Abstract
The effect of supplemental lighting on strawberry growth and anthracnose disease response of three strawberry (Fragaria ×ananassa) cultivars was evaluated in two greenhouse trials, and the effect on strawberry anthracnose pathogens (Colletotrichum sp.) was evaluated in the laboratory. The objective of the greenhouse trials was to determine the effect of various intensities of the red and blue light emitting diode (LED) light treatment on strawberry plant vigor, injury, and disease development. In these trials, the duration of supplemental light treatments was split into two 4-hour periods: dawn and dusk. The intensity of the red and blue LED bulbs was set using an adjustable dial at 1 or 3 in trial 1 and at 1, 5, or 10 in trial 2. Illuminance and photosynthetic photon flux densities of the light treatments ranged from lows of 402 lx and 5 μmol⋅m–2⋅s–1 (blue LED 1) to highs of 575 lx and 25 μmol⋅m–2⋅s–1 (red LED 1 + blue LED 3) in trial 1, and from lows of 4213 lx and 81 μmol⋅m–2⋅s–1 (red LED 1) to highs of 7051 lx (red LED 5) and 194 μmol⋅m–2⋅s–1 (red LED 10) in trial 2. Lower light intensities in trial 1 resulted in no significant differences as a result of light treatments in relative chlorophyll content, plant vigor ratings, or disease severity ratings (DSRs). However, plant injury ratings were significantly greater in plants in the wide-spectrum fluorescent (WSF) plus ultraviolet B (UVB) light treatment compared with the other treatments. Under the higher light intensities in trial 2, there were more significant effects among light treatments. Relative chlorophyll content of plants in the WSF + UVB, WSF, and red LED 1 treatments was significantly greater than that of plants in the red LED 10 treatment; however, plants in the red LED 10 treatment had the greatest injury ratings. Detached leaves from plants in the red 5 LED and red 10 LED treatments inoculated with Colletotrichum gloeosporioides received the greatest DSRs, and leaves from plants in the red LED 1 and WSF treatments received the lowest DSRs. In the laboratory, five days of exposure to supplemental lights did not prevent the growth of isolates of three species of Colletotrichum pathogens even though the intensity of the LED lights was set at their highest intensity. However, growth of isolates exposed to the WSF + UVB light treatment was slowed.
Light is necessary for plants to create nutrition, grow, and reproduce; and various wavelengths of light have different effects on plant growth and general plant health. Most wavelengths are beneficial, but some can be harmful at certain intensities. Supplemental lighting in greenhouses and high tunnels often include light-emitting diodes (LEDs) as a means of extending daylength and increasing seasonal strawberry production, and in some cases, plant growth chambers only use LEDs as a lighting system. Compared with fluorescent tubes, LEDs have the added benefit of producing narrow bands of specific wavelengths that can influence certain growth processes selectively in plants [e.g., red (630–700 nm) wavelengths are often used to optimize photosynthesis]. Red and far-red (222 nm) wavelengths are important for shoot/stem elongation, phytochrome responses, and changes in plant anatomy (Schuerger and Brown 1997); red wavelengths also inhibit flower bud initiation in fall flowering strawberry cultivars (Takeda et al. 2008); and blue (435–500 nm) wavelengths enhance plant quality, plant growth, and biomass by promoting chlorophyll biosynthesis, stomatal opening, enzyme synthesis, phototropism, and photosynthesis (Johkan et al. 2010; Muneer et al. 2014).
