Answer: Without seeing the fruit, it's hard to accurately diagnose the problem. It sounds as though you have a couple of things going on here. The white spots and underbelly problem is called belly rot and it's a fungal disease; the bitterness is caused by wide fluctuations in soil moisture. At this point in time (near the end of the growing season) I don't think there's much remedial action you can take. The fungus in question is soil-borne, meaning it can splash up onto your plant with rainfall or with water from the sprinkler. You might try covering the soil directly beneath your cucumber plants with several inches of compost or straw, or even several thicknesses of newspapers to keep water from splashing up onto the plants. A mulch will also help slow water evaporation and keep the soil more consistently moist. When you water your cucumbers, water slowly and deeply, then allow the top of the soil to dry out before watering again. If you decide to grow cucumbers next year, be sure to plant them in a different area of the garden so they don't have the same problems as this crop.
Best wishes with your garden~
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