Overwatering frequently contributes to stress in drought-tolerant plants

This area of Georgia is known for hot, dry summers. Even with these conditions, there are flowers that can grow and thrive.

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Common examples include lantana, verbena, portulaca and annual vinca.

These plants are mostly pest-free. but can have insect and disease problems if they are under stress. A common stress is overwatering. I have seen numerous samples of annual vinca with disease problems this summer due to overwatering.

Annual vinca (Catharanthus roseus ) is commonly used for summer color in landscapes. It is also called Madagascar periwinkle or just vinca. This can be confusing because there are other plants in the genus vinca that are perennial vines, which are also called vinca or periwinkle. These perform best in shady areas. It thrives in sunny areas, well-drained soils, and is fairly drought-tolerant.

There are a number of diseases that will affect annual vinca. The most common are Phytophthora stem blight and root rot. Aerial stem blight and root rot are caused by Phytophthora nicotianae and occasionally other species.

Dark brown to black lesions will form on stems and branches, causing the portions above to wilt and die back.

Symptoms include yellowing and scorching of leaves, poor growth and stunting of plants, wilting and death. Plants with root rot have reduced root systems and individual roots tend to slough off the outer tissue, leaving the inner core behind.

The second most common disease on annual vinca is pythium root rot. The pathogen that causes this disease is closely related to Phytophthora species, so root rot symptoms are similar. This pathogen doesn't cause branch blight, only root rot and damping off of seedlings.

The next disease attacks the leaves of the plant. These fungi cause spots on the leaves. The fungi Alternaria alternata and Ulocladium sp cause the spots on foliage, stems and petioles. The symptoms first appear on lower leaves and stems. If left unchecked, it moves up. Spots caused by both fungi are small, ranging from the size of a pinhead to one-eighth-inch in diameter. As they enlarge, light and dark bands might alternate within the lesion, giving it a target-spot appearance. As the spots increase in number, leaves turn yellow and drop from the plants.

Rhizoctonia stem and root rot sometimes can cause stem rots of vinca plants and seedlings. Also, this fungus can cause root rots, but is less commonly encountered than the stem rot. Plants affected by stem rot turn yellow, wilt and collapse. Death by root rot is generally slower and more subtle. Affected plants are stunted, their roots have brown lesions, and leaves turn yellow and plants wilt even when soil moisture is sufficient.

Gray mold (Botrytis blight) is another common disease in vinca beds. This disease, caused by Botrytis cinerea , is seen during cool, moist spring weather, especially in heavily fertilized landscape plants. The pathogen usually builds up on dead plant parts and disease develops when these come into contact with living tissue. Symptoms progress rapidly and can include leaf spots and blights, stem cankers, stem rots and damping-off of seedlings.

To control these diseases, certain management practices are needed. The first is to watch how much water is being applied to these plants. Vinca doesn't need to be planted in the same bed with plants that have high water requirements. Frequent watering, even in moderate to dry sites, can make conditions favorable for these diseases. Annual vinca and vinca species are fairly drought tolerant, so water only as needed. When rainfall is lacking, deep irrigation is needed once, or possibly twice per week, depending on soil type, exposure and weather conditions.

Also, avoid excessive fertilizer. To help prevent root rot, it is also important to provide excellent drainage.

When preparing a plant bed, thoroughly dig up the whole area. Adding organic materials, such as composted pine bark, to the soil will help increase drainage due to improved soil structure.

To help reduce leaf spot diseases and gray mold, it is best to remove old plants from the bed area and turn the material on top of the soil 6 to 8 inches deep.

Charles Phillips is a retired Columbia County Extension Service agent and operates Hort Consulting. He can be reached at cphillipshort@comcast.net.