Assessing calcium treatments for botrytis blight in petunias

Previous studies have demonstrated the efficacy of calcium (Ca) spray applications derived from Ca chloride for reducing botrytis (Botrytis cinerea) infection severity on petunia (Petunia ×hybrida) flowers. A recent study examines the effects of six Ca sources for their efficacy in reducing Botrytis blight on petunia flowers and their potential to cause spray damage or phytotoxicity.

The research is published in the journal HortTechnology.

Botrytis (Botrytis cinerea), a necrotrophic plant pathogen, infects more than 200 hosts worldwide, causing significant losses within agricultural systems. Bedding plant producers experience problems with Botrytis blight in all stages of production, from propagation to postproduction. When flowers are present on bedding plants, such as petunia (Petunia ×hybrida), botrytis infects the flowers rapidly under conducive environmental conditions, resulting in the disease referred to as Botrytis blight or gray mold. Botrytis blight is a significant issue during transportation of bedding plants to the retail environment, which may take up to 2 days.

Based on previous work, several sources of calcium (Ca) have been evaluated for improving plant quality or reducing disease severity on vegetable, fruit, and ornamental crops. Exploring greater rates of Ca is important to determine whether additional benefits could be obtained in terms of heightened disease management. Greater Ca concentrations may have negative consequences in regard to spray damage, so it was also necessary to address this issue.

Because bedding plants have flowers of varying ages on individual plants, the researchers hypothesized that spray damage may be related to flower age, so this factor was included in the study. Therefore, the objectives of the study were to compare Ca sources for their effect on Botrytis blight of petunia flowers 1) to determine whether there was any benefit from applying greater Ca concentrations than those reported previously, 2) to identify the risk of spray damage that may result from Ca application, and 3) to evaluate the potential for spray damage as flowers age.

Two experiments were conducted to quantify the effect of different Ca sources for their potential spray damage effects and their efficacy for reducing Botrytis blight severity on petunia flowers of different ages. The first experiment examined the potential spray damage to petunia flowers after spray treatments of six Ca sources applied at each of three rates. The second experiment examined the same Ca products and rates for their reduction of Botrytis blight severity after inoculation of petunia flowers with botrytis spores.

Spray damage severity varied with flower age. Flowers that were 0 d old (i.e., unopened flower buds) were the least susceptible to spray damage from any Ca source. The only Ca source to cause significant damage on flower buds was Ca-EDTA, and the damage occurred at 1250 and 2000 mg⋅L^–1 Ca. All sources, except for Ca-EDTA, can be sprayed on flower buds without any damage at the highest rate tested in this study.

Therefore, spray applications can have greater Ca concentrations before open flowers are present; however, after flowers are open, Ca concentrations in the spray solution should be decreased because the flower petal tissue becomes more susceptible to spray damage. Flowers exhibited the greatest amount of spray damage on 3-, 5-, and 7-d-old flowers. Because individual petunia flowers survive for ∼9 d, if spray damage were to occur, these flowers would senesce and the plants would be completely undamaged ∼1 week after spraying. Thus, the persistence of the spray damage is relatively limited.

Several Ca sources provided similar reductions in Botrytis blight; however, it is important to consider spray damage potential as well. The results from the study suggest that calcium chloride is the most effective source of those tested for reducing Botrytis blight severity and causing the least amount of spray damage. Flower age should also be taken into consideration because the potential for damage increases as the age of the flower increases. Calcium sources containing EDTA should not be used while flowers are present on petunia plants because of the high risk of spray damage.

The use of calcium as a tool for botrytis management of floriculture crops has become a standard practice across the floriculture industry in the U.S., Central and South America. This paper highlighted the specific responses of different calcium sources. Calcium chloride has proven to be effective with minimal risk of phytotoxicity. Growers now incorporate calcium chloride into the regular spray programs and target applications immediately prior to harvest or in the postharvest environment to protect plant tissues from botrytis infection.

As fungicide resistance continues to create challenges for growers, calcium chloride applications provide an effective alternative to conventional chemical fungicides. Calcium chloride is safe and inexpensive to apply. OMRI-listed forms of calcium chloride are now available.

Drs. James Faust and Guido Schnabel are professors in the Plant and Environmental Sciences Department, Clemson University. Katie Bennett Jent leads the R&D team at Metrolina Greenhouses in Huntersville, NC.

More information:
Katherine Bennett et al, Evaluation of Calcium Sources for the Management of Botrytis Blight on Petunia Flowers, HortTechnology (2023). DOI: 10.21273/HORTTECH05097-22

Provided by American Society for Horticultural Science