Supplemental light quality, especially red, blue, green, and violet wavelengths, has been shown to influence plant disease. For example, exposure to red LED light (625–740 nm) (Jones 2020)] at an intensity of 12 μmol⋅m–2⋅s–1 photons reduced the severity of downy mildew in basil (Patel et al. 2016), cucumber plants at the four-leaf stage grown under red light were more resistant to the powdery mildew pathogen and expressed a stronger level of defense genes than plants grown under white light (Wang et al. 2010), pretreatment of broad bean leaflets with red light for 24 h before inoculation suppressed Alternaria leaf spot lesion development and induced resistance to leaf spot disease (Rahman et al. 2003), and broad bean leaflets inoculated with the gray mold pathogen (Botrytis cinerea) and exposed to red light for 48 h induced a relatively high-molecular weight, water-soluble, heat-stable, and fungi-specific antifungal substance (Islam et al. 1999). On detached grape leaves inoculated with the gray mold pathogen, exposure to combinations of blue (450–495 nm) and red LED light inhibited lesion development, induced an accumulation of disease-fighting polyphenols known as stilbenes, and slowed disease development by upregulating defense-related genes (Ahn et al. 2015). Imada et al. (2014) determined that development of gray mold symptoms in detached tomato leaves inoculated with B. cinerea spores was reduced significantly by irradiation with 405-nm light (violet light, 380–435 nm), and that mycelial growth of B. cinerea was inhibited by light at 405 and 415 nm. Kudo et al. (2011) reported that green light (480–570 nm) irradiation inhibited strawberry anthracnose incidence, promoted growth of plants and enlargement of fruit, and was effective against spider mites in fields.
Electromagnetic wavelengths, such as ultraviolet (UV), which are invisible to humans, can impact plant health and reproduction crucially, both negatively and positively (Sampson and Cane 1999; Teramura and Sullivan 1994). UV light is comprised of three narrower bands of shorter wavelength electromagnetic radiation: UVA and ultraviolet UVB, which reach Earth’s biosphere; and ultraviolet C, which is totally absorbed by Earth’s upper atmosphere and only occurs at the troposphere layer through artificial means. At increased irradiances, UVB (280–320 nm) is known to harm plants (Pancotto et al. 2005; Rousseauxy et al. 2004). However, ambient UVB or a brief exposure to supplemental UVB can benefit plants by suppressing foliar and fruit diseases. In strawberry, for instance, UVB lessens powdery mildew symptoms on fruit, improves fruit color, stimulates the production of antifungal substances, and upregulates genes that produce enzymes capable of conferring disease resistance (e.g., chalcone isomerase and synthase, phenylalanine ammonia lyase, and β-1,3-glucanase) (Kanto et al. 2009, 2014). Likewise, night irradiation of greenhouse roses with UVB was more effective than daytime treatments for suppressing powdery mildew disease while minimizing damage to plants (Suthaparan et al. 2012, 2016).
Many other plant pathogens could be controlled fully or partially using visible and invisible light sources. Anthracnose diseases of strawberries cause substantial losses on crops grown in warm, humid areas (Howard et al. 1992; Louws et al. 2019; Peres et al. 2017; Smith 2008). One of these diseases is anthracnose crown rot or Colletotrichum crown rot, the primary causative agent of is Colletotrichum gloeosporioides. Effective fungicides are available for control of anthracnose crown rot on strawberries, but management becomes difficult when environmental conditions favor the pathogen’s development. In addition, overuse of fungicides with the same mode of action without sufficient rotation often results in fungal pathogens such as Colletotrichum sp. developing resistance (Dowling et al. 2020; Smith et al. 2013).
Smith et al. (2022) reported a harmful effect of high-intensity red LED irradiation on strawberries grown in a greenhouse (i.e., higher plant injury scores, lower plant vigor scores, and lower relative chlorophyll content values) and a beneficial effect of wide-spectrum fluorescent (WSF) + UVB light treatment (i.e., reduced disease severity ratings in strawberry plants inoculated with a C. gloeosporioides isolate). However, greater intensity light treatments in their study, especially high-intensity red LED light, led to nutrient deficiencies and reduced vigor. In response to these paradoxical findings that the high intensity of supplemental light can thwart fungal pathogens, but still damage the host plant, a follow-up greenhouse study was designed with two main objectives. The first objective was to investigate the effect of supplemental lighting at reduced irradiances and durations on strawberry plant growth and C. gloeosporioides infection. The second objective was to determine the effect of supplemental lighting on the growth of four isolates from three species of strawberry anthracnose pathogens: C. gloeosporioides, Colletotrichum acutatum, and Colletotrichum fragariae. Supplemental light treatments for both objectives included UVB, WSF light, red and blue LED lights, alone and in combination at various intensities.
