Pythium arrhenomanes causal agent of root rot on yellow maize in Mexico
Here we show that Pythium arrhenomanes is a causal agent of root rot on maize. The pathogen was isolated from diseased root tissue collected from a conventional maize crop in Michoacan, during 2015. For taxonomic identification, the obtained isolate was used for morphological characteristics and molecular analysis (ITS and COXII genes). Pathogenicity tests confirmed that P. arrhenomanes causes root rot on maize plants.
KeywordsPythium arrhenomanes Zea mays Root rot Pathogenicity
Samples of diseased plants were collected and taken to the laboratory. The isolation of the pathogen was performed cutting small transversal sections of the crown, approximately of 0.5 cm2 with pathogen symptoms, that were disinfested with a solution of 10% commercial chlorine (Clorox®) during 30 s, rinsed with distilled sterile water, dried with sterile filter paper, placed on potato dextrose agar (PDA) media, and incubated at 24 °C for 24 h. When mycelial growth was observed isolate purification was performed, each mycelium was transferred on water agar for purification and each grown hyphal tip were further transferred on PDA. The isolate was transferred to cryovials with distilled water and kept at 15 °C with the code CPV-669. The isolate was identified in terms of morphological characterization and molecular phylogenetics, followed by pathogenicity tests.
Pathogenicity tests were performed with maize seedlings of the cultivar Overland. Seeds were washed three times with sterile distilled water for 10 min to remove fungicides and insecticides. Seeds were germinated in Falcon tubes (50 mL) with 27 g of the substrate Sunshine Mix #3. Prior to use substrate was sterilized at 120 °C/15 lb. for 30 min. The tubes with the seeds were placed in a growth chamber at 27 °C with a photoperiod of 12/12 h (day/night) and watered with 10 mL every 72 h. The inoculum was prepared by pre-inoculating axenic cucumber tissue (3 mm thick and 9 mm in diameter) with P. arrhenomanes from a four day old PDA mycelial plug, that was incubated at 26 °C for 24 h. Fourteen days after seed germination one cucumber disk with mycelia of P. arrhenomanes was placed on the base of the stem of twenty individual plants and covered with substrate. Other twenty control plants were inoculated with PDA discs without P. arrhenomanes. Five days after inoculation the plants started to show symptoms of reduction of leaf length and plant height, (Fig. 2 a). Ten days after inoculation the pathogen was reisolated from diseased root tissue. The isolates showed the same morphological characteristics as P. arrhenomanes. The isolated was deposited in the National Collection of Genetic Resources of Mexico under the registry number: CM-CNRG 5416.
According to the morphological and molecular characteristics the isolate was identified as P. arrhenomanes. This pathogen was reported for the first time causing symptoms of root rot on corn in the District of Columbia, Canada by Drechsler (1928). P. arrhenomanes is pathogenic on different cereal crops including wheat and rice (Vanterpool 1930; Van Buyten et al. 2013). In Mexico this oomycete has only been reported pathogenic on sugar cane (Riess and Flores 1976; Fernández-Pavía et al. 2015). To the best of our knowledge this is the first report of P. arrhenomanes causing wilting, root necrosis and death of maize plants in Mexico.
We thank PhD Jose de Jesus Luna Ruiz for providing us the maize seeds and PhD Héctor Javier Villegas Moreno for facilitating the photo taking procedure.
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