mealybugs are difficult to control especially if they are feeding on roots. The waxy coating protects them from insecticides. If only a few - try washing them off, or on a weekly basis, use a Q-tip to coat them with rubbing alcohol. If plants are heavily infected - consider destroying the plants, if replanting, use new potting soil, or use suggested insecticides according to all label directions and precautions.

Whitefly (Insect)

Symptoms & Life Cycle: Severe white fly infestations may cause mottling and yellowing of leaves as well as reductions in vegetable yields. Whiteflies are covered in a white waxy powder and adults are about the size of small "dandruff" particles. Whiteflies flutter around the plant when disturbed but soon return to the plant to continue feeding. Whiteflies found damaging cultivated plants in Illinois are not cold hardy this far north. They are shipped in as eggs on flowers and vegetable transplants grown in the South or they survive indoors on houseplants, herbs, and greenhouse crops. Eggs are very tiny and are on the upper leaf side. Adults as well as the tiny, clear oval nymphs tend to feed on the lower leaf surface.

Control: Use suggested insecticides according to all label directions and precautions. On vegetables make sure to follow pre-harvest interval. If there is no pre-harvest interval listed, do not use that insecticide. Find another that is labeled for controlling whitefly on vegetable.

Two-Spotted Spidermite (Insect-relative)

Symptoms & Life Cycle: The two-spotted spidermite is greenish to greenish-yellow with two dark spots on its back. It is also known as the red spidermite in Europe where it is red. But it is rarely red in the United States. Spidermites make single strands of silk, cause stippling (tiny yellow to whitish spots due to feeding of the foliage), and can cause the leaves to turn brown and die when infestations are severe. The webbing is frequently washed away by rains and blown away by the wind. The females overwinter in leaf litter or under the bark of several different types of trees and shrubs. Reproduction can occur throughout the growing season as long as the weather is warm enough. From egg to adult takes a short time, so in Illinois twenty or more generations per growing season is common.

Control: Beneficial; mites may feed on them. Predator mites usually crush red and two-spotted spidermite and other harmful spidermites tend to crush green. Look for the webbing or hold a white sheet of paper under the plant and rap the plant vigorously on the paper. Watch closely for tiny mites crawling while the debris will not move. Use a miticide, insecticidal soap or summer spray oil only if the moving mites are the "bad" mites. Follow all label directions and precautions.