Sycamores and ash trees provide a lot of the shade San Jose yards enjoy. Their broad canopies cool patios through the hottest months, and their size alone makes them feel like a permanent part of the landscape.
So it’s unsettling when one of these trees starts dropping leaves in April or May, months before fall color has any business showing up. One common cause of this is anthracnose, a group of fungal diseases favored by the Bay Area’s cool, wet spring weather.
A single bad spring rarely does lasting harm. But when a tree drops its leaves early two, three, or more years running, that repeated stress can add up and leave it more vulnerable to other problems.
But don’t worry — we’ll explain everything you need to know about anthracnose in sycamores and ash trees below.
Anthracnose: One Name for Many Fungal Diseases
The term anthracnose describes fungal diseases that cause similar damage, including dark leaf blotches, blighted new growth, and, in more severe cases, cankers on twigs.
Each anthracnose-causing fungus typically affects one host or a small group of related trees. That means the disease affecting your sycamore differs from the one affecting your ash, even when the symptoms look similar from the ground.
On sycamores, anthracnose is associated with Apiognomonia veneta. On ash trees, including the highly susceptible Modesto ash, it is associated with Apiognomonia errabunda. Other fungi cause anthracnose on oaks, maples, elms, and other landscape trees across California.
Why San Jose’s Spring Weather Favors Anthracnose
San Jose’s Mediterranean climate brings most of the region’s rain between late fall and early spring, followed by a long, dry summer. When lingering spring rain overlaps with bud break, the tender new leaves and shoots on sycamore and ash remain wet long enough for anthracnose fungi to infect them.
Even a short stretch of cool, wet weather can contribute to an outbreak, while a warm, dry spring tends to limit the disease. That’s why the same tree may look healthy one year and alarmingly bare the next.
The difference often has more to do with spring weather than a major change in the tree’s overall health.
How Anthracnose Spreads in Spring
Anthracnose fungi overwinter in infected twigs, small cankers, and fallen leaves.
When spring moisture arrives, they produce microscopic spores that travel in splashing rain or irrigation water to newly opening buds, tender shoots, and expanding leaves. If those spores land on wet plant tissue, they can germinate and cause new infections.
When cool, wet conditions continue, infected leaves and twigs can produce additional spores that spread the disease to more new growth. As the weather becomes warmer and drier, usually by late spring in San Jose, disease activity slows considerably.
The fungi then survive in infected tissue and fallen debris, ready to begin the cycle again when favorable conditions return.
Trees Most at Risk from Anthracnose
Anthracnose does not affect every sycamore or ash with the same intensity. Spring weather still influences how severe an outbreak becomes, but knowing what kind of sycamore or ash you have can help you understand its underlying vulnerability.
Modesto ash (Fraxinus velutina ‘Modesto’) is particularly susceptible to anthracnose. American sycamore (Platanus occidentalis) and California sycamore (Platanus racemosa) are also vulnerable, along with the ‘Yarwood’ cultivar of London plane (Platanus × acerifolia ‘Yarwood’).
Several ash and London plane cultivars offer greater resistance. These include evergreen or Shamel ash (Fraxinus uhdei), Moraine ash (Fraxinus holotricha), and Raywood ash (Fraxinus oxycarpa). Among London planes, the ‘Bloodgood,’ ‘Columbia,’ and ‘Liberty’ cultivars are less susceptible.
But even these cultivars may develop symptoms during an unusually cool, wet spring. Disease resistance is also only one consideration when selecting a new tree; mature size, water needs, and available growing space matter, too.
An ISA-certified arborist can help you choose and plant a tree that suits your property and is less likely to develop recurring anthracnose problems.
Signs and Symptoms of Anthracnose
Anthracnose does not look exactly the same on every tree. The pattern of leaf damage differs between sycamore and ash, while the severity can change considerably depending on the tree’s condition and the spring weather.
Watch for these common signs of anthracnose on sycamore and ash trees:
- Leaf Blotches: On sycamore leaves, brown or black lesions often follow the major veins. Ash leaves tend to develop more irregular patches of dead tissue.
- Distorted Young Leaves: Leaves infected while still expanding may curl or become misshapen, with portions of the leaf turning brown and dying.
- Blighted Shoots: Tender new shoots may wilt or die, sometimes leaving clusters of dead leaves attached to the tree.
- Early Leaf Drop: Heavily infected leaves can fall in spring or early summer, often beginning in the lower canopy and moving upward.
- Twig Cankers and Dieback: In more severe cases, sunken lesions can develop on twigs and small branches.
Can Sycamore and Ash Trees Recover from Anthracnose?
Most otherwise healthy, established sycamore and ash trees recover from a single anthracnose outbreak.
After losing infected leaves in spring, they will often produce a second flush of growth as the weather becomes warmer and drier. A single anthracnose outbreak is usually more alarming to look at than damaging to the tree’s long-term health.
Repeated defoliation, however, is more concerning. A tree that must replace lost leaves year after year draws on the energy reserves it would otherwise use for growth, defense, and recovery from other stresses. Persistent twig dieback or cankering also deserves attention.
One rough spring is rarely worth losing sleep over. But a pattern of heavy leaf loss across several consecutive years is worth a closer look.
Treating and Managing Anthracnose
Once anthracnose symptoms appear, the damaged leaves cannot be restored, and the infection cannot be effectively controlled for that growing season. Management instead focuses on reducing future infections and helping the tree recover from premature leaf loss.
Depending on the tree, the severity of the outbreak, and its history of recurring symptoms, an arborist may recommend some combination of these strategies:
- Remove Infected Debris: Rake and dispose of fallen leaves and twigs during the growing season and again in fall. This reduces some of the infected material in which anthracnose fungi can overwinter, although sanitation cannot eliminate every source of spores.
