Cedar-Hawthorn Rust

What is Cedar-Hawthorn Rust?

Cedar-hawthorn rust is a unique disease caused by the fungus Gymnosporangium globosum. This pathogen requires two distinct host plants to complete its lifecycle: one among the Juniperus species and the other within the Rosaceae family, commonly hawthorns (Crataegus spp.). The disease can affect the leaves, fruits, and young twigs of these plants.

Main plants affected by Cedar-Hawthorn Rust include:

• Apple trees
• Cedar trees
• Crabapple trees
• Hawthorn trees
• Serviceberry trees

Cedar-Hawthorn Rust: How Does it Spread

The disease begins in the juniper, where the fungus overwinters as galls. During wet or humid spring weather, these galls swell and produce gelatinous spore horns filled with teliospores. These spores are then released and carried by the wind to susceptible plants in the Rosaceae family, like hawthorns.

Upon reaching the Rosaceae host, the teliospores germinate and penetrate the plant’s tissues, typically the leaves, fruit, and young twigs, causing infections that manifest as orange-yellow leaf spots or lesions on the fruits. This phase of the disease cycle produces another type of spore called basidiospores.

These basidiospores are subsequently released and carried by the wind back to juniper plants, infecting young twigs and branches, creating new galls and thus completing the cycle. It takes two years to produce a spore-bearing gall.

The process relies heavily on weather conditions, especially humidity and temperature, for spore release and germination. The disease spreads most effectively during wet, warm spring weather, facilitating the release of spores and their subsequent travel between host plants. Additionally, the proximity of the alternate hosts plays a critical role; the closer the junipers are to susceptible Rosaceae plants, the more likely the disease is to spread.

Regions Impacted

The disease is prevalent in regions across North America, especially in the eastern and central United States, where host plants coexist. The distribution coincides with the range of the host species, extending into areas where humidity levels support fungal growth.

What are the Symptoms?

Cedar-hawthorn rust presents a variety of symptoms on its two host groups. Here are the symptoms commonly observed:

On Junipers

• Galls: The most distinctive symptom is the formation of galls on twigs and branches. These galls are initially small and hard, becoming larger and more prominent with time, sometimes reaching the size of a golf ball or larger. They are typically brownish and may blend in with the bark.
• Gelatinous Horns: In the spring, particularly during wet weather, these galls produce orange-brown, gelatinous structures called “telial horns” or “spore horns.” These horns release spores that are spread by the wind to Rosaceae hosts.

On Rosaceae Hosts (e.g., Hawthorns, Apples, Crabapples)

• Leaf Spots: One of the primary symptoms is the appearance of bright orange-yellow spots on the upper surface of leaves. These spots may also have a darker red border.
• Fruit Lesions: The disease can affect fruits, resulting in small, circular, orange to yellowish spots, often surrounded by a red halo. Severely infected fruits may become deformed or drop prematurely.
• Twig and Branch Lesions: Less commonly, the fungus can cause cankers on twigs and small branches, potentially leading to dieback.

These spots contain black pimple-like fruiting bodies that produce spores.

The disease typically doesn’t kill its hosts but can weaken them and make them more susceptible to other pests and diseases. In aesthetic terms, it can significantly impact the ornamental value of affected trees, especially those in the Rosaceae family.

How to Control and Prevent Cedar-Hawthorn Rust?

Controlling and preventing Cedar-Hawthorn Rust requires a multi-faceted approach that includes cultural and chemical strategies. Here’s how you can manage this fungal disease:

Cultural Controls:

• Resistant Varieties: Plant resistant varieties of hawthorns, apples, or other susceptible plants. Many cultivars have been bred for resistance to rust diseases. Resistant hawthorn varieties include:
  • Crataegus crus-galli
  • Crataegus flava
  • Crataegus laevigata 
  • Crataegus intricate
  • Crataegus monogyna
  • Crataegus phaenopyrum
  • Crataegus pruinosa
  • Crataegus viridis ‘Winter King’
• Planting: Avoid planting susceptible hawthorns within a two-mile radius of junipers.
• Sanitation: Remove and destroy affected juniper branches and Rosaceae leaves and fruits. Since the fungus overwinters in galls on junipers or as lesions on leaves and fruits of Rosaceae, removing these can reduce the source of new infections.
• Proper Spacing: Ensure plants have adequate spacing to improve air circulation. Good airflow can help foliage dry more quickly, reducing the likelihood of infection.
• Watering Practices: Water plants at the base to avoid wetting the foliage. Moist leaves can encourage fungal growth.

Chemical Controls:

• Fungicides: Apply fungicides to protect susceptible plants during the period when they are vulnerable to infection. This is usually in the spring when junipers are producing spores. Effective fungicides may include those containing copper, mancozeb, or propiconazole. Always follow the manufacturer’s instructions for application rates and timings.
• Timing: The timing of fungicide applications is critical. Begin when the gelatinous spore horns are forming on junipers and continue as per the fungicide’s guidelines or until the weather dries.

Integrated Approach:

• Use a combination of the above methods for an integrated disease management approach. Monitor plants regularly and adjust strategies based on the season’s disease pressure.

Remember, it’s essential to correctly identify the disease before beginning any treatment. Misidentification can lead to ineffective control measures. In some cases, especially for valuable or extensively landscaped areas, consulting with a professional arborist or extension service can be very beneficial.