Potted poinsettia (Euphorbia pulcherrima) is one of the most important greenhouse ornamental crops in the United States, with an estimated wholesale value of $191 million in 15 top-producing states (USDA-NASS 2019). Because it is one of the most popular holiday flowers worldwide, limiting the losses of poinsettia plants from disease is critical to production (Lookabaugh et al. 2020). Pythium aphanidermatum is a recurrent disease and the predominant Pythium species causing poinsettia root rot disease, thereby significantly affecting poinsettia production in greenhouses across the United States (Lookabaugh et al. 2020; Múnera et al. 2019). Under favorable environmental conditions, P. aphanidermatum causes stunting, root rot, wilting, defoliation, chlorosis, and, in severe cases, plant death (Lookabaugh et al. 2017).
Soilless substrate can be conducive to Pythium root rot because it has limited microbial activity (Stephens and Stebbins 1985); however, greenhouses may purchase plantlets or cuttings that are infected but asymptomatic from propagation greenhouses (Moorman 1986). When Pythium successfully intrudes into greenhouses, it can infect the whole greenhouse and become a source of primary inoculum (Krasnow and Hausbeck 2017). Because of the monocultural and humid Pythium-favorable environment of greenhouses, mycelium is easy to survive and reproduce, thus making Pythium an intractable problem (Krasnow and Hausbeck 2017).
Replacing peatmoss, a commonly used soilless substrate, for poinsettia production with biochar provides several benefits, including mitigating climate change, increasing plant yield, and protecting wildlife habitats (Alexander et al. 2008). Biochar is a carbon-rich byproduct of pyrolysis (a main method for biofuel production), which is a process of thermochemical biomass decomposition under an oxygen-depleted or oxygen-limited environment with a specific period of time and temperature conditions (Demirbas and Arin 2002; Lehmann 2007). Several studies have shown that biochar can replace peatmoss-based substrate for greenhouse plant production such as tomato (Solanum lycopersicum), pepper (Capsicum annuum), mint (Mentha spp.), basil (Ocimum basilicum), Easter lily (Lilium longiflorum) and poinsettia (Guo et al. 2018a; Huang et al. 2019; Yan et al. 2020; Yu et al. 2020). Replacing peatmoss-based substrate with biochar has been proven to reduce the environmental concerns associated with peatmoss, such as rare wildlife habitat destruction, wetland ecosystem disturbance, and climate change interference (Alexander et al. 2008). Additionally, incorporating biochar in the substrate could reduce the initial investment for growers; because the price of peatmoss has been increasing, growers’ profits, especially when transportation costs are considered, have been hindered (Gu et al. 2013).
Biochar can replace peatmoss for poinsettia plant production (Guo et al. 2018b) and has the potential to suppress plant diseases in different plant-pathogen systems. For instance, incubating sandy soil for 20 d and adding 1.33% (weight/weight) corn straw biochar (pH 9.73) in the container before transplanting suppressed pepper blight disease because of the improvement of soil chemical properties and increased beneficial microorganisms (Wang et al. 2019). Other studies with biochar-amended soil control disease caused by Pythium spp. were also reported with biochar at relatively low rates (≤3% weight/weight) (Jaiswal et al. 2019).
In most cases, biochar provides synergistic effects with other components, and Trichoderma spp. has been reported as a reliable biological control agent for a wide range of pathogens, including P. aphanidermatum (Manoharachary and Nagaraju 2020). For instance, T. asperellum was proven to suppress tomato damping-off caused by P. aphanidermatum (Kipngeno et al. 2015). It has been shown that the efficacy of spent mushroom substrate against cucumber damping-off caused by P. aphanidermatum was related to the presence of Trichoderma spp. in the substrate (Al-Malikya et al. 2018).
To date, there are not enough studies focusing on biochar suppressing plant disease development, and the biochar incorporation rate is relatively low (range, 0.5%–3%). The highest rate of biochar used in the phytopathogenic system was 50% (by volume) for testing its effects on Pythium ultimum with different crops (Gravel et al. 2013).
The potential mechanisms of how biochar may influence plant disease include both direct and indirect influences on pathogens. For example, the chemical compounds of biochar affect pathogen growth; the physicochemical properties of biochar improve soil nutrients availability and abiotic conditions; the physical properties of biochar help absorb toxins and enzymes produced by pathogens, thus reducing virulence; the presence of biochar induces systemic resistance into host plants; and the physical properties of biochar enhance abundance and/or activities of beneficial microbes (Bonanomi et al. 2015; Graber et al. 2014).
