A cool, wet spring may be welcome in Marin County, but it can be hard on some of the area’s oaks and sycamores. As new leaves emerge, brown blotches can spread along the veins, young foliage may curl and distort, and leaves can begin dropping well before fall.
The culprit may be anthracnose, a group of fungal diseases that thrive when wet weather coincides with tender new growth. California sycamores are particularly susceptible, but several of Marin’s native oaks can also develop the disease.
The damage can look dramatic, especially when a sycamore loses much of its first flush of leaves. But anthracnose usually doesn’t cause lasting damage to otherwise healthy oaks and sycamores.
Below, we’ll explain why Marin’s climate favors anthracnose, which trees are most vulnerable, the signs to watch for, and what you can do to help an affected tree recover.
What Is Anthracnose?
Anthracnose is a broad term for a group of fungal diseases that affect the leaves, twigs, and shoots of many deciduous and evergreen trees. Different fungi cause anthracnose on different hosts, so the disease affecting an oak isn’t necessarily the same one affecting a sycamore.
These fungi primarily attack tender, developing tissue, causing dark lesions, distorted leaves, shoot dieback, and premature leaf drop. The severity of the damage depends on the tree species, its overall health, and — importantly — the weather conditions present as new growth emerges.
That connection between weather and new growth helps explain why anthracnose can become especially noticeable during Marin’s cool, wet springs.
Why Anthracnose Thrives in Marin’s Climate
Anthracnose outbreaks depend heavily on timing. The fungi that cause the disease are most successful when cool, wet weather coincides with the emergence of tender new leaves and shoots in spring.
Rain and prolonged moisture create favorable conditions for spores to germinate and infect developing tissue. Cool weather can also slow leaf development. That leaves susceptible new growth exposed to infection for longer.
That combination can occur readily in Marin County, particularly in cooler, moister areas influenced by coastal weather and fog. Conditions can vary considerably across the county, however, with warmer inland areas experiencing a different disease environment than cooler coastal neighborhoods.
As spring gives way to warmer, drier weather and leaves mature, anthracnose activity typically declines. That helps explain why a tree can look dramatically affected early in the season and then begin recovering as conditions change.
How Anthracnose Spreads
Anthracnose doesn’t disappear when the growing season ends. On deciduous trees, the fungi can overwinter in infected twigs and fallen leaves. On evergreen hosts such as coast live oak, they may persist year-round on leaves and woody tissue.
When wet conditions return, the infected tissue releases microscopic spores. Rain and splashing water carry those spores to emerging leaves and shoots, where new infections can develop.
And the cycle may not stop there.
If wet weather continues, newly infected leaves can produce another generation of spores, allowing anthracnose to spread to additional new growth.
Marin Oaks and Sycamores Most at Risk for Anthracnose
Anthracnose doesn’t affect every Marin oak or sycamore equally. Some species are particularly vulnerable, while others show considerably more resistance to infection.
California sycamore (Platanus racemosa) is one of the standouts. This Marin native is very susceptible to anthracnose, and repeated infections can leave its canopy looking thin, distorted, or ragged.
Among Marin’s native oaks, coast live oak (Quercus agrifolia) and California black oak (Quercus kelloggii) are considered susceptible.
Other familiar local oaks fare better. Valley oak (Quercus lobata), canyon live oak (Quercus chrysolepis), and blue oak (Quercus douglasii) are considered resistant, although susceptibility can vary among individual trees.
| Tree | Relative Susceptibility to Anthracnose |
| California sycamore (Platanus racemosa) | Very susceptible |
| Coast live oak (Quercus agrifolia) | Susceptible |
| California black oak (Quercus kelloggii) | Susceptible |
| Valley oak (Quercus lobata) | Resistant |
| Canyon live oak (Quercus chrysolepis) | Resistant |
| Blue oak (Quercus douglasii) | Resistant |
Knowing the species you’re caring for can help put anthracnose symptoms in context. Our guide to trees commonly found throughout Marin County can help you learn more about these oaks, California sycamores, and other trees growing in the region.
