The cool, wet spring has created perfect conditions for leaf and stem diseases on woody ornamentals this year. American Sycamore (Platanus occidentalis) and London Planetree (Platanus x acerifolia) are showing distinctive bud and shoot failure and leaf blights due to sycamore anthracnose disease caused by the fungus, Apiognomonia veneta. Infected trees are busy producing new buds that will form new leaves and twigs once weather becomes warmer and drier.
Sycamore anthracnose overwinters in the twigs and branches, becoming active in the spring while the tree is still dormant. During wet springs, fungal fruiting structures produce spores that infect emerging shoots and developing leaves. Emerging shoots and leaves rapidly wilt and die, causing great concern to the observer. Most of the time, infected trees produce a healthy amount of new growth by mid summer and can literally grow out of the disease once the weather becomes warmer and drier.
For newly planted Platanus sp. trees or trees that are experiencing other forms of stress (e.g. other pest pressure, root zone issues), it may take longer for a healthy set of foliage to appear after spring infection. Daconil, Dithane and Manzate fungicides are sometimes used to help protect stressed specimens. Where there is a history Sycamore anthracnose is the area, you may want to choose anthracnose-tolerant cultivars of London Planetree (a hybrid of American sycamore and Oriental sycamore, Platanus orientalis). Try ‘Bloodgood’ Planetree (Platanus x acerifolia ‘Bloodgood’) or Exclamation! Planetree (Platanus x acerifolia ‘Morten Circle’).
Malformed and stunted leaves with brown lesions on ash (Fraxinus spp.) is likely due to ash anthracnose, caused by the fungus, Gnomoniella fraxini. Ash anthracnose is common during cool, wet springs and is not a cause for concern in tree health. Infected trees will usually flush out again, masking the symptomatic leaves in the next few weeks.
Malformed and stunted leaves with brown lesions on oak (Quercus spp.) is likely due to oak anthracnose, caused by the fungus, Apiognomonia quercina. Oak anthracnose is common during cool, wet springs and is not a cause for concern in tree health. Infected trees will usually flush out again, masking the symptomatic leaves in the next few weeks.
Several fungal pathogens are responsible for maple anthracnose such as: Aureobasidium apocryptum (syn. Kabatiella apocrypta), Discula campestris and Colletotrichum gleosporoides. In addition, there is also maple leaf blister, caused by the fungal pathogen, Taphrina. Both leaf diseases are common this spring and can even be found on the same leaf.
These can be common on silver, red and (to a lesser extent) sugar maples and can be difficult to distinguish. Just like the previous diseases, those on maple will likely be masked by a new flush of leaves in the next few weeks.
Keep a note of trees that are experiencing moderate to severe symptoms and ensure that they receive supplemental irrigation if (and I mean “IF”) weather turns hot and dry.