yellow streaked and spotted phal leaves

[NYCorchidman]

Orchid Culture - Genus

This is a good website with pictures and description of the main orchid diseases.

Many perfectly healthy looking phals carry virus affecting other plants. I know, it sounds really crazy but that is the danger of virus as many infected plants are asymptomatic.
This is why it is so important to practice sanitary and caution anytime working with orchids.

So yeah, virused but asymptomatic orchids can suddenly exhibit symptoms for some reasons.

The different between fungal and microfungus, well, it can be difficult because there are so many different fungal disease. The most common fungal disease that affect phals are probably the crown rot and the black rot of the roots.
The microfungus shows up on leaves as small brown sunken spots that get larger and larger eventually killing the leaves in a matter of just one or two weeks. I don't think yours look like it though on the second look. The leaf surface look even, right?

Only systemic fungicide will take care of microfungus. Many people find it easy to just toss the affected plants unless they are highly valuable. This is good because the disease is highly infectious. Many growers wrongly assume this disease as something else like mites or virus and do not toss or treat. As a result, they lose considerably amount of phals to this disease when it hits.

So I think we can safely narrow down the cause between viral or physiological. or something else we don't know.

I know. it's a headache. I hate diseases.