Rhytisma: Fungal Disease | Treatment Guide
Rhytisma spp. are fungi that cause tar spot on maple leaves. Infection typically begins in late spring with small, yellowish spots that darken over the summer, developing into raised, black, shiny "tar-like" stroma. While severe infestations can lead to premature leaf drop, the trees rarely suffer serious damage—it is primarily an aesthetic issue. The pathogen overwinters in fallen leaves, from which it releases spores in the spring. It most commonly affects Norway maple (Acer platanoides), sycamore maple (Acer pseudoplatanus), and field maple (Acer campestre), as well as other maple species.
What is Rhytisma?
Rhytisma is a fungal condition with moderate severity that affects plants. Rhytisma spp. are fungi that cause tar spot on maple leaves. Infection typically begins in late spring with small, yellowish spots that darken over the summer, developing into raised, black, shiny "tar-like" stroma. While severe infestations can lead to premature leaf drop, the trees rarely suffer s...
Rhytisma is a fungal disease with moderate severity. This condition is contagious and can spread between plants.
Unlike bacterial infections, Rhytisma spreads slowly but is harder to eradicate. Without treatment, affected leaves won't recover. Without isolation, nearby plants can become infected within days.
🧪 Natural vs Chemical: Which Treatment Works for Rhytisma?
🌱 Natural Solutions
- ✓ Regularly rake and remove fallen leaves in autumn and early spring; dispose of them in green waste or hot compost only, and do not leave them under the tree over winter.
- ✓ Perform light thinning cuts to ensure the crown dries faster after rainfall.
- ✓ Water at the base of the plant and avoid wetting the foliage.
- ✓ Apply 3–5 cm of fresh mulch after raking to suppress spore dispersal from the soil.
- ✓ Plant less susceptible maple varieties in airy, sunny locations whenever possible.
- ✓ During high-risk periods, apply registered foliar biopreparations based on Bacillus subtilis or B. amyloliquefaciens.
💉 Chemical Solutions
- ✓ If the infestation is severe and recurs annually on valuable specimens, apply early spring preventive treatments using copper-based products (copper oxychloride or hydroxide) or sulfur from the bud-swell stage through leaf expansion, with a possible follow-up after 10–14 days during rainy weather.
- ✓ Avoid spraying during high heat and perform a test on a small section of the crown to rule out phytotoxicity.
🛡️ How to Prevent Rhytisma?
TLDR: Prevent Rhytisma with 6 essential care practices. Regular monitoring is key for early detection.
- ✓ Remove all fallen maple leaves from beneath the tree every autumn.
- ✓ Maintain crown airflow through light, regular pruning.
- ✓ Avoid overhead watering to keep foliage dry.
- ✓ Apply a thin layer of mulch after clearing leaves to limit pathogen splashing.
- ✓ Avoid over-fertilizing with nitrogen, as lush, soft tissue is more susceptible to infection.
- ✓ Monitor young leaves in May and June; if symptoms are frequent, plan preventive measures for the following season.
🔗Related Diseases
Rust
Rust fungi (Pucciniales) are fungal diseases that form rust-colored, usually orange to brown pustules with powdery spore dust on leaves, stems, or fruits. Typical symptoms include small spots that develop into cushions or rings; leaves turn yellow, curl, and drop prematurely. The fungi spread primarily in damp weather and dense foliage. Many ornamental and crop plants are affected, frequently including roses, mallows (hollyhocks), pears (pear rust), and currants. Some rust fungi require two host plants for their life cycle, such as pear and certain juniper species.
Cherry Witches' Broom
Taphrina wiesneri is a fungal disease affecting cherry trees (Prunus species, including sweet, sour, and ornamental varieties). The fungus overwinters in buds and bark, infecting young leaves and shoots during cool, wet spring weather. Infected leaves become thickened, puckered, and curled, often displaying reddish or yellow blotches and a fine whitish bloom on the surface. Severely affected leaves may drop prematurely. Small twigs can develop 'witches' brooms,' which are dense clusters of weak, distorted shoots.
Powdery Mildew
MediumPowdery mildew is a common fungal disease caused by various fungi species including Erysiphe, Podosphaera, Oïdium, and Leveillula. It affects over 10,000 plant species worldwide. The disease thrives in warm, dry climates with high humidity and appears as a distinctive white powdery coating on plant surfaces.
Root Rot
HighRoot rot is a serious fungal disease that affects the root system of plants, causing them to decay and die. It is primarily caused by overwatering, poor drainage, or soil-borne fungi such as Pythium, Phytophthora, Rhizoctonia, and Fusarium. The disease thrives in waterlogged conditions where roots are deprived of oxygen, making them susceptible to fungal infection.