Because of the complexity of the biochar-plant-media-microorganisms system, it is difficult to decipher which mechanism is responsible for biochar impact disease development in any given phytopathogenic system (Graber et al. 2014). Except for the chemical compound mechanism, which can be identified and measured separately by removing the physical and chemical properties of biochar and their influences on pathogen and host plants, other mechanisms are difficult to identify and measure separately. We conducted an in vitro test and greenhouse trial to identify which mechanism is involved in the biochar-poinsettia-P. aphanidermatum system and test the effects of biochar on poinsettia root rot disease development.
Material and Methods
Biochar-amended media and biochar water extracts
Mixed hardwood biochar (HB) (Proton Power Inc., Lenoir City, TN, USA) was mixed with peatmoss-based commercial substrate (CS) (Jolly Gardener C/20; Oldcastle Lawn & Garden Inc., Atlanta, GA, USA) at 20% (HB20) and 40% (HB40) (by volume). The CS used in this study contained 80% Canadian Sphagnum peatmoss with the rest being perlite (CS100). The HB was a byproduct of fast pyrolysis of mixed hardwood; it had a pH of 10.1 and an electrical conductivity (EC) of 1058 μS⋅cm−1 (Webber et al. 2018; Yu et al. 2019). The HB used in this study contained 67.3% of particles larger than 2.0 mm (Yan et al. 2020).
The water extracts were obtained according to Gravel et al. (2013). Briefly, the mixtures were mixed with deionized (DI) water at a ratio of 1:1 (by volume) and shook for 24 h. The mixture was filtered through filter paper, and 25 mL extract was collected and sterilized for the in vitro test. The same amount of DI water was used as the control.
The biochar-amended potato dextrose agar (PDA) media was prepared by adding the sterilized mixture water extracts in the 25% PDA sterilized solution before media hardening. The control medium contained the same amount of sterilized DI water only.
Plant material, trichoderma, and P. aphanidermatum
Root shield Plus-WP (BioWorks, Victor, NY, USA) was used as a biological control agent during this study and contained two active strains of Trichoderma (T. harzianum strain T-22 and T. virens strain G-41). P. aphanidermatum was isolated from an infected poinsettia plant. Cultures of pathogens were isolated and maintained in the dark on a cornmeal agar selective for the culture of oomycete organisms (Al-Sheikh 2010).
In vitro test
Biochar-amended extracts and pathogen growth.
where A1 is the area of pathogen growth in biochar-amended media and A2 is the area of pathogen growth in the control.
Biochar-amended extracts, Trichoderma, and pathogen growth.
The dual confrontation technique (Sinclair and Dhingra 1995) with slight modifications was used during this trial. A drop of Trichoderma containing solution (mixed at the manufacture rate) was paired with a 5-mm plug of actively growing P. aphanidermatum at equal distances opposite to each other in a 100-mm-diameter petri dish containing biochar-amended 25% PDA. Then, the petri dishes were placed in a dark environment at room temperature. Radial mycelium growth was measured 4 d later. The percentage inhibition of pathogen growth was calculated using the following formula (Nawaz et al. 2018):
where A1 is the area of pathogen growth in biochar-amended Trichoderma added media and A2 is the area of pathogen growth in the control.
Greenhouse trial
Poinsettia (Euphorbia pulcherrima ‘Prestige Sunrise Red’) cuttings were inserted in commercial propagation media (BM2 Berger; Saint-Modeste, Quebec, Canada) on 5 Jun 2020. After the roots were established in the media, uniform cuttings were transplanted into 6-inch azalea pots (depth, 10.8 cm; top diameter, 15.5 cm; bottom diameter, 11.3 cm; volume, 1330 mL) filled with biochar-amended mixes on 21 Jun 2020. At transplanting, slow-release fertilizer Osmocote Plus (15N–4P–10K; Scotts-Sierra Horticultural Products Company, Marysville, OH, USA) was applied at rates suggested by the manufacturer.
The Trichoderma-containing product was applied 4 weeks after transplanting (WK4) at the rates recommended by the manufacturer on 23 Jul 2020. P. aphanidermatum was inoculated by taking a 5-mm-diameter agar plug from the margin of actively growing mycelium and placed on surface of the substrate contacting the plant stem at a rate of five plugs with plastic inoculation loops (VWR, Radnor, PA, USA) on 31 Jul 2020.
Substrate physical and chemical properties.
The physical properties of the biochar mixes, including total porosity, container capacity, air space, and bulk density, were measured according to the North Carolina State University Porometer manual (Fonteno et al. 1995) and previously shown by our study. The leachate pH and EC were measured with a portable EC/pH meter (Hanna Instrument, Woonsocket, RI, USA), followed by the pour-through method (LeBude and Bilderback 2009).
Plant growth.
Plant height and two canopy widths (width 1, the widest width; width 2, width perpendicular to width 1) were measured biweekly starting at WK4, when Trichoderma was applied. The plant growth index (GI) was calculated according to the formula: GI = plant height/2 + (width 1 + width 2)/4 (Guo et al. 2018b). On 25 Aug 2020, at the end of this experiment, poinsettia plant shoots were harvested. After being dried at 80 °C in an oven until a consistent weight was reached, the shoot dry weight (SDW) was measured.
