Abstract
Soybean (Glycine max) seedlings with symptoms of Pythium damping-off were collected in northeastern Iowa soybean fields and processed for isolation of the causal agents on both potato dextrose agar (PDA) and pimaricin-, ampicillin-, rifampicin-, and pentachloronitrobenzene (PARP)-containing media. Isolates were identified based on morphological characteristics, growth rates, along with sequence data for the nuclear rDNA ITS1–5.8S-ITS2 region (ITS barcode). Nine isolates were identified via NCBI BLASTn search of sequences available in GenBank: one isolate of Pythium orthogonon; three isolates of P. inflatum; two isolates of P. ultimum var. ultimum; one isolate of P. torulosum; and two isolates of P. ultimum var. ultimum or P. ultimum var. sporangiferum. Pathogenicity of all the nine isolates, along with a positive control (P. irregulare), was tested in greenhouse conditions on soybean variety Pioneer 22T61R. Soybean seeds were planted in potting mixture inoculated with Pythium inoculum fermented on sterilized proso millet grains. The Pythium spp. were subsequently re-isolated from symptomatic plants. Average incidence of Pythium damping-off across isolates was 27.4% but varied among isolates, ranging from 1.2 to 79.8%. Among the Pythium spp. collected in this single location, the most aggressive isolate was selected to test the efficacy of seed treatments using foliar fungicides in artificially-inoculated field conditions. Out of the eight tested foliar fungicides, six of them significantly suppressed damping-off compared with the untreated control. The average yield advantage of foliar fungicides as seed treatments was 0.23 mt (metric ton)/ha (ranged from 0.15 to 0.31 mt/ha) over the untreated control, with a corresponding economic advantage of $90.69 (range $60.5 to $123.9/ha) based on soybean price at $397/mt as of September 30, 2017. Our findings suggest a potential for using foliar fungicides as seed treatments to control Pythium damping-off, and provide an alternative solution for managing resistance to metalaxyl/mefenoxam seed treatments in soybean production.
Highlights
Worldwide more than 200 pathogens affect soybean (Glycine max [L.] Merr.), of which at least 35 are considered economically important (Hartman 2015)
Pythium isolates collected in Iowa comprise several different species or subspecies Out of 16 isolates that produced oospores and sporangia (Fig. 2), and displayed culture morphology of Pythium spp., nine were selected for ITS barcoding due to differences in culture morphology
ITS sequences obtained from these isolates matched Pythium sequences in GenBank: “C1–2” had 99% (855/864 bases) identity with Pythium orthogonon HQ643723; isolates “D2–1”, “D1– 2”, and “A2–1” had 100% (822/822, 814/814, and 823/ 823 bases, respectively) identity with P. inflatum AY598626; isolates “2–3” and “1–4” had 100% (876/876 and 870/870 bases, respectively) identity with P. ultimum var. ultimum KU210728; isolate “− 4” had 100% (826/826 bases) identity with P. torulosum AY598624; isolate “-?” had 100% identity with both P. ultimum var. ultimum KU211001 (662/662 bases) and P. ultimum var. sporangiferum KT429653 (659/659 bases); and isolate “-??” had 100% identity (886/886 bases) with the sequence obtained from isolate “-?”
Summary
Worldwide more than 200 pathogens affect soybean (Glycine max [L.] Merr.), of which at least 35 are considered economically important (Hartman 2015). In Iowa these include several species in the genus Pythium (Oomycota) that cause damping-off and root rot symptoms in soybean seedlings. Pythium spp., which have very wide host range including grasses and a wide variety of dicotyledonous plants (Waterhouse and Waterston 1964), cause problems in stand establishment, reduce seedling emergence, kill emerged seedlings, and reduce plant vigor (Kirkpatrick et al 2006; Broders et al 2009). Infection by Pythium spp. leads to seed rot and the premature weakening and death of developing seedlings, referred to as damping-off (Kirkpatrick et al 2006). Other important soil-borne pathogens that cause soybean seedling diseases include species of the oomycete genus Phytophthora (Erwin and Ribeireo 1996), as well as the fungal genera Rhizoctonia (Yang 1999), and Fusarium (Díaz Arias et al 2013)
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