Impatiens Diseases

Impatiens face diseases that are common in ornamental plants, especially those grown in poor conditions. Impatiens require shady spots in the garden with soil that retains moisture, yet also drains well. The delicate and fleshy stems of most impatiens varieties do not tolerate waterlogged soils or very cold weather in general. 

Both Botrytis and Rhizoctonia Crown Rot are active in the cooler months of spring and fall. The bacteria that causes Leaf Spot also likes wet and humid weather but can be active at most temperatures. Impatiens can also be infected by viruses transmitted by a primary pest such as an aphid or mite. 

Botrytis

By far the most common fungal disease in Impatiens is Botrytis, also known as Gray mold. Gray mold thrives in damp, cool conditions and most often occurs in the early spring or fall months. The fungus is an opportunistic pathogen, attacking soft tissue damaged by mechanical force. The soft stems and tender foliage of Impatiens are easily damaged, allowing the fungus to invade and grow. 

As the plant material dies, gray or black mold spores develop, giving the affected area a fuzzy look. Left untreated the fungus attacks healthy tissue and can spread quite quickly. Affected flower buds will turn brown and fail to open, eventually becoming covered in the fuzzy dark mold spores. 

botrytis-colonized-flowers-may-fall-onto-leaves-allowing-the-fungus-to-gain-entry-into-these-tissues.jpg

Photo by Jay W. Pscheidt, Oregon State University, 2002

Identifying Botrytis 

The infection starts in injured areas of the soft tissue. Impatiens have juicy stems and tender leaves that can become easily damaged by accident or from other pests feeding on them. The damaged area turns brownish and will have a somewhat watery look. Eventually, the gray mold spores will begin to grow and give the damaged area a fuzzy, smoky look.

Treating Botrytis

Good garden hygiene will prevent Botrytis from spreading to other parts of the affected plant and surrounding plants. Remove any damaged plant parts as quickly as possible. Spent flowers left on the soil or in a pot can also attract fungal spores. Fungal diseases are spread easily through the use of overhead watering. Avoiding wetting the plant as much as possible by using drip irrigation or soaker hoses in appropriate areas. Make sure that the soil drains well and that air can circulate well around all plants. Fungicidal sprays are rarely effective for control and only work as a preventative treatment.

Rhizoctonia Crown Rot

Fungal Crown Rot is a common disease in almost all ornamental and edible plants. Rhizoctonia is a damping-off disease that attacks the base of the plant where the roots attach to the top growth. Rhizoctonia causes decay of plant tissue either at the seedling stage when plants do not have strong root systems or later as a result of damage from another pathogen or pest. Damping-off fungi are present in most soils but are only active during periods of high humidity and overly wet soil.

Identifying Rhizoctonia Crown Rot

Rhizoctonia Crown Rot makes itself known in two ways. Either seedlings seem to fall over and rot at the soil line soon after germinating, or the top growth of larger seedlings becomes dull and matte in appearance with limp foliage and weakened stems. Seedlings that are growing in wet, cold soils are more likely to be attacked. Warmer growing conditions with more light and less watering will allow seedlings to become large enough and strong enough to resist the Rhizoctonia fungus.

plants-have-collapsed-due-to-rhizoctonia-sp.jpg

Photo by Stacy Fisher, 1990, Oregon State University

Treating Rhizoctonia Crown Rot

Garden and greenhouse hygiene is the best way to combat damping off diseases. Use fresh or sterilized soil for seed starting. Give the seedlings added heat from below with heat mats. This will speed up germination in tropical annuals like Impatiens so that the seeds do not rot in cold, wet soil. Avoid overhead watering of seedlings. Place the seed trays in a tub of water to irrigate from the bottom. Start seeds directly in the garden only when the soil temperature has warmed to at least 55 degrees Fahrenheit in the spring. Quickly remove and dispose of diseased seedlings or plants. 

