In order of likelihood.

The mottling pattern of Downy Mildew is the most commonly confused. It's a fungus that often attacks cucumbers first. The pathogen overcame the genetic resistance claimed on all the old OP's 20 years ago.

Angular leaf spot is the second one. It has a narrower host range. And often will infect the cucumbers and melons but leave others alone.

There are also whitefly transmitted viruses like CYSDV or CGMMV which are more common in the subtropics or greenhouses. CGMMV is seedborne but easily tested so most larger suppliers do it regularly. The symptoms for both start on the lower leaves.

Vector feeding preference: aphids transmit CMV, ZYMV, PRSV and WMV. They sometimes have are shown to have feeding preference based upon leaf colour or species.

Seedborne disease - the only serious virus that is significantly seedborne in cucumbers is CGMMV. ZYMV has been shown to be weekly seedborne low rates in Cucurbita pepo. SqMV is transmitted in cantaloupe seed.

The only one you are likely to run into in OP seed is ZYMV in C. pepo. This is because it is endemic to the seed growing regions but infects the seed at low levels. Hybrids for commercial available almost all have ZYMV resistance either from GMO or interspecific crosses with C. moschata "Nigerian" and are not routinely tested. OP seed is grown as cheaply as possible and is more likely to be infected with seedborne diseases.

So what do you do:

Plant your cucumbers in a different location and watch them closely. If signs of disease show up, post a pic and tag me. I can send you down the right path.

I personally have transitioned to more modern hybrids with stacked disease packages. I like the parthenocarpic gyneceous ones because they do not require pollination to set fruit. They also yield a lot more.