The Dogwoods
The dogwoods are in full bloom, and you hardly need to slow down to notice that some of them don’t look just right. Contrary to what some have suggested, I think it is not dogwood anthracnose (caused by the fungus Discula distructiva) but the much less serious spot anthracnose (caused by the fungus Elsinoe corni). It is quite easy to confuse the two outside of a lab. And I am basing my windshield diagnosis on the generally accepted idea that dogwood anthracnose is not present in the central piedmont of N.C. A windshield diagnosis is by no means scientific and science is built on people proving that generally accepted ideas were wrong. (We now agree that the earth is round and revolves around the sun.) So I’ll be glad to accept disagreement and ask for evidence.
Meanwhile, spot anthracnose is very common in our part of the world. Its severity waxes and wanes largely with the weather. Plant diseases require a combination of ingredients to thrive: a susceptible host (such as dogwood), a pathogen (such as Elsinoe corni), an environment suitable for pathogen growth (spring-like temperatures and abundant moisture), and time sufficient for disease development. Since we have the host and the pathogen is very common locally, then we find that sometimes in spring the damp weather provides abundant opportunity for the disease and it only requires time for it to develop.
The pathogen is present but doesn’t thrive in the cold of winter. It survives in previously infected twigs, buds, and berries on the tree waiting for favorable conditions. In most years development of disease in spring is minimal. But in some years, the temperature is right and moisture is adequate for extensive development of the characteristic tiny spots with purple borders and lighter centers. They are very obvious on the white flowers. They are less obvious on the leaves, but we’re sure to find them there later if we look. Under suitable conditions the spots may enlarge or be so numerous that they grow together leaving just a brown blotch. Bracts or leaves may develop a puckering around the spots.
Fortunately, the disease is only cosmetic and has little negative effect on the health of the tree (unless it is in poor health otherwise). Unfortunately, dogwoods are primarily appreciated for their cosmetic contribution to the landscape. Anything that makes them less attractive gets our attention. But the damage is done at this point. As the temperature has become suitable for dispersal of spores, rain has cooperated to splash through the canopy, facilitating dispersal and infection. There is little we can do at this point that would make any difference. The time to initiate use of fungicides was before the buds began to swell. We can’t cure it now.
According to NC State plant pathologist, Dr. Kelly Ivors, what you can do is “prune out and destroy dead and dying twigs and branches and rake up fallen leaves to help reduce potential sources of inoculum and improve tree appearance. It is also advisable to prune out water sprouts which develop on the trunk or main scaffolding limbs since they are very susceptible to infection from anthracnose. Prune only under dry conditions and sterilize pruners with alcohol between pruning cuts.” Good sanitation is especially important for trees infected with spot anthracnose. Unfortunately, the weather next spring may have a trump card to play with greater impact on the pathogen than options available to us now.
alcooke
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Al Cooke
Extension Agent, Horticulture
Chatham County Center
N.C. Cooperative Extension
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