Evaluation of Biological Agents for Control of Macrophomina Root Rot and Powdery Mildew in Flowering Dogwood (Cornus florida L.)

in HortScience

Six biological control agents (BCAs) (two bacteria, two fungi, and two yeasts) that were previously shown to be effective against powdery mildew (Erysiphe pulchra) were tested for efficacy against Macrophomina phaseolina root rot on flowering dogwood (Cornus florida) in the greenhouse. Two of the bacterial isolates, Stenotrophomonas sp. (B17A) and Serratia sp. (B17B), were effective in controlling both macrophomina root rot and powdery mildew, similar to fungicide control thiophanate methyl, when roots were drenched with the six BCAs individually. In addition, the two bacterial BCAs improved plant growth with respect to stem diameter, stem length, dry weight, and green foliage compared with fungicide-treated plants or nontreated controls grown in sterile soil. These results confirm previous results in which B17A and B17B suppressed powdery mildew and also promoted plant growth in flowering dogwood. Although macrophomina root rot has been previously reported as a potential problem in flowering dogwood, especially in field conditions, simultaneous infection with macrophomina root rot and powdery mildew has not been previously reported. This study confirmed that M. phaseolina infection was characterized by stubby roots and black root lesions, and plants infected with both powdery mildew and macrophomina root rot had smaller root mass compared with fungicide-treated plants. Neither of the two pathogens killed their host plants, but compounded infections significantly reduced the plant root system and plant growth. The efficacy of the two bacterial isolates in controlling both powdery mildew and macrophomina root rot suggests their potential utilization in controlling both diseases in dogwood nursery production and in other plants that are hosts to both powdery mildew and macrophomina root rot. Plant growth promoted by the two BCAs may be attributed to powdery mildew and macrophomina root rot control, but comparisons between fungicide-treated plants and control plants not inoculated with BCAs or root rot pathogen suggested that the two BCAs may play a role as bio-stimulants in growth enhancement. These results also suggest that the two biocontrol agents are not phytotoxic to dogwood.

Contributor Notes

We would like to thank Richard Hall for his critical review of this manuscript. The research was partly funded by USDA-NIFA capacity building grants, project no. TENX-2010-02399.

Corresponding author. E-mail: mmmbaga@tnstate.edu.

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    (A) Effect of six biocontrol agents (BCA) (B17A, B17B, F13, F16, Y4, and Y14), compared with the fungicide thiophanate methyl (fungicide) and nontreated control (water) on dogwood (Cornus florida) seedlings planted in heat sterilized soil infested with Macrophomina phaseolina and then exposed to airborne spores of Erysiphe pulchra, with treatment applied by soil drenching. (B) Overall plant growth and powdery mildew severity in plants treated with bacterial BCA (B17A and B17B) and fungicide (fungicide) compared with control plants with macrophomina root rot and no BCA (Control+Mp) and control plants grown in sterile soil with no BCAs and no macrophomina root rot (Control).

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    Powdery mildew disease severity on Cornus florida plants grown in soil infested with Macrophomina phaseolina (Mp) and drenched with six biocontrol agents (B17A, B17B, F16, Y14, Y4, and F13), thiophanate methyl, and nontreated control (Control+Mp) and nontreated control with no macrophomina and no biocontrol (control). Disease severity on a 0 to 5 scale in which 0 = no infection; 1 = 1% to 10%; 2 = 11% to 25%; 3 = 26% to 50%; 4 = 51% to 75%; and 5 = 76% to 100% of plant covered with powdery mildew symptoms. *Significantly different from nontreated control (Control) and Control+Mp at P < 0.05 in each experiment. Standard error bars at P < 0.05

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    Roots of flowering dogwood seedlings grown in soil infested with Macrophomina phaseolina and drenched with selected biocontrol agents and the fungicide thiophanate methyl in the greenhouse: (A) root discolorations and (B and C) stubby roots with no feeder roots compared with (D) healthy roots with plenty of feeder roots in effective treatments (B17A, B17B, Fungicide, and control with no macrophomina), (E) root hairs (arrows), and (F) larger root mass in plants treated with effective biocontrol agents B17A, B17B, and fungicide.

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    Stem height and stem diameter as a measure of plant growth in Cornus florida seedlings grown in soil infested with Macrophomina phaseolina and drenched with biocontrol agents (B17A, B17B, F13, F16, Y4, and Y14) and fungicide thiophanate methyl (Fungicide), compared with nontreated control with macrophomina (Control+Mp) and plants grown in sterile soil with no M. phaseolina (Control). *Significantly different from nontreated control with macrophomina (Control+Mp). **Significantly different from Control and Control+Mp at P < 0.05 in each experiment. Standard error bars at P < 0.05

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    Oven dry weight of Cornus florida seedlings grown in soil infested with Macrophomina phaseolina and treated with biocontrol agents (B17A, B17B, F13, F16, Y4, and Y14) and compared with the fungicide thiophanate methyl (Fungicide), nontreated control (Control+Mp), and plants grown in sterile soil with no macrophomina (Control). No data were taken in Expt 1. Standard error bars at P < 0.05.

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