Evaluation of Poinsettia Cultivars for Resistance to Pythium Root Rot Caused by Pythium aphanidermatum

Lookabaugh, E. C.; Whipker, Brian E.; Shew, B. B.

doi:10.21273/horttech03817-17

Cited by 4 publications

(2 citation statements)

References 17 publications

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“…After 5 d of pathogen inoculation, disease symptoms were observed and recorded at 5-d intervals. The disease severity was recorded using a scale of 0 to 4 according to Lookabaugh's work with modifications (Lookabaugh et al 2017) Areas under the disease progress curves (AUDPCs) were calculated using the disease severity obtained at different times after inoculation:…”

Section: Greenhouse Trialmentioning

confidence: 99%

“…Pythium aphanidermatum is a recurrent disease and the predominant Pythium species causing poinsettia root rot disease, thereby significantly affecting poinsettia production in greenhouses across the United States (Lookabaugh et al 2020;M unera et al 2019). Under favorable environmental conditions, P. aphanidermatum causes stunting, root rot, wilting, defoliation, chlorosis, and, in severe cases, plant death (Lookabaugh et al 2017).…”

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confidence: 99%

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Biochar and Trichoderma Reduce Containerized Poinsettia Root Rot Caused by Pythium aphanidermatum

Ong

Ueckert

et al. 2023

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Poinsettia (Euphorbia pulcherrima) root rot disease caused by Pythium aphanidermatum is a significant problem for poinsettia production. Biochar can replace peatmoss, a commonly used substrate, and has many benefits, including reducing soilborne diseases. A greenhouse study was conducted to test the suitability of mixed hardwood biochar (HB) for greenhouse potting mix and its potential to reduce poinsettia root rot disease. Mixed HB was blended with the commercial substrate (CS) at 0%, 20% and 40% [CS100 (control), HB20, HB40, respectively, by volume]. Half of the poinsettia plants were inoculated with P. aphanidermatum, and the other half were not. Trichoderma spp. were inoculated to half of the pathogen-inoculated and pathogen-free plants. The plant growth index (GI), shoot dry weight (SDW), disease severity (DS), and disease incidence (DI) were measured. In the absence of the pathogen, plant growth in HB20 and HB40 had similar GI and SDW with the control. In the presence of the pathogen, plants grown in HB20 had significantly higher SDW but lower DS and DI than the control. HB20 could replace peatmoss-based substrate to reduce poinsettia root rot disease caused by P. aphanidermatum.

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Section: Greenhouse Trialmentioning

confidence: 99%

mentioning

confidence: 99%

Biochar and Trichoderma Reduce Containerized Poinsettia Root Rot Caused by Pythium aphanidermatum

Ong

Ueckert

et al. 2023

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Fitness Attributes of Pythium aphanidermatum with Dual Resistance to Mefenoxam and Fenamidone

et al. 2018

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Pythium aphanidermatum is the predominant species causing Pythium root rot on commercially grown poinsettias in North Carolina. Resistance to mefenoxam is common in populations of P. aphanidermatum but resistance to fenamidone and other quinone outside inhibitor fungicides has only just been reported in greenhouse floriculture crops. The in vitro sensitivity to the label rate of mefenoxam (17.6 μl active ingredient [a.i.]/ml) and fenamidone (488 μl a.i./ml) was determined for 96 isolates of P. aphanidermatum. Isolates were assigned to four fungicide phenotypes: mefenoxam-sensitive/fenamidone-sensitive (MefS, FenS), mefenoxam-sensitive/fenamidone-insensitive (MefS, FenR), mefenoxam-insensitive/fenamidone-sensitive (MefR, FenS), and mefenoxam-insensitive/fenamidone-insensitive (MefR, FenR). In all, 58% of isolates were insensitive to one (MefR, FenS = 36% and MefS, FenR = 16%) or both fungicides (MefR, FenR = 6%). A single point mutation in the cytochrome b gene (G143A) was identified in fenamidone-insensitive isolates. Mycelial growth rate at three temperatures (20, 25, and 30°C), in vitro oospore production, and aggressiveness on poinsettia were evaluated to assess relative fitness of sensitive and insensitive isolates. Isolates with dual insensitivity to mefenoxam and fenamidone had reduced radial hyphal growth at 30°C and produced fewer oospores than isolates sensitive to one or both fungicides. Isolates sensitive to both fungicides produced greater numbers of oospores. Aggressiveness on poinsettia varied by isolate but fungicide phenotype was not a good predictor of aggressiveness. These results suggest that populations of P. aphanidermatum with dual resistance to mefenoxam and fenamidone may be less fit than sensitive populations under our imposed experimental conditions but populations of P. aphanidermatum should continue to be monitored in poinsettia production systems for mefenoxam and fenamidone insensitivity.

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Evaluating Fungicide Selections to Manage Pythium Root Rot on Poinsettia Cultivars with Varying Levels of Partial Resistance

2021

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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.

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Evaluation of Poinsettia Cultivars for Resistance to Pythium Root Rot Caused by Pythium aphanidermatum

Cited by 4 publications

References 17 publications

Biochar and Trichoderma Reduce Containerized Poinsettia Root Rot Caused by Pythium aphanidermatum

Biochar and Trichoderma Reduce Containerized Poinsettia Root Rot Caused by Pythium aphanidermatum

Fitness Attributes of Pythium aphanidermatum with Dual Resistance to Mefenoxam and Fenamidone

Evaluating Fungicide Selections to Manage Pythium Root Rot on Poinsettia Cultivars with Varying Levels of Partial Resistance

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