Materials and Methods
Light treatments.
Six supplemental light treatments included blue and red LEDs, separately and in combination, a WSF light, and a WSF plus UVB light combination. These supplemental light treatments were evaluated for their effect on strawberry plant growth and plant response to anthracnose infection and took place in two trials in a greenhouse at the US Department of Agriculture–Agricultural Research Service (USDA-ARS) Thad Cochran Southern Horticulture Laboratory in Poplarville, MS (lat. 30.84°N, long. 89.5°W). This study used the same equipment and similar procedures to expand on the light levels used by Smith et al. (2022). Lighting systems included arrays of LumiGrow Pro325 HV LEDs (Emeryville, CA), GroLux 20W T12 Wide Spectrum Fluorescent bulbs (Sylvania, Wilmington, MA), and Philips Broadband UVB TL 20W/12 RS bulbs (Hamburg, Germany). Light arrays were suspended ∼1.2 m above potted strawberry plants. The intensity adjustment setting of the LumiGrow Pro 325 LED lights ranged from the lowest intensity setting of 1 to the highest intensity setting of 10. The intensity of each of the individual red and blue LED lights was set to 1, 3, 5, or 10. The four LED treatments in trial 1 included red LED 1, blue LED 1, blue LED 3, and red LED 1 + blue LED 3. In trial 2, the four LED treatments were red LED 1, red LED 5, red LED 10, and red LED 5 + blue LED 5.
Trial 1 ran for 28 weeks (from April until early October) and trial 2 ran for 16 weeks (from December until mid-March). Light treatments across each of three greenhouse benches occurred in randomized complete blocks (bench = block), with benches and treatment replicates (n = 3 replicates per light treatment) kept separated by 1.2-m-high by ∼1.5-m square barriers constructed from rigid 2.5-cm insulation board (Smith et al. 2022). Strawberry plants received 8 h supplemental light treatments each day for 4 h before sunrise and 4 h after sunset. During the day from 8:00 AM to 8:00 PM, plants received ambient light only. For the WSF + UVB light treatment in both trials, plants were exposed to WSF for 8 h during the day and were also exposed to UVB irradiation for 3 h beginning 1 h after WSF exposure ended.
Strawberry cultivars.
Strawberry plants came from commercial nurseries and were maintained at the USDA-ARS location in a greenhouse kept at 28 °C during the day and 12 °C at night (±10 °C) for at least 4 weeks before beginning each trial. Plants were grown in 9-cm pots containing a 1:1 mixture of sand:Jiffy-Mix (JPA, West Chicago, IL), watered via drip emitters, and fertilized every 14 d with Miracle-Gro (24–8–16, N-P-K) Water-Soluble All Purpose Plant Fertilizer (Marysville, OH) at a rate of 40 mL/pot of a 2.5-mL/4-L solution. Each plant was pruned back to the four youngest leaves before the start of each trial and every 6 weeks thereafter during trial 1. Each replication consisted of plants of three cultivars: Chandler (n = 2 plants), Sweet Charlie (n = 2 plants), and Strawberry Festival (n = 3 plants).
Light quality and plant growth data.
A portable handheld spectrometer (Spectromaster C-7000; Sekonic Corporation Nerima-Ku, Tokyo, Japan) was used to measure the photosynthetic photon flux density (PPFD; in micromoles per square meter per second), illuminance (in lux), and peak wavelength (in nanometers) of the various LED and WSF bulbs. A soil plant analysis development (SPAD) chlorophyll meter (SPAD-502; Konica Minolta Sensing, Osaka, Japan) was used to determine the relative chlorophyll content—an indicator of foliar nitrogen level (Wang et al. 2012)—of each plant. Using the same rating protocols published by Smith et al. (2022), plant vigor was rated visually by one observer on a subjective scale that ranged from 1 (least vigorous) to 5 (most vigorous); plant injury was rated visually on a subjective scale, which ranged from 0 (no damage) to 5 (extreme damage).
Growth of fungal isolates and preparation of inoculum.
