I have a rich soil combination of sandy loam (basic to my yard), my own organic compost, ground oak leaves, peat moss, wood chips, and dynadirt (treated human waste from the Denton waste treatment plant)--all tilled together into an 9-inch raised bed. One can easily dig down deep into the loose, black soil with bare fingers. The bed lies along a part of my back fence, and is under a filtered-light canopy of tall oak trees. This bed is the latest addition to a full shade garden running the length of my back property. The garden has all kinds of shade-tolerant plants, including Japanese maples, Dogwoods, Hydrangeas, about 60 azaleas, 15 varieties for ferns, vibernums, Elephant ears, Caladiums, Pittosporums, and the like. They look healthy to me.
Last Fall I planted 10 5-foot plants and shrubs of various kinds in this bed: azaleas, sweet olive, camelia, and gardenia. Even though the bed was prepared for about 3 months before I put plants in it, when I planted them I primed the holes with peat moss, and subsequently have kept the plants moist. I haven't observed any forms of scale, mites, or aphids in the area. The soil Ph is slightly acid after a light application of soil sulphur. I sprinkled Ironite very lightly over the soil, too. We do have moles, but they haven't been a problem. I've also captured an opossum within the past few weeks who was digging many places, but I haven't observed digging in this bed. I've used BT to control the Armyworms that descend from the oak trees in Spring (comparatively few of them this year) and also applied DE regularly to the Impatiens to control pillbugs.
After looking great until a month ago, three different adjacent plants now are dying of a wilt that begins on one side of the plant and gradually kills the entire plant. The afflicted plants are a camelia, azalea, and a sweet olive. The new growth on a limb will begin to wilt, followed by the gradually drying of the whole limb. Then the wilt moves to the next limb. The other plants in the bed and in the rest of the garden seem fine, except that I lost a new azalea in a front yard bed (similar conditions) that turned completely brown. Two other new azaleas next to it show a tiny amount of die-back at the tips of their leaves, but don't appear to be dying.
I thought I might have a drying problem with the peat moss repelling water from the roots, but I applied a wetting agent (dishwashing liquid) to the soil after I observed the initial wilt. I've also applied a light dormant oil during the cool part of the day (we've had a cool Spring). Neither has helped, and the plants continue to lose limbs. I've used no other chemicals, other than high nitrogen fertilizer, but I've not applied that to this particular bed. I water deeply at least once a week, and I have a sprinkler system that lets me mist the area 4 or 5 times a week to keep the hydrangia blooms in other areas of the garden from drooping.
Can you offer me some advice on this problem?