Geranium Diseases

Perennial Geraniums and annual Pelargonium species are susceptible to many of the same diseases which are most often fungal or bacterial in nature. Mildews, rusts of the leaves and stems, damping off of newly sprouted seedlings, and root rots are all found in garden soils and only become active when the temperature, humidity, and health of the plant are at the correct levels. Fasciation, Leaf spots, and viral diseases are often caused by a previous attack from insects or mites that act as vectors to transmit the disease from plant to plant.  

Pythium Root Rot

Pythium and related root rot diseases are capable of causing serious (and most often deadly) damage to virtually all flowering plants. The fungus is common in garden soil and can be transplanted to other locations through root divisions and transplants. Typically, Pythium root rots on soft-stemmed geraniums are most active during warm and humid weather or in greenhouses that lack good air circulation. The Fungal spores enter the plant through damage to stems and leaves or previous damage to the plant crown by other pests such as slugs, snails, or other small animals. 

Identifying Pythium Root Rot

Portions of the top growth will appear wilted like an under-watered plant. The leaves turn yellow, brown, or purplish falling off the plant. Stems of cuttings will turn brown or black rotting from the soil level upward and completely fail to strike roots. 

Treating Pythium Root Rot On Geraniums

Garden sanitation is key to protecting vulnerable cuttings and seedlings from infection. Clean any pruning tools between plants, or even cuts. Do not reuse old potting mix or garden soil for potting up cuttings and starting seeds. Good drainage from containers and in garden beds along with removing spent plant material at the end of the growing season reduces the amount of fungal spores overwintering in the soil. Good air circulation in greenhouses helps to reduce a wide range of diseases and is absolutely necessary. 

Botrytis

Botrytis, also called Grey Mold, requires a very specific range of atmospheric conditions to be active in geraniums. Greenhouse-grown annual Geraniums are more susceptible due to their sot stems and larger flower buds. Damage from improper pruning, accidental crushing of stems, or other mechanical injury allows the fungal spores to enter the plant and begin to develop.

Warm and humid temperatures are also required with the pathogen being active with as little as 6 hours of humidity over 90%. The spores develop quickly and are easily spread through contact between plants or overhead watering spreading the spores. 

Identifying Botrytis

Affected leaves or stems will have a watery appearance in areas that have been previously damaged. Stems, flowers, and foliage are all susceptible to infection. As the infection progresses a fuzzy white to grey mold forms, killing off the affected plant part.  

Treating Botrytis On Geraniums

Remove affected plant material or whole plants if needed. Air circulation is very important in an enclosed climate like a greenhouse. Avoid watering plants overhead. Drip irrigation or soaker hoses keep the foliage dry either in a greenhouse or in a garden border. Remove all spent flowers on the annual Geraniums regularly to prevent infection. 

Impatiens Necrotic Spot Virus

Impatiens Necrotic Spot Virus (INSV) is passed between plants by a host insect called a Western Flower Thrip. This insect pest is more common in greenhouses or other enclosed growing situations. The virus is transmitted when the Thrips feed on foliage and can be quickly spread through large numbers of plants.

Controlling Thrip infestations is the best way to control INSV. Annual Pelargonium species are affected the most although any plants raised in warm and humid conditions in a greenhouse are susceptible to infection. INSV may be more of an issue in climates where winter temperatures rarely fall below freezing, 32F (oC).

Identifying INSV

Leaves are stippled with yellow or brownish spots where the Thrips have fed and/or laid eggs. Patterning of the leaves with either yellow or white lines and splotches, loss of color in the veins of the leaves, unusual color patterning of the flowers (color breaking), or death of flowers and foliage are the most prominent symptoms.  

Treating INSV On Geraniums

Monitor and control outbreaks of Western Flower Thrips in greenhouse growing situations. Remove any affected plant parts to prevent the spread to other portions of the plant. Ensure any new plant material is not infected with the virus or Thrips. Disinfect pots and tools if an outbreak does occur. Grow GEraniums from seed for infection-free plants. Reduce host weeds around the garden and greenhouses to reduce the number of Thrips or other vector insects. 