Disease assessment.
After 5 d of pathogen inoculation, disease symptoms were observed and recorded at 5-d intervals. The disease severity was recorded using a scale of 0 to 4 according to Lookabaugh’s work with modifications (Lookabaugh et al. 2017). A scale of 0 to 4 (0 = healthy plants; 1 = slightly stunted or wilted; 2 = chlorosis, moderate stunting, and/or defoliation; 3 = wilting and/or severe stunting; 4 = dead plants) was used to indicate the disease effects on plants shown in Fig. 1. The disease severity (DS) index was calculated by the following formula:
Scale of 0 to 4 (0 = no symptom, 4 = dead plant) used to rate disease severity of poinsettia root rot caused by P. aphanidermatum during this study. No plant was dead in this study.
Citation: HortScience 58, 8; 10.21273/HORTSCI17203-23
Areas under the disease progress curves (AUDPCs) were calculated using the disease severity obtained at different times after inoculation:
With the AUDPC formula,
Experimental design and maintenance
The in vitro test was arranged as a randomized complete block design with five blocks. Within each block, we randomly applied the treatments to the petri dish from the 4 × 2 factorial design with biochar treatments (water extracts of CS100, HB20, HB40, and DI water) and Trichoderma application (with or without). The greenhouse experiment was a randomized complete block design with eight blocks. Inside the greenhouse, we assigned an area of 1.6 m × 1.2 m as a block on the benches. Within each block, we randomly applied our treatments to the pots from the 3 × 2 × 2 factorial design with biochar treatments (CS100, HB20, and HB40), Trichoderma application (with or without), and pathogen (nonpathogen or pathogen-inoculated).
All pots were placed in a P. aphanidermatum-permitted greenhouse at Texas A&M University (College Station, TX, USA). Plants were watered as needed throughout the experiment. The average greenhouse temperature, relative humidity, and dew point were 30.2 °C, 77.2%, and 25.0 °C, respectively.
Data collection and analysis
Image J (version 1.53a) was used to estimate the area of pathogen growth during the in vitro test. All data were analyzed using a one-way analysis of variance and R program software (R version 3.5.1; RStudio). For the greenhouse trial, nonpathogen and pathogen-inoculated treatments were analyzed separately. All means were separated using Dunnett’s test when treatments were significantly different from the control at P ≤ 0.05 or the least significant difference (LSD) when treatments were significantly different from each other at P ≤ 0.05.
Results
Biochar-amended extracts and trichoderma on pathogen growth
In the absence of Trichoderma, extracts of biochar-amended mixtures had significantly lower inhibition effects on P. aphanidermatum growth than the CS extracts (Figs. 2 and 3A). All the extracts of the mixes stimulated (indicated by the positive inhibition rate) P. aphanidermatum growth.
Inhibition percentage of Pythium aphanidermatum growth on 25% potato dextrose agar (PDA) amended with 25 mL of liquid extracts from the peatmoss-based commercial substrate (CS100), hardwood biochar (HB) 20% (HB20) mix, and 40% (HB40) mix (by volume) without (A) and with (B) the addition of Trichoderma. Data are the mean of n = 5. Values followed by the same letters indicate no significant difference from each other according to the least significant difference (LSD) multiple test at P ≤ 0.05.
Citation: HortScience 58, 8; 10.21273/HORTSCI17203-23
Pythium aphanidermatum grown on 25% potato dextrose agar (PDA) amended with 25 mL of liquid extraction from peatmoss-based commercial substrate (CS100), hardwood biochar (HB) 20% (HB20) mix, 40% (HB40) mix, and deionized water (DI water) in the absence (A) and presence (B) of Trichoderma (on the right, small circle) after 2 d in a dark environment.
Citation: HortScience 58, 8; 10.21273/HORTSCI17203-23
In the presence of Trichoderma, extracts of biochar-amended mixtures had similar effects on P. aphanidermatum growth compared with the CS extracts (Figs. 2 and 3B). All the mixture extracts suppressed P. aphanidermatum growth (indicated by the negative inhibition rate).
Substrate physical and chemical properties
Most of the physical properties of the mixes were within the recommended range (Yeager et al. 2007), except for the bulk densities in all treatments, which were lower than the recommended value (Table 1). The HB20 and HB40 mixes had a significantly lower total porosity and higher pH than the control (CS100). The HB40 mixes had significantly higher BD and lower EC than the control.
Substrate physical properties including total porosity (TP), container compacity (CC), air space (AS), bulk density (BD), and chemical properties including pH and electrical conductivity (EC).
