LEDs are increasingly available and more efficacious (PPFD per watt, PPFD/W) than incumbent high-intensity discharge (HID) light sources, making them attractive for horticulture (e.g., Gomez and Izzo, 2018; Jones, 2018; Massa and Norrie, 2015). LED products for the horticulture market are predicted to further improve in terms of increased efficacy and decreased cost over the next 5 years (Pattison et al., 2016).
For a given PPFD, many available LED horticultural luminaires enable growers to reduce the electrical power required for photosynthetic crop production from top lighting in controlled environments relative to incumbent HID luminaires (Radetsky, 2018). Two types of narrowband LED spectra are commonly available in commercial horticultural luminaires: blue LEDs (λmax ≈ 450 nm) and red LEDs (λmax ≈ 660 nm). The present study used commercial horticultural LED luminaires to examine the ability of red and blue LEDs to reduce DM disease pressure on basil commonly grown in controlled environments.
Basil DM, caused by Peronospora belbahrii, is a devastating disease that can cause economic havoc for sweet basil growers worldwide, leading up to 100% yield losses. Some resistant and/or tolerant varieties of sweet basil are available (e.g., U.S. Patent 10159212; Simon et al., 2018), but many desirable commercial varieties are still susceptible to DM. Basil DM spreads via windborne spores and infected seeds or seedlings (Farahani-Kofoet et al., 2012; Garibaldi et al., 2004).
Peronospora belbahrii sporulate at night (Yarwood, 1937); however, sporulation can be inhibited if DM-infected leaves are exposed to sufficient amounts of broadband (Cohen et al., 1978; Yarwood, 1937) or narrowband light spectra (Cohen, 1976; Cohen et al., 2013; Patel et al., 2016) at night. Continuous nighttime light exposures ranging from 3.7 to 240 µmol·m−2·s−1 have been shown to inhibit sporulation on DM-infected leaves (Cohen, 1976; Cohen and Eyal, 1977, 1980; Cohen et al., 1978, 2013; Nordskog et al., 2007; Patel et al., 2016). In addition, intermittent broadband light spectra from 40 to 200 µmol·m−2·s−1 have been shown to suppress DM sporulation by 90% or more (Cohen et al., 1978; López-López et al., 2014). Cruickshank (1963) found that brief, intermittent light treatments (<1 h light with <2 h dark) using broadband spectra at moderate irradiances were more effective at suppressing sporulation of Peronospora tabacina (tobacco DM pathogen) than longer intermittent light treatments (≥2 h light with ≥3 h dark) at the same irradiance (9.8 µmol·m−2·s−1) and for the same cumulative darkness duration. On the basis of this literature, to reduce DM sporulation, darkness duration must be limited and/or the irradiance must be high.
Because many growers will have already installed LED luminaires with blue and red LEDs in their controlled environments, the present study was designed to investigate the added value of using commercially available red (λmax = 670 nm) and blue (λmax = 458 nm) LED top lighting at night to suppress basil DM sporulation. In the context of maximizing lighting energy efficiency, the present study was designed to compare the efficacy of continuous nighttime narrowband LED exposures with shorter nighttime narrowband LED exposures for suppressing basil DM sporulation.
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