Suppression of Powdery Mildews by UV-B: Application Frequency and Timing, Dose, Reflectance, and Automation.

Disease-suppressive effects of nighttime applications of ultraviolet-B (UV-B) were investigated at two irradiance levels (1.6 or 0.8 W/m2) in strawberry and rosemary plants inoculated with Podosphaera aphanis or Golovinomyces biocellatus, respectively. Plants were exposed to each irradiance level every third night for either 9 or 18 min, every night for either 3 or 6 min, or three times every night for either 1 or 2 min. Thus, over time, all plants received the same cumulative dose of UV-B, and severity of powdery mildew was reduced by 90 to 99% compared with untreated controls in both crops. Use of polished aluminum lamp reflectors and UV-B reflective surfaces on greenhouse benches significantly increased treatment efficacy. An automated apparatus consisting of an adjustable boom with directed airflow was used to move UV-B lamps over greenhouse benches at 25 or 50 cm/min. Directed airflow moved leaves on the subtending plants to better expose upper and lower surfaces to UV-B but directed airflow actually decreased the efficacy of UV-B treatments, possibly by dispersing conidia from lesions before they were exposed to a lethal dose of UV-B. Results indicate broad applicability of nighttime applications of UV-B to suppress powdery mildews, and that cumulative UV-B dose is an overriding factor determining efficacy. Finally, enhanced suppression on shaded or obscured tissues is more likely to be affected by reflective bench surfaces than through attempts to physically manipulate the foliage.

Suppression of Cucumber Powdery Mildew by Supplemental UV-B Radiation in Greenhouses Can be Augmented or Reduced by Background Radiation Quality.

This study demonstrates that the spectral quality of radiation sources applied with ultraviolet-B (UV-B; background radiation) affects the suppression of cucumber powdery mildew (Podosphaera xanthii) by UV-B. Suppression provided by daily UV-B exposure of 1 W/m2 for 10 min was greatest in the presence of red light or by a complete lack of background light, and powdery mildew suppression was least in the presence of ultraviolet-A (UV-A) or blue radiation compared with plants exposed only to 16 h of daily natural light supplemented with high-pressure sodium lamps that supply broad-spectrum radiation with peaks in the yellow-orange region. Exposure of powdery mildew-inoculated plants to supplemental red light without UV-B, beginning at the end of the daylight period, also reduced disease severity; however, supplemental blue light applied in the same fashion had no effect. Daily application of UV-B at 1 W/m2 beginning on the day of inoculation significantly reduced the severity of powdery mildew to 15% compared with 100% severity on control plants. Maximum suppression of powdery mildew was observed following 15 min of exposure to UV-B (1.1% severity compared with 100% severity on control plants) but exposure time had to be limited to 5 to 10 min to reduce phytotoxicity. There was no additional disease suppression when plants were exposed to UV-B beginning 2 days prior to inoculation compared with plants exposed to UV-B beginning on the day of inoculation. UV-B inhibited germination, infection, colony expansion, and sporulation of P. xanthii. The results suggest that efficacy of UV-B treatments, alone or in combination with red light, against P. xanthii can be enhanced by exposure of inoculated plants to these wavelengths of radiation during the night, thereby circumventing the counteracting effects of blue light and UV-A radiation. The effect of UV-B on powdery mildew seemed to be directly upon the pathogen, rather than induced resistance of the host. Night exposure of plants to 5 to 10 min of UV-B at 1 W/m2 and inexpensive, spectral-specific, light-emitting diodes may provide additional tools to suppress powdery mildews of diverse greenhouse crops.

Suppression of Powdery Mildew (Podosphaera pannosa) in Greenhouse Roses by Brief Exposure to Supplemental UV-B radiation.

Ultraviolet (UV)-B (280 to 315 nm) irradiance from 0.1 to 1.2 W m-2 and exposure times from 2 min to 2 h significantly suppressed powdery mildew (Podosphaera pannosa) in pot rose (Rosa × hybrida 'Toril') via reduced spore germination, infection efficiency, disease severity, and sporulation of surviving colonies. Brief daily exposure to UV-B suppressed disease severity by more than 90% compared with unexposed controls, and severity was held at low levels as long as daily brief exposures continued. Selective removal of wavelengths below 290 nm from the UV lamp sources by cellulose diacetate filters resulted in significant reduction of treatment efficacy. Exposure of plants to 2 h of UV-B during night for 1 week followed by inoculation with P. pannosa did not affect subsequent pathogen development, indicating that the treatment effect was directly upon the exposed pathogen and not operated through the host. Following 20 to 30 days of exposure, chlorophyll and flavonoid content was slightly higher in plants exposed to the highest UV-B levels. Brief daily exposure to UV-B for 5 min at 1.2 W m-2 or 1 h at 0.1 W m-2 substantially reduced mildew severity without significant phytotoxicity, and may represent a useful nonchemical option for suppression of powdery mildew in greenhouse roses and, possibly, other crops.

Specific Light-Emitting Diodes Can Suppress Sporulation of Podosphaera pannosa on Greenhouse Roses.

When rose plants bearing colonies of Podosphaera pannosa were placed in a wind tunnel, the number of conidia trapped was directly proportional to intensity of daylight-balanced (white) light from 5 to 150 µmol m-2 s-1. Illumination of samples using blue (420 to 520 nm) light-emitting diodes (LEDs) increased the number of conidia trapped by a factor of approximately 2.7 over white light but germination of conidia under blue light was reduced by approximately 16.5% compared with conidia germination under white light. The number of conidia trapped under far-red (>685 nm) LEDs was approximately 4.7 times higher than in white light, and 13.3 times higher than under red (575 to 675 nm) LEDs, and germination was not induced compared with white light. When mildewed plants were exposed to cycles of 18 h of white light followed by 6 h of blue, red, far-red light, or darkness, light from the red LEDs reduced the number of conidia trapped by approximately 88% compared with darkness or far-red light. Interrupting the above dark period with 1 h of light from red LEDs also reduced the number of conidia trapped, while a 1-h period of light from far-red following the 1 h of light from red LEDs nullified the suppressive effect of red light. Our results indicate that brief exposure to red light during the dark interval may be as effective as continuous illumination in suppressing powdery mildew in greenhouse rose plant (Rosa × hybrida).

Continuous Lighting Reduces Conidial Production and Germinability in the Rose Powdery Mildew Pathosystem.

The effect of day length on production and germinability of conidia and severity of disease caused by Podosphaera pannosa, the causal agent of rose powdery mildew, was studied. Whole potted plants or detached leaves of Rosa interspecific hybrid 'Mistral' were inoculated with P. pannosa and exposed to 0, 12, 18, 20, 22, or 24 h of artificial light per day in growth chambers equipped with mercury lamps. Increasing duration of illumination from 18 to 20 to 24 h per day reduced production of conidia by 22 to 62%. Exposure to 24 h of illumination per day also strongly reduced disease severity compared with 18 h. Our results suggest that increasing day lengths from 18 h per day to 20 to 24 h may suppress the disease significantly and, thereby, reduce the need for fungicide applications against powdery mildew.