Identification of Fusarium commune causing meltdown issue in zinnia cut flowers

This report is funded by the ASCFG Research Foundation.
Ravi Bika and Fulya Baysal-Gurel Tennessee State University, Otis L. Floyd Nursery Research Center, McMinnville, Tennessee

Commercial cut flower growers in the eastern and southeastern United States have for many years experienced a postharvest issue with zinnias, commonly called “zinnia meltdown”. The most frequent symptom is bent stems, usually just below the flower head. Meltdown reduces yields of cut flower stems, and therefore, income for the grower. The project started in 2017 with an online survey that reported 29 responses from 17 states: Connecticut (1), Georgia (3), Illinois (2), Iowa (1), Louisiana (1), Maine (1), Maryland (1), Massachusetts (1), Mississippi (2), New Jersey (4), New York (1), North Carolina (3), Ohio (2), Tennessee (2), Texas (1), Utah (1), and Virginia (1), as well as Ontario (1) in Canada. According to the survey results, ‘Benary’s Giant’ was the series most susceptible to the zinnia meltdown issue.

The next step was to identify possible causes of zinnia meltdown. Fifteen zinnia cut stems with flowers exhibiting meltdown symptom were collected from a flower farm in Lincoln County, Tennessee. Small sections of zinnia stems and leaves were isolated from the symptomatic zinnia samples and plated on potato dextrose agar (PDA) and Fusarium-selective media. The plates were incubated for five to seven days in laboratory ambiance (21 °C, 60% RH and 12-h fluorescent light and dark cycle).

The morphological characterization (pigmentation, growth pattern, shape and size of micro and macro conidia) and molecular analysis confirmed that Fusarium commune was the causal agent for zinnia meltdown issue.

To confirm the pathogen ‘F. commune’, a pathogenicity study was performed on three zinnia cultivars: ‘Benary’s Giant Golden Yellow’, ‘Benary’s Giant Pink’, and ‘Benary’s Giant Lime’ at vegetative stage (two weeks after transplantation) or flower bud stage (one month after transplantation).

The fungal conidial suspension (inoculum) was prepared by flooding a 10-14 day old culture of F. commune. Three different methods of inoculation were tested on the vegetative and flower bud stages: drench (25 mL of conidial suspension was applied to the substrate near zinnia plant root area); stem injection (30 μL of conidial suspension was injected into zinnia stem using a 1-mL syringe with a disposable needle); and foliar spray (conidial suspension was sprayed on zinnia plant [including leaves, stem and flower bud] until runoff using a handheld sprayer). Control plants received sterile distilled water.

Zinnia stems were harvested when the outer petals of the flowers were fully expanded, and displayed under laboratory conditions. Similar symptoms, such as stem bending just below the flower, were observed on inoculated zinnia cut flowers of all three cultivars two days after harvesting.

Fusarium commune was re-isolated from the infected flower stems of all three cultivars of zinnia, but not from the non-inoculated zinnia flower stems. We observed that the zinnia stem colonization by F. commune was statistically similar in all three tested cultivars regardless of plant growth stage and method of inoculation. The typical meltdown issue observed in zinnia cut flowers in postharvest condition might have been due to vascular occlusion; the microconidia of F. commune may form a cluster in vascular tissue, hindering the water uptake that can lead to bending of the stem below the flower during postharvest vase life.

In conclusion, the morphological and molecular analysis, as well as pathogenicity tests, confirmed that Fusarium commune is the causal organism for zinnia meltdown in Tennessee. The next steps in this project would be to screen cultivars for sensitivity to F. commune, and identify possible sources, such as irrigation water, transplants, soils etc. of F. commune in the zinnia production area. Additionally, we are still accepting infected zinnia samples from other states, and conducting pathogenecity studies on the zinnia meltdown issue.

So, what can be done to prevent zinnia meltdown? Control methods are still being developed, but growers report that it helps to be sure buckets and cutters are clean, and fresh solutions are used. Growers report success with using commercial hydration or slow-release chlorine right after harvest.