Crataegus (Hawthorn): Pests and Diseases to Watch Out For

Aphids

Description: Aphids are small, pear-shaped insects, often green but sometimes black, yellow, or pink. They have long antennae and two tube-like structures called cornicles protruding from their hind end. They’re notorious for sucking plant sap, causing various issues. 

Symptoms: Infested plants exhibit yellowed, curled, or distorted leaves. Aphids leave behind a sticky substance called honeydew, which can lead to sooty mold growth. Stunted growth and reduced yields are also common. In some cases, they transmit plant viruses, causing further damage and irregular patterns on leaves.

Treatment Indoors: Treat aphids on indoor plants by wiping them off with a damp cloth or spraying with a mixture of water and a few drops of mild dish soap. For persistent infestations, apply neem oil or insecticidal soap directly to the affected areas. In most cases, the treatment will have to be repeated multiple times. Imidacloprid granules put onto the soil will also control aphids.

Treatment Outdoors: Blast aphids off outdoor plants with a strong jet of water from a hose. Encourage natural predators like ladybugs, lacewings, damsel bugs, or hoverflies. If necessary, spray affected plants with neem oil or a ready-to-use insecticidal soap, ensuring complete coverage, especially under leaves where aphids congregate. 

Borers

Description: Borers are insects that spend a portion or all of their larval stage inside the stems, trunks, branches, or fruits of plants, where they feed and grow. Borers often prefer stressed, weakened, or older trees.

Symptoms: Infestations are evidenced by entry and exit holes in the trunk or branches, frass (sawdust-like material), wilting foliage, branch dieback, canopy thinning, and overall decline in plant health. Severe infestations can lead to structural weakness and potentially the death of the plant.

Treatments: Maintain tree health to prevent infestations. Prune and destroy infested branches, use physical barriers, pheromone traps, beneficial insects, nematodes or fungi that target borer larvae, or insecticides for severe cases. Consult an arborist for advanced infestations or preventive care.

Fall Webworm

Description: Fall webworms (Hyphantria cunea) are hairy, white caterpillars that grow up to an inch long and transform into white moths. Despite their appearance, fall webworms are generally not harmful to the overall health of the tree or shrub.

Symptoms: Trees infested with fall webworms will have conspicuous, large, silken webs at the branch tips, usually enveloping leaves. The caterpillars feed inside these protective tents, causing leaf damage.

Treatments: Physical removal, affected branches pruning, beneficial predators (birds), Bt (Bacillus thuringiensis)- A biological insecticide that targets caterpillars.

Japanese Beetle

Description: Japanese beetles (Popillia japonica) are metallic green pests with bronze wings, approximately 0.5 inches long. They are active during the day and commonly found on plants and trees in summer.

Symptoms: Affected plants display skeletonized leaves, as beetles consume the tissue between veins. They also feed on flowers and fruits, causing significant aesthetic and functional damage to a wide variety of plants.

Treatments: Hand-picking, beneficial nematodes, floating row covers, knockdown sprays, natural predators, insecticides.

Lace Bugs

Description: Lace bugs are tiny, winged insects with ornate, lace-like wings. Typically under 1/8-inch long, these pests are often found on the underside of leaves, sucking the sap out of plants.

Symptoms: Lace bugs can cause a range of damage to plants, including stippling, yellowing, and wilting of leaves, as well as stunted growth and reduced vigor. The damage is typically most severe on the undersides of leaves, where the insects feed and lay their eggs.

Treatments: For minor infestations, remove and discard damaged leaves. Horticultural oil or insecticidal soaps can deter lace bugs. Encourage natural predators like ladybugs, which feed on them. In severe cases, consider systemic insecticides. Regularly check and treat the undersides of leaves.

Leaf Miner

Description: Leafminers are the larvae of certain insects, like moths, flies, or beetles. They live inside and feed on the leaf tissue, creating distinctive tunnels or mines that can affect the tree’s ability to photosynthesize and overall health.

Symptoms: Visible signs include serpentine-like tunnels or blotches on leaves, discoloration, and, in heavy infestations, premature leaf drop. The damage is mostly aesthetic but can weaken the plant if it’s extensive.

Treatments: Treatment involves pruning and disposing of affected leaves, row covers, or netting, applying insecticidal soap and neem oil, or using systemic insecticides if the infestation is severe. Encouraging natural predators, such as spiders, lacewings, ladybugs, soldier beetles, and parasitic wasps, can also help manage leafminer populations.