Symptoms of Anthracnose
Anthracnose symptoms vary somewhat by tree species, but the disease often leaves clues in the foliage. Watch for irregular brown or black blotches, especially along leaf veins, as well as curled or distorted leaves and premature leaf drop.
The timing can provide another clue. Symptoms often appear on new growth following cool, wet spring weather, sometimes making a healthy-looking tree seem to decline surprisingly quickly.
Anthracnose Symptoms on Sycamores
On California sycamores, anthracnose can affect a large number of leaves. Infected foliage may develop brown lesions along the veins, while young leaves can become distorted or die before reaching full size. Severe infections may also cause shoot and twig dieback.
The result can be dramatic. A heavily affected sycamore may lose much of its first flush of leaves and develop a sparse-looking canopy. It may even appear dead or dying before producing another round of foliage as conditions become less favorable for the disease.
Anthracnose Symptoms on Oaks
Oak anthracnose also causes irregular brown lesions, often developing along or between major leaf veins. Infected leaves may curl, become distorted, or drop prematurely.
Symptoms can vary depending on the oak species and severity of the infection. And because leaf browning, thinning canopies, and branch dieback can have many causes, it’s important not to assume that every struggling Marin oak has anthracnose.
Some of these symptoms can also occur with more serious diseases, including sudden oak death.
What Anthracnose Means for Your Tree
A nearly bare sycamore in late spring can be a startling sight. Fortunately, appearance doesn’t always tell the whole story when anthracnose is involved.
Sycamores that lose their first flush of leaves often produce new foliage as temperatures rise and conditions become drier. Oaks can also recover from leaf damage and premature defoliation when the tree is otherwise healthy.
Most established oaks and sycamores can tolerate a single bout of anthracnose without lasting harm. Repeated infections, however, deserve more attention.
Repeated defoliation can weaken a tree over time, while twig and branch infections can contribute to dieback. Trees already under stress from drought, root damage, poor growing conditions, or other health problems may also have a harder time recovering.
So, the condition of the whole tree matters as much as the severity of one spring outbreak.
A mature tree that produces healthy new growth may need little more than monitoring and supportive care. Ongoing defoliation, significant dieback, or a canopy that fails to recover may point to a problem that warrants a professional tree health evaluation.
Managing Anthracnose in Oaks and Sycamores
Oaks and sycamores are generally resilient trees, and a single anthracnose outbreak may require little more than monitoring and good cultural care. But when infections are severe or keep returning, a few management practices can help reduce disease pressure and support recovery.
- Clean Up Fallen Leaves and Twigs: Rake and dispose of infected debris during the growing season and in fall. This helps reduce the amount of fungal material available to produce spores the following spring.
- Prune Infected Growth: When practical, infected twigs and branches can be removed during the dormant season. Proper pruning can also improve sunlight and air circulation through the canopy, helping foliage dry more quickly.
- Keep Foliage Dry: Avoid sprinkler irrigation that repeatedly wets the canopy. Anthracnose fungi need moisture to spread and infect new growth, so reducing unnecessary leaf wetness can make conditions less favorable for disease.
- Support Overall Tree Health: Proper watering, mulching, and protection from root or trunk damage can help reduce additional stress while a tree recovers.
- Consider Preventive Treatments: Fungicides may sometimes be considered for valuable trees with a history of severe anthracnose, but these treatments are preventive rather than curative and must be applied before infection occurs.
In many cases, especially with established sycamores, the best approach may simply be to clean up infected debris, support the tree’s health, and let it recover as warmer, drier weather arrives.
When to Call a Professional Arborist
Not every case of anthracnose requires professional treatment. If an otherwise healthy tree loses some leaves in spring and produces healthy new growth as the weather dries, monitoring may be all that’s needed.
But it’s worth having an arborist take a closer look when:
- Defoliation Happens Repeatedly: Losing a significant amount of foliage year after year can put additional stress on a tree and may indicate that more active management is warranted.
- Dieback Is Increasing: Dead twigs and branches throughout the canopy deserve attention, particularly when the problem continues after the initial spring outbreak.
