CONTROL OF DOWNY MILDEW IN GREENHOUSE-GROWN CUCUMBERS USING BLUE PHOTOSELECTIVE POLYETHYLENE SHEETS

Six types of polyethylene sheets with or without a blue pigment, havin g an absorption peak at the yellow part of the spectrum (580 nm), in c ombination with three levels of UV-B (280 to 320 nm) absorbance, were investigated for their effects on sporangial production and colonizati on of Pseudoperonospora cubensis on cucumbers in growth chambers. The effect of these photoselective sheets on the epidemiology of downy mil dew in greenhouse-grown cucumbers has been investigated in several loc ations. The addition of the blue pigment to the films resulted in a si gnificant inhibition of colonization and sporangial production of P. c ubensis, whereas filtration of the UV spectrum enhanced the colonizati on but had no effect on the sporangial production. The appearance of t he first symptom-bearing plants was delayed under the blue covers, and consequently, a significant reduction in the disease incidence of dow ny mildew was recorded under all blue sheets at each corresponding lev el of UV-B transmittance in five different field experiments through f our seasons. Regardless of the differences in disease incidence, there were no significant differences among the yields that were obtained u nder the various sheets, probably due to the lower photosynthetically active radiation transmissivity of the blue films. The optimal feature s required for a desirable commercial sheet are discussed.