There are several fungi that can affect your garden. While there are many, this page we focus on four of the main problem causers in this area of Texas.

Black Spot

What is it?:  Black spot is one of the most common and important diseases of roses throughout the world. It is caused by the fungus Diplocarpon rosae. Black spot will cause a general weakening of the plant so that progressively fewer and fewer blooms are formed if the disease is left unchecked. Plants so weakened are increasingly subject to winter injury. Infected leaves show black spots especially on the upper leaf surface. Yellowing of the leaf begins surrounding the spots and the entire leaf may yellow and eventually drop off. The fungus may also infect the canes where lesions appear purple at first and later black. As is true with most fungi, this fungus requires free water for infection to occur. The spores must be wet for at least 7 hours before they will germinate. A temperature of 65^oF is best for spore germination and the disease develops most rapidly at about 75^oF. Temperatures of 85^oF and above inhibit the spread of the disease.

What can you do? 

 1) Rake and discard all fallen leaves because they are the main source of spores in the spring.

 2) Also prune and discard any obviously infected canes.

 3) Avoid wetting the foliage especially during dark cloudy days.

 4) Grow plants in an open sunny location to promote rapid drying of the foliage.

 5) Do not plant in dense plantings and avoid windbreaks to allow good air circulation.

6) Roses vary in their resistance to this disease. Use resistant varieties for low maintenance plantings.

 7) Removing infected leaves as they arise early in the season may help to retard the rate of disease spread.

8) Use horticultural cornmeal or a bacterial fungicide to treat the disease.

Blossom end Rot

What is it?: A serious disorder of tomato, pepper and eggplant, blossom end rot is an environmental disease (not a microorganism) caused by insufficient calcium input when fruits are forming. It may also be a result of rapid growth from excessive fertilization, high salt levels, drought or uneven soil moisture. Blossom end rot may occur on both green and ripe fruits and is identified by a sunken, brown leathery spot on the blossom (bottom) end. In many cases, secondary pathogens, which appear as black, felt-like growth, invade the affected area and cause total destruction of the fruit. Blossom end rot does not spread from plant to plant.

What can you do?: Since the disease is physiological in nature, organic fungicides are useless as control measures. Choose resistant varieties whenever possible. Prevent problems by keeping soil evenly moist and by spraying plants with kelp or calcium as a foliar treatment. Adding bone meal, fish fertilizer and dolomitic lime (will change pH), all of which contain high levels of calcium, to the soil at planting time usually prevents the problem from occurring. A calcium spray may also be used. Cultivate around plants as little as possible in dry weather to avoid drying of the soil, and be careful not to damage roots during cultivation. Mulch plants to conserve moisture and to provide a more uniform water supply. Avoid large amounts of nitrogen, which hinders the uptake of calcium by plants.

Powdery Mildew

What is it?: Common on many plants and easily recognized, powdery mildew is a fungal disease found throughout the United States. It is caused by a variety of closely related fungal species, each with a limited host range. (The fungi attacking your roses are unlikely to spread to your lilacs). Low soil moisture combined with high humidity levels at the plant surface favors this disease. Powdery mildew starts on young leaves as raised blister-like areas that cause leaves to curl, exposing the lower leaf surface. Infected leaves become covered with a white to gray powdery growth, usually on the upper surface; unopened flower buds may be white with mildew and may never open. Leaves of severely infected plants turn brown and drop. Crepe Myrtles, zucchini, roses and zinnia are especially susceptible.

What can you do?: Plant resistant plant varieties in sunny locations whenever possible. Prune or stake plants to improve air circulation. Keep the fallen and diseased foliage picked off the plant and from the ground. Make sure to disinfect your pruning equipment (one part bleach to 4 parts water) after each cut. Use a thick layer of mulch to cover the soil after you have raked and cleaned it well. The mulch will prevent the fungus spores from splashing back up onto the leaves. Washing foliage with Plant Wash occasionally in mid-morning may disrupt the daily spore-releasing cycle. Water only in the morning so plants have a chance to dry during the day. Drip irrigation and soaker hoses will help keep the foliage dry. Apply sulfur or copper-based fungicides regularly to prevent infection of susceptible plants.

Common Rust

What is it?: There are more than 5,000 known species of rust on plants. Common rust (Phragmidium spp.) is a fungal disease that affects roses, snapdragons, bluegrass and woody perennials. It is most common on mature plants, and symptoms can be seen primarily on the surfaces of lower leaves. Early on, look for small, white, slightly raised spots on the undersides of leaves and sometimes on the stems. After a short period of time, these spots become covered with rust-colored spore masses. Later, leaf postules may turn yellow-green and eventually black. Severe infestations will deform and yellow leaves and cause leaf drop.



What can you do?: Pick off and destroy the infected leaves, and rake under the plants frequently to remove all fallen debris. Water in the early morning hours (avoiding overhead watering if possible) to give plants time to dry out during the day. Do not compost infected leaves or stems and thoroughly clean up garden areas in the fall to reduce over wintering sites for the fungal spores. Prune plants and remove weeds to improve air circulation.
Sulfur or copper powders/sprays should be applied regularly, starting when foliage begins to develop and continuing throughout the growing season. Spray early in the day, and avoid applications during hot weather. This will not kill the rust, but prevent the spores from germinating.