Leaf Spot

Leaf Spot diseases on ornamental plants can be caused by fungal or bacterial pathogens or damage from insect pests. Geraniums most often are affected by a bacteria called Pseudomonas syringae which causes damage mainly to the foliage. Perennial Geraniums typically are able to survive outbreaks for many years with nothing more than a good cutting back through the growing season to refresh the foliage.

Annual Geraniums may be so affected that their growth becomes stunted with early leaf drop and reduction in flower production. Periods of high humidity and warm temperatures aid in the spread of the bacteria as does poor air circulation of container-grown plants in a greenhouse.

Identifying Leaf Spot

Affected foliage yellows and browns along the edges and may have sunken or raised spots. Early leaf drop is common on greenhouse-raised and container-grown plants. Lower leaves are more affected and will show more spotting than new foliage. 

Treating Leaf Spot On Geraniums

Growing disease-free and certified plants is the best way to avoid outbreaks of Leaf Spot. Avoid overhead watering or watering late in the day when foliage is less likely to dry thoroughly. Remove all affected plant material as soon as possible. Do not reuse pots without thoroughly disinfecting them first. Use a solution of 1 part bleach to 10 parts water to clean containers and pruning tools. Rotate plants in the landscape every few years to avoid a build-up of the Leaf Spot bacteria in the soil. 

Fasciation

Unusual growth of the stem or flowerheads in annual Geraniums is common and most often caused by a virus or bacterial infection. Stems growing together as a flat mass, multiple flowerheads fusing into one, or unusually shaped leaves are the most common signs of fasciations. Some plants are genetically predisposed to fasciation which does not affect healthy growth. 

Identifying Fasciation

Unusual formations on the stems, flowers, or leaves are common. Look for stems that grow together to form a flattened and sturdy branch, flowerheads that are larger or smaller in size or have what look like a doubled petal appearance. The rest of the plant is typically growing normally and thriving. 

Treating Fasciation On Geraniums

Cut out the affected plant parts with sterilized tools to avoid further contamination incase the fasciation is caused by a bacteria or virus. Do not take cuttings from affected plant stems. Purchase plants that are guaranteed disease-free or resistant.

Powdery Mildew

Powdery Mildew is most often seen in the garden during the early spring and the late summer. The weather conditions for powdery mildew are specifically long stretches of cool nighttime temperature, low humidity, and warm daytime temperatures.

Unlike most other fungi, Powdery Mildew does not require moisture to spread and easily moves between plants on windy days. Geraniums can be quickly infected with the powdery spores although their overall growth vigor is rarely reduced. All parts of the plant are affected with the leaves often first showing signs of infection. 

Identifying Powdery Mildew

White or greyish spots on the top or bottom of leaves. As the fungi ages the spots become large blotches that completely cover the leaf surface. Stems and flower buds eventually become covered in the powdery fungus. 

Treating Powdery Mildew On Geraniums

Remove affected foliage as soon as the white spots appear. Monitor plants during periods of optimal weather. Avoid over-fertilizing with high nitrogen feeds which promote dense foliage growth that is susceptible to pests and disease. Prune out excessive foliage if necessary to allow for good air circulation around the plant. Wet down plants early in the day to allow excess moisture to dry during the day. 

Rust

Rust is a very common fungal disease that is typically species-specific and rarely deadly. Periods of high humidity or overhead watering of plants help to spread the spores and can wash them into the soil where they can remain viable for many years. Wide-leaved zonal annual Pelargoniums are the most affected by rust, causing mainly cosmetic damage late in the summer. Ivy-leaved Pelargoniums and the hardy perennial Geraniums are rarely if ever affected making them much better choices to grow in gardens with a history of rust disease.

