broad beans - scorched? black edged leaves?

[matowakan]

I have noticed this morning that some of my broad beans have black edged leaves and I was wondering if this is down to scorching?
They are outside and otherwise seem healthy.
I am unsure what to do if it is scorch.
The black bits seem dry and are not spotted nor do I think they are chocolate brown in colour,def a blackish grey .
Some of the leaves are just at the edges whilst others seem to have it on the majority of the leaf.
Out of the 8 plants I have it is only 2 that have this problem
Any advice ?

[matowakan]

Thanks for reply,
I checked for blackfly but not that.
They are in a fairly exposed site but there has been no forecasted frost for this area for a while,I have been checking the weather every night for a couple of months.
I have now picked off the leaves with the black on so hopefully they will be ok but will keep an eye on them