I have a pretty unique opportunity going right now in one my test plots. The plot has a fairly decent run of Zythia going, at the same time one of the test materials is causing a slight burn on the leaves (it's an unregistered oil), so we have the chance to compare the two.
Those of you have called me concerning Zythia know that I look for the little brown to black spots representing fruiting bodies (thank you Steven Koike!) in the centers of the necrotic areas caused by the fungus to confirm it. Usually there are concentric rings of growth also, and while these aren't diagnostic they for sure point out that symptom in question is being caused by a living organism.
Contrast that with phytotoxic burn of the leaves and other plant parts. No fruiting bodies for sure, nor any rings of growth. Too, if one is familiar with how spray droplets disperse on the leaves, it should be a small matter to trace back areas of burn to where water would gather. Veins, edges of leaves and low spots on the foliage are common.
Let's go to the pictures to make my point, shall we?
Attached Images:
View from the top, distinct areas of tan colored burns distributed around the most exposed leaves.
Spray burn close up. Distinct tan area at leaf edge.
Leaf severly infected with Zythia fungus. Note the abundance of dark colored fruiting bodies towards the center of the spot.
Comparing phyto burn (leaf on the left) with Zythia (leaf on the right).
Challenge question. Which one is the phyto? Spoiler alert - the answer is now in the comments below.