I've always had a few leaves looking like yours, including this year - being cukes it could be almost anything, including sun scorch if exposed to full sunlight. Once the leaf is damaged it is prone to fungal attack - I just cut them off to avoid encouraging the spread of botrytis in the GH.
As long as the plant is still growing strongly and producing I wouldn't be unduly worried unless the number of unhealthy looking leaves overtakes the number of healthy ones.