Alternaria leaf spot usually seen in seedlings and older leaves. The main symptoms are appearance of yellow color spots. The center of the spots may be covered with sooty black masses of spores.
https://pnwhandbooks.org/node/2646/print

Whereas Itersonilia blight exhibit gray-green discoloration and wilting of the tips of the leaves.
http://www.apsnet.org/publications/pl...

Another way to identify the disease is looking for spores. Do you have microscope facility?
Itersonilia perplexansis is a basidiomycete fungus produces ballistospores (bilaterally symmetrical, lunate). The Alternaria spores are dark and multi-celled (image attached).
http://www.publish.csiro.au/dn/Fullte...