RESUMO
Potato (Solanum tuberosum) plants showing blackleg and soft rot symptoms were collected at a commercial vegetable farm near Newmanstown, PA in August 2021 (Fig. S1). The incidence of potato blackleg in the unirrigated field was about 5 to 8%, but approximately 30% in the irrigated field. The diseased stems were cut into 5 cm and surface disinfected. The stem segments were placed into a 50-mL tube containing 15 mL of sterile water for 15 min for bacterial release. The bacterial suspension was streaked on crystal violet polypectate (CVP) (Hélias et al. 2012) plates and incubated at 28°C for 48 h. Three single colonies produced pits on CVP were picked and purified. Genomic DNA of all three isolates were extracted using the FastDNA Spin Kit (MP Biomedicals, Santa Ana, CA). Polymerase chain reaction (PCR) was performed using all three extracted DNAs as a template with the primer pairs gapA 7F/938R (Cigna et al. 2017), recA F/R (Waleron et al. 2001), dnaA F/R (Schneider et al. 2011) and dnaX F/R (Slawiak et al. 2009) targeting the gapA, recA, dnaA and dnaX genes, respectively. Isolate 21PA01 was further studied as a representative isolate. PCR amplicons derived from both forward and reverse primers were sequenced and analyzed using the BLAST algorithm against the NCBI database (https://www.ncbi.nlm.nih.gov). The regions of gapA (GenBank accession No. ON989738), recA (ON989739), dnaA (OP121183), and dnaX (OP121184) had 99.86%, 100%, 98.88%, and 100% identities with Pectobacterium brasiliense strains S1.16.01.3M (MN167062.1), BL-2 (MW721598.1), IPO:4132 (CP059956.1), and BL-2 (MW721603.1), respectively. A phylogenetic maximum-likelihood tree of the concatenated genes with the length of 2551 bp was constructed to visualize the relationship among different species of Dickeya and Pectobacterium. As a result, 21PA01 was in a single monophyletic cluster with other Pectobacterium brasiliense reference strains (Fig. S2 C). To confirm the pathogen, Koch's postulates were performed. Seed pieces of potato 'Lamoka' were planted in potting mix in one-gallon plastic pots in a greenhouse. Three weeks after emergence, the stems of three plants were each injected with 10 µL of bacteria suspension of either 21PA01 at 107 CFU/mL, P. parmentieri ME175 in tryptic soy broth (TSB) at 107 CFU/mL or TSB at 2 cm above the soil line. Seven days after inoculation, stems inoculated with 21PA01 and ME175 showed black and rotten symptoms, whereas the TSB-injected control plants remained symptomless. In addition, 'Lamoka' tubers were inoculated by placing 10 µL 21PA01 and ME175 suspensions at 107 CFU/mL, and TSB in a 1-cm-deep hole poked in a tuber separately and then sealed with petroleum gel, followed by incubation in a moist chamber at 22 °C for 4 d. The 21PA01 and ME175 inoculated tubers showed soft rot symptoms, but the TSB treatment had no symptoms. Bacterial colonies were isolated from the infected stems and confirmed by the DNA sequences as described above. PCR result was negative on control plant samples. Both stem and tuber inoculation trials were repeated two times, and the results were consistent. Thus, 21PA01 was identified as Pectobacterium brasiliense. To our knowledge, this is the first report of P. brasiliense infecting potatoes in Pennsylvania, USA, although it has been reported somewhere else (van der Merwe et al. 2010, Zhao et al. 2018). This could be a new species in Northeastern US.
RESUMO
Arbuscular mycorrhizal (AM) fungi are considered to be a key group of soil organisms for assessments of soil biological properties and developing relationships among crop production management practices, soil properties, crop performance, and ecosystem services. In a field study of cover crop treatments established during the transition from small grains to corn (Zea mays L), we assessed multiple measures of AM fungal responses to the management treatments: soil propagule numbers, biomass via lipid biomarkers, and root colonization extent. Our objectives were to determine response variables that reliably distinguished cover crop treatments and formed consistent relationships with grain yield, plant biomass, and mineral nutrient concentrations of the following corn crop. The number of soil AM fungal propagules and amount of the NLFA biomarker C16:1cis11 measured on fall-collected soils most consistently and significantly responded to fall cover crop treatments. Neither of these measures of soil inoculum potential was strongly related to measures of crop performance. The PLFA biomarker C16:1cis11 was marginally responsive to cover crop but did not strongly relate to crop performance parameters. Corn root colonization by AM fungi was not significantly affected by cover crop treatment, but significant negative relationships were found between root colonization and grain N concentration and plant biomass at maturity. In contrast, a significant positive relationship between root colonization and plant N concentration at the 6-leaf stage was found. Understanding the relative effectiveness and limitations of AM fungal response variables will inform their application in field studies of agricultural management practices.
