The following are some diseases now active to be on the watch for in the landscape.
CEDAR-QUINCE RUST is present on Washington hawthorn, Crataegus phaenopyrum. Symptoms include old swollen twigs from last year and swollen deformed fruit (very evident now). Fungicide control is rarely needed. The same fungus infects serviceberry-- seen this past week. The alternate host is juniper, especially ground cover species. Cedar-quince rust rarely causes any serious damage to the host, just loss of fruit display and dead shoots, which can detract from overall aesthetics.
POWDERY MILDEW is present in many perennials. Look for infection on peony, Monarda (Bee-balm), garden phlox and other susceptible plants. It is controlled well with a number of fungicides (apply at the first sign of disease) as well as hort oil, neem oil, and potassium bicarbonate products.
SCLEROTINA SHOOT BLIGHT OF FORSYTHIA was diagnosed twice last week-- from a homeowner sample and one from my yard. Look for dead shoots that often have an orange discoloration. The fungus infects the blossoms and flower stalks, then grows into the twigs and kills them for some distance. The black sclerotia (fungus fruiting bodies) develop inside the infected twigs and are the best diagnostic sign at this point. Prune and destroy dead twigs and stems.
OAK LEAF BLISTER caused by the fungus Taphrina is a common disease during wet seasons of many oaks in the red oak group but mostly pin oak and red oak. The fungus overwinters in infected buds and infects the newly emerging leaf tissue. Symptoms include scattered circular raised areas of varying sizes up to ½ inch over the upper surface of the infected leaves causing a depression on the corresponding lower leaf surface. Lesions are yellow-green when new and brown with age. This disease causes no defoliation and rarely requires chemical control.
Information from Bob Mulrooney, Extension Plant Pathologist, UD