Vmachnig, do you have any single trunk specimens at all? And if so, have any of them died? Can you do a bit of forensics here and check to see if the ones with multiple leaders are in fact multiple stems from a single trunk or multiple (cluster) trees intertwined?
You state that you have not seen any pests but used insecticide anyway. Please do not do that. You are killing any beneficials that might be around and upsetting a natural balance that is necessary to maintain a healthy landscape.
I really don't think this looks like herbicide damage in the fact that I believe you would see a different pattern to the die back rather than selective, as pointed out by Urban Forester. But I encourage you to minimize use of that product as well. Maintaining a moderately thick, constantly decomposing mulch layer in conjunction with proper irrigation are the two best cultural practices you can do for any tree.
If this is a natural dieback from the overcrowded situation, the remaining trees may be fine and in fact better in the long run.
Sylvia
You state that you have not seen any pests but used insecticide anyway. Please do not do that. You are killing any beneficials that might be around and upsetting a natural balance that is necessary to maintain a healthy landscape.
I really don't think this looks like herbicide damage in the fact that I believe you would see a different pattern to the die back rather than selective, as pointed out by Urban Forester. But I encourage you to minimize use of that product as well. Maintaining a moderately thick, constantly decomposing mulch layer in conjunction with proper irrigation are the two best cultural practices you can do for any tree.
If this is a natural dieback from the overcrowded situation, the remaining trees may be fine and in fact better in the long run.
Sylvia
