when you examine the whole garden, is there plenty of those ''v'' shaped chlorosis/necrosis leaf spots?
what ever it is ( and it definitely suggests verticillium very much ), do not fertilize any more- nitro would be like an atomic disease- fuel here, sterilize your hands and equipment when working with plants, do not work on plants on humid weather, etc.... and take one of those plants for a test somewhere
if you still can't make a positive lab- test, you may pick one of the plants which has only started developing symptoms and drench it with 2-3l of
1% copper oxychloride + 0.25% mankozeb mixture ( just as a test, both are organic fungicides and shouldn't make your honey kill you
). the test would most probably show some fito- toxicity ( mostly depends on the flowering stage of the plant ), but if it's verticillium it should also contain the disease to some point ( not fully, it would continue developing after some time ). anyway, such test could help you confirm/exclude this bugger if you don't have any other way to do it.
br,
ivan