This looks like excess fertilizer which leads to too much salt in the leaf. Cucumber are very susceptible to this. I am posting some excellent content from the New South Wales Government in Australia. http://www.dpi.nsw.gov.au/agriculture...
"Excess fertiliser (excess salt)
Cucumbers are sensitive to excess fertiliser, which increases soil salinity and causes toxicity, usually from nitrogen. Plants grown under saline conditions, where the soil or growing medium has high electrical conductivity (EC), are stunted and produce dark green, dull, leathery leaves that are prone to wilting. A narrow band of yellow necrotic tissue is often present on leaf edges. This can affect leaf expansion, causing a slight downward cupping of the leaf. After a serious water stress, the oldest leaves may develop a uniform pale green chlorosis and small necrotic areas within the leaf. If water supply is maintained, leaves may only develop a band of pale green tissue around their edges. Plants are likely to wilt in warm weather.
Leach the soil or growing medium with fresh water until the excess fertiliser is removed. Adjust the fertiliser program to ensure that rates do not exceed crop needs."
You mentioned that you gave extra boron
The same source says this about Boron toxicity
"Cucumber plants are very sensitive to high boron levels. In fact, boron compounds have been used as herbicides.
Boron toxicity is indicated by yellowing between the veins of older leaves. This is followed quickly by the development of small, brown necrotic spots, which eventually join to form large areas of dead tissue. At the same time, the newer leaves become chlorotic and are distorted because of damage in the bud. Few female flowers may develop.'
You should check for these brown spots.
I think you should try washing the soil out to remove the access fertilizer.