Plant growth
There were no main factor (biochar and Trichoderma) interactions among any of the growth parameters for any of the treatments with or without pathogens (Table 2). For the nonpathogen treatments, the SPAD at 8 weeks after transplanting (WK8) and SPAD at 10 weeks after transplanting (WK10) were both significantly influenced by biochar. For the pathogen-inoculated treatments, biochar significantly influenced GI at WK10, SPAD at WK8, SPAD at WK10, and SDW.
Summary of the statistical significance of treatment factors on the growth index (GI) at 4, 6, 8, and 10 weeks after transplanting (GI WK4, GI WK6, GI WK8, and GI WK10, respectively), SPAD at 8 and 10 weeks after transplanting (SPAD WK8 and SPAD WK10, respectively), and shoot dry weight (SDW).
The biochar rates had no significant impacts on the SDW of nonpathogen poinsettia plants with or without Trichoderma (Fig. 4). Biochar had no significant influences on any of the GI values; furthermore, Trichoderma had no significant impact on any the GI values (Fig. 5). The Poinsettia plants grown in the biochar mixes (both HB20 and HB40) had significantly lower SPAD at WK8, but this had no significant impact on SPAD at WK10. Trichoderma did not significantly influence SPAD at WK8 or SPAD at WK10 (Fig. 6).
The effect of the biochar rate (A) and Trichoderma application (B) on shoot dry weight for nonpathogen treatments. CS100 = peatmoss-based commercial substrate; HB20 and HB40 = 20% and 40% (by volume) mixed hardwood biochar-amended mixes, respectively.
Citation: HortScience 58, 8; 10.21273/HORTSCI17203-23
The effect of the biochar rate (A) and Trichoderma application (B) on the growth index at weeks 4, 6, 8, and 10 after transplanting (WK 4, WK6, WK8, and WK10, respectively) for nonpathogen treatments. CS100 = peatmoss-based commercial substrate; HB20 and HB40 = 20% and 40% (by volume) mixed hardwood biochar-amended mixes, respectively.
Citation: HortScience 58, 8; 10.21273/HORTSCI17203-23
The effect of the biochar rate (A) and Trichoderma application (B) on SPAD at weeks 8 and 10 after transplanting (WK 8 and WK10, respectively) for nonpathogen treatments. CS100 = peatmoss-based commercial substrate; HB20 and HB40 = 20% and 40% (by volume) mixed hardwood biochar-amended mixes, respectively. *, **Significantly different from the control (CS100) according to the Dunnett test at P ≤ 0.05 and P ≤ 0.01, respectively.
Citation: HortScience 58, 8; 10.21273/HORTSCI17203-23
The pathogen-inoculated poinsettia plants grown in the HB20 mixes had significantly higher SDW compared with those grown in CS100 (Fig. 7A). Trichoderma had no significant influence on the SDW (Fig. 6B). Neither biochar nor Trichoderma had significant impacts on any of the GI values (Fig. 8). Poinsettia plants grown in the HB40 mixes had significantly lower SPAD values at WK8 and WK10 compared with those grown in CS100, but Trichoderma had no significant influence on the SPAD (Fig. 9).
The effect of the biochar rate (A) and Trichoderma application (B) on shoot dry weight for pathogen-inoculated treatments. CS100 = peatmoss-based commercial substrate; HB20 and HB40 = 20% and 40% (by volume) mixed hardwood biochar-amended mixes, respectively. ***Significantly different from the control (CS100) according to the Dunnett test at P ≤ 0.001.
Citation: HortScience 58, 8; 10.21273/HORTSCI17203-23
The effect of the biochar rate (A) and Trichoderma application (B) on growth index at 4, 6, 8, and 10 weeks after transplanting (WK4, WK6, WK8, and WK10, respectively) for pathogen-inoculated treatments. CS100 = peatmoss-based commercial substrate; HB20 and HB40 = 20% and 40% (by volume) mixed hardwood biochar-amended mixes, respectively.
Citation: HortScience 58, 8; 10.21273/HORTSCI17203-23
The effect of the biochar rate (A) and Trichoderma application (B) on SPAD at 8 and 10 weeks after transplanting (WK8 and WK10, respectively) for pathogen-inoculated treatments. CS100 = peatmoss-based commercial substrate; HB20 and HB40 = 20% and 40% (by volume) mixed hardwood biochar-amended mixes, respectively. **, ***Significantly different from the control (CS100) according to the Dunnett test at P ≤ 0.01 and P ≤ 0.001, respectively.
Citation: HortScience 58, 8; 10.21273/HORTSCI17203-23
Disease parameters
Five days after inoculation, poinsettia plants grown in all the treatments showed Pythium poinsettia root rot disease symptoms (Fig. 10A). Among all treatments, HB20 treatments maintained low disease severity throughout the experiment. The disease severity of HB20 mixes at 5, 10, 15, 20, and 25 d after inoculation were 10.9%, 10.9%, 18.8%, 21.9%, and 21.9%, respectively, lower compared with the control (CS100). However, disease severity of the HB40 treatments at 15, 20, and 25 d after inoculation were higher than those of the control by12.5%, 4.7%, and 1.6%, respectively. Trichoderma did not affect disease severity over the course of the experiment (Fig. 10B).
