Wilting and dying forsithia bushes
We have a forsythia hedge (the old-fashioned, pale yellow cultivar) in which several bushes have died showing wilting and browning symptoms starting with yellowing leaves localized on one or a few branches. Have treated with triazole fungicides without much success. Guidance please?
I am assumiing that you do not see galls on the stems of your forsythia since you did not mention them. Therefore your hedge is likely to be infected by the fungus Sclerotinia sclerotiorum. Prune out all infected branches. Clean your pruners with a 70% alcohol solution between cuts and dispose of all debris. Clean up and dispose of all of the leaf litter from around the plants now and in every coming season. To minimize recurrence of infection, avoid overhead watering as the fungus is readily spread by splashing water. Maintain good air circulation among the branches. This can be done by removing the oldest branches, cutting them to the ground. Your old-fashioned Forsythia is meant to be an open, arching shrub. If you are maintaining it as a closely trimmed and shaped hedge the resulting density of the stems and branches are making it especially susceptible to fungal diseases. Added to that, the excessive rain this season has provided an excellent moist, humid environment for fungi to flourish. This fungus can survive in soil for many years, therefore do not compost the cuttings and litter.
Using a fungicide may help control the spread of the disease, but it will not "cure" the affected branches. The best time to apply fungicide is prior to the appearance of symptoms, in this case as soon as the leaves appear. Be sure to follow the product directions for use and personal protection and to apply to the upper and lower surfaces of the leaves. You can then repeat this application throughout the season following the product directions. Consider applying thiophanate methyl + flutolanil fungicide such as SyStar. See http://extension.psu.edu/pests/plant-diseases/all-fact-sheets/forsythia-diseases for additional information from our Cooperative Extension colleagues at Penn State University.
I sincerely hope that you are able to save your Forsythia hedge. As a last resort you might consider cutting the shrubs down to within 18 inches of the group and sanitizing the area by cleaning away ALL debris. This is a drastic but not uncommon measure that can be taken to rejuvenate Forsythia. Be sure to treat the new growth with a fungicide as described above.