Herbaspirillum seropedicae promotes maize growth but fails to control the maize leaf anthracnose

Abstract

Herbaspirillum seropedicae is an endophytic diazotrophic bacterium and a plant growth promoting bacteria. Colletotrichum graminicola causes the anthracnose, one of the most destructive maize diseases worldwide. The main objective of this work was to evaluate the effects of H. seropedicae SmR1 strain on the plant growth and leaf anthracnose of maize plants grown in substrate amended or not amended with humic substances. In the first assay, plants were pre-treated with H. seropedicae and inoculated with C. graminicola at 7, 14 and 21 days after treatment (DAT). In the second assay, plants were treated with H. seropedicae, grown in substrate amended with humic substances and inoculated at 3 and 7 DAT. The anthracnose severity was assessed by measurement of necrotic and chlorotic leaf area, and bacteria were quantified in leaves by quantitative PCR. H. seropedicae did not affect the disease severity in maize leaves, although it efficiently colonized the leaf tissues and it promoted maize leaf growth. Humic substances improved H. seropedicae colonization in maize.

Keywords: Anthracnose; Biocontrol agent; Colletotrichum graminicola; Diazotrophic bacteria; Herbaspirillum seropedicae; Plant growth promoting bacteria.

Fig. 1

qPCR standard curves for H. seropedicae quantification. a Cq versus log DNA copy number. Curve generated using DNA isolated from pure culture of H. seropedicae SmR1 as template. b Cq versus log CFU. Curve generated using DNA isolated from 100 µL of each serial dilution of H. seropedicae culture mixed with 100 mg of crushed in vitro plant-leaf tissues as template

Fig. 2

H. seropedicae DNA quantification in leaves of P30F53 Zea mays, after inoculation with C. graminicola 3 and 7 days after treatment (DAT) with H. seropedicae and with or without humic substances. The groups represent: −H.s − Humics (non treated with H. seropedicae, without humics), −H.s + Humics (non treated with H. seropedicae, with humics), +H.s − Humics (treated with H. seropedicae, without humics) and +H.s + Humics (treated with H. seropedicae, with humics): aH. seropedicae DNA copy number/g leaf (fresh weight) using standard curve generated with DNA isolated from H. seropedicae pure culture. bH. seropedicae CFU/g leaf (fresh weight) using standard curve generated with DNA isolated from serial dilution of H. seropedicae culture mixed with crushed plant-leaf tissues. Values presented as mean ± standard deviation (n = 8). Lowercase and capital letters indicate statistical difference using Kruskal–Wallis test (a = 0.05) at 3 and 7 DAT, respectively