Guignardia/Phyllosticta shows up in 2 different ways: diamond shaped spots, and little black dots, there's both. What's "funny" is the line pattern but it's not incompatible.
The trouble with viruses is that they weaken the plant so there can be multiple diseases at the same time (and I don't believe all are accounted for already) and if there's co-infection with the current CYMV and ORSV, the result can be difficult to describe, and even then can be mistaken for something else.
You guessed right from a series of observations but it's the test that did the trick. The observations could have been "wrong" for a virus diagnose.
I'm currently geeking in the phytopathology domain, and the more I go thru, the more I see we don't know that much in fact. And precise diagnosis is difficult, in fact, we should be happy to tell without a lab that this is bacteria, that fungus and the other there virus infected. It works not that bad for bacterias and fungi, but certainly not for viruses.
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