It's a potash deficiency guys. A little hard to correct if they're already well into production. You might try a really weak brew of potassium permanganate. (AKA Condies Crystals) Available from some nurseries or a chemist if all else fails. Use a level tablespoon to a gallon of water, or half this rate to be less volatile. This is not an organic practice and may cause some effect on earthworms. If your plants
are already past their prime this will be a waste of time. It often happens toward the end of a crop as potassium is used by the plant
. As a nutrient it is leached from the soil by watering and so between that and the plant
using it , it can deplete over the season.
Next year, add some sulphate of potash to the soil when you're preparing the ground prior to sowing. Also if you add zeolite, a natural mineral, the potassium adheres to it at a molecular level and prevents the leaching.
Hope this info helps.