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Anthracnose may be the culprit

  • March 7, 2020
  • James Campbell
  • Last spring, my ancient and beloved oak tree started dropping leaves as if it decided to switch from evergreen to deciduous. Adjacent oaks looked healthy and full of leaves, but when I looked up at mine, I saw more sky than canopy. The bark on the trunk looked healthy and showed no obvious signs of sudden oak death. This is when I learned about anthracnose.

    Anthracnose is the menacing name for certain leaf spot and twig blight diseases. There are many different (but closely related) anthracnose fungi and each fungus is specific to a particular host plant.  Anthracnose infects both deciduous and evergreen trees as well as shrubs. In some regions, it can infect turfgrass, cacti, succulents, fruits and vegetables. Despite the differing fungal causes, the way anthracnose spreads is the same.

    Anthracnose fungi produce numerous spores when they become active in the spring. These microscopic spores spread via rain or sprinkler water to new growth. After entering new leaves or newly expanded twigs, they germinate. If moist conditions prevail, a successive generation of spores will form within the leaf spots and spread throughout the canopy. Young leaves and shoots are generally the most susceptible while mature leaves tend to be resistant to infection. Leaf symptoms are often most severe on lower and inner branches.

    Shade trees such as sycamore, maples, elms and oaks are often host to anthracnose.

    In Marin county, all our native oaks can get anthracnose. Black oak seems to be most susceptible due in part to the fact that this is the first oak to break leaves. Valley and blue oak can also get anthracnose if it is a cool, damp spring. Coast live oak may also be affected, but their tough leathery leaves seem less susceptible than other species.

    On leaves, look for tan, brown or black tar-like spots. These spots can darken as they age and may also expand. Leaves can also become brown, curled, deformed or dead. A severe anthracnose infection of an oak will cause die back of current season twig growth. The time most people notice the anthracnose infection in their oaks is when the leaf drop begins. It will start in the spring and can continue into the summer. This can cause defoliation of between 50-75% of the leaves on an oak. When it happened to me, I thought my oak tree was dying.

    In most instances, anthracnose does not cause permanent damage to mature trees. When symptoms develop or become severe, anthracnose can’t be effectively controlled during the current season. Once the weather becomes dry and leaves mature, the spread of the disease will end, and the tree will begin to replace lost leaves. Generally, the disease is cyclical with several years between severe infections. Good sanitation helps, so be sure to rake and dispose of fallen leaves and twigs and prune native oaks by removing only small twigs and dead branches.

    While succulents may be some of the easiest plants to grow, they have a propensity to develop fungal diseases, including anthracnose, when moved out of their natural habitats. In my foggy, shady garden, succulents are the other place I have noticed anthracnose disease.  Anthracnose infects succulents that are grown in insufficient light, overwatered or watered using overhead irrigation. There will be brown lesions on the leaves or crown and there may be active pink, red or orange spore pads in the lesions. The remedy is simple: remove the infected plant to avoid further infection to other plants, remove overhead watering sources, or reduce watering.

    While it may look and sound scary, anthracnose is usually nothing to fear. To learn more about anthracnose and to see what other plants might become infected, visit the University of California Pest notes page: http://ipm.ucanr.edu/PMG/PESTNOTES/pn7420.html