Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
I recently received a question on why cucumbers were turning yellow and not staying green. If cucumbers are not over-mature then the main cause is poor leaf cover over fruits.
In high heat or with water stress, cucumber leaves wilt during the day and fruits become exposed to full sunlight. A process similar to sunburn occurs where chlorophyll is damaged in the fruit and fruits become yellow. A similar effect can occur when downy mildew affects leaves and there is a loss of leaf cover.
In addition, our research on loamy sand soils has shown the importance of nitrogen and sulfur to maintain fruit color and leaf cover.
Three years of research were conducted from 2011-2013 on the effect of fertility programs on pickling cucumber yield and quality. Processors were concerned that fruits were not as well colored in Eastern and Southern production compared to the Midwest and North Central states.
Color of pickles was regulated largely by N rate: the higher the rate, the more green color on the pickles. This was an interesting result as it was thought that extra N might cause added leaf cover and therefore more shade resulting in lighter fruit, but the opposite effect occurred. Treatments with additional sulfur when added as ammonium sulfate had darker pickles in some trials but not in others.
Yellowing in pickling cucumber fruit due to a loss of leaf cover because of disease (downy mildew).
Higher nitrogen can improve pickle color; however, this effect is variety dependent. The variety ‘Expedition’ responded to N more than the variety ‘Vlaspik’. There may be critical N rates for varieties to achieve highest color. However, this needs to be balanced with potential for delaying flowering and fruit set, excessive foliage production, and increasing the potential for disease incidence. Nitrogen sources did not vary in their effect on yield but they did vary in their effect on fruit color. We tested ammonium sulfate, urea, UAN solution and UAN solution with sulfur. The addition of ammonium sulfate as part of the nitrogen source was shown to increase the amount of dark colored areas in pickle fruits in some trials but not in others. By including a portion of the nitrogen source as ammonium sulfate, the same color improvement may be obtained without using higher total nitrogen rates.