Landscape - Spring Diseases

Periodic rainy days and nights have been occurring recently in Delaware. Wet weather leading to prolonged leaf wetness as new foliage is emerging is very favorable for many landscape tree diseases. Symptoms of scab, rust, and anthracnose diseases are already beginning to appear in many landscapes.

Apple scab. Unsprayed, susceptible flowering crabapples and some backyard apples are already showing symptoms of leaf infections by the scab fungus, Venturia inaequalis. Long (15-18 hours) periods of leaf wetness are were favorable for new apple scab infections in. If trees were not already infected, expect symptoms developing from recent infections to appear in 10-12 days.

Cedar apple rust and cedar quince rust. The causal fungi, Gymnosporangium juniperi-virginianae and Gymnosporangium clavipes are still active on infected cedar trees. Symptoms on flowering crabapple and apple are already visible in some locations.

Fire blight (Erwinia amylovora). The primary infection season on flowering pears and crabapples and backyard apples and pears is over since most blooms have dropped flower petals by now. Depending on location, landscapes in Delaware were exposed to one or two fire blight primary infection periods based on favorable weather conditions this spring. If fire blight infections occurred earlier this season, symptoms in the fruiting spurs should be visible now. Look for blossom blight symptoms in the form of blackened, dead fruitlets in the fruit spurs throughout the tree. If there are not too many of them, infected clusters can be broken out by hand. Shoot blight symptoms, if they are going to occur, will appear in the coming weeks.

Anthracnose diseases. Where cool, rainy weather has prevailed, it is likely that sycamore (Apiognomonia veneta), oak (Apiognomonia quercina), maple (Discula sp. or Kabatiella apocrypta), and dogwood (Discula destructiva) anthracnose diseases will be active.

Adapted for Delaware from "SPRINGTIME DISEASES IN THE LANDSCAPE" By John Hartman in the May 5, 2008 edition of the Kentucky Pest News from the University of Kentucky College of Agriculture.