Dying branches on a red azalea
Individual branches began dying on a 35 yr-old red azalea that has bloomed beautifully for over 35 years. Image 2094 and 2095 show some of the new die-off. The large bare spot on IMG_2092 shows the space left by many dead branches that died this spring as well as a few new dead branches. How can I arrest this gradual die-off? These images were taken after i pruned it in may this year. History: For many years the azalea suffered from lace-bugs (on which a soap spray didn't work) which I controlled with malathion. About four years ago, I had to treat a nearby dwarf Alberta spruce with Bayer 3-in-1 insect control and found that it also controlled the lace bugs so I discontinued the malathion. The azalea gets full sun in the afternoon. The soil is not compacted. It's been pruned by hand into the ball shape you see, cutting the shoots and about every 5 years more severely to control size--always by hand, The azalea was purchased from, and planted by a local (Howard County) nursery. It's about 3.5 ft in height, and is the only one of three like it, planted at the same time, with this die-off problem.
Based on your photos the azalea may be subject to a canker disease called Botryosphaeria dieback. Symptoms are scattered dying branches on an otherwise healthy plant Cankers are more common on stressed plants due to poor maintenance or site problems. The pathogen can infect all ages of stem tissue through wounds, pruning cuts, and leaf scars. Heat, drought stress, and winter injury can increase disease incidence. Your azalea looks like it is planted in a tough site. It is located in a sunny area near a driveway which can absorb and reflect heat. This makes the area hotter. Azaleas are prone to lacebugs in a sunny area.
AT this point, all you can do is prune dead wood or branches. Make sure mulch is no thicker than several inches and keep away from the base of the stems. Keep the shrub well watered during dry periods. Azaleas grow best in morning sun and afternoon shade.
Here is our publication on this issue page 5 http://extension.umd.edu/sites/default/files/_images/programs/hgic/Publications/HG51_IPM_Azaleas_and...