Foliar and root applications of the rare sugar tagatose control powdery mildew in soilless grown cucumbers

Powdery mildew is a constant threat to cucumber production in soilless systems requiring the intensive use of chemical fungicides to limit yield losses. However, the toxicity risks for the growers and the food safety issues linked to fungicide residues represent a major concern for crops with a frequent and extended harvest period, like cucumber, and create an urgent need of eco-friendly alternatives. The objective of this study was to assess the effectiveness of foliar and root applications of the rare sugar tagatose, against naturally occurring powdery mildew (Podosphaera xanthii and Golovinomyces cichoracearum) on cucumber plants grown in a soilless system under commercial-like greenhouse conditions. Foliar and root applications of tagatose reduced powdery mildew severity and incidence under conditions of low severity in untreated plants, but tagatose and the chemical standard strategy lost their effect in the control of powdery mildew when the disease severity was increasing steadily. The combination of sulphur with foliar applications of tagatose effectively reduced powdery mildew under high severity conditions. Overall, under high disease pressure, foliar and root applications of tagatose were more effective than the standard chemical treatment reducing powdery mildew disease incidence and severity. Foliar application of tagatose directly inhibited P. xanthii conidial germination. After root application, tagatose was translocated to the leaves guaranteeing a systemic protection with no accumulation in cucumber fruits. Root applications of tagatose represent a novel strategy for cucumber protection against powdery mildew and reduce fungicide applications.