Pear Sawfly

Description:  The pear sawfly, or Caliroa cerasi, is an insect closely resembling a wasp in its adult form. It lays eggs on the undersides of leaves, leading to slug-like larvae, greenish-black and shiny, known for their voracious appetite for the soft tissue of leaves.

Symptoms:  Infested trees exhibit leaves with a skeletonized pattern as the larvae consume the leaf’s surface, leaving only the veins untouched. This damage first appears as small, irregular transparent patches on leaves, which can escalate to significant defoliation, impacting the tree’s health and fruit production.

Treatments:  Physically removing and destroying the larvae can be effective for mild infestations. Water spray can knock pear sawfly caterpillars off of their host plant. Insecticidal soap or horticultural oils can disrupt the larvae’s feeding. More severe infestations might require chemical controls. 

Scale Insects

Description: Scale insects are small, sap-sucking pests, often resembling bumps on plant stems, leaves, or fruit. They’re known for their protective outer coating, which shields them from predators and environmental factors, making them a resilient adversary for many plant species.

Symptoms: Infested plants show signs like yellowing leaves, stunted growth, and sooty mold development, resulting from the sticky “honeydew” excreted by the insects. Severe infestations may lead to leaf drop, branch dieback, or even plant death if not addressed.

Treatment Indoors: Treat scale insects on indoor plants by lightly scraping these pests with a fingernail. Scales are susceptible to many insecticides, such as insecticidal soap, neem oil, canola oil, or pyrethrins. Apply to infested areas, ensuring thorough leaf coverage. Repeat treatments as needed, and consider isolating affected plants to prevent spread. 

Treatment Outdoors: For outdoor plants, manually remove scale insects using a soft brush or cloth soaked in soapy water. Spray affected plants with neem oil or horticultural oil, targeting the undersides of leaves and stems where scales reside. Encourage beneficial insects like ladybugs, lacewings, soldier beetles, and parasitic wasps that prey on scale insects.

Spider Mites

Description: Spider mites are minuscule spider-like pests notorious for damaging plants by feeding on the sap within the leaves and stems. They thrive in hot, dry conditions and can multiply rapidly, often creating fine webs on infested plants, similar to spiders.

Symptoms: Affected plants exhibit yellow, stippled leaves from the mites’ feeding, progressing to bronze or reddish discoloration with heavy infestation. Webbing is a definitive sign, and severe cases can lead to leaf loss, stunted growth, and plant death, especially when left unchecked.

Treatment Indoors: Increase humidity to deter spider mites in indoor plants. For sturdy plants, forcefully spray with water, especially under leaves, to remove mites and webs. Apply insecticidal soap or neem oil for stubborn cases, ensuring full coverage. Repeat weekly until the infestation is under control.

Treatment Outdoors: Forcefully spray outdoor plants with water to dislodge spider mites and disrupt their webs. Treat with insecticidal soap, neem oil, or a sulfur-based insecticide covering all leaf surfaces for persistent issues. Weekly treatments may be necessary for several weeks to manage mite populations effectively. Introducing or encouraging natural predators like ladybugs and lacewings can also help control mite populations.

Tent Caterpillar

Description: Tent caterpillars are hairy larvae of certain moth species. They’re recognizable by their distinct markings and the silk webbing or “tents” they construct in tree branches, which they use as shelter during the early spring months.

Symptoms: Trees infested with tent caterpillars display extensive webbing in branches. The caterpillars voraciously consume leaves, leading to defoliation, weakened trees, and a diminished aesthetic appeal. The tents are most noticeable in the spring.

Treatments: Early detection allows for the manual removal of tents. Introduce natural predators like birds or beneficial insects. Apply Bacillus thuringiensis (Bt), an organic pesticide that targets caterpillars without harming beneficial insects. Chemical Pesticides should be used judiciously, targeting the early larval stage for best results.

Fire Blight

Description: Fire blight is a destructive bacterial disease caused by Erwinia amylovora, affecting members of the Rosaceae family like rose, apple, serviceberry, or hawthorn. The pathogen thrives in warm, humid conditions, rapidly spreading through splashing rain, insects, and pruning tools.

