Is disease induced by flooding representative of nursery conditions in rhododendrons infected with P. cinnamomi or P. plurivora?

Authors: MESTAS, ANGIE - Oregon State University; Weiland, Gerald - Jerry; Scagel, Carolyn; Grunwald, Niklaus - Nik; Davis, Elizabeth; Mitchell, Jesse; Beck, Bryan

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/12/2021
Publication Date: 3/27/2022
Citation: Mestas, A., Weiland, G.E., Scagel, C.F., Grunwald, N.J., Davis, E.A., Mitchell, J.N., Beck, B.R. 2022. Is disease induced by flooding representative of nursery conditions in rhododendrons infected with P. cinnamomi or P. plurivora? Plant Disease. 106(4):1157-1166. https://doi.org/10.1094/PDIS-06-21-1340-RE.
DOI: https://doi.org/10.1094/PDIS-06-21-1340-RE

Interpretive Summary: Soil moisture influences how Phytophthora pathogens cause root rot in nurseries growing rhododendron plants in containers. Most research experiments will flood the soil of container plants with water to ensure that root rot develops. However, the degree of flooding used in experiments does not usually occur in nurseries. Instead, rhododendrons are either maintained in containers that can drain freely or they may periodically sit in a shallow pool of water if drainage is poor. Two experiments were conducted to evaluate how three soil moisture treatments influence root rot caused by P. cinnamomi and P. plurivora. Rhododendrons were either flooded for 48 hours (flood), placed in a shallow saucer kept continuously filled with water (saucer), or allowed to freely drain to 75% container capacity (75% CC). In general, all treatments were conducive to root rot and P. cinnamomi often caused more disease than P. plurivora. However, the amount of root rot in the flood treatment was either similar to or greater than that in the saucer treatment, but was always greater than that on plants in the 75% CC treatment. Results are important because they show that flooding is an efficient and rapid way of inducing root rot that is representative of the amount damage that occurs in nurseries.

Technical Abstract: The degree of flooding commonly used to induce disease in Phytophthora root rot studies rarely occurs in container nurseries. Instead, over irrigation and poor drainage result in plants periodically sitting in shallow pools of water. Rhododendron plants were grown in a noninfested substrate or substrate infested with Phytophthora cinnamomi or P. plurivora to determine whether root rot induced by flooding represents disease that occurs under simulated nursery conditions when plants are in a shallow pool of water (saucers) or are allowed to freely drain and maintained at ~ 75% container capacity (75% CC). P. cinnamomi generally caused more disease than P. plurivora and all water treatments were conducive to root rot. In experiment 1, the amount of disease due to flooding was similar to that in the saucer treatment (75% CC not tested) while in experiment 2, flooding often caused more rapid and severe disease than the saucer or 75% CC treatment. Pathogens differed in their response to water treatments. P. cinnamomi caused more disease in treatments with >90% substrate moisture for a short (flood) or long duration (saucer), while P. plurivora was less capable of causing disease when soil moisture was maintained >90% with substrate moisture was maintained at a more moderate level (flood, 75% CC). Our results indicate that it is not necessary to flood plants to induce disease under experimental conditions and that disease induced by flooding can represent disease in container nurseries when containers are in pools of water or maintained ~75% CC. In addition, our results suggest that P. cinnamomi may be a more aggressive pathogen than P. plurivora in nursery conditions where drainage is poor; however, both species are capable of causing a similar amount of disease under better irrigation management.