HortScience (Jul 2023)
Biochar and Trichoderma Reduce Containerized Poinsettia Root Rot Caused by Pythium aphanidermatum
Abstract
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.
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