Symptoms: They include wilting and blackening of shoots, resembling fire damage, hence the name. Flowers and fruits may also appear water-soaked before turning black. Infected branches exhibit a “shepherd’s crook” shape, and cankers ooze a sticky, amber liquid during wet conditions.

Treatments: Prune infected branches 12 inches (30 cm) below visible damage, and sterilize tools between cuts. Avoid excessive nitrogen fertilizer, which encourages susceptible new growth. Apply bactericides (like copper-based products) during high-risk periods, though they’re preventive, not curative. Resistant varieties are fundamental in areas prone to fire blight

Cedar-Hawthorn Rust

Description: Cedar-hawthorn rust is a fungal disease caused by Gymnosporangium globosum, involving a complex life cycle between juniper and some members of the Rosaceae family (hawthorn, apple, crabapple, pear, quince, and serviceberry). It’s characterized by distinctive galls on junipers and orange, spotty lesions on the leaves and fruit of the Rosaceae host.

Symptoms: Affected Rosaceae hosts exhibit bright orange-yellow leaf spots, often with a tiny black dot in the center, which can merge and lead to defoliation. Junipers develop bulbous galls that, in moist conditions, exude orange, jelly-like spore masses.

Treatments: Treatment involves cultural practices like removing infected parts or nearby alternate host plants, applying fungicides in the early stages of infection or as a preventive measure during the wet season, avoiding planting susceptible hosts within a two-mile radius of junipers and choosing rust-resistant plant varieties for new plantings.

Cedar-Quince Rust

Description: Cedar-quince rust, caused by Gymnosporangium clavipes, is a fungal disease affecting junipers and Rosaceae family plants, including hawthorn, apple, crabapple, quince, and others. It’s characterized by its unique life cycle, requiring two host species to proliferate.

Symptoms: On fruit trees, quince rust manifests as swollen, distorted fruits and cankers on twigs, while junipers exhibit galls producing orange spore masses.

Treatments: Treatment involves pruning infected parts, applying fungicides during initial spore release, and planting resistant varieties. Maintaining distance between host plants also helps control spread.

Apple Scab

Description: Apple scab, caused by the fungus Venturia inaequalis, is a prevalent disease affecting apple trees, including some related species like hawthorns. The fungus thrives in cool, wet conditions, initiating its life cycle on fallen leaves and fruit from the previous season.

Symptoms: Symptoms start as pale yellow or olive-green spots on the upper surface of leaves, eventually turning black and velvety. Infected leaves may curl and fall prematurely. On fruits, it manifests as dark, corky spots, deforming them and sometimes causing premature dropping.

Treatments: Management strategies include planting resistant varieties, thorough fall cleanup of debris, and adequate plant spacing for air circulation. Fungicidal sprays may be necessary, starting at bud-break and continuing until conditions no longer favor the disease. Always follow label instructions for any chemical controls to ensure effectiveness and safety.

Leaf Spot Diseases

Description: Leaf spot diseases are caused by various fungi or bacteria that manifest as dark spots on plant foliage. These pathogens thrive in humid conditions, leading to aesthetic damage and, in severe cases, compromised plant health due to extensive defoliation.

Symptoms: Symptoms include irregular, brown, black, or yellow spots on leaves, often surrounded by a yellow halo. Severely affected leaves may turn yellow and drop prematurely. In advanced stages, leaf spot diseases can lead to significant defoliation, weakening the plant and reducing its aesthetic appeal.

Treatments: Cultural control methods are primary: remove and destroy affected leaves, ensure proper spacing for air circulation, and avoid overhead watering. Fungicides can be used as a preventive or treatment for severe cases; apply according to label directions, considering the specific pathogen responsible for the disease.

Powdery Mildew

Description: Powdery mildew is a common fungal disease affecting a wide range of plants. It manifests as white or grayish powdery spots, primarily on the upper surfaces of leaves and stems. Though it can mar a plant’s appearance, it’s often more of a cosmetic issue than a fatal one.

Symptoms: Powdery white or grayish-white fungal growth on the leaves, stems, and sometimes fruit and flowers. Distorted or stunted growth, yellowing or curling of leaves, stunted or reduced fruit or flower production, reduced plant vigor.

Treatments: Increase air circulation, reduce overhead watering, and prune affected areas. Apply fungicides or homemade solutions like baking soda sprays. Neem oil and horticultural oils can also suppress the spread. Ensure good garden hygiene to prevent recurrence.