A number of methods are used in potato programs to achieve successful crosses. The cut stem technique is used in many of them and it usually achieves good results. However sometimes berries turn black, produce sterile seed and drop off prematurely. No pathogen was associated with the disease. Six water sources were compared including potable water, water from a deep tubular well, water from a superficial well, Hoagland’s nutrient solution, distilled and deionized water. Flowering stems of cv Yungay were cut, pollinated with pollen from clones R 128.6 and placed in bottles with water from different sources. Hoagland’s nutrient solution and water sources with highest boron content produced lowest incidence of blackened berries. An additional experiment testing 6 macrometabolic and 6 micrometabolic elements including B produced the following results: Treatments containing B produced normal berries; whereas treatments without B produced around 90% of blackened berries and/or drop off. It was thus demonstrated that this abnormality was caused by B deficiency which could be corrected by addition of 2 ppm of boric acid. Solutions containing greater than 5 ppm caused phytotoxicity in leaves of flowering stems. © 1991, Springer. All rights reserved.