The vibrant, colorful blooms of the chrysanthemum, often called “mums,” are a highlight of the late-season garden, but their appeal extends to a host of unwanted visitors. These popular ornamental plants are susceptible to various pests and pathogens that can quickly diminish their display. Understanding the specific type of damage present is the fastest and most reliable way to identify the culprit. Treating a plant without first accurately diagnosing the problem often wastes time and resources while allowing the issue to worsen. This systematic guide helps distinguish between chewing insects, sap-sucking pests, and common plant diseases affecting your mums.
Start Here: Categorizing the Damage
The first step in diagnosing any plant issue is a thorough inspection, focusing on categorizing the primary symptom rather than trying to spot a specific pest. Look closely at the leaves, stems, and flowers to determine the general type of injury. Damage usually falls into three distinct categories that point toward different groups of organisms.
The most literal form of “eating” is characterized by missing plant tissue, such as large, irregular holes or severed stems. The second category involves discoloration, stunting, or distortion, typically caused by smaller organisms that pierce the tissue and extract fluids. Look for symptoms like speckling, yellow mottling, or leaves that are curled. The final category includes spots, dusty coatings, or sudden wilting, which are visual signs of a disease caused by fungi, bacteria, or viruses.
Culprits That Chew Leaves and Flowers
Pests that physically consume plant material leave behind clear evidence of tissue loss.
Slugs and Snails
Slugs and snails are notorious for their voracious appetites, leaving large, ragged holes in the leaves and sometimes consuming entire seedlings. The tell-tale sign of their presence is a silvery, slimy trail left on the foliage or surrounding soil. Hand-picking these mollusks at night or applying iron phosphate baits are effective control measures.
Caterpillars
Caterpillars, the larvae of moths and butterflies, are significant chewers that can cause rapid defoliation. They often leave behind small, dark pellets of excrement, known as frass, which accumulate below their feeding area. Some species tie leaves together with silk before feeding. Removing affected plant parts can manage small infestations, while biological controls containing Bacillus thuringiensis (Bt) target the larvae specifically.
Earwigs
Earwigs feed on the soft tissues of flowers and leaves, creating jagged, irregular holes, often near the base of the plant. These insects are nocturnal and tend to hide in dark, damp places during the day. To manage earwigs, you can trap them using rolled-up damp newspaper or a small can filled with oil and soy sauce.
Pests That Suck Sap and Cause Distortion
Pests that pierce plant cells and extract sap cause damage that results in significant plant stress and distortion.
Aphids
Aphids are small, soft-bodied insects that cluster on new growth and the undersides of leaves. Their feeding activity causes leaves to curl, yellow, or become stunted. They excrete honeydew, a sticky substance that supports the growth of black sooty mold fungus, further marring the plant’s appearance.
Mites and Thrips
Two-spotted spider mites are tiny arachnids that thrive in hot, dry conditions. They feed on chlorophyll-containing cells, leading to fine stippling or bronzing on the upper leaf surface. Severe infestations are identified by the fine, silken webbing they spin across the foliage. Thrips are minute insects that feed by rasping surface cells, resulting in silvery streaks or mottled patches. Their feeding on developing flower buds can cause blooms to become discolored or deformed.
Control Methods
Controlling these pests often involves using targeted, contact-based methods to reach them where they hide.
- Aphids can be dislodged with a strong jet of water.
- Insecticidal soap or horticultural oil sprays are necessary for entrenched infestations, especially to coat the undersides of leaves.
- Horticultural oils are effective against mites because they smother the pests.
- Natural predators like ladybugs and lacewings can provide long-term biological control.
Identifying Common Chrysanthemum Diseases
When damage symptoms do not involve tissue loss or sap extraction, the problem is likely a plant disease caused by a pathogen.
Fungal Infections
Powdery mildew is a common fungal infection that appears as a white or grayish, dusty coating on the upper surfaces of leaves, stems, and buds. This disease reduces photosynthesis and is exacerbated by poor air circulation and high humidity. Cultural practices like proper plant spacing are important for prevention.
Chrysanthemum rust manifests as small, raised pustules that are orange or reddish-brown on the undersides of the leaves. These pustules release powdery spores and cause corresponding yellow spots on the upper leaf surface, leading to premature leaf drop. Leaf spot diseases, caused by various fungi such as Septoria, result in yellowish spots that gradually turn dark brown or black, typically starting on the lower leaves.
Management and Viral Issues
Managing these diseases begins with sanitation, such as removing and destroying infected leaves and fallen plant debris to reduce the source of fungal spores. Watering plants at the base early in the day, rather than overhead, helps keep foliage dry and limits spore germination. If cultural controls are insufficient, fungicides containing active ingredients like chlorothalonil or myclobutanil can be applied to protect healthy new growth.
Viral diseases, often spread by sucking insects, cause symptoms like stunted growth, yellow mottling, or ring spots on the leaves. Infected plants must be removed and destroyed immediately as there is no chemical cure.