Characterization of Pythium Species Associated With Greenhouse Floriculture Crops in Michigan

Abstract

Michigan ranks third in the United States for the wholesale value of floriculture products, with an estimated value of $375.7 million. Seedling damping-off and root and crown rot are commonly caused by Pythium spp. and are important problems for greenhouse growers. Pythium spp. associated with Michigan’s floriculture crops were characterized as a means to improve current management strategies. During 2011 and 2012, potted poinsettias with root rot symptoms were sampled from nine greenhouses located in Kent, Kalamazoo, and Wayne counties. In 2013, from the same three counties, symptomatic geranium and snapdragon bedding plants were sampled from 12 greenhouses. Additionally, symptomatic hibiscus and lantana plants were sampled at one greenhouse facility. Isolates were confirmed to be Pythium spp. via morphology and sequencing of the ITS region. A total of 287 Pythium spp. isolates were obtained from poinsettias and 726 isolates from geranium, snapdragon, hibiscus, and lantana. Seven Pythium spp., and a group of isolates determined as Pythium sp. 1 were identified. The most prevalent species were P. irregulare, P. ultimum, and P. aphanidermatum. A subset of isolates was chosen for pathogenicity and mefenoxam sensitivity testing. Six of the species were virulent to germinating geranium seeds. Most P. ultimum and P. cylindrosporum isolates tested were intermediate to highly resistant to mefenoxam, whereas most P. aphanidermatum isolates were sensitive. This study suggests that Pythium spp. recovered from Michigan greenhouses may vary depending on the host, and that mefenoxam may not be effective to control P. ultimum or P. cylindrosporum.

Integrating Host Resistance and Plant Protectants to Manage Pythium Root Rot on Geranium and Snapdragon

“…Plant protectants and host resistance were evaluated as a means to manage highly virulent Pythium spp. commonly associated with floriculture crops in Michigan (Del Castillo-M unera and Hausbeck, 2014). There are relatively few effective products regis-tered to control Pythium root rot in floriculture crops (Garz on et al, 2011).…”

“…Mefenoxam sensitivity was determined based on radial mycelial growth on corn meal agar (CMA) amended with 100 ppm of mefenoxam. An isolate was determined as resistant if radial mycelial growth was >50% compared with the radial mycelial growth in unamended CMA (Del Castillo-M unera and Hausbeck, 2014;Olson and Benson, 2011).…”

Influence of pH and Etridiazole on Pythium Species

“…Catastrophic losses can occur if plants develop symptoms near the time of retail sale (Hausbeck et al, 1987;Moorman, 1986). Numerous Pythium species cause disease on ornamental crops; however, P. ultimum, P. aphanidermatum, and P. irregulare are isolated frequently from symptomatic plants in commercial production (Del Castillo M unera and Hausbeck, 2016;Moorman et al, 2002). Pythium species are ubiquitous in natural environments, and maintaining production facilities free of the pathogen is difficult.…”

“…Historically, these fungicides have provided effective control of pythiaceous organisms when applied as soil drenches (Benson, 1979;Raabe et al, 1981;Stephens and Stebbins, 1985) or mixed into the potting medium (McCain and Byrne, 1966). Resistance to mefenoxam has developed in greenhouse populations of Pythium and Phytophthora because of the site-specific mode of action of the fungicide and selection pressure from the repeated use of this active ingredient (Del Castillo M unera and Hausbeck, 2016;Lamour et al, 2003;Moorman et al, 2002). In Michigan, greater than 35% of Pythium species isolates collected from greenhouses were resistant to mefenoxam (Del Castillo M unera and Hausbeck, 2016).…”

“…Resistance to mefenoxam has developed in greenhouse populations of Pythium and Phytophthora because of the site-specific mode of action of the fungicide and selection pressure from the repeated use of this active ingredient (Del Castillo M unera and Hausbeck, 2016;Lamour et al, 2003;Moorman et al, 2002). In Michigan, greater than 35% of Pythium species isolates collected from greenhouses were resistant to mefenoxam (Del Castillo M unera and Hausbeck, 2016). Control failures have been reported in greenhouses and nurseries (Ferrin and Rohde, 1992;Moorman and Kim, 2004) and may become more common if resistant isolates are accidently moved among greenhouses with prefinished plants and propagative material (Moorman et al, 2002).…”