Absent an obvious cause like a fungus or leaf -striping insect this kind of die off can be an available nitrogen issue. Note the word "available;" a chicken manure based product that is not well composted can have tons of nitrogen but until it interacts with a good source of carbon it can be locked away. When it does interact, ( if it hasn't before hitting your soil) it can reach some series temps. I recently measured soil treated with a bag of an organic amendment at 150° F ten inches into the soil over the course of a week. Chicken manure can also be too "hot" because in the process of breaking down it creates ammonia, which is not good for most plants if applied directly to growing roots. Finally , you might want to amend with some woody compost, especially if you tend toward sand and shell. The woody matter interacts with the nitrogen heavy chicken waste to release the nitrogen, but also provides a home for helpful mycorrhizae ...