The effect of the biochar rates (A) and Trichoderma (B) on disease severity for pathogen inoculate treatments. CS100 = peatmoss-based commercial substrate; HB20 and HB40 = 20% and 40% (by volume) mixed hardwood biochar-amended mixes, respectively. The same letter indicates no significant difference from each other according to the least significant difference (LSD) multiple comparison test at P ≤ 0.05 on the same day.
Citation: HortScience 58, 8; 10.21273/HORTSCI17203-23
Biochar mixes had a significant impact on disease incidence, especially the HB-amended (HB20 and HB40) mixes (Fig. 11A). Compared with the CS100 treatments, HB20 treatments reduced the disease incidence by 31.3% starting at 5 d after inoculation. The HB40 mixes, however, increased disease incidence at 15 and 20 d after inoculation by 12.5% and 6.3%, respectively. The application of Trichoderma did not significantly reduce the disease incidence for the entire duration of the experiment (Fig. 11B).
The effect of the biochar rates (A) and Trichoderma (B) on disease incidence for pathogen-inoculate treatments. CS100 = peatmoss-based commercial substrate; HB20 and HB40 = 20% and 40% (by volume) mixed hardwood biochar-amended mixes, respectively. The same letter indicates no significant difference from each other according to the least significant difference (LSD) multiple comparison test at P ≤ 0.05 on the same day.
Citation: HortScience 58, 8; 10.21273/HORTSCI17203-23
The HB20 mixes had significantly lower AUDPCs than the CS100, whereas the AUDPC of HB40 was similar to that of CS100 (Fig. 12A). The HB20 reduced the AUDPC value by 13.6, but the HB40 slightly increased the AUDPC value by 3.9 (Fig. 12A). Trichoderma did not significantly reduce the AUDPC during the entire experiment (Fig. 12B).
The effect of biochar types and rates (A) and Trichoderma (B) on the area under the disease progress curve. CS100 = peatmoss-based commercial substrate; HB20 and HB40 = 20% and 40% (by volume) mixed hardwood biochar-amended mixes, respectively. The same letter indicates no significant difference from each other according to least significant difference (LSD) multiple comparison test at P ≤ 0.05.
Citation: HortScience 58, 8; 10.21273/HORTSCI17203-23
Discussion
Biochar chemical compound and pathogen inhibition
The chemical compounds in biochar could have a direct influence on disease development. Some types of biochar contain chemicals compounds such as ethylene glycol and propylene glycol, hydroxypropionic and hydroxybutyric acids, benzoic acid and o-cresol, quinones (resorcinol and hydroquinone), and 2-phenoxyethanol, which could adversely affect microbial growth and survival. Compounds such as methoxyphenols, phenols, carboxylic acids, furans, and ketones, which could form during pyrolysis process, have a suppressive impact on microbial activity (Graber et al. 2010; Klinke et al. 2004). During the in vitro test, in which the pathogen growth was unaffected by biochar physically properties, all the extracts of the biochar mixtures stimulated P. aphanidermatum growth, similar to the findings of Jaiswal et al. (2017), who reported that eucalyptus wood biochar or greenhouse pepper plant waste biochar extracts did not inhibit Fusarium oxysporum growth. However, in contrast, the study by Gravel et al. (2013) showed that softwood biochar mixed with organic potting mix extracts suppressed P. ultimum growth. In this study, the pH of CS in the presence of Trichoderma and the stimulating effects of extracts of biochar-amended mixes on P. aphanidermatum were reversed because of the suppressive effects of Trichoderma on P. aphanidermatum.
Biochar properties and disease development
When a biochar is added to the substrate, it may profoundly influence the complex rhizosphere–root–media–pathogen system by its physical and chemical properties such as nutrient content, water-holding capacity, redox activity, adsorption ability, pH, and contents of toxic and hormone-like compounds, which can affect the disease triangle factors both directly and indirectly (via the influence on the rhizosphere microbiome). In turn, the direct and indirect impacts of biochar on the environment, host plant, pathogen, and rhizosphere microbiome can have domino effects on plant and disease development (Graber et al. 2014). According to Bolton (1980), P. aphanidermatum grows best when the substrate pH ranges from 5.5 to 6.5. The pH of the CS used in this study was 6.8, which is slightly higher than the optimal range for pathogen growth, could be the reason why its disease severity was higher than that of HB20. The biochars with high pH (>9) could contain some phenolic groups, such as phenolic acid and alkali, which mainly exist as organic anions (Yuan and Xu 2012). The high pH and buffer capacity of many biochars could also reduce the toxic acids near plant roots (Graber et al. 2014).