Colletotrichum spp. used in these studies included an isolate of C. acutatum (Goff), two isolates of C. fragariae (CF63 and CF75), and an isolate of C. gloeosporioides (CG162) (Smith and Black 1990). Each isolate was initiated from silica gel cultures maintained at the USDA-ARS in Poplarville, MS, and grown on 1:1 oatmeal potato dextrose agar at 20 to 28 °C under fluorescent lights with a 12-h photoperiod. Inoculum for detached leaf inoculations was prepared as a conidial suspension from 7- to 14-day-old cultures in sterile deionized water and adjusted to a concentration of 1.5 × 106 conidia/mL by diluting with sterile deionized water containing 1 drop/L H2O of the surfactant (Tween-20; Sigma Chemical Co., St. Louis, MO).
Detached leaf inoculation.
The effect of light treatments on the response of strawberry plants to infection by the anthracnose pathogen, C. gloeosporioides isolate CG162, was assessed using a detached leaf assay as described by Smith et al. (2022). A young, fully developed, blemish-free leaf was removed from each plant and the petiole inserted into a 10 × 150-mm test tube filled with sterile deionized water. A hand pump mister was used to inoculate the adaxial surfaces of the leaves to the point of runoff with a conidial suspension (1.5 × 106 conidia/mL) of C. gloeosporioides. After inoculation, leaves were placed immediately Rin a dew chamber at 100% relative humidity and 30 °C, incubated in the dark for 48 h, then transferred to sealed, clear plastic containers at 100% relative humidity and 30 °C with continuous fluorescent light for an additional 12 d. Disease symptoms were assessed visually 7 and 14 d after inoculation (DAI) using a disease severity rating (DSR) scale of 0 (no visible disease symptom on any leaflet), 1 through 4 (increasing disease symptom severity), and 5 (total area of leaflets necrotic) (Miller-Butler et al. 2018). Leaves were collected and inoculated after 13, 19, and 28 weeks of exposure to light treatments in trial 1, and after 13 weeks of exposure to light treatments in trial 2.
Laboratory studies with three Colletotrichum species.
The effect of the six light treatments reported in the greenhouse study by Smith et al. (2022) was evaluated in the laboratory on the growth of isolates from three Colletotrichum species in culture. The light intensity of the variable-spectrum LumiGrow Pro 325 LED grow lights was set to the highest setting of 10 for each of the red, blue, and white LEDs and for each of the three spectra in the combination LED treatment (combination 10 LED). These four LED fixtures, a WSF fixture, and a WSF plus a UVB fixture were each suspended 100 cm above culture plates on separate light carts in the laboratory. Inoculum of the strawberry anthracnose pathogens (C. acutatum isolate Goff, C. fragariae isolates CF75 and CF63, and C. gloeosporioides isolate CG162) was prepared from cultures grown in the laboratory at 28 °C on potato dextrose agar plates for 10 to 14 d. Two inoculum types were used to initiate colonies of each fungal isolate: mycelial blocks (i.e., 4-mm-diameter mycelial circles cut with a cork borer and placed in the center of each plate with the mycelial side down) or conidial drops (i.e., three 10-μL drops of a 104 conidia/mL suspension placed onto each plate). The experimental design included treating the three plates of each inoculum type for each of the four fungal isolates with each light source. As a control, three additional plates of each inoculum type were covered with an opaque material to shield fungi from light treatments (i.e., dark treatment). Plates in each light treatment were exposed for 17 h each day, followed by 7 h of darkness. In addition to 17 h of exposure to WSF light, plates in the WSF + UVB treatment were exposed to UVB light for 3 h preceded by a 1-h dark period and followed by a 3-h dark period. For each plate, as a measure of fungal growth, colony diameter was measured twice (up and down, left to right) after 3 and 5 d of exposure to light treatments.
Statistical analysis.
The experimental design of each greenhouse trial was a randomized complete block with two main effects (light treatment and cultivar) and three replications. The experimental design of the laboratory trial was a randomized complete block with two main effects (light treatment and isolate) and three replications. Analysis of variance was used to determine the effects of light treatments and isolate on fungal growth, and the effects of light treatments and cultivars on light quality, plant vigor, plant injury, SPAD, and DSRs. Main effect means were separated by Fisher’s protected least significant difference method at P < 0.05 using SAS statistical software (version 8.2; SAS Institute, Cary, NC). If the Pr > F value of the light treatment by cultivar interaction was P < 0.05, analyses of variance and mean separation were rerun by cultivar to perform the contrasts.
