Poplar dying - unsure of disease

[Big J]

I've planted a fair few Poplar cuttings this year. These are in a park in the town. They've been doing really well, but three of them have suddenly died (not at the same time) when in apparently good health. 

 

Growth has been vigorous, they are planted by a stream and completely unstressed. The tree next to this one is a bushy 4ft tall with no I'll health but this one has succumbed.

 

Any help is much appreciated. 

 

 

[Will C]

Can’t help with what’s killed him but another time clear the turf back a couple of ft and give them some chip or other mulch. Grass is the biggest competitor to a young tree for moisture and nutrients. 
 

end of lesson on sucking eggs 😂🤐