Biochar and plant growth
Because the effects of biochar and Trichoderma applications on plants can be complex and difficult to explain, and because of the two rates of biochar and multiple variables, a principal component (PC) analysis was used to depict variables shaped by different biochar rates with nonpathogen and pathogen-inoculated (Fig. 13) plants. For the nonpathogen plants, 80% of the variability was explained by the first two components (Fig. 13A). PC1 accounted for 51% variance, with biochar mixes (HB40-TN, HB40-TY, HB20-TY) differing from the CS mixes (CS100-TY and CS100-TN). Biochar-amended mixes were associated more with the early GI (GI at WK4). PC2 accounted for 29% variance, thus distinguishing the CS100 and HB40 mixes from HB20 mixes. The CS and HB40 mixes tended to be affiliated with the plant yield (SDW), early and later GI values (GI at WK4, GI at WK6, and GI at WK10), and SPAD at WK8.
Principal component analysis (PCA) depicting the relationships between selected variables and treatment factors with nonpathogen (A) and pathogen-inoculated (B) plants. Selected variables are displayed by arrows and include plant growth parameters: growth index after 4, 6, 8, and 10 weeks of transplanting (GI WK4, GI WK6, GI WK8, and GI WK10, respectively), SPAD values after 8 and 10 weeks of transplanting (SPAD WK8 and SPAD WK10, respectively), and shoot dry weight (SDW). Treatment factors are displayed by filled grey circles: peatmoss-based substrate (CS100) and biochar-amended mixes at different rates (by volume; HB20, and HB40) with (TY) or without Trichoderma (TN).
Citation: HortScience 58, 8; 10.21273/HORTSCI17203-23
For the pathogen-inoculated plants, 95% of the variability was explained by the first two components (Fig. 13B). PC1 accounted for 80% variance, thus differentiating HB40 and CS100-TN from HB20 and CS100-TY. Treatment with HB20 and CS100-TY were associated more with yield (SDW), early and later GI values, and SPAD values. PC2 accounted for 15% variance, distinguishing HB20-TN, HB40-TY, and CS100-TN from HB20-TY, HB40-TN, and CS100-TY. Treatment with HB20-TN, HB40-TY, and CS100-TN tended to be associated with the early GI (GI at WK4), plant yield (SDW), and SPAD values.
Treatment factors determine plant disease development
For the pathogen-inoculated plants, 89% of the variability was explained by the first two components (Fig. 14). PC1 accounted for 73% variance, thus differentiating HB20 from HB40 and CS treatments. The CS100 and HB40 treatments were positively associated with all the disease parameters, whereas the HB20 treatments were negatively associated with them. PC2 accounted for 16% variance, thus distinguishing HB20-TY, HB40-TN, and CS100-TN treatments from the other treatments. Treatment HB20-TY, HB40-TN, and CS100-TN tended to be affiliated with later DIs (DI 3–5) and DSs (DS 4–5) and AUDPCs, whereas HB20-TN, HB40-TY, and CS100-TY appeared to be related to early DI values (DI, 1–2) and DS index value (DS index, 1–2).
Principal component analysis (PCA) depicting the relationships between selected variables and treatment factors with pathogen-inoculated plants. Selected variables are displayed by arrows and include disease parameters: disease severity after 5, 10, 15, 20, and 25 d of transplanting (DS1, DS2, DS3, DS4, and DS5, respectively), disease incidence after 5, 10, 15, 20, and 25 d of transplanting (DI1, DI2, DI3, DI4, and DI5, respectively), and area under the disease progress curve (AUDPC). Treatment factors are displayed by filled grey circles: peatmoss-based substrate (CS100) and biochar-amended mixes at different rates (by volume; HB20 and HB40) with (TY) or without Trichoderma (TN).
Citation: HortScience 58, 8; 10.21273/HORTSCI17203-23
Conclusions
The mixed HB blended with commercial peatmoss-based substrate at 20% (by volume) could significantly reduce the Pythium aphanidermatum poinsettia root rot disease severity and disease incidence with or without the addition of Trichoderma. The extracted solution from biochar did not contribute to its disease inhibition capability because it stimulated pathogen growth during the in vitro test. In the absence of the pathogen, mixed HB could replace commercial peatmoss-based substrate at 20% and to 40% (by volume) in container poinsettia plant production without any negative impact on growth. In the presence of the pathogen, 20% (by volume) of mixed HB could replace commercial peatmoss-based substrate for container poinsettia plants and reduce of disease severity and incidence.
