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Abstract Phytophthora cinnamomi, which causes the disease root rot, is an oomycete pathogen that is damaging to woody plants, including many horticulturally important groups, such as Rhododendron. Infecting the root of plants, Phytophthora cinnamomi inhibits water uptake, leading to root damage, wilting, and increased rates of plant mortality. Some observations suggest that P. cinnamomi infection corresponds to changes in leaf coloration, though whether this indicates a plant stress response or plant damage is generally unknown. We used leaf color analysis to test for differences in leaf discoloration between plants inoculated with the pathogen and control plants. We demonstrate a significant link between leaf discoloration in Rhododendron species and Phytophthora cinnamomi inoculation. This method was most useful when mortality was not exceptionally high, and analyzers must consider mortality as well as leaf damage in quantifying effects of the pathogen. Plants with leaf discoloration were 3.3 times more likely to die 2 weeks from our leaf census than plants with no leaf discoloration (P =0.005). This method is particularly inexpensive to implement, making it a valuable alternative to multi-spectral or hyperspectral imaging, especially in contexts such as horticulture and citizen science, where the high speed and low-cost nature of this technique might prove valuable. Species used in this study: root rot disease pathogen (Phytophthora cinnamomi Rands); Rhododendron atlanticum (Ashe) Rehder; Rhododendron brachycarpum D.Don ex G.Don; Rhododendron kiusianum Makino; Rhododendron maximum L.; Rhododendron minus Michx.; Rhododendron calendulaceum (Michx.) Torr.; Rhododendron kaempferi Planch.; Rhododendron keiskei Miq. Chemicals used in this study: Fosal Select Aliette/aluminum phosphite. 

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.