- Phytophthora root rot decreases availability and quality of rhododendrons produced in the USA. Symptoms of Phytophthora root rot include root necrosis, leaf chlorosis, stunting, and permanent wilt. The purpose of this thesis was to better understand the impact of root damage, soil moisture, nitrogen fertilizer application, and pathogen species on root rot development.
In chapter 2, a greenhouse experiment with container rhododendrons was conducted to assess if Phytophthora root rot induced by flooding in research studies is representative of disease progression under nursery conditions, whether physical damage to roots increases disease severity, and whether these factors affect root rot caused by Phytophthora cinnamomi and P. plurivora. Rhododendron ‘Boule de Neige’ (trial 1) and ‘Scintillation’ (trials 2 and 3) with either low or high amounts of physical root damage were grown in a soilless substrate infested with P. cinnamomi or P. plurivora, then subjected to two water treatments for 18 weeks after inoculation (WAI) (trials 1 and 2) and 24 WAI (trial 3). Plants were either flooded for 48 h then maintained at or below container capacity or not flooded and kept in saucers of water to maintain substrate moisture ≥ 95% container capacity. The flooded treatment is typical of methods used to induce Phytophthora root rot in experimental conditions, whereas the plants in saucers were considered representative of nursery conditions when container plants sit in a shallow pool of water. Root rot and mortality were observed in both the flood and saucer treatment. In general, root rot in plants inoculated with P. cinnamomi was more severe than in plants inoculated with P. plurivora. There were few differences in disease induced by flooding compared to disease induced by saucers in plants inoculated with P. cinnamomi. In contrast, there was more disease in plants inoculated with P. plurivora in the saucer treatment compared to the flood treatment. Across all pathogen and water treatments, there were no differences in disease between low and high physical root damage.
P. cinnamomi was generally a more aggressive pathogen than P. plurivora and the experimental method of flooding inoculated plants to induce disease produces similar results as placing inoculated plants into a saucer water. It was not necessary to flood for 48 h to induce disease under experimental conditions. However, root rot damage occurred in noninoculated plants of the saucer treatment so future experiments should continue to use the flood treatment to induce disease. Future research should also compare both the flood treatment and the saucer treatment to an inoculation method that maintains soil moisture around 60 to 70% container capacity, as a high soil moisture content can change soil properties and alter host physiology.
In chapter 3, a greenhouse experiment was conducted to assess the impact of increasing N fertilizer rate on the progression and severity of Phytophthora root rot of rhododendron caused by P. cinnamomi and P. plurivora. Rhododendron catawbiense ‘Boursault’ was grown with no (0 g N/ pot), low (1.04 g N/pot), and high (3.12 g N/pot) N (urea) incorporated into the container substrate and infested with either P. cinnamomi or P. plurivora for 18 weeks after inoculation (WAI). Application of N changed plant physiology and promoted greater biomass and leaf greenness when plants were not inoculated with Phytophthora. Nitrogen application enhanced N, K, Mg, P, S, and Mn uptake in low and high N treatments. The results suggest that differences in growth between plants in the no N treatment and those fertilized with N were primarily driven by N availability and its influence on the uptake of other nutrients and water. The severity of Phytophthora root rot was greater in plants inoculated with P. cinnamomi but not in plants inoculated with P. plurivora with increased N. While mortality of plants inoculated with P. plurivora was greater from no N to high N, the difference was not significant. Root rot and mortality was greater with added N compared to the no N application rate (from 10 to 50% and 0 to 10% in P. plurivora plants in trials 1 and 2 respectively, and from 20 to 70% and 30 to 90% in P. cinnamomi plants in trials 1 and 2 respectively). On average, inoculation with P. plurivora did not restrict uptake of any nutrient, but inoculation with P. cinnamomi restricted uptake of Mg, Mn, and Cu. Although reducing N application would reduce disease severity, the quality of plants infected with either pathogen would be suboptimal as shoot mortality still occurred in the no N treatment. Decreasing plant N status only slowed the disease. As nurseries may apply high amounts of N during production to promote fast growth and economic production, it is important to understand how N affects growth and disease. In order to understand how N (both rate and form) affects disease, more research is needed on different rhododendron cultivars sensitivity to N and effects of N in vitro on P. cinnamomi and P. plurivora. The effect of increased N via foliar application should also be examined, as disease severity may not be greater with added N if N is applied away from the site of infection.
In general, plants inoculated with P. cinnamomi had more severe disease than plants inoculated with P. plurivora in both experiments. Although there was no difference in disease severity as a result of physical root damage, root rot was just as severe, or more severe in the saucer treatment than the flood treatment for P. cinnamomi and P. plurivora inoculated plants, as well as in the noninoculated control plants. Disease progression and severity was greater with increased N in plants inoculated with P. cinnamomi but not P. plurivora. Understanding the impact of root damage, soil moisture, nitrogen fertilizer application, and pathogen species on root rot development requires further research. These factors affect disease development and management in the nursery industry and how pathology studies are conducted.