This is a long shot but deficiency in either calcium or boron can cause the death of the growing point in cucumbers, but I am sure you would see symptoms in other plants. Calcium is common in most soils (though poor compost mixes can be deficient) but uneven watering schedules can cause calcium uptake problems. Hard water contains calcium as well so it is even more unlikely in these areas (unless using rain water). Boron is common in plant matter so good compost should be fine, or mixing home compost into soil. You should be able to find 'Borax' or similar to rectify a deficiency.