Results
Light quality.
Illuminance was lower at the start of trial 1 than at the end of the trial for all light treatments, and was significantly lower in the blue LED 1, blue LED 3, and red LED 1 light treatments than in the red LED 1 + blue LED 3, WSF, and WSF + UVB light treatments at both readings (Table 1). PPFD was greatest in the red LED 1 + blue LED 3 light treatment and lowest in the blue LED 1 light treatment at both readings. As expected, peak wavelength was highest in the red LED 1 light treatment and lowest in the blue LED 1, blue LED 3, WSF, and WSF + UVB light treatments. In trial 2, illuminance was highest in the red LED 5 light treatment and lowest in the red LED 1, red LED 5 + blue LED 5, WSF, and WSF + UVB light treatments (Table 1). PPFD was greatest in the red LED 10 light treatment and least in the red LED 1, WSF, and WSF + UVB light treatments. The peak wavelengths of light treatments that included red LED light spectra fell within the red range (625–740 nm) of the visible spectrum and were higher than the mean peak wavelength of the WSF and WSF + UVB light treatments (Table 1).
Spectrometer light quality readingsi taken at sunrise near the start and 1 h after sunrise near the end of trial 1 and at 9:00 AM on an overcast day at the start of trial 2.
Relative chlorophyll content.
No significant differences in SPAD readings occurred among the light treatments after 5 or 18 weeks of exposure in trial 1 (Table 2); however, SPAD readings did vary among cultivars. For instance, after 5 and 18 weeks, ‘Chandler’ plants had a lower SPAD value than ‘Sweet Charlie’ plants. With the higher illumination rates in some treatments during trial 2, SPAD values in the WSF + UVB light treatment were significantly greater than those in the red LED 10, red LED 5, and red LED 5 + blue LED 5 light treatments (Table 2). Among the three cultivars in trial 2, SPAD values were greater from ‘Sweet Charlie’ plants than from ‘Chandler’ plants.
Relative chlorophyll content of plants measured using a soil plant analysis development (SPAD) chlorophyll meter 5 and 18 weeks after the initiation of trial 1 and 2 weeks after the initiation of trial 2.
Plant vigor and injury.
There was no significant difference in plant vigor scores as a result of light treatment or cultivar in trial 1 or trial 2 (Table 3). Nonsignificant interactions between light treatment and cultivar for plant vigor scores indicated that each cultivar grew similarly well under each of the six light treatments in both trials. Plant injury after 13 weeks of exposure to light treatments in trial 1 was the least in plants in the red LED 1 + blue LED 3 treatment, and greatest in plants in the WSF + UVB light treatment (Table 3). Greater plant injury ratings on plants in the WSF + UVB light treatment could be a result of the harmful effect of 3 h of UVB exposure each night. In trial 2, after 2 weeks of exposure to the light treatments, plants in the WSF light treatments showed less injury than plants in the WSF + UVB, red LED 5, and red LED 10 light treatments, and after 8 weeks of exposure, plants in the red LED 10 light treatment had greater plant injury scores than plants in all other light treatments (Table 3). Nonsignificant interactions between light treatment and cultivar for plant injury scores at each evaluation indicated that each strawberry cultivar resisted damage caused by each of the light treatments in both trials similarly (Table 3).
Vigor ratings 8 weeks after initiation of trial 1, injury ratings 8 and 13 weeks after initiation of trial 1, and vigor and injury ratings 2 and 8 weeks after initiation of trial 2.
Detached leaf inoculations.
Thirteen, 19, and 28 weeks after initiation of trial 1, detached leaves from each plant were inoculated with C. gloeosporioides isolate CG162 and rated for anthracnose symptoms 7 and 14 DAI. There were no significant differences resulting from light treatment in DSR at any evaluation date; however, there were significant differences resulting from cultivar (Table 4). DSRs were significantly greater for ‘Sweet Charlie’ leaves than for ‘Chandler’ and ‘Strawberry Festival’ leaves at each evaluation.
