Plant diseases caused by organisms in the genus Phytophthora negatively impact nursery stock, field crops, tree crops, and forest systems (Erwin and Ribeiro, 1996; Hansen et al., 2008). Phytophthora diseases are also widespread and damaging to woody plants that are commonly found in managed landscapes. In trees, Phytophthora pathogens can cause fine root disease, root collar or crown rots, and trunk or stem lesions that are often referred to as “bleeding cankers” (Erwin and Ribeiro, 1996). Physiologically, stem canker-causing Phytophthora species (e.g., P. cinnamomi and P. cactorum) are known to kill phloem, leading to plant death through girdling, and also to colonize and block xylem, leading to altered plant water relations (Brown and Brasier, 2007).
Chemical and non-chemical management options are increasingly being sought to preserve valuable infected specimen trees and protect non-infected hosts. Chemical products containing phosphorous acids or derivatives have often been found most effective and are widely recommended for use against Phytophthora bleeding cankers (Garbelotto et al., 2009; Weiland et al., 2009). Systemic induced resistance (SIR) is the mechanism underlying Phytophthora disease reduction or prevention after treatment with phosphorous acids (Daniel and Guest, 2006; Daniel et al., 2005; Jackson et al., 2000). As a systemic treatment, this material has also been effective in reducing plant damage caused by several fungal pathogens (Agostini et al., 2003; Elliott and Edmonds, 2008; Percival and Noviss, 2010).
Recently, Elad et al. (2010) showed that incorporation of biochar into potting mix of pepper (Capsicum annuum cv. Maccabi) and tomato (Lycopersicum esculentum cv. 1402) reduced the disease severity caused by two foliar pathogens and damage from a broad mite pest (Elad et al., 2010). In addition, Harel et al. (2012) showed that incorporation of biochar reduced the damage caused by three foliar pathogens of strawberry (Fragaria ×ananassa cv. Yael). Results in both studies were attributed to biochar-induced systemic resistance resulting from the reduction in disease caused by pathogens exhibiting both necrotrophic and biotrophic strategies and reduction of damage caused by an arthropod pest. These effects may have been caused by direct interactions between the plant and the biochar or may be the result of biochar-related alterations in the soil microbial community (Elad et al., 2011; Kolton et al., 2011; Warnock et al., 2007). If SIR was the cause of the observed decrease in disease severity, similar results may be possible in defense against Phytophthora pathogens. The potential for biochar incorporation as a disease management option has only recently been suggested and research to date is limited (Elad et al., 2011; Lehmann et al., 2011).
The objective of the present study was to determine if biochar amendment of a soilless potting media can reduce the development and impact of stem lesions caused by Phytophthora spp. on common nursery and landscape hosts and to determine if an optimal rate of biochar amendment exists beyond which benefits are reduced. By testing an aboveground disease and providing all plants with ample nutrients and moisture, any reduction in disease severity resulting from biochar incorporation can reasonably be attributed to an increased systemic resistance in the plant rather than a direct effect of biochar on the pathogen. Separate experiments on two host–pathogen systems were conducted to test the hypothesis of induced resistance and to determine the potential effectiveness for this soil amendment treatment on two combinations of plant host and Phytophthora pathogen.
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