Coleus Diseases

The diseases that occur most often on Coleus plants are a result of improper growing conditions or mechanical damage from poor pruning or pest damage. Downy Mildew is the most common disease that strikes Coleus plants, especially when they have been planted a bit too early when nighttime temperatures are well below 50F (10C) and the relative humidity is high.

Botrytis and Pythium Root Rot are two infections that need to have just the right temperature and humidity level to be active. Virus diseases can be genetically present or spread by host pests such as other garden weeds or insect carriers like the impatiens necrotic spot virus that is spread between Coleus plants by greenhouse and garden thrips feeding on their fleshy leaves.

Downy Mildew

Downy Mildew in the garden is not easy to miss. Early on, the disease is spurred by low night-time temperatures and high humidity. As the mildew spreads over more of the leaf it restricts the plant's ability to photosynthesize and eventually reduces the overall vigor. There is a wide range of plants that are susceptible to downy mildew and it will quickly spread in areas where the air circulation is poor between plants.

Once temperatures heat up and the humidity drops the spores are no longer able to move between plants on their own. Watering by soaker hoses and drip irrigation is preferable for keeping the plant's leaves dry and reducing the chance of spreading spores through overhead watering.

Identifying Downy Mildew

Early spring and late summer are prime periods to be on the lookout for the telltale white spots that eventually grow until whole leaves are covered. The spots may have a brownish or grey look at first, becoming white eventually. Leaves will quickly turn brown and drop early.

Treating Downy Mildew On Coleus

Remove all affected foliage and soft stems of coleus. Cutting well below damaged areas, just above a set of leaves will encourage your Coleus to re-grow bushy and strong. Fungicidal sprays are only useful for protecting undamaged foliage on larger plants. The downy mildew spores can spread quickly on a Coleus plant making the sprays ineffective. Remove heavily infected plants and dispose of them in the trash, not your home compost bin.

Plant or move Coleus plants outdoors only when nighttime temperatures are reliably above 50F. This is typically when the tomato and pepper plants can be planted. Avoid overwatering Coleus that is planted in fully shaded locations and make sure to plant modern sun-loving cultivars in no more than partial shade. 

Impatien Necrotic Spot Virus (INSV)

Even though this virus is called Impatien Necrotic Spot Virus it is commonly found on many different annuals. Both edible and ornamental. Most often INSV is found in commercial greenhouses where populations of Thrips can easily spread the virus in a short period.

Thrips only take 10 days to complete a life cycle and can absolutely explode in an undermanaged greenhouse situation. Some thrips are too small to see with the naked eye, but the stippling of flowers and leaves is a dead giveaway that they are enjoying your plants.   

Identifying INSV

The symptoms can be widely different between species of plants. Typically Coleus will show signs of foliage browning, drooping, and drying out. Spots that look like bullseyes and black spots are also common symptoms of Coleus. When left untreated the whole plant can collapse and die. 

Treating INSV On Coleus

Remove all affected foliage if the disease is caught early. Plants that have too much damage will need to be pulled out and disposed of. Do not dispose of the plant material in your home compost. Inspect any new plants well for symptoms of disease before planting. The thrips that carry INSV thrive in greenhouse settings where plants are grown close.

Weed around garden beds or near greenhouses to remove the hosts for thrips and other pests. The thrips that carry this virus easily overwinter in plant debris so be sure to practice good garden hygiene by removing all spent top growth of plants that have been affected during the season. 

Pythium Root Rot

Pythium Root Rot can lay dormant in garden or container soils for many seasons without showing any signs of infection. As soon as the conditions are just right, cool and very wet weather that keeps soils wet longer than normal, the disease will take advantage of any susceptible plants nearby.

Healthy plants typically show very little damage and can resume good growth after the soils dry and the temperatures warm. Plants that have been weakened from other poor planting practices or are not planted in the right location are more likely to be attacked and suffer heavy root and stem loss. 

Identifying Pythium Root Rot

Pythium Root Rot causes loss of the root ends and newest root shoots. The plants begin to have issues taking up moisture and nutrients and will eventually decline in growth. The whole plant will droop and begin to wilt even though it is well-watered or fertilized. The roots will look black, brown, or non-existent as they have already rotted away. The base of the main trunk or stem may also be turning black or brown near and below the soil line. 

Treating Pythium Root Rot On Coleus

Soil drainage is absolutely crucial for keeping plants from contracting root rot diseases. Reduce watering frequency during periods of cool weather and resist the urge to over-fertilize early or late in the growing season. Good garden hygiene and rotation of crops is a great way to reduce the instances of many diseases, especially soil-borne types like Pythium.

Remove all spent top growth and root balls at the end of the growing season if the disease is starting to show signs. Rotate where you grow Coleus in a garden bed to prevent the build-up of any disease. Use fresh potting mix each year to pot up annuals and perennials.2

Botrytis

Also called grey mold, Botrytis is a fungus that spreads by spores and is most common in warm and damp conditions. Infections typically begin as the result of a damage somewhere on the plant from dull pruning shears or broken branches with rough edges or dead plant material that is the result of improper deadheading. Spores are spread quickly from overhead watering and can travel between plants that grow close together such as in a greenhouse growing situation.

Identifying Botrytis

Damaged parts of the plant will begin to turn brown or black as it dies. THe spores multiple quickly in the wound and eventually form a grey fuzzy covering. Leaf spots also form on infected foliage and will grow to cover the whole leaf surface. 

Treating Botrytis On Coleus

Avoiding overhead watering and giving plants plenty of space to grow helps to keep infections from Botrytis to a minimum. Avoid and remove damaged plant material quickly. Sanitize pruning tools between cuts on plants and between plants. Air circulation is paramount in a greenhouse growing situation. SPace plants well and use a gentle fan to move air around if necessary. Good garden hygiene at the end of the season will keep spores from overwintering amongst any spent plant material.

Common Coleus Diseases Chart

Sources:

“Coleus Disease and Disorder Guide”, E-Gro Alert. www e-gro.org