Can anyone speculate on what this may be caused by?

[Ray]

Not my plant, and it appears to be in pretty good shape otherwise,

 

[big923cattleya]

Could it possibly be viral in nature? The symptoms are in such straight lines running the way that the veins would be laid out within the leaf.

 

[kman]

I've seen something similar in the past on Zygopetalum and Dinema, and written it off to fertility issues. Always seemed to clear up.

 

[gonewild]

Could be magnesium deficiency.

 

[Ray]

I do not know the feeding regimen of that grower, but I have asked, but he stated this is a plant he has had for about 10 years.

FWIW, my plants have gotten K-Lite (12-1-1) for the last 13 years, and I have yet to see any such deficiency.

 

[gonewild]

I corrected my comment error from potassium to magnesium deficiency.

 

[TyroneGenade]

Manganese deficiency?

"Manganese (Mn) deficiency is a plant disorder that is often confused with, and occurs with, iron deficiency. Most common in poorly drained soils, also where organic matter levels are high. Manganese may be unavailable to plants where pH is high." From Wikipedia. Perhaps it is time to repot?

 

[IvoPhal]

I don’t think that is feeding problem or some nutrient deficiency. On the left side the color is pretty dark green. If we have more in information from the owner …

 

[Ray]

I think he may be poisoning his plants.

He fertilizers at about 25 ppm N most waterings; plain water once a month, but he only pours about a cup of solution per watering, with no flooding/flushing.

Editorial comment about nutrient deficiencies:
I think most of the published diagnostics don't work well with orchids. They are so slow growing that deficiencies tend to be expressed as slower growth, rather than outward symptoms.

 

[Ozpaph]

what does the underside of the leaf look like? Old or new growths?