Verbena Diseases

Verbena is one of the more reliable annuals for blooming during the hottest part of the summer. The saturated colors of the flowers help to extend color well into the fall until the first frosts occur. Only a few diseases affect Verbena and are easy to control. Powdery mildew, Botrytis, and Pythium Root Rot are all caused by fungal spores that lay dormant until the right atmospheric conditions occur.

Good garden hygiene at the end of a growing season is crucial for controlling diseases of the soil as the spores can survive over winter easily no matter if you have snowy cold winters or a relatively mild and warm climate year-round. 

Powdery Mildew

Cool and humid temperatures are ripe for the spreading of Powdery Mildew between plants in the garden. Verbena is particularly susceptible when grown in the close confines of a mixed seasonal container. Powdery mildew starts out as a few specks of white scattered around a leaf or two. When plants are watered overhead, the spores are easily transferred to other plants of the verbena plant or its neighbors.

Sterilize pruning equipment after cutting out infected foliage to prevent spreading the fungus between plants. Often this disease is an issue in the spring or late summer when the evenings are much cooler than the daytime temperatures.

Identifying Powdery Mildew

Small white dots on the tops of leaves. After a few days, whole leaves may be covered in powdery white mold sores that are easily transferred to surrounding plants and the garden soil. Most plants that are growing healthy are not adversely affected by Powdery Mildew and will make a full recovery. 

Treating Powdery Mildew On Verbena

Remove all visibly affected foliage and stems. Improve air circulation by moving containers of infected plants away from each other or removing some plants to make more space in a container. Overwatering is an issue, especially during periods of cool and humid weather. Let the containers dry between waterings and make sure drainage is excellent. Keep the foliage as dry as possible and water early in the day to allow complete drying of foliage.

Do not plant susceptible plants like cucumbers, squash, and other cucurbits near Verbena to reduce the chance of transferring spores. Use fresh potting mix every year in containers as the spores can live in soil until the right combination of atmospheric conditions comes together.  

Botrytis

Grey mold is the common name given to the Botrytis fungus. This disease occurs mainly in warm and humid conditions and can quickly infect damaged parts of plants, especially ones with fleshy stems such as the annual types of Verbena. Young plants and seedlings are highly susceptible to long-term damage by Botrytis as are plants growing in artificial conditions like a greenhouse, heated or unheated.

Left untreated the whole plant will succumb to the fungus and die. Diseased plant material not cleaned up can overwinter the spores as freezing temperatures only slow their growth. When temperatures rise in the spring the fungus will reappear.

Identifying Botrytis

Damaged parts of the plant may begin to show spots of brown or black. Quickly the spores grow to cover the initial infection site with a fuzzy grey mold that is easily spread by air or overhead watering.  Moving containerized plants displaying grey mold can spread the microscopic spores between plants and to different parts of a garden. 

Treating Botrytis On Verbena

Smaller plants may need to be completely removed and disposed of if the mold has covered large parts. More mature plants may recover fine when the affected portions are pruned away. Increase air circulation between plants if possible. Do not bring damaged plants into the garden if they display any signs of Botrytis disease. Clean up dead and infected debris from your garden beds to keep the spores from overwintering.

Watering with drip irrigation and soaker hoses is best for keeping foliage as dry as possible. Be careful to not damage plants when potting them up or transplanting them for the season. Remove any damaged plant material with clean cuts from sharp pruning tools. Clean pruning tools between cutting jobs or even individual cuts if working on a diseased plant.  

Pythium Root Rot

The water molds that cause Pythium Root Rot naturally occur in most garden soils. When conditions are just right they are able to affect poorly growing or young plants easily. Often the damage is not visible until the whole plant is drooping and close to death. The roots can be decayed so much that there isn't a visible root ball underground. The base of the stem and the growth cron at or just below soil level will also show signs of infection. 

Identifying Pythium Root Rot

The roots and/or the base of the main stem will turn brown or black. The infected roots are not able to reproduce at the normal rate making the root ball decline in size. The diminished root ball is not able to support vigor top growth and the stems and leaves begin to yellow, turn brown, and eventually die. 2-4 sentences on how to spot and tell whether the disease is impacting the plant. Mature plants that are re-planted into fresh compost and better-draining situations usually recover their root growth.

Treating Pythium Root Rot On Verbena

Improve drainage in garden beds. Use containers that have plenty of bottom drainage. Do not reuse potting mixes from year to year. The water mold for Botrytis can survive in dry conditions for years. Clean nursery pots well before using them for planting. Reduce watering frequency during humid periods to allow for soils to dry. Remove all infected plant material as quickly as possible and dispose of it in the trash, not your home compost pile.  

Common Verbena Disease Chart

Sources:

“Verbena Diseases”, Penn State University Extension. www.extension.psu.edu