Storing Peonies – Disease Free
After WWII and the scientific advances in plant physiology and life cycles that enabled the floriculture industry to grow seasonal crops year round, peonies gradually fell out of favor. The GIs coming back from the war moved off the farm, and their wives opted for newer plant varieties instead of the more traditional peonies.
But peonies are slowly regaining the popularity they had in years past. They are most commonly used for decorating graves on Memorial Day, though a few progressive florists and customers are using them for wedding work and other arrangements.
Besides being seasonal, peonies are also unique in that they are one of the few floriculture crops best grown outside in permanent plantings. I have yet to see a peony flower grown in a container that can compare with one produced by a five-year-old field plant. Field-grown flowers have bigger, more robust blooms. Some growers have experimented with erecting temporary hoop houses over plantings to hasten the bloom and take advantage of earlier markets, like Mother's Day. Most growers, though, rely on the natural course of the season.
Presently, peonies grown in the Northern Hemisphere are available from late April until June, and those grown in the Southern Hemisphere from mid-October through December. These seasons can, however, be extended by cold storage of the flowers. Many cultivars can be stored reliably for two weeks and others four weeks or longer.
One problem with cold storage is disease, especially Botrytis sp. Ohlander and Watson (1951) evaluated the effectiveness of several fungicidal chemicals on three peony cultivars placed in long-term cold storage. They found that one cultivar, 'Felix Crousse', performed better than the others and that the paradichlorobenzene treatment gave the best disease suppression. Unfortunately, most of the chemicals they evaluated are no longer in use, necessitating more research on new products.
Available Peony Fungicides
In the past three harvest seasons, I have conducted studies to determine the effectiveness of various disease control methods and chemicals on peony flowers placed in long-term cold storage. The methods I have evaluated include methyl jasmonate; calcium salts; a standard fungicide, Daconil; and an organic fungicide, Aspire.
Methyl jasmonate is currently being evaluated by many postharvest researchers on a variety of horticultural crops. Methyl jasmonate has been found to elicit numerous plant responses, among which are the postponement of senescence and resistance to disease (Parthier, 1991). Meir, et al. (1998) found methyl jasmonate suppressed Botrytis cinerea on infected fresh-cut roses. The method of application for methyl jasmonate is different from most fungicidal chemicals. Only a small amount is needed, as its vapor or gas creates the beneficial effect. Methyl jasmonate can be applied to an absorbent cotton pad and placed with the flowers in cold storage, an easy way to provide continuous disease control.
Calcium is involved in the maintenance of strong cell walls and integrity of cell membranes. Increased calcium levels in plants also slow the development of some diseases (Stall, 1963). Gerasopoulos and Chebli (1999) were able to improve the vase life of gerbera daisies by using calcium chloride in preharvest sprays or postharvest dips and injections. Volpin and Elad (1991) reported that cut roses pulsed with calcium sulfate and calcium nitrate solutions were less susceptible to botrytis infection.
Many peony growers live in areas where moisture is prevalent, and they have to spray to suppress disease incidence in the field and to help with disease resistance in stored peonies. There are a variety of chemicals growers can and do use in these instances. Daconil and Aspire are simply representative of what is available. Daconil is a very common fungicide readily available in most garden centers. Aspire is a new postharvest fungicide that is environmentally friendly.
Table 1 is a summary of two years of data on the effectiveness of disease control treatments and the inherent susceptibility of some cultivars. Nine herbaceous, cut-flower -type peony cultivars have been evaluated. Four disease control treatments and an untreated control had their effectiveness evaluated on the following nine cultivars:
• 'Bridal Shower,' white double
• 'Duchess de Nemours,' white double
• 'Dr. Alexander Fleming,' pink double
• 'Felix Supreme,' red double
• 'Mrs. FDR,' pink double
• 'Walter Faxon,' pink double
• 'Sarah Bernhardt,' pink double
• 'Shawnee Chief,' red double
• 'Snow Mountain,' white bomb
Treatment methods were as follows:
• Methyl jasmonate: about 0.2 ml or 3 drops with estimated concentration of 130 mM mÑ3 applied to an absorbent cotton pad placed in the storage bag;
• Calcium salt: Two-hour pulse at room temperature of a 100 mM calcium chloride solution;
• Aspire: Prestorage spray of Aspire at the label-prescribed rate of 0.16 oz. per gallon; or
• Daconil: Prestorage spray of Daconil at the label-prescribed rate of 1.5 fl. oz. per gallon.
Ten stems of each cultivar were used for each treatment. Each stem was tracked during the study. After treatment application, stems were placed in 2-gallon, self-sealing polyethylene bags and placed in cold storage at 2-3¡ C. They were evaluated after 4, 8 and 12 weeks. Evaluations included determining disease incidence, deciding whether disease lesions were present and estimating the extent of the disease by determining what percent of area was covered by the disease. Leaves, sepals, guard petals and petals were evaluated.
Peony cultivars were variable in their response to disease prevention treatments and their susceptibility to disease. Table 1 at left summarizes the results of these studies on susceptibility and treatments. Untreated 'Mrs. FDR' and 'Snow Mountain' had lower disease incidence levels, whereas untreated 'Duchess de Nemours,' 'Dr. Alexander Fleming' and 'Felix Supreme' had high incidence levels. Growers interested in storing peonies for later markets should choose cultivars that are less susceptible to disease. Trying to remedy an inherent problem with protective treatments is not a very good strategy.
Methyl jasmonate treated flowers of 'Bridal Shower' had less disease incidence than the other disease control treatments and the untreated flowers; whereas, both flowers and leaves treated with calcium chloride had greater disease incidence than the other treatments. The fungicide, Daconil, reduced disease incidence for 'Felix Supreme' leaves, but not flowers, while other treatments were worse than no treatment. Although disease incidence for untreated 'Mrs. FDR' flowers was lower compared to the other cultivars, both fungicides were beneficial in reducing disease incidences even more. 'Snow Mountain' leaves treated with Aspire had less disease incidence. Disease incidence was the same or worse with the disease control treatments for 'Duchess de Nemours,' 'Dr. Alexander Fleming,' 'Sarah Bernhardt,' 'Shawnee Chief' and 'Walter Faxon.'
Selecting 'Duchess de Nemours' and 'Dr. Alexander Fleming' for long-term cold storage is not recommended, as these two cultivars appear to be more susceptible to disease and the control measures evaluated were not effective. 'Felix Supreme' also exhibited high susceptibility to disease but was responsive to treatments. Ideally, selecting cultivars that were both less susceptible to disease and responsive to disease control treatments would be the best recommendation.
Prestorage fungicidal sprays should be part of a total disease control program that includes:
• Field clean up and removal of dead leaves and stems in the fall, as they harbor disease spores;
• A series of pre-bloom fungicidal sprays. The number depends on climatic and weather conditions, the site and what the season is like. More sprays are needed when there is free water on the plants; and
• Harvesting only dry flowers. Try to avoid harvesting flowers if they are wet from rain, dew or fungicide sprays. Never pack, send or store flowers wet.
More information on other current peony research at KSU and the agricultural experiment station reports from 1995 to 2000 are available at www.oznet.ksu.edu/library/hort2/commerc.htm.