The lack of rain this winter has become very alarming and the possibility of a third consecutive year of extreme drought is getting closer to being a reality. However, there is, believe it or not, a silver lining to the absence of spring rains.
Really? I’m looking for any bit of positive news I can get!
The lack of spring rains has impacted the spread of foliar funguses such as Anthracnose often called leaf, shoot, bud, or twig blight. It attacks trees and shrubs throughout the United States. It can turn the beautiful, newly created bright green leaves of spring (like the ones in this picture) into deformed leaves.
Anthracnose? What is it? I can barely pronounce it!
Anthracnose is pronounced just like it is written An-thrac-nose and no, it is not that deadly powder found in a mailbox! That‘s called anthrax! Anthracnose is a group of diseases that cause leaf death either as small spots, irregular areas of the leaf or on the whole leaf. Anthracnose can also distort the leaves and may cause them to drop prematurely.
Does it attack all tree species?
Fortunately, no. It mostly attacks ash, elms, oaks, and sycamores. The most susceptible species are Modesto ash and sycamores.
Will it kill my tree?
Twig or leaf blight does not seriously harm a healthy tree unless the defoliation occurs repeatedly or branch dieback is extensive, or if the tree is already suffering other health issues. Anthracnose can cause cankers on more susceptible species such as Chinese elm. Cankers are dead areas on twigs, branches, or even the trunk that may or may not be surrounded by callus tissue, causing them to become girdled and die. But in most instances, anthracnose does not warrant any treatment except to wait for the disease to die out when the weather turns warm.
Okay, I’m still waiting to hear the good news.
The good news has to do with how the disease is spread. Anthracnose fungi overwinter on infected twigs. Reproductive spores are produced in the spring and are spread by splashing and wind-driven rain. If it is moist during the new growth season (which is the month of March and April in the Diablo Valley area), these spores germinate and infect new twigs and foliage. This cycle continues as long as cool and wet weather is present. However, the fungus cannot survive in hot, dry weather so once it stops raining, and the leaves mature, the spread of the disease will end. The tree may drop infected leaves and replace those lost leaves with new growth. And cause you to ask, “Why am I raking leaves in May?” However, if the weather is dry in early spring (like it has been in 2022), then there is no mechanism (spring rains) to initiate the disease. Therefore, the damage done by anthracnose should be way less than it would be during a normal rainy, wet spring! Good news!
What if it starts raining in April or May?
That’s always a possibility and something we all can hope for! Given all the drought-stressed trees that are dying, the benefits of some really heavy rains would far outweigh the damage caused by the fungus.
When our normal rainy spring returns, how can I manage anthracnose on my sycamore or Modesto ash?
There are a couple of things you can do to help limit the effects of anthracnose. One way is through pruning. Remove and dispose of infected twigs during fall and winter. Thin throughout to increase airflow and sunlight. Prune back adjacent trees to increase sunlight and airflow knowing the disease favors a moist environment. Second, rake and dispose of fallen leaves and twigs during the growing season and in fall to limit the number of spores that could be activated during spring.
Is there a chemical treatment for anthracnose?
There are several fungicides available that provide varying degrees of anthracnose control on Modesto ash only. However, these fungicides only work on healthy trees as a preventive measure. They don’t get rid of existing infections. And these treatments are tricky to use. It is crucial to get adequate spray coverage and the timing has to be just right for it to be effective. You need to spray just as the buds are beginning to open in spring and before it rains. However, if it rains during the application, additional applications may be required at intervals of about 2 weeks to protect new growth.
Another management tool – plant a new tree
One of the first suggestions in managing any tree disease is to plant a species that is resistant to it. This is an option of last resort if you have a mature tree. We mention it because if you are thinking of re-landscaping and you like the characteristics of your ash or sycamore, there are cultivars that are more resistant to anthracnose that you can plant in their place.
So there’s your silver lining….or not?
No one knows what the future holds. Who knows, maybe we will receive extensive rains in May or maybe even into June, in which case if your Modesto ash starts dropping deformed, green leaves you will know why. Or we may not get another drop of rain until next winter. In which case, your tree is going to need every healthy leaf it can produce to survive the drought.