Leaf discoloration in Rhododendron species exposed to Phytophthora cinnamomi corresponds with future mortality

Phytophthora cinnamomi, also known as root rot, is an oomycete that is particularly damaging to the plant
world. Infecting the root of plants, Phytophthora cinnamomi inhibits water uptake in plants, leading to
increased rates of plant mortality. Rhododendron species are not impervious to the infestation of root rot, so,
as a popular plant among gardeners, decreasing susceptibility to and identification of Phytophthora
cinnamomi is beneficial to plant longevity. In this study, phosphite treatment and soil microbial communities
are used to potentially prevent root rot from infecting the eight tested Rhododendron species. It is
hypothesized that the phosphite treatment will directly attack the oomycete, as well as improve the defense
system of the plants themselves. Rhododendrons treated with the live soil microbiota are predicted to be less
susceptible to root rot due to increased resilience to disease from the presence of soil biota, potentially
including mutualists such as mycorrhizal fungi. Since Phytophthora cinnamomi primarily affects the roots of
plants, it is difficult to detect without uprooting those suspected of being diseased, which causes unnecessary
and potentially fatal stress on the plant. This is why we used color analysis software to find a link between
root rot infection and leaf color. Since Phytophthora cinnamomi decreases water uptake, plants that are
infected will begin to wilt, and their leaves will begin to change color. Discovering a significant link between
leaf color in Rhododendron species and Phytophthora cinnamomi infection has given a new diagnostic
measure that will cause significantly less stress to the plant and will lead to better plant longevity outcomes.
Our data also suggests both preventative measures and treatment options for certain Rhododendron species
infected with P. cinnamomi, through the use of a combination of phosphite treatments and live soil biota
presence. Our results differ by species, which we further analyzed through the utilization of specific leaf area
measurements. Using this data, we were able to link our results to current theory, such as growth-defense
tradeoffs and implications of tolerance versus resistance.

樟疫霉（Phytophthora cinnamomi），俗称根腐病，是一类对植物界危害极强的卵菌。该病原菌侵染植物根系后，会抑制植株水分吸收，进而提高植物死亡率。杜鹃花属（Rhododendron）物种并非根腐病的免疫宿主，而作为广受园艺从业者喜爱的常见观赏植物，降低对樟疫霉的易感程度并实现其快速鉴定，有助于延长植株寿命。
本研究采用亚磷酸盐处理与土壤微生物群落调控两种策略，尝试预防根腐病侵染供试的8种杜鹃花属植物。研究假设：亚磷酸盐处理可直接靶向杀灭该卵菌，同时强化植物自身的防御系统；而经活体土壤微生物群处理的杜鹃花植株，因土壤生物（包括菌根真菌等互利共生微生物）带来的病害抗性提升，预计对根腐病的易感程度更低。
由于樟疫霉主要侵染植物根系，若不拔除疑似染病植株便难以完成检测，这会对植物造成不必要且可能致命的应激损伤。为此，本研究借助色彩分析软件，探寻根腐病侵染与叶片颜色变化之间的关联。鉴于樟疫霉会抑制水分吸收，染病植株会逐步出现萎蔫与叶片变色症状。本研究成功在杜鹃花属物种的叶片颜色与樟疫霉侵染之间建立了显著关联，由此提出一种对植株应激伤害极小的新型诊断方法，有望改善植株存活表现并延长寿命。
本研究数据还显示，联合使用亚磷酸盐处理与活体土壤微生物群，可为部分感染樟疫霉的杜鹃花属物种提供有效的预防与治疗方案。研究结果因物种而异，我们通过比叶面积（specific leaf area）测量对该物种间差异展开了进一步分析。依托本研究数据，我们可将实验结果与现有理论（如生长-防御权衡、耐病性与抗病性的相关影响）建立关联。