References Cited
Al-Malikya BS, Jamela DS, Kareema T, Al-Sadaawyb AK. 2018. Effect of spent mushroom substrate (SMS) on cucumber damping off and root rot caused by Pythium aphanidermatum in greenhouse. Asian J Microbiol Biotechnol Environ Sci. 20:S1–S5.
Al-Sheikh H. 2010. Two pathogenic species of Pythium: P. aphanidermatum and P. diclinum from a wheat field. Saudi J Biol Sci. 17:347–352. https://doi.org/10.1016/j.sjbs.2010.05.001.
Alexander P, Bragg N, Meade R, Padelopoulos G, Watts O. 2008. Peat in horticulture and conservation: The UK response to a changing world. Mires Peat. 3:1–8. http://www.mires-and-peat.net/.
Bolton AT. 1980. Effects of temperature and pH of soilless media on root rot of poinsettia caused by Pythium aphanidermatum. Can J Plant Pathol. 2(2):83–85. https://doi.org/10.1080/07060668009501443.
Bellini A, Ferrocino I, Cucu MA, Pugliese M, Garibaldi A, Gullino ML. 2020. A compost treatment acts as a suppressive agent in Phytophthora capsici–Cucurbita Pepo pathosystem by modifying the rhizosphere microbiota. Front Plant Sci. 11:885. https://doi.org/10.3389/fpls.2020.00885.
Bonanomi G, Ippolito F, Scala F. 2015. A “black” future for plant pathology? Biochar as a new soil amendment for controlling plant diseases. J Plant Pathol. 97(2):223–234.
Demirbas A, Arin G. 2002. An overview of biomass pyrolysis. Energy Sources. 24:471–482. https://doi.org/10.1080/00908310252889979.
Fonteno W, Hardin C, Brewster J. 1995. Procedures for determining physical properties of horticultural substrates using the NCSU Porometer. Horticultural Substrates Laboratory. North Carolina State University, Raleigh, NC, USA.
Graber E, Frenkel O, Jaiswal A, Elad Y. 2014. How may biochar influence severity of diseases caused by soilborne pathogens? Carbon Manag. 5:169–183. https://doi.org/10.1080/17583004.2014.913360.
Graber ER, Meller Harel Y, Kolton M, Cyrtyn E, Silber A, Rav David D, Tsechansky T, Borenshtein M, Elad Y. 2010. Biochar impact on development and productivity of pepper and tomato grown in fertigated soilless media. Plant Soil. 337:481–496. https://doi.org/10.1007/s11104-010-0544-6.
Gravel V, Dorais M, Ménard C. 2013. Organic potted plants amended with biochar: Its effect on growth and Pythium colonization. Can J Plant Sci. 93:1217–1227. https://doi.org/10.4141/cjps2013-315.
Gu M, Li Q, Steele PH, Niu G, Yu F. 2013. Growth of ‘Fireworks’ gomphrena grown in substrates amended with biochar. J Food Agric Environ. 11:819–821. www.world-food.net.
Guo Y, Niu G, Starman T, Gu M. 2018a. Growth and development of Easter lily in response to container substrate with biochar. J Hortic Sci Biotechnol. 1:80–86. https://doi.org/10.1080/14620316.2018.1444514.
Guo Y, Niu G, Starman T, Volder A, Gu M. 2018b. Poinsettia growth and development response to container root substrate with biochar. Horticulturae. 4:1. https://doi.org/10.3390/horticulturae4010001.
Huang L, Yu P, Gu M. 2019. Evaluation of biochar and compost mixes as substitutes to a commercial propagation mix. Appl Sci (Basel). 9:4394. https://doi.org/10.3390/app9204394.
Jaiswal AK, Elad Y, Paudel I, Graber ER, Cytryn E, Frenkel O. 2017. Linking the belowground microbial composition, diversity and activity to soilborne disease suppression and growth promotion of tomato amended with biochar. Sci Rep. 7:44382. https://doi.org/10.1038/srep44382.
Jaiswal AK, Graber ER, Elad Y, Frenkel O. 2019. Biochar as a management tool for soilborne diseases affecting early stage nursery seedling production. Crop Prot. 120:34–42.
Kipngeno P, Losenge T, Maina N, Kahangi E, Juma P. 2015. Efficacy of Bacillus subtilis and Trichoderma asperellum against Pythium aphanidermatum in tomatoes. Biol Control. 90:92–95. https://doi.org/10.1016/j.cropro.2019.02.014.
Klinke HB, Thomsen A, Ahring BK. 2004. Inhibition of ethanol-producing yeast and bacteria by degradation products produced during pre-treatment of biomass. Appl Microbiol Biotechnol. 66:10–26. https://doi.org/10.1007/s00253-004-1642-2.
Krasnow CS, Hausbeck MK. 2017. Influence of pH and etridiazole on Pythium species. HortTechnology. 27:367–374. https://doi.org/10.21273/HORTTECH03633-16.
