Black spots on weeping willow leaves are overwhelmingly caused by fungal infections, with anthracnose from the fungus Marssonina salicicola being the most frequent offender. Cool, rainy springs create ideal conditions for this pathogen, which produces dark spots along leaf veins and can defoliate large portions of a tree in a bad year. Other fungi and even bacteria occasionally produce similar-looking symptoms, though, and the distinction matters when deciding how to respond.
Anthracnose and the Marssonina Fungus
The classic pattern goes like this: spring arrives with plenty of rain and mild temperatures, and within weeks your weeping willow’s leaves develop purplish-brown to black spots, especially along the veins. The leaves curl, distort, and drop prematurely. This is anthracnose, caused by the fungus Marssonina salicicola (known in its sexual stage as Drepanopeziza sphaeroides). A first formal report of the disease on weeping willow documented small necrotic spots that were purplish-brown or black, often running along leaf veins, with leaves becoming distorted before falling.1Plant Disease. First Report of Anthracnose on Weeping Willow in Greece
Anthracnose doesn’t stop at the leaves. Small, sunken cankers with a dark brown rim and paler center often form on young green shoots and leaf stalks. Under humid conditions, the fungus produces masses of spores from tiny fruiting structures on the leaf surface, which rain then splashes to nearby foliage. This is why outbreaks tend to be worst in years with extended wet weather. The same Greek report linked the outbreak to spring rainfall of roughly 70 to 105 mm combined with temperatures around 20°C, conditions that essentially let the fungus complete infection cycle after infection cycle throughout the canopy.1Plant Disease. First Report of Anthracnose on Weeping Willow in Greece
Other Fungi That Produce Dark Leaf Spots
Anthracnose gets the most attention, but at least a few other fungal pathogens can mark willow leaves with conspicuous dark lesions.
Rhytisma salicinum causes a condition known as black spot blight. Rather than the scattered small spots typical of anthracnose, Rhytisma tends to produce tar-like, raised black blotches that look almost painted onto the leaf surface. It’s a relative of the tar spot fungi that commonly disfigure maple leaves. Surveys of willow diseases in European mountain environments detected Rhytisma salicinum alongside rust infections on several Salix species.2European Journal of Plant Pathology. Foliar diseases of willows (Salix spp.) in selected locations of the Karkonosze Mts. (the Giant Mts)
Elsinoë is a less well-known genus that can also be responsible. Researchers in China isolated Elsinoë murrayae from weeping willow leaf spots and confirmed through inoculation studies that it could infect mature leaves of Salix babylonica (weeping willow), S. fragilis, and S. suchowensis. It could not infect Salix matsudana, demonstrating that even among close relatives, susceptibility to a given pathogen varies.3Journal of Phytopathology. Molecular identification and characterization of Elsinoë murrayae (Synonym: Sphaceloma murrayae) from weeping willow
A more aggressive pathogen is Glomerella miyabeana, which produces a Colletotrichum form as its asexual stage. In New Zealand, researchers established this fungus as the cause of both black canker on stems and leaf spots on willows.4European Journal of Forest Pathology. Black canker and leaf spot of Salix in New Zealand caused by Glomerella miyabeana (Colletotrichum gloeosporioides) Unlike Marssonina, Glomerella can girdle stems and kill entire branches, making it a concern beyond cosmetic leaf damage.
When Bacteria Cause the Spots
Not every dark mark on a willow leaf is fungal. During unusually wet growing seasons, bacterial pathogens can produce strikingly similar symptoms. A study during the exceptionally moist 1985–86 growing season in New Zealand documented terminal and side shoot blackening, dieback, leaf spots, and stem cankering on field-grown willows. Researchers isolated several bacterial species from infected tissue, but only Pseudomonas syringae pv. syringae proved pathogenic when re-inoculated onto healthy willows.5European Journal of Forest Pathology. Characterisation of bacteria from poplars and willows exhibiting leaf spotting and stem cankering in New Zealand
Bacterial leaf spot tends to be more opportunistic than its fungal counterparts. It shows up during periods of sustained wetness, often following physical damage to leaves from wind, hail, or insect feeding. The spots themselves may look water-soaked or greasy early on before darkening, which is one way to distinguish them from fungal spots that usually start out as well-defined dark lesions from the outset. The distinction matters for treatment because bacterial infections don’t respond to standard fungicides.