Disease severity ratingsi (DSRs) 7 and 14 d after inoculation (DAI) of detached leaves in trial 1 with Colletotrichum gloeosporioides isolate CG162.
In trial 2, detached leaves were collected and inoculated after 13 weeks of exposure to light treatments. DSRs 14 DAI after were generally less than in trial 1, and there were significant interactions between light treatments and cultivars 7 and 14 DAI (Table 5). DSRs at 7 DAI of leaves from plants in the red LED 5 and WSF light treatments were significantly greater than those of leaves from plants in the red LED 1, red LED 10, and WSF + UVB light treatments. However, by 14 DAI, the DSRs of leaves from plants in the red LED 5, red LED 5 + blue LED 5, red LED 10, and WSF + UVB light treatments were the greatest.
Light × cultivar interaction of disease severity ratingsi (DSRs) of detached leaves from plants in trial 2ii at 7 and 14 d after inoculation (DAI) with Colletotrichum gloeosporioides isolate CG162.
Growth of Colletotrichum isolates in culture.
In addition to the greenhouse trials, we investigated the effect of light treatments on the growth of three species of anthracnose pathogens in the laboratory using the same six light treatments reported previously (Smith et al. 2022) in which each of the LED lights was set at its highest intensity. Fungal colonies were initiated on agar plates using either drops of a conidial suspension or a block of mycelium, incubated under the six light treatments, and colony growth was measured 5 DAI. Colonies initiated from conidial drops grew significantly slower when exposed to WSF + UVB light compared with all other light treatments, indicating that 3 h of exposure to UVB irradiation damaged but did not kill the fungus (Table 6). There was no significant difference in the diameters of colonies initiated from mycelial blocks after exposure to light treatments for 5 d. The diameter of colonies grown in the dark equaled the mean diameter of colonies exposed to all other light treatments. Among the isolates, C. acutatum isolate Goff initiated from conidial drops grew the slowest. There was no significant interaction between light treatment and isolate 5 DAI (Table 6). None of the light treatments halted growth of any of the pathogens, but the WSF + UVB light treatment did impede the growth of colonies from conidial drops.
Main effects of light treatment and Colletotrichum isolate on colony growth 5 d after cultures were initiated from either conidial drops or mycelial plugs.
Discussion
The effect of supplemental lights on plant growth and disease response of strawberries was evaluated in a greenhouse, and on the growth of isolates from three species of strawberry anthracnose pathogens in the laboratory. The first objective was to test the theory that, by reducing the intensity of the red and blue LEDs from levels reported previously by Smith et al. (2022), slower disease development would be maintained, but at light intensities less harmful to strawberry plants. Thus, plant vigor should increase as foliar injury and disease decreased. To achieve this, intensities of the blue and red lights were lowered from a top setting of 10 on the adjustable control to 1 and 3 in trial 1, and to 1, 5, and 10 in trial 2. In addition, plant exposure to LED light arrays was cut from 17 h per day to 8 h (4:00 AM to 8:00 AM and 4:00 PM to 8:00 PM) during both trials. A significant decrease in spectral output of the LED lights was achieved in this study compared with that of Smith et al. (2022). Illuminance of the red and blue LED lights went from 16,159 lx and 18,444 lx, respectively (Smith et al. 2022), to an intensity that was about 50× and 45× less in trial 1 of our study. Irradiance levels of the LEDs were set higher in trial 2, yet their illumination was still 3× less intense than that reported by Smith et al. (2022). Consequently, PPFD was reduced similarly by about 30-fold for the red and blue LEDs in trial 1 and ∼2-fold in trial 2 compared with that of Smith et al. (2022). Such reductions in illumination and exposure times were made to examine the possibility of reducing the cost of the light treatment, maintaining the benefit of disease suppression, and mitigating damage that higher light levels might inflict on strawberry plants. In fact, when comparing our study’s light levels with the higher LED levels in Smith et al. (2022), the reductions in light intensity and reduction of plant exposure to LEDs resulted in healthier plants with a greater chlorophyll content, improved vigor, and lower plant injury rating. In Smith et al. (2022), shorter wavelengths (blue, ambient, WSF, UVB) tended to elevate SPAD, whereas the longer wavelengths associated with red light or its combination with blue light tended to lower relative chlorophyll content in plants, but not by much. WSF + UVB, WSF, and red LED 1 light treatments produced slightly greener plants when compared with plants subjected to the red LED 10 light treatment. A greater chlorophyll content based on SPAD values did not necessarily translate into more vigorous strawberry plants. However, when light intensities were lowered in trial 1 of our study, light treatments had no appreciable effect on chlorophyll content of strawberry leaves, and hence any of these treatments should pose no danger to a plant’s capacity for photosynthesis.
