Investigating Chemical and Biological Control Applications for Pythium Root Rot Prevention and Impacts on Creeping Bentgrass Putting Green Rhizosphere Bacterial Communities

Pythium root rot (PRR) is a disease that can rapidly devastate large swaths of golf course putting greens, with little recourse once symptoms appear. Golf courses routinely apply preventative fungicides for root diseases, which may be altering the rhizosphere microbiome leading to unintended impacts to plant health. A multi-year field trial was initiated on a ‘T-1’ creeping bentgrass (Agrostis stolonifera L. cv. ‘T-1’) putting green in College Park, Maryland to evaluate preventative PRR management for disease suppression and impacts to rhizosphere bacterial communities. Fungicides commonly used to prevent PRR and a biological fungicide were repeatedly applied to experimental plots throughout the growing season. Rhizosphere samples were collected twice annually from each plot to evaluate rhizosphere bacterial communities through amplicon sequencing and monitor biological control organism populations via qPCR. Cyazofamid was the only treatment to suppress PRR in both years compared to the control. Fosetyl-Al on a 14 d interval and Bacillus subtilis QST713 also reduced PRR severity in 2019 compared to the non-treated control. Treatments did not significantly alter bacterial communities, however seasonal environmental changes did. Repeated rhizosphere targeted applications of B. subtilis QST713 appear to have established the bacterium into the rhizosphere, as populations increased between samples, even after applications stopped. These findings suggest that QST713 may reduce pathogen pressure when repeatedly applied and can reduce fungicide usage during periods of low PRR pressure.

Identification of metalaxyl and ethaboxam insensitive Pythium sylvaticum pathogenic to pulse crops in Montana, USA.

Pythium root rot and damping-off caused by Pythium spp. are important diseases of pulse crops. In a 2016 pathogen survey from dry pea growing fields in Montana, along with commonly known causal agents P. ultimum and P. irregulare, an isolate identified as P. sylvaticum (LPPY17) was isolated from the rhizosphere of a diseased pea plant collected from Valley County, MT. Root rots and damping-off caused by P. sylvaticum have not previously been reported for chickpea, pea, and lentil crops. The isolate LPPY17 was tested for fungicide resistance in vitro, and results indicated a reduced sensitivity to metalaxyl and ethaboxam containing fungicides. LPPY17 was also tested for pathogenicity on chickpea, pea, and lentil seedlings in the greenhouse, and the results from the study revealed LPPY17 is capable of causing both root rots and damping off. Due to the potential pathogenicity and reduced fungicide sensitivity of this species, in the future it will be important to monitor for P. sylvaticum in pulse root rot surveys and diagnostics, as management options may be different from other common Pythium spp.

Management of Pythium myriotylum in Tobacco Transplant Production Greenhouses

Pythium root rot is a common disease that can threaten tobacco seedling production in greenhouses. However, management tools are limited in tobacco transplant production greenhouses. To identify additional Pythium control options, oomyceticide treatments (ethaboxam, mefenoxam, and copper ethanolamine complex) and non-oomyceticide (ultraviolet light and copper ion) water treatments were compared with etridiazole and an untreated control on TN 90LC tobacco seedlings inoculated with P. myriotylum in greenhouses. All the treatments in oomyceticide trials were applied to the bay water once before inoculation, when seedling roots had extended into the water. The inoculum was applied immediately before seeding in non-oomyceticide trials, where etridiazole was applied to bay water once, two weeks after seeding, as a positive control. Non-oomyceticide treatments were applied three times: 24 hours before, two weeks after, and four weeks after seeding. At the end of the tobacco transplant production season, ethaboxam and mefenoxam significantly (P<0.05) reduced root rot incidence and severity by as high as 100%, compared with the untreated control. Ethaboxam and mefenoxam also significantly (P<0.05) reduced oospores produced in infected root tissues, while significantly (P<0.05) increasing root length and weight. Ultraviolet radiation and copper ion treatments had no significant effects on tobacco seedling root length or weight compared with the untreated control, although the copper ion treatments significantly (P<0.05) reduced root rot severity and oospores produced in root tissues. Similar to etridiazole, ethaboxam and mefenoxam consistently reduced the AUDPC of Pythium root rot, but copper ion treatments only reduced AUDPC significantly (P<0.05) in one trial.