- The Tree Isn’t Recovering: A canopy that remains sparse or fails to produce healthy new foliage as conditions improve may be dealing with more than a routine anthracnose outbreak.
- You’re Not Sure It’s Anthracnose: Leaf browning, canopy thinning, and dieback can have other causes. An accurate diagnosis is especially important for Marin oaks, which can be affected by other serious diseases.
- The Tree Is Large, Mature, or Especially Valuable: An arborist can help determine whether preventive or supportive care makes sense before recurring disease or other stresses take a greater toll.
An ISA-certified arborist can evaluate the entire tree, distinguish anthracnose from other potential problems, and recommend a management plan based on the species, severity of the infection, and overall condition of the tree.
Anthracnose FAQ
Still have questions about anthracnose on your Marin oak or sycamore? Here are a few quick answers to common homeowner concerns.
Does anthracnose come back every year?
It can. Anthracnose fungi can overwinter in infected plant tissue, but disease severity varies from year to year depending largely on spring weather conditions.
Will my sycamore grow new leaves after anthracnose?
Usually. Established sycamores that lose their first flush of leaves often produce new foliage as temperatures rise and conditions become drier.
Should I rake up leaves infected with anthracnose?
Yes. Removing and disposing of infected leaves and twigs can reduce the amount of fungal material that remains around the tree and produces spores the following spring.
Can I prevent anthracnose next spring?
You can reduce the risk, but complete prevention isn’t always possible. Good sanitation, proper pruning, and avoiding irrigation that wets the foliage can help make conditions less favorable for infection. Fungicides may also be helpful in some cases.
Does anthracnose mean my tree is dying?
Usually not. Anthracnose can make a tree look severely affected, but otherwise healthy oaks and sycamores often recover as weather conditions become less favorable for the disease.
Can anthracnose spread from an oak to a sycamore?
Not necessarily. Anthracnose is caused by several different fungi, and individual fungi generally infect a particular host or a small group of closely related trees.
Can Treemasters cure anthracnose?
There’s no treatment that can reverse damage already caused by anthracnose. Treemasters can diagnose the problem, assess your tree’s overall health, and recommend preventive or supportive care when it’s warranted.
Treemasters Tree and Garden Care: Protecting Marin’s Oaks and Sycamores
Anthracnose can make a tree look alarming in spring, but the right response depends on the species, severity of the infection, and overall health of the tree. Sometimes, monitoring and supportive care are enough. Other trees may benefit from closer evaluation and a longer-term management plan.
Treemasters Tree and Garden Care has been caring for Marin County trees for more than 35 years. If your oak or sycamore is dropping leaves, developing dieback, or simply doesn’t look right, our team can help identify the cause and recommend the next steps.
Contact Treemasters Tree and Garden Care to schedule an evaluation for your Marin County trees.
Sources & References
- UC Integrated Pest Management — “Anthracnose”
Supports anthracnose identification, host susceptibility, symptoms, and management, including susceptibility information for California sycamore and several California oak species.
https://ipm.ucanr.edu/legacy_assets/PDF/PESTNOTES/pnanthracnose.pdf - UC Integrated Pest Management — “Anthracnose Diseases”
Explains the anthracnose disease cycle, including overwintering, spore production and spread during wet weather, infection of new growth, and the decline of disease activity as conditions become warmer and drier.
https://ipm.ucanr.edu/home-and-landscape/anthracnose-diseases/ - UC Agriculture and Natural Resources — “Defoliated Sycamore Trees?”
Provides California-specific guidance on sycamore anthracnose, including symptoms, spring weather conditions, defoliation and recovery, sanitation, irrigation practices, and the limitations of fungicide treatment.
https://ucanr.edu/blog/pests-urban-landscape/article/defoliated-sycamore-trees - UC Agriculture and Natural Resources — “A Field Guide to Insects and Diseases of California Oaks”
Provides California-specific information on anthracnose and other leaf and twig diseases affecting native oaks, including coast live oak, California black oak, valley oak, blue oak, and canyon live oak.
https://ucanr.edu/sites/default/files/2022-03/365212.pdf