Reductions in spectral emissions reduced LED efficiency to control anthracnose in trial 1, in which no significant differences occurred among DSRs on detached leaves from plants inoculated with C. gloeosporioides isolate CG162 resulting from light treatments at any evaluation. In trial 2, after 13 weeks of exposure to the treatment lights, detached leaves from the red LED 5, red LED 10, red LED 5 + blue LED 5, and WSF + UVB light treatments had greater DSRs 14 DAI than leaves from plants in the RED LED 1 light treatment. No differences occurred in the DSR among leaves from the three cultivars in trial 2, but significant interactions between light treatment and cultivar 7 and 14 DAI indicated that some cultivars may benefit more from certain light treatments in terms of reducing disease severity.
Isolates of three species of Colletotrichum pathogens were exposed to supplemental lights in the laboratory, but none of the light treatments (including WSF + UVB) halted the growth of any of the isolates, even though the intensity of the LED lights was set at the highest intensity. This suggests that any effect of the lights on disease symptoms was a result of an induced resistance of the plant, rather than a direct effect on the pathogen. Increased light intensity, as was the case for trial 2, reduced plant vigor and increased injury, but without any benefit of lessening disease severity. In our study, injury was greatest on plants treated with WSF + UVB, compared with Smith et al. (2022), in which leaves from plants receiving red LED light at the maximum level of 10 were injured the most. Among the cultivars in our study, ‘Chandler’ plants exposed to red light at the lowest intensity level of 1 received the lowest DSR score, suggesting that red light may reduce disease symptoms in some cultivars. However, more cultivars need to be evaluated to verify such specificity. Although lights at the intensities in our study had little impact on disease, they did promote healthier plants at least in terms of their relative chlorophyll content.
To maintain or improve plant health, LED-based supplemental lighting systems are commonly installed in greenhouses and grow rooms. Examples of supplemental lights include white light or a single broad spectrum consisting of multiple colors, or narrower spectra generated by red, white, or blue LEDs, along with invisible spectra associated with the UV bands. LED lights provide finer control over the wavelengths received by plants and hence give growers more targeted control over spectra affecting vegetative growth, flowering stages, or both. Some lighting systems provide greater output control whereby firmware and apps connected via Wi-Fi can attenuate the spectra generated, so that growers can tailor emissions to their crop needs. However, for such systems to work safely on plants, more studies are needed to determine the impact of these intense lights on various aspects of crop health and physiology. Thus, strawberry growers can make a more informed decision on the most appropriate supplemental lighting system and spectra needed for each phase of strawberry production, such as vegetative growth, flowering, pollination, and fruiting (Smith et al. 2022).
Demand for LED- and lamp-based supplemental lighting in greenhouse agriculture is likely to expand in certain regions of the globe. In the United States, strawberry production in greenhouses, high tunnels, and grow rooms is a tiny fraction of that of field-produced strawberries. By contrast, in 2011 in Japan, almost 90% of strawberry production occurred in greenhouses [5257 greenhouse ha vs. 6020 ha total (Kubota and Kroggel 2013)]. Consumer demand for locally grown, high-quality fruit is increasing the demand for greenhouse production of strawberries in these regions (Kubota 2021). As greenhouse production of strawberries increases in all areas of the United States, as well as other countries, data will be needed on the effect of various lighting options on plant growth and pest pressure.
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