Is Disease Induced by Flooding Representative of Nursery Conditions in Rhododendrons Infected with Phytophthora cinnamomi or P. plurivora?

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). Generally, P. cinnamomi 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 either a short (flood) or long duration (saucer), while P. plurivora was less capable of causing disease when soil moisture was maintained >90% than when 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 at ~75% CC. In addition, our results suggest that P. cinnamomi is 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 more typical irrigation management.

Irrigation Frequency and Volume has Little Influence on Phytophthora Root Rot in Container-grown Rhododendron

“…Phytophthora cinnamomi was isolated approximately twice as often from inoculated, asymptomatic plants than P. plurivora [14/86 (16%) versus 9/88 (10%) at 1 ppg and 23/49 (49%) versus 7/26 (27%) at 100 ppg, respectively]. This is similar to observations from previous studies, where a number of inoculated plants would become infected, but never develop symptoms within the timeframe of the experiment (Mestas et al 2022, Sacher et al 2021, Weiland et al 2018. It is unknown what effect these asymptomatic infections will have on long-term plant health (Weiland 2021).…”

“…This confirms our previous research showing the plant health consequences of low Phytophthora inoculum levels (Weiland et al 2018) and the ability of both Phytophthora species to cause severe disease when soil inoculum levels are ! 50 ppg (Mestas et al 2022, Sacher et al 2021, Weiland et al 2018. It also confirms that P. cinnamomi can be more aggressive than P. plurivora as a root rot pathogen on rhododendron (Mestas et al 2022, Sacher et al 2021, Weiland et al 2018.…”

“…50 ppg (Mestas et al 2022, Sacher et al 2021, Weiland et al 2018. It also confirms that P. cinnamomi can be more aggressive than P. plurivora as a root rot pathogen on rhododendron (Mestas et al 2022, Sacher et al 2021, Weiland et al 2018. Given the prevalence of P. cinnamomi and P. plurivora in the PNW rhododendron nursery industry and their potential for causing severe disease (Weiland et al 2020), both species should continue to be included in Phytophthora root rot disease control studies.…”

“…Irrigation treatment were altered in trial 1 based on daily measurements and in trials 2 and 3 based on average daily measurements. All plants were flooded 2 WAI (trial 1 and 2) or 3 WAI (trial 3) to encourage zoospore release and infection according to previous methods (Mestas et al 2022, Sacher et al 2021, Weiland et al 2018 Substrate environmental variables [VWC, substrate temperature (T sub ), and electrical conductivity (EC)] were monitored differently among trials (T sub and EC data not presented). In trial 1, substrate measurements were taken five times per week at mid-day using a hand-held probe (GS3 probe with ProCheck meter, Decagon Devices, Inc., Pullman WA) from each irrigation treatment in control plants from 2 WAI to 12 WAI and plants inoculated with P. cinnamomi at 100 ppg from 6 WAI to 12 WAI (n¼10).…”

“…Phytophthora infection is favored by abundant SM because the zoospores produced by the pathogen need water to move to the roots and infect the host (Blaker and MacDonald 1981, Hoitink et al 2014, Krebs 2013. Therefore, root rot is often more severe when SM is high (Blaker and MacDonald 1981, Krebs 2013, Ownley and Benson 1991, Mestas et al 2022. In blueberry fields, irrigation practices that reduce water saturation are associated with less root rot by P. cinnamomi (Bryla and Linderman 2007, Vargas et al 2015, Yeo et al 2017.…”