Farmers’ fertilizer strategies include preventive soil or foliar application of boron in fields with a history of boron deficiency. The aim of the study presented here was to obtain more precise information about the timing of fertilizer application taking into consideration the development of deficiency symptoms. Because the appearance of boron deficiency in field trials is very uncertain, pot experiments were performed in the greenhouse. Different fertilizer amounts applied to the soil at sowing caused plants to develop deficiency symptoms in some of the treatments. These symptoms appeared at different development stages and with different intensity. The deficiency treatments were used to study the effect of timing foliar sprays with sodium borate (times were: before, at, after or before-and-after beginning of symptom appearance). Severe boron deficiency reduced white sugar yield to 20 % of the maximum. Foliar fertilizer application was not sufficient to restore yield or quality completely under conditions of boron deficiency. Foliar spraying not later than the time of symptom appearance reduced white sugar yield by almost a half. If foliar application was delayed until symptoms had developed clearly, white sugar yield was less than 40 % of the maximum. Boron deficiency irrevocably affects plant development before symptoms appear.