Identifying Rust

Small dots of orange or rusty red on the top sides of the leaves. The spots may grow into larger splotches across the whole leaf surface. Yellowing and browning of dead leaves which will drop off the plant. 

Treating Rust On Geraniums

Remove infected plant material being careful to not remove more than ⅓ of the foliage at one time. Remove the whole plant if more than ½ of the foliage and stems are affected. Buy varieties and cultivars that are rust-resistant. Avoid overhead watering when infection is present. This washes the spores down the plant allowing them to attack more parts of the plant and be washed into the surrounding soil.

Infrequent but deep watering is preferred as it reduces the humidity around the foliage and reduces the chances of overwatering, especially during humid weather. Dispose of infected plant material in the trash or community composting bins. Home composting rarely reaches high temperatures for the amount of time necessary to completely kill off rust pathogens. 

Southern Bacterial Wilt 

Southern Bacterial Wilt (Ralstonia solanacearum) is a disease common in garden soils although it primarily strikes geraniums grown in greenhouses or other artificial environments. Bacterial Wilt presents very similar to an early or late blight on food crops in the garden. Plants seem to all of a sudden fail even though they have been properly watered or fertilized.

One particular strain of Southern Bacterial Wilt is known to infect potatoes and other food crops and is on a quarantine watchlist for plants being exported from the United States. The strain that is likely to affect homegrown annual Geraniums is not as invasive and can be controlled in a greenhouse by using sterilized containers and fresh potting mixes from year to year. 

Identifying Southern Bacterial Wilt

Foliage that is wilted or drooping even though the plant is well watered and fertilized. Wedge-shaped discoloration on the leaf surface that is yellow, tan, or brown. Older leaves are affected first with new growth still looking healthy and vibrant. 

Treating Southern Bacterial Wilt On Geraniums

The most common mode of transmission of Southern Bacterial WIlt is by using infected cuttings for propagation or using a closed hydroponic growing system where the bacteria is moved between plants. Always use soil-free potting mixes that are clean and fresh. Resist the urge to reuse seed-starting potting mixes from year to year without sterilizing them first. Remove affected foliage and stems as soon after diagnosis as possible. Good hygiene practices in the greenhouse are invaluable for stopping most diseases.

Clean benches and tables with excess soil or spent plant material. Seasonal disinfecting of all surfaces in between growing seasons removes fungal spores and bacteria. Keep weeds away from the outside of the greenhouse to prevent possible disease vectors from overwintering. Only use certified disease-free material for propagating.  

Damping Off

Damping off can quickly kill off whole crops of emerging seedlings seemingly overnight. Cool temperatures and high humidity are the perfect setting for damping off to infect plants. Seed-grown plants and softwood cuttings are most likely to suffer from damping off disease in the early stages when the soil needs to be consistently moist. Low lighting also can be a factor making dedicated grow lights a must for starting seeds that are healthy and strong from the start. Reusing contaminated pots and potting mix is most often how damping off is spread. 

Damping off can happen at any time of the year as it is a disease that strikes plant starts and cuttings. 

Identifying Damping Off

A white mold may start to grow on the surface of the soil in a seedling flat or pot. The base of a seedling or cutting will start to turn brown or black at the soil level. The stems fall over from the base.

Treating Damping Off On Geraniums

Sterilize all potting and seed starting materials and tools between crops. Use fresh, peat-free seed starting mix for each seedling crop. Grow seedlings in a warm location with either a grow light overhead or in a spot with bright direct lighting for more than 12 hours a day. Heat mats may be needed to keep the soil temperatures around 65-70F consistently until the seedlings emerge. Use room temperature water when watering seedlings. Do not fertilize seedlings or cuttings until the true leaves appear.

Common Geranium Disease Chart

Sources:

“Geranium Pests in Gardens and Landscapes”, the University of California Statewide Integrated Pest Management Program. www.ipm.ucanr.edu