What types of diseases attack geraniums and how to treat and prevent them
Diseases of geraniums (genus pelargonium) are sometimes an unpleasant part of growing these beautiful balcony flowers. It's always good to know what kind of disease you're up against, so we've picked out the most common geranium plant disorders, along with pictures of affected geraniums:
Geranium leaf edema
Leaf edema is a physiological problem of geraniums that is not caused by microorganisms or pests. Leaf edema of geraniums occurs when soil and air humidity are high and when the air temperature is lower than the soil temperature. In these conditions, plants can develop this disorder due to increased water absorption paired with a low rate of evaporation of water from the leaves. The result is excessive enlargement, cracking and subsequent cell death in the geranium leaves. The dead cells turn yellow to brown in color and can be seen as protrusions on the leaf undersides. Leaf edema of geraniums can be effectively prevented by ensuring sufficient drainage of excess water from the pots, ensuring sufficient air circulation around the plant and reducing the frequency of watering.
Alternaria leaf spot
Leaf spot can be caused by a number of viruses, the most common being alternaria leaf spot (alternaria alternate). Viral leaf spot manifests as brown, yellow or whitish spots on the leaves. Once the geraniums are infected with this viral disease, they cannot be saved. All you can do is separate the infected plants from the healthy ones and dispose of them as quickly as possible to prevent the spread of infection.
Bacterial diseases are most often manifested by brown spots on the underside of the leaves or brown tips on the lower leaves. These diseases are caused by different species of bacteria (xanthomonas campestris, pseudomonas cichorii, ralstonia solanacearum, and others) that survive in undecomposed plant debris and attack geraniums through contaminated tools, water and touch. Infection of the plant may not be apparent in colder weather, but as warmer days arrive, infected plants will show the first symptoms. Prevention is particularly important for this type of geranium disease: buying only healthy plants, propagating from cuttings of healthy plants, and disinfecting working tools. Any infected plants should be removed immediately.
Gradual darkening and rotting of the root system of geraniums or the lower part of the stems of planted cuttings (blackleg) is usually caused by fungi of the genus Pythium. This disease can be prevented by using a sterile potting medium and taking cuttings only from healthy plants. Any infected plants should be burned or composted.
Rust of geraniums
Rust is manifested as pale spots on the upper side of the leaves and rusty spots on the underside. This geranium disease is caused by the fungus puccinia pelargonii-zonalis and mainly attacks zonal geraniums during rainy years.
Disease prevention in geraniums
All diseases of geraniums can be prevented by keeping the plants in good health. Healthy plants are not as susceptible to disease, so it’s critical to use a nutritious potting medium, water in moderation, and ensure sufficient drainage and good air circulation. It is important to buy only healthy plants and to propagate cuttings only from healthy geraniums to avoid spreading infection. Any affected plant parts should be burned or composted.