- Prune Diseased Growth: During fall or winter, prune out infected twigs and dead or diseased branches. Professional pruning can also improve air circulation without triggering the dense watersprouts that often follow overly aggressive cutting.
- Keep Foliage Dry: Avoid overhead irrigation or sprinklers that repeatedly wet the leaves. When planting new trees, allow enough space for sunlight and air circulation to help foliage dry more quickly.
- Support the Tree’s Recovery: Provide appropriate water during dry periods and maintain a 2- to 4-inch-thick layer of organic mulch over the root zone, keeping it away from the trunk. For a severely affected tree, properly timed fertilization may encourage healthy new growth after the leaves open and spring rains have ended. However, fertilization is not automatically necessary for every tree with anthracnose.
- Consider Preventive Fungicides: Fungicides protect healthy new tissue rather than curing existing infections, so timing at bud break and ahead of wet weather is critical. They have shown the strongest results on Modesto ash; their effectiveness has not been established on other tree species in California. Complete coverage of a large, mature tree can also be difficult and may not be practical.
Removal is rarely necessary for anthracnose alone. It may enter the conversation only when repeated defoliation, extensive dieback, structural concerns, or additional health problems have left the tree in serious decline.
If anthracnose keeps returning or the tree is developing substantial dieback, our pest and disease control specialists can confirm the diagnosis and determine whether preventive treatment or supportive care makes sense.
Frequently Asked Questions About Anthracnose
Early leaf drop can be alarming, even when the tree is likely to recover. These answers address some of the most common concerns San Jose homeowners have about anthracnose on sycamore and ash trees.
Will anthracnose kill my sycamore or ash tree?
Usually not. A single anthracnose outbreak rarely causes lasting harm to an otherwise healthy, established sycamore or ash tree. Repeated annual defoliation, extensive cankering, or persistent branch dieback is more concerning.
Why does my tree look so much worse some years than others?
Spring weather is the main reason. Cool, wet conditions allow anthracnose fungi to infect tender new growth, while warm, dry weather limits disease activity. The same tree can therefore show dramatically different levels of damage from one year to the next.
Is early leaf drop from anthracnose the same as normal fall color?
No. Normal fall leaf drop is part of the tree’s seasonal transition into dormancy. Anthracnose-related leaf loss occurs in spring or early summer, often after leaves develop brown blotches, curling, or distortion. An otherwise healthy tree may then produce another flush of foliage.
Can I stop anthracnose from coming back every spring?
Not entirely. Removing infected debris, keeping foliage dry, and improving air circulation can reduce the conditions that favor anthracnose, but they cannot eliminate every source of fungal spores. Symptoms may return when another cool, wet spring creates favorable conditions.
Should I fertilize a tree that has anthracnose?
Not automatically. A severely affected tree may benefit from properly timed fertilization after its leaves open and spring rains have ended, but fertilization is not necessary for every tree recovering from anthracnose. An arborist can evaluate the tree’s overall condition and growing environment before recommending it.
Can fungicide spraying guarantee my tree stays symptom-free?
No. Fungicides can protect healthy tissue before infection, but they cannot repair leaves that already show symptoms. Their effectiveness depends on accurate timing and thorough coverage, and results have been strongest on Modesto ash rather than sycamore and other California landscape trees.
When is it time to call an arborist about anthracnose?
Consider an assessment if your tree experiences substantial leaf loss, develops twig or branch dieback, fails to produce healthy replacement foliage, or shows significant symptoms in consecutive years. An arborist can also determine whether anthracnose — or another disease, pest, or environmental problem — is responsible.
Anderson’s Tree Care Specialists: Anthracnose Diagnosis and Care in San Jose
Blotchy leaves and unexpected spring leaf drop can make a sycamore or ash tree look as though it is in serious trouble. Often, the tree needs time and supportive care rather than drastic action. But recurring defoliation, twig dieback, or poor replacement growth deserves a closer look.
Anderson’s team of ISA-certified arborists can determine whether anthracnose or another problem is affecting your tree, evaluate its overall condition, and recommend care suited to the tree and your property. Contact Anderson’s Tree Care Specialists today to schedule an assessment.
Sources & References
- UC Statewide Integrated Pest Management Program – “Anthracnose,” a peer-reviewed pest note covering identification, life cycle, host susceptibility, and management for anthracnose in California landscape trees.
https://ipm.ucanr.edu/home-and-landscape/anthracnose/ - National Weather Service, San Francisco Bay Area / NOAA – “Climate of San Jose,” a technical memorandum describing San Jose’s Mediterranean precipitation pattern and wet-season timing.
https://www.weather.gov/media/mtr/climate/SJC_CLIMATE.pdf - UC Agriculture and Natural Resources – “Wet Weather Tree Diseases,” a UC ANR blog post explaining how rain-splash spreads anthracnose and other fungal diseases in California landscapes.
https://ucanr.edu/blog/pests-urban-landscape/article/wet-weather-tree-diseases - International Society of Arboriculture, Journal of Arboriculture – “Insects Have Defoliated My Tree—Now What’s Going to Happen?” a peer-reviewed article on how repeated defoliation stresses trees and increases vulnerability to secondary pests.
https://auf.isa-arbor.com/content/4/8/169 - UC Statewide Integrated Pest Management Program – “Sycamore,” a UC IPM reference page on sycamore care and disease susceptibility, including cultivar-level detail on anthracnose resistance.
https://ipm.ucanr.edu/PMG/GARDEN/PLANTS/sycamore.html