Leaf Spot

Leaf Spot is caused by bacteria in the Pseudomonas, Xanthomonus, and Erwinia families. The bacteria attack the soft tissue of herbaceous plants, resulting in blights, soft spots, and overall wilting. The top growth of affected Impatiens can completely die back if left untreated.  These bacteria are found in almost all soils and only become active when weather conditions are right or plants are weakened by improper growing conditions. Wet, cool weather is ideal for bacterial Leaf Spots to grow and spread. 

Identifying Leaf Spot 

Small areas of infection on leaves start out looking like they are soaked in water or oil. The areas enlarge and turn dark brown to black. The spots grow larger, merging into one another until the whole leaf dies back and drops off the plant. 

Treating Leaf Spot

Remove infected leaves as quickly as possible to lessen the chance of Leaf Spot spreading. Clear out any fallen leaf litter during and at the end of the growing season. Avoid overhead watering and wetting the foliage. Allow enough space between Impatiens for good air circulation. Remove plant parts that are damaged by other pests to lessen the chance of bacteria entering the plant.

Viruses

Necrotic Spot Virus (NSV) and Tomato Spotted Wilt Virus (TSWV) are the two most common viruses to affect Impatiens. Both of these diseases are transmitted by a pest and are therefore considered secondary infections. Sap-sucking pests such as aphids, thrips, and mites are typical virus vectors. Weedy areas around or in gardens can harbor virus vectors as well as other infected plants. Often, viruses damage but do not kill the plant. The weakened plants can be attacked by other more dangerous pests or diseases.

Identifying Viruses

Affected leaves may have spots with concentric rings or appear to be brown and watery. Stippling of the leaves is also common along with yellowing of all but the veins. Affected flowers may be stunted, discolored, deformed, or fail to open at all. Overall growth may be stunted in infected Impatiens.

Treating Viruses

Young and annual plants are most severely impacted by viruses. In all cases, viruses cannot be cured. Remove affected plants so that further spreading of the disease is kept to a minimum. Inspect surrounding plants and weeds for any aphids or mites carrying the disease. Remove all weeds to keep these pests from overwintering. Remove any spent plant debris in the garden or in the container where the Impatiens were grown.  

Fasciation

Fasciation can be spread by a virus or bacteria, or result from a mutation of the plant's genes. Bacterial and viral Fasciation happens when plants are damaged, allowing the pathogen to enter soft tissue. The pathogen is usually found in water, soil, or on infected debris left in the garden over the winter. 

Identifying Fasciation

Fasciation can appear as flattened stems, two or more flower buds or stems fused together, dense tufts of leaves on single nodes, and shortened stems and flower stems. Mostly the disease only causes cosmetic damage. In very rare instances plants may die from Fasciation.

Treating Fasciation

Impatiens that exhibit signs of fasciation can be left in place for the rest of the growing season if no other signs of disease are present. Disposing of the affected plants is the best way to treat an outbreak. Clean all pruning tools thoroughly after each use to help stop the spread of any virus or bacterial agent. Avoid overhead watering or pruning back plants while wet from rain or watering. This will prevent any virus or bacteria in the area from entering the plant tissue. Do not use cuttings from distorted growth for propagation. 

Impatiens Disease Chart

Disease

Identifying

Treating

Rhizoctonia Crown Rot


Seedlings rotting at soil level, older plants turning a dull matte color and drooping

Remove affected plants, do not reuse infected soil, reduce watering and avoid overhead watering during cold temperatures

Leaf Spot

Brownish areas on leaves that eventually enlarge and turn black, early leaf drop

Remove leaf litter to keep the bacteria from spreading, improve air circulation, avoid overhead watering

Botrytis

Watery spots on previously damaged plant tissue, fuzzy gray or black mold growing on damaged plant parts

Remove damaged plant material immediately, remove spent flowers, avoid overhead watering

Viruses

Spots of yellowing on leaves, dying back of foliage

Clear vector host weeds from the growing area, take out infected plants, only buy plants that are certified virus-free

Fasciation

Distorted or fused stems, flattened areas of plant tissue, flower buds with unusual development patterns

Remove any large areas of distorted growth, sterilize pruning tools between uses, and avoid heavy pruning of wet plant material

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Author Robbin Small - Published 7-30-2023