Evaluating fungicide selections to manage Pythium root rot on poinsettia cultivars with varying levels of partial resistance

Pythium aphanidermatum is the predominant species causing Pythium root rot of commercially grown poinsettia (Euphorbia pulcherrima Willd. ex Kotzch) in North Carolina. Pythium root rot is managed primarily through a combination of sanitation practices and preventative fungicide applications of mefenoxam or etridiazole. Insensitivity to mefenoxam is common but growers continue to rely on it due to lack of inexpensive and efficacious alternatives. This research was conducted to identify alternative fungicides for Pythium root rot control and evaluate their efficacy on poinsettia cultivars with varying levels of partial resistance. Greenhouse studies were conducted to assess efficacy of fungicide treatments in seven poinsettia cultivars inoculated with a mefenoxam-sensitive isolate of P. aphanidermatum. One study examined control with a single fungicide drench made at transplant and a second study examined repeat fungicide applications made throughout the experiment. Treatments containing etridiazole, mefenoxam, fenamidone, and cyazofamid provided control of Pythium root rot across all cultivars in both experiments whereas Fosetyl-al, potassium phosphite, and Trichoderma spp. failed to offer satisfactory control. Azoxystrobin, pyraclostrobin, and propamocarb reduced disease on some cultivars but failed to control Pythium root rot on highly susceptible cultivars. Four isolates of P. aphanidermatum cultured from plants growing in commercial greenhouses were evaluated for in vitro sensitivity to fungicides labeled for Pythium root rot control at four rates. Etridiazole, fosetyl-al, and potassium phosphite completely inhibited mycelial growth, whereas isolates varied in response to mefenoxam, cyazofamid, propamocarb, fenamidone, azoxystrobin, and pyraclostrobin in vitro. Twenty-one additional isolates then were evaluated at label rates of these fungicides. Seven isolates were insensitive to label rates of all three quinone outside inhibitors (QoIs) and one isolate was insensitive to the QoIs and mefenoxam. These results provide guidelines for selecting fungicides to maximize control of Pythium root rot on poinsettia cultivars.

Sanitation of Float Trays for the Management of Pythium Species in Tobacco Float Systems

Pythium root rot is an economically important disease threatening greenhouse production of tobacco seedlings. Although methyl bromide was historically used for tray sanitation, the phase-out of the fumigant from agricultural use has left few options for growers to produce disease-free transplants. Steam sanitation at 80°C for 30 min has shown control of disease caused by Rhizoctonia solani and has been adopted for use to manage Pythium spp. This study evaluates other steam temperatures and time durations to effectively manage Pythium spp. in float-tray systems. Naturally infested trays steamed at 63, 71, and 77°C for 30 min significantly reduced Pythium spp. from trays compared with TriSan wash and CC-15 dip treatments. Float trays inoculated with Pythium spp. that were steamed at 70 and 80°C for 2 h 30 min, respectively, also significantly reduced Pythium spp. survival. Other fungi, likely saprophytic or beneficial organisms, were not significantly impacted by any steaming treatment.

Rapid Sand Filtration of Recycled Irrigation Water Controlled Pythium Root Rot of Poinsettia in Greenhouse

Pythium species incite crown and root rot and can be highly destructive to floriculture crops in greenhouses, especially when irrigation water is recycled. This study assessed the performance of rapid filtration of recycled irrigation water for controlling pythium root rot of poinsettia (Euphorbia pulcherrima) in greenhouses. Two greenhouse experiments investigated the effect of filter media type (sand and activated carbon), fungicide application (etridiazole), and pathogen inoculum source (infested growing media and infested irrigation water). Rapid sand filtration consistently controlled pythium root rot of poinsettia. Significant improvements in height, weight, root rot severity, and horticultural quality were observed for the plants in the sand filter treatment, compared with the inoculated control plants. However, the activated carbon filter removed essential nutrients from the irrigation water, resulting in plant nutrient deficiency and consequently leaf chlorosis, thus reducing plant weight, height, and horticultural quality. The etridiazole application did not completely prevent root infection by Pythium aphanidermatum, but plant weight, height, and horticultural quality were not negatively affected. P. aphanidermatum spread from infested growing media to healthy plants when irrigation water was recycled without filtration. Rapid sand filtration appears to have the potential to limit the spread of P. aphanidermatum that causes root rot of greenhouse floriculture crops.