Farm Progress

Tree Talk: What did a wet 2016 mean for your trees? Be on the lookout for these possible diseases.

Fredric Miller

June 14, 2017

4 Min Read
sunshine in forest

The spring and summer of 2016 were very wet, with record-setting rainfall that broke a 101-year record for July and August, accompanied by high humidity. This combination of precipitation and humidity provided an ideal environment for foliar fungal leaf diseases such as apple scab and anthracnose.

Last summer, apple scab was found on common apple and crabapple, with dark green to olive green blotches on the leaves. Heavy scab levels can cause premature leaf drop by midsummer, as seen in 2016. Last year was a bit different, as most crabapples did not refoliate because of the continued high humidity and precipitation throughout July and August, which resulted in high inoculum levels (spores) well into early fall. Isolated years of heavy apple scab are usually not detrimental to trees, but repeated, multiple-year defoliations can predispose plants to invasion by borers, such as the flat-headed apple tree borer, and cankers. Fungal foliar sprays can be applied, but require a very regimented schedule.

Another group of diseases, anthracnose, was very common recently, and not just on sycamores. Anthracnose is a general term for many foliar diseases attacking a wide range of hosts, including but not limited to sycamore, maple, oak, ash and dogwood. Anthracnose is a foliar disease, infecting the foliage and causing black necrotic areas. Most anthracnose fungal species are host-specific.

Weather conditions promoting anthracnose are 50 to 55 degree F temperatures along with high humidity and rainfall. The fungus may also infect twigs.

There are differences in susceptibility within hosts. For example, white oaks are more susceptible to oak anthracnose compared to red oaks. In the case of sycamore anthracnose, the fungus also infects the twigs, resulting in stem cankers. Spores produced from fruiting bodies associated with twig cankers have a short trip from the twig to the new foliage, making leaf infection much more severe. In addition, twig infection may result in witch’s brooms with short internodes and a “bushy” growth habit. The witch’s brooms are easy to see during the winter months.

Several tree diseases and abiotic factors may resemble anthracnose. Early in the growing season, late-spring freezes and frosts may kill new growth. All the new leaves will be affected, and the entire leaf will probably be brown and may be killed. In addition, frost damage will extend across a wide variety of species and be very apparent in low-lying areas with cold air drainage. New growth will look normal.

Most foliar fungal leaf diseases, including anthracnose, are not lethal to trees. However, repeated defoliation events like in 2016 can lead to tree stress and predisposition to secondary lethal agents.

Oak wilt, on the other hand, is lethal to oaks, and trees must be treated to ensure survival. Anthracnose may be confused with oak wilt later in the season. Be sure to properly diagnosis the problem before employing management options. Listed below are some general diagnostic tips for comparing oak anthracnose and oak wilt. The only way to be absolutely sure is to send in samples to a plant clinic to confirm which fungus is involved. Keep in mind, a tree could have both oak wilt and anthracnose at the same time.

Oak wilt signs and symptoms
• red oaks very susceptible, but all oaks can be killed
• spreads through root grafts and bark beetles
• caused by vascular wilt fungus and produces fungal mats under bark
• red oaks can die within one year; white oaks may take several years
• leaves turn brown from tip and edges
• premature leaf drop and defoliation occur

Oak anthracnose signs and symptoms
• infects twigs, buds and leaves; distorts and kills leaves
• considered a minor stress; trees usually recover
• most severe on white oaks; red oaks mildly affected
• most prevalent during cool, wet springs and summers
• leaves have irregular, water-soaked blotches that start along veins
• leaves become distorted, cupped and drop from tree

Management options for oak wilt
• early diagnosis required; prevention best
• do not prune from April to June
• break root grafts between healthy and diseased trees
• injections of fungicides may be effective for white oaks with less than 30% crown dieback
• prune out infected branches
• split and dry oak firewood; chip and burn small branches

Management options for oak anthracnose
• fungicide injection treatments every two to three years may be effective, if warranted
• foliar fungicide foliar sprays every two weeks may be effective, if warranted
• use host plant resistance; plant Ovation and Exclamation London plane trees, which are less-susceptible
• reduce other stresses

Miller is a horticulture professor at Joliet Junior College, and a senior research scientist in entomology at The Morton Arboretum in Lisle. Email your tree questions to him at [email protected].


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