How to Tell the Causes Apart Without a Lab
If you’re standing under your weeping willow looking at spotted leaves and wondering what’s going on, the pattern, texture, and timing of the spots can help you narrow the culprit.
- Anthracnose: Small, scattered purplish-brown to black spots concentrated along leaf veins. Leaves curl and distort. You may see small sunken cankers on green twigs and leaf stalks. Spots sometimes merge into larger blotches in severe infections.
- Rhytisma black spot: Raised, shiny, tar-like patches that are typically larger and fewer than anthracnose spots. The surface has a distinctive crusty texture, almost like a scab.
- Bacterial leaf spot: Spots that appear water-soaked or greasy before darkening. Often accompanied by shoot-tip blackening and branch dieback, especially after a prolonged wet spell.
- Glomerella/Colletotrichum: Leaf spots paired with aggressive cankers on stems. Branch dieback is prominent. Damage tends to be more severe than with Marssonina alone.
Timing also helps. Anthracnose and bacterial spots tend to appear in spring and early summer when rain and moderate temperatures converge. Rhytisma spots usually become obvious later in summer, with the characteristic black fruiting bodies maturing by autumn. If spots are showing up in April or May and the weather has been wet, anthracnose is the leading suspect. If dark blotches appear midsummer and look tar-like, think Rhytisma.
Why Wet Springs Make Everything Worse
The single biggest environmental driver behind black spot outbreaks on weeping willows is moisture. Fungal spores need water to travel, germinate, and penetrate leaf tissue. Bacterial pathogens similarly exploit wet surfaces to enter through natural openings and wounds. A dry spring can keep infection minimal even when spores are present from the previous year, while a rainy one turns the same tree into a disease incubator.
This explains a pattern that many weeping willow owners notice: some years the tree looks terrible, with spotted and dropping leaves by late spring, while other years it sails through the season nearly unblemished. The tree didn’t suddenly become resistant or susceptible. The weather changed.
Weeping willows are especially vulnerable because of their architecture. Those long, cascading branches create a dense, humid microclimate underneath. Air circulation is poor, and morning dew lingers on interior leaves long after the outer canopy has dried. Combine that habit with the tendency to plant weeping willows near ponds, streams, and other water features, and you’ve created an environment the pathogens love. The tree’s own growth form conspires against it.
Treatment and Prevention
Here’s the honest reality about treating black spots on weeping willows: for most homeowners, the disease is cosmetic and the tree will recover on its own. Weeping willows are vigorous growers. Even after a heavy anthracnose year that strips half the leaves by June, most established trees push out a fresh flush of foliage and are fine by late summer. That said, repeated severe infections year after year can weaken a tree, and there are practical steps worth taking.
Sanitation
The most effective measure costs nothing. Marssonina and other foliar fungi overwinter in fallen leaves and in cankers on infected twigs. Raking up and disposing of fallen leaves in autumn removes a major reservoir of spores that would otherwise reinfect the tree the following spring. If you’ve noticed cankers on small branches, pruning them out during dormancy further reduces the pathogen load. Burn or bag the debris rather than composting it at home, because backyard compost piles rarely reach temperatures high enough to kill fungal spores reliably.
Pruning for Airflow
Thinning the canopy helps leaves dry faster after rain, which shortens the window during which spores can germinate. This is especially worthwhile for weeping willows because their natural form creates such a dense interior. You don’t need to fundamentally change the tree’s silhouette. Removing crossing branches, dead wood, and some interior growth opens things up enough to make a real difference. Do pruning work during dry weather to avoid introducing pathogens through fresh cuts.
