Research Update: Plant growth promoting bacteria used to control gray mold

This report is funded by the ASCFG Research Foundation.
Kaylee A. South and Michelle L. Jones I The Ohio State University DC Kiplinger Floriculture Crop Improvement Program

Plant growth promoting bacteria as biocontrol agents

Microorganisms, including bacteria and fungi, play an important role in plant growth and overall health. These microorganisms can be pathogens that cause disease, they can have no effect on the plant, or some can even be beneficial. Plant growth promoting bacteria (PGPB) can benefit plants by improving nutrient availability and plant uptake or increasing tolerance to diseases and environmental stresses. Some PGPB can be used as biocontrol agents for the control of plant diseases such as gray mold, which is caused by the pathogenic fungus Botrytis cinerea. We have previously identified several PGPB that can reduce disease symptoms caused by Botrytis in petunias (South et al., 2020a). Botrytis infects many different crops and can result in large postharvest losses of cut flowers. PGPB are a promising control method for Botrytis in cut flowers (Figure 1).

Figure 1. Plant growth promoting bacteria (PGPB) are being evaluated for the biocontrol of fungal pathogens in cut flowers.

Background and Objective

Previously, four beneficial strains of Pseudomonas were evaluated for the biocontrol of Botrytis in lisianthus (Eustoma grandiflorum) cut flowers. Application of Pseudomonas chlororaphis 14B11 resulted in reduced disease severity compared to the untreated flowers (South et al., 2020b). The objective of the current study was to evaluate additional PGPB and calcium chloride (CaCl2) treatment for the biocontrol of Botrytis in various species of cut flowers.

Treating the flowers with bacteria or CaCl2 (Figure 2)

1. Ranunculus (Ranunculus asiaticus ‘Elegance Salmon’) were obtained from Sunny Meadows Flower Farm (Columbus, OH). The flowers were separated into treatment groups to apply the bacterial and CaCl2 treatments.
2. Each flower was treated with either an individual strain of bacteria (6 total strains) or a 600 ppm solution of CaCl2 using trigger spray bottles. Control flowers were untreated (no bacteria or CaCl2). All bacteria were applied at the same final concentration (108 cell/mL).
3. Flowers sat for one day after being treated.
4. Flowers were then inoculated with Botrytis cinerea (104 spore/mL) using a hand pressure sprayer.
5. The flower stems were cut to 30 cm, placed in vases with deionized water, and covered with plastic bags to create optimal conditions for Botrytis. This experiment was set up as a randomized complete block design in an interior room with fluorescent lighting. The average temperature during the experiment was 21.8°C (71.3°F).
6. The flowers were removed from the bags 2 days after the Botrytis inoculation and disease severity evaluations began. A rating scale was developed specifically for ranunculus (Figure 3) on a scale from zero (no disease symptoms) to 9 (bent stem and symptoms and signs of Botrytis are present OR all petals fell off). Rating for the flowers continued until 6 days after Botrytis inoculation.

Figure 2. Method used for the evaluation of PGPB for the biocontrol of Botrytis in ranunculus.
Figure 3. Flowers were rated daily on a scale of 0 to 9 based on symptoms and signs of Botrytis.
Figure 4. The average disease severity ratings 4 days after Botrytis inoculation of ranunculus treated with PGPB or CaCl2. Control flowers were untreated (sprayed with only the LB liquid growing media with no bacteria or CaCl2).

PGPB found to reduce disease severity

The daily disease severity ratings were used to determine if disease was reduced in ranunculus flowers treated with one of the PGPB or the CaCl2 treatments compared to the control (untreated) flowers. On day 4, all six bacterial treatments and the CaCl2 treatment reduced disease severity compared to untreated control flowers (Figure 4). The greatest reduction in disease was seen with Pseudomonas protegens AP54. CaCl2 and Pantoea agglomerans MBSA-3BB1 were the next most effective treatment for reducing Botrytis severity. Five days after Botrytis inoculation, flowers treated with PGPB strain MBSA-3BB1 had less gray mold and associated damage than the control flowers (Figure 5).

Figure 5. Ranunculus treated with PGPB strain MBSA-3BB1 compared to ranunculus treated with CaCl2 or the control (untreated) 5 days after Botrytis inoculation.


The bacterial and CaCl2 applications in this study resulted in a decrease in gray mold caused by Botrytis in ranunculus cut flowers. These treatments were also evaluated in stock (Matthiola incana ‘Katz Purple’). Pantoea agglomerans MBSA-3BB1 treatment resulted in the greatest decrease in disease severity compared to the untreated control flowers in the study with stock. Future experiments are evaluating additional application methods to improve the efficacy of the bacterial treatments. PGPB as biocontrol agents provide growers with promising tools for the control of Botrytis.


South, K. A., Peduto Hand, F., and Jones, M. L. (2020a). Beneficial bacteria identified for the control of Botrytis cinerea in petunia greenhouse production. Plant Dis. 104(6), 1801-1810. doi:10.1094/pdis-10-19-2276-re.

South, K.A. Chapin, L.J., Jones, M. L. J. (2020b). Biocontrol of Botrytis in cut flowers. The Cut Flower Quarterly. 32, 28–30.