Cucumbers turn orange when they grow excessively ripe before harvesting, explains Veggie Gardener. The cucumbers first turn yellow, and if left on the vine, they quickly develop a vibrant orange hue. This happens because chlorophyll levels decrease past the point of peak ripeness. Orange cucumbers are very bitter and unsuitable for human consumption.
The only way to prevent cucumbers from turning yellow and orange is to harvest them at the proper time. Ripe cucumbers have firm flesh with a medium-green rind and feel heavy for their size. Most varieties ripen between 50 and 70 days after planting.
Size is also an important indicator of ripeness. Each cucumber variety has a different optimal size and quickly develops a bitter flavor if allowed to grow larger. Some cucumbers, such as those used for pickling, are naturally smaller than other varieties. Consequently, gardeners must know what type of cucumber they have planted and the target size for ripe specimens in that category.
The most common cause of orange and yellow cucumbers is over-ripening, but the discoloration is sometimes a symptom of the Cucumber Mosaic Virus. According to Gardening Know How, the Mosaic Virus produces soft, mushy cucumbers with mottled patches and curled, withered leaves. This incurable virus also affects peppers. When a cucumber displays symptoms of the Mosaic Virus, the best course of action is to remove it from the garden.