Irrigation frequency and volume has little influence on Phytophthora root rot in container-grown rhododendron

Authors: Weiland, Gerald - Jerry; Scagel, Carolyn; Grunwald, Niklaus - Nik; DAVIS, ELIZABETH - Retired ARS Employee; Beck, Bryan; Mitchell, Jesse

Submitted to: Journal of Environmental Horticulture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/9/2022
Publication Date: 6/30/2022
Citation: Weiland, G.E., Scagel, C.F., Grunwald, N.J., Davis, E.A., Beck, B.R., Mitchell, J.N. 2022. Irrigation frequency and volume has little influence on Phytophthora root rot in container-grown rhododendron. Journal of Environmental Horticulture. 40(2):67-78. https://doi.org/10.24266/2573-5586-40.2.67.
DOI: https://doi.org/10.24266/2573-5586-40.2.67

Interpretive Summary: Phytophthora pathogens often cause more severe root rot disease in heavily irrigated or water-logged conditions. We therefore evaluated whether reducing irrigation could reduce the ability of Phytophthora plurivora and P. cinnamomi to cause root rot on rhododendron. We inoculated plants with either a low or high level of inoculum and then irrigated to maintain soil moisture at 70% container capacity (1X), at one half volume of the 1X treatment (0.5X), or at two times the volume of the 1X treatment for one week followed by no irrigation until soil moisture reached <50% container capacity. We then evaluated disease symptoms weekly and evaluated root rot, pathogen presence, plant biomass, and nutrient uptake at the end of the experiment. Both pathogens caused mild root rot at the low inoculum level and severe root rot at the high inoculum level. However, reducing irrigation did not lessen the amount of root rot caused by either pathogen. Instead, severe root rot often led to increased soil moisture as the roots became progressively compromised in their ability to take up water. Results show that reducing irrigation after infection has occurred does little to control root rot. Instead, root rot control efforts should focus on preventing infection in the first place.

Technical Abstract: Phytophthora pathogens, also known as water molds, are expected to cause more severe epidemics in heavily irrigated or waterlogged conditions. We therefore evaluated whether reducing irrigation frequency and volume alters the ability of Phytophthora plurivora and P. cinnamomi to cause root rot on rhododendron. Our treatments included noninfested media or media infested with 1 or 100 propagules per gram (ppg) of potting media. Plants were irrigated to maintain a substrate moisture of >70% container capacity (1.0X), one-half volume of 1.0X (0.5X), or two times the volume of 1.0X at each irrigation event for one week, followed by no irrigation, until soil moisture reached <50% container capacity. Aboveground disease symptoms (chlorosis, stomatal conductance, wilting, and plant death) were evaluated weekly and root rot, pathogen presence, plant biomass, and nutrient uptake were measured at the end of each trial. Both pathogens generally caused mild disease at 1 ppg and severe disease at 100 ppg. Reducing irrigation did little to lessen disease caused by either pathogen once infection had occurred. Instead, severe root infection at 100 ppg often led to increased SM and root rot across all irrigation treatments as roots became progressively compromised in their ability to take up water. Results show that reducing irrigation after infection has occurred is unlikely to effectively control root rot. This work provides novel insights into water management for control of Phytophthora infection in container nurseries.