Fungicide Sprays
Copper-based fungicides and chlorothalonil are the products most commonly recommended for anthracnose on ornamental trees. The key is timing: fungicides work preventively, not curatively. You need to spray before infection occurs, which typically means applying at bud break in spring and repeating once or twice more at roughly two-week intervals during wet weather. For a large weeping willow, this is a significant commitment, and professional application may be necessary to reach the full canopy.
For most homeowners with a single landscape tree that gets moderate anthracnose, fungicide spraying is overkill. It makes more sense when the tree has been severely defoliated multiple years in a row, when the tree is young and still establishing, or when it’s a high-value specimen. Bacterial leaf spot doesn’t respond to fungicides, though copper sprays may offer some suppression since copper has bactericidal properties. Reducing overhead irrigation and improving drainage around the tree are more practical strategies for bacterial problems.
Watering and Feeding
A healthy tree resists infection more effectively than a stressed one. Willows are heavy water users and generally don’t need supplemental irrigation once established near a water source, but trees planted in drier landscape positions benefit from deep watering during dry spells. Balanced fertilization in early spring supports vigorous new growth that replaces spotted leaves. Avoid heavy nitrogen late in the season, because lush, soft growth produced in late summer is more susceptible to fungal infection heading into autumn.
Species Susceptibility and Choosing Alternatives
Not all willows are equally prone to leaf spot diseases. The Elsinoë study from China illustrated this clearly: the pathogen successfully infected weeping willow (Salix babylonica), crack willow (S. fragilis), and shrub willow (S. suchowensis) but completely failed on Salix matsudana.3Journal of Phytopathology. Molecular identification and characterization of Elsinoë murrayae (Synonym: Sphaceloma murrayae) from weeping willow That kind of variation matters if you’re choosing a tree for a landscape where disease has been a persistent problem.
Salix babylonica, the classic weeping willow, is unfortunately one of the more disease-prone species in the genus. Its dense, pendulous canopy traps humidity, its rapid growth produces soft tissue that pathogens penetrate easily, and it gravitates toward moist sites where fungal pressure is highest. If you want the weeping form with better disease tolerance, hybrids between S. babylonica and more resistant species sometimes deliver. The commonly planted golden weeping willow (‘Chrysocoma’) and some S. × sepulcralis selections have a reputation for holding up somewhat better, though controlled resistance trials specifically for anthracnose are scarce in the published literature. For anyone planting a new tree and frustrated by recurring disease, asking a local extension office about regionally adapted cultivars is worth the effort.
When Black Spots Signal a Bigger Problem
For an established weeping willow, a year of black spots followed by early leaf drop is the equivalent of a bad cold. The tree has plenty of stored energy to bounce back. The calculus changes in a few scenarios, though.
Young trees in their first few years after planting lack the carbohydrate reserves of mature specimens. Severe defoliation in a young weeping willow can set back establishment significantly, and repeated years of heavy infection may even kill it. Young trees deserve more aggressive protection with sanitation and potentially fungicide.
When cankers girdle stems and kill back entire branches, the disease has moved beyond cosmetic leaf spotting into structural damage. This is more characteristic of Glomerella/Colletotrichum infections than Marssonina.4European Journal of Forest Pathology. Black canker and leaf spot of Salix in New Zealand caused by Glomerella miyabeana (Colletotrichum gloeosporioides) If you’re seeing cankers enlarging year after year on major limbs, a certified arborist should evaluate whether the tree can be saved through targeted pruning or whether the structural damage has gone too far.
If your tree is losing leaves well before autumn every single year and never seems to fill back in properly, the disease and stress may be feeding each other: infection weakens the tree, and a weakened tree resists the next infection less effectively. Breaking that cycle usually requires a combination of aggressive leaf cleanup, corrective pruning, soil improvement, and possibly a few seasons of preventive fungicide applications to give the tree breathing room to regain its vigor. The good news is that weeping willows, for all their disease susceptibility, are remarkably resilient growers when given even a modest advantage.