Stress Physiology of Phytophthora-canker Pathogens in Landscape Trees: impacts, mechanisms, and mitigation through biochar amendment
Zwart, Drew C.
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Soil-amendment with biochar is thought to confer multiple benefits to plants including induction of systemic resistance to plant pathogens. Pathogens in the genus Phytophthora cause damaging diseases of woody species throughout the world. The objectives of the experiments described in the following chapters were to determine 1) what are the physiological impacts of stem cankers caused by Phytophthora cactorum in Acer rubrum seedlings, 2) if biochar amendment induces resistance to canker causing Phytophthora pathogens and how this resistance is related to the amount of biochar amendment in two common landscape tree species: Quercus rubra (L.) and Acer rubrum (L.), and 3), how does biochar improve resistance against stem lesions. Inoculation of A. rubrum seedlings with P. cactorum resulted in a dramatic and sustained reduction in carbon assimilation rates and stomatal conductance, and a temporary reduction in photosynthetic efficiency compared to non-inoculated control plants. Foliar starch levels did not support the hypothesis that the reduction of photosynthesis was the result of reduced carbohydrate transport and subsequent feedback inhibition. Water use efficiency and stem water potential data suggested a slight moisture stress in the inoculated group compared to the control group, which may have contributed to the photosynthetic decline. Early reductions were probably not due to water shortage and are most likely caused by pathogen-derived signals, or hormonal responses in the plant caused by wounding and pathogen inoculation that led to reduced stomatal aperture and a carbon dioxide supply-related reduction in photosynthesis. Amendment of potting media with 5 % biochar by volume significantly reduced the expansion of necrotic cankers in both test species combinations. Further, in A. rubrum seedlings, biochar amendment was shown to significantly improve photosynthetic performance compared to non-amended, inoculated seedlings. Analysis of foliar defense compounds did not clarify the potential mechanism underlying the reduction in disease impact in amended seedlings. However, combining two years of data clearly indicated that amendment of potting mix with 5 % biochar by volume significantly improved plant physiological performance and reduced the size and impact of necrotic lesions caused by P. cactorum in A. rubrum seedlings. Further studies will be needed in order to determine the exact mechanism underlying the induced resistance against lesion development in biochar amended seedlings. Analysis of defense compounds immediately adjacent to the infection site, rather than a foliar analysis, will help to clarify the impact of biochar amendment on the plant defense response. Analysis of plant phenolic content over time as opposed to at the conclusion of the experiments may also shed light on the process of resistance induction following biochar amendment.
- Forestry