LeBude A, Bilderback T. 2009. The pour-through extraction method: A nutrient management tool for nursery crops. AG–717–W, 1-8. NC State Univ. Coop. Ext. Bul. In. North Carolina State University, Raleigh, NC, USA.
Lehmann J. 2007. A handful of carbon. Nature. 447(7141):143–144. https://doi.org/10.1038/447143a.
Lookabaugh EC, Kerns JP, Shew B. 2020. Evaluating fungicide selections to manage Pythium root rot on poinsettia cultivars with varying levels of partial resistance. Plant Dis. 105(6):1640–1647. https://doi.org/10.1094/PDIS-04-20-0807-RE.
Lookabaugh EC, Whipker B, Shew BB. 2017. Evaluation of poinsettia cultivars for resistance to pythium root rot caused by Pythium aphanidermatum. HortTechnology. 27:805–812. https://doi.org/10.21273/HORTTECH03817-17.
Madden LV, Hughes G, Van Den Bosch F. 2007. The study of plant disease epidemics. American Phytopathological Society, St. Paul, MI, USA.
Manoharachary C, Nagaraju D. 2020. Trichoderma: Boon for agriculture, p 87–112. In: Manoharachary C, Singh HB, Varma A (eds). Trichoderma: Agricultural applications and beyond. Springer, Cham, Switzerland. https://doi.org/10.1007/978-3-030-54758-5_4.
Moorman G. 1986. Increased plant mortality caused by Pythium root rot of poinsettia associated with high fertilization rates. Plant Dis. 70:160–162.
Múnera JDC, Belayneh B, Ritsvey A, Koivunen EE, Lea-Cox J, Swett CL. 2019. Enabling adaptation to water scarcity: Identifying and managing root disease risks associated with reducing irrigation inputs in greenhouse crop production–A case study in poinsettia. Agric Water Manage. 226:105737. https://doi.org/10.1016/j.agwat.2019.105737.
Nawaz K, Shahid AA, Bengyella L, Subhani MN, Ali M, Anwar W, Iftikhar S, Ali SW. 2018. Diversity of Trichoderma species in chili rhizosphere that promote vigor and antagonism against virulent Phytophthora capsici. Scientia Hortic. 239:242–252. https://doi.org/10.1016/j.scienta.2018.05.048.
Nelson P. 2012. Root substrate. Greenhouse operation and management (7th ed). Prentice Hall, Upper Saddle River, NJ, USA.
Sinclair JB, Dhingra OD. 1995. Basic plant pathology methods (2nd ed). CRC Press, Boca Raton, FL, USA. https://doi.org/10.1201/9781315138138.
Stephens C, Stebbins T. 1985. Control of damping-off pathogens in soilless container media. Plant Dis. 69:494–496.
USDA-NASS. 2019 Census of Horticultural Specialties. Agricultural Statistics https://www.nass.usda.gov/Publications/AgCensus/2017/Online_Resources/Census_of_Horticulture_Specialties/index.php.
Wang G, Govinden R, Chenia HY, Ma Y, Guo D, Ren G. 2019. Suppression of Phytophthora blight of pepper by biochar amendment is associated with improved soil bacterial properties. Biol Fertil Soils. 55:813–824. https://doi.org/10.1007/s00374-019-01391-6.
Webber CL III, White PM Jr, Gu M, Spaunhorst DJ, Lima IM, Petrie EC. 2018. Sugarcane and pine biochar as amendments for greenhouse growing media for the production of bean (Phaseolus vulgaris L.) seedlings. J Agric Sci. 10:58. https://doi.org/10.5539/jas.v10n4p58.
Yan J, Yu P, Liu C, Li Q, Gu M. 2020. Replacing peat moss with mixed hardwood biochar as container substrates to produce five types of mint (Mentha spp.). Ind Crops Prod. 155:112820. https://doi.org/10.1016/j.indcrop.2020.112820.
Yeager T, Gilliam C, Bilderback TE, Fare D, Niemiera A, Tilt K. 2007. Best management practices: Guide for producing container-grown plants. Southern Nursery Association, Marietta, GA.
Yu P, Li Q, Huang L, Niu G, Gu M. 2019. Mixed hardwood and sugarcane bagasse biochar as potting mix components for container tomato and basil seedling production. Appl Sci (Basel). 9:4713. https://doi.org/10.3390/app9214713.
Yu P, Li Q, Huang L, Qin K, Niu G, Gu M. 2020. The effects of mixed hardwood biochar, mycorrhizae, and fertigation on container tomato and pepper plant growth. Sustainability. 12:7072. https://doi.org/10.3390/su12177072.
Yuan H, Xu K. 2012. Effects of biochars generated from crop residues on chemical properties of acid soils from tropical and subtropical China. Soil Res. 50:570–578. https://doi.org/10.1071/SR12118.
