When I receive three calls within a few days regarding a particular problem with a particular tree species I take it as a clue that it is something to write about.
That’s what happened last week regarding Chinese elms. Three different property owners in different parts of Napa became concerned when they saw their trees shedding leaves and looking like they were dying when they should look fresh and green at this time of year.
Fortunately the news was not nearly as bad as they were afraid it might be.
When one thinks about dying elms it’s easy to jump to worrying that it might be Dutch elm disease, the fungal disease that has wiped out millions of elms in North America and is active here in Napa County. It is good to be aware of this disease and take preventative action if you have susceptible trees. But Chinese elms as a species are “resistant” to Dutch elm disease, according to three good reference books. Personally, I have never seen a Chinese elm show symptoms of this infection.
So, what’s going on?
Chinese elm (Ulmus parvifolia), can be recognized by its broad spreading and sometimes weeping structure, small leaves, thin flaky bark and sometimes evergreen habit in contrast to the Dutch elm disease susceptible American and English elms, which are fully deciduous, taller and more upright at maturity. Chinese elm is naturally resistant to Dutch elm disease, probably because the species originated in regions where Dutch elm disease was present. Over time, natural selection produced elms resistant to Dutch elm disease.
But no tree is absolutely free of pests and diseases.
What we are seeing on the Chinese elms this spring is anthracnose, another type of fungal disease that tends to infect and kill, for the most part, leaves, buds and twigs.
Anthracnose is a general term for a large group of fungi that cause similar diseases in many plants. Locally we see it in several other tree species including sycamore, oak, ash, and dogwood.
Anthracnose — derived from “coal” and “malady” — has classic symptoms on Chinese elms: dark spots on new leaves, which fall from the trees in spring.
Why is it suddenly so prevalent this spring? The “disease triangle” must have been completed:
- Susceptible tree species: Chinese elm is naturally susceptible (although the ‘Drake’ variety is resistant).
- Presence of the pathogen: The spores of the fungus are present in the environment and they live over the winter on the stems.
- Environment conducive to infection: The spores are released during spring rainfall.
It is likely that the most recent late rainfall combined with the right temperature range completed the disease triangle, resulting in an unusually heavy outbreak of the disease.
Fortunately, as described in one reference, “The disease is arrested during dry summers and even severely blighted parts seem to recover as the result of growth from buds that would normally remain dormant until the next spring.” (Diseases of Trees and Shrubs, Sinclair, Lyon and Johnson).
A close look at infected trees reveals that the twigs and small branches are pliable and alive.
Be aware that the fungus may over-winter on evergreen leaves, fallen leaves and stems. It sometimes causes large cankers of limbs and trunks, which can be excised with a chisel.
If moisture and temperature conditions are just right again next spring, we could see the cycle repeating itself.
Bill Pramuk is a registered consulting arborist. Visit his website, billpramuk.com, email questions to email@example.com, call him at 236-2884 or visit Bill Pramuk, consulting arborist, on Facebook.
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