Postharvest Control of Botrytis cinerea Infections on Cut Roses Using Fungistatic Storage Atmospheres

The effectiveness of fungistatic atmospheres for postharvest control of Botrytis cinerea Pers. infections on cut rose flowers (Rosa hybrids L.) was investigated. Storing cut ‘Sonia’, ‘Royalty’, and ‘Gold Rush’ roses at 2.5C with 10% CO2 for 5 days, followed by 2 days of cold storage in air, reduced the number of B. cinerea lesions that developed on inoculated and noninoculated flower petals by 77% and 82%, respectively, compared to cold storage for 7 days in air. Higher CO2 concentrations and longer CO2 treatment times reduced disease severity further, but resulted in unacceptable leaf discoloration on some cultivars. No deleterious effects of CO2-enriched storage atmospheres on flower quality, weight gain, or vase life were observed. Storage at 2.5C for 7 days in 2 μl SO2/liter reduced B. cinerea infections on inoculated and noninoculated flowers by 53% and 43%, respectively. No deleterious effects on flower quality, weight gain, or vase life were observed. Higher SO2 levels reduced disease severity further, but caused bleaching of the petal margins and necrosis around leaf wounds. Botrytis cinerea is a serious pathogen of rose flowers and other cut flower crops. Infections first appear as water-soaked spots or flecks on the flower petals. As the lesions coalesce, the infected petals turn brown and wither. Eventually, the entire flower may rot off at the receptacle. B. cinerea infections often cannot be detected at harvest, but develop rapidly under the moist conditions encountered during storage and transit. Such infections cause major postharvest losses and are considered a limiting factor in the storage and shipment of cut flowers (Carre, 1984; Cline and Bardsley, 1984; McCain and Welch, 1982). Low storage temperatures slow the development of B. cinerea infections (Maude, 1980), but do not always provide adequate control for long-term storage or when inoculum loads are high. At present, many rose growers dip cut flowers in fungicides to prevent postharvest development of B. cinerea infections, but this practice leaves unsightly residues on the flowers and foliage (McCain and Welch, 1982). Fungistatic atmospheres are used to control B. cinerea infections on certain commodities during storage and shipment (Maude, 1980). The atmospheres can be applied in storage rooms or shipping containers and do not leave the unsightly residues associated with most fungicide dip treatments. Carbon dioxideenriched atmospheres are used to minimize Botrytis rot of strawberry fruit during truck shipment (Harvey, 1982). Phillips et al. (1985) found that atmospheres containing 10% to 30% CO2 significantly reduced Botrytis flower rot of roses stored at 10 to 12C for 6 days, but Joyce and Reid (1986) reported foliar damage to ‘Sonia’ roses after storage for 7 days at 1C in CO2 concentrations as low as 7%. Sulfur dioxide-enriched atmospheres are used to control defor publication 5 July 1988. This research was supported, in part, by al Science Foundation Graduate Fellowship (P.E.H.), by a grant from ph H. Hill Memorial Rose Foundation (J.J.M.), and by the generous of roses by the Ninomiya Nursery Co., Richmond, Calif. The cost of g this paper was defrayed in part by the payment of page charges. stal regulations, this paper therefore must be hereby marked adversolely to indicate this fact. e student, Dept. of Vegetable Crops. Present address: Dept. of Hor, The Pennsylvania State Univ., University Park, PA 16802. r, Dept. of Vegetable Crops. r, Dept. of Environmental Horticulture. te Professor, Dept. of Plant Pathology. diluted to 1000/ml in deionized water, and sprayed onto the cay of table grapes caused mainly by B. cinerea during storage and transoceanic shipment (Nelson, 1985). Longley (1933) reported damage to roses stored in SO2 concentrations of 5 μl·liter –1 or higher. Concentrations of 200 to 5000 μl·liter typically are used for weekly fumigation of grapes, but continuous exposure to low levels (<10 μl·liter) is also effective for controlling decay (Dahlenburg et al., 1979). The objectives of the present study were to determine the feasibility of using SO2or CO2enriched storage atmospheres for postharvest control of B. cinerea infections on cut rose flowers. Materials and Methods Botrytis cinerea conidia were washed from 9to 12-day-old cultures of three separate isolates that were grown as described by Hammer and Marois (1988). The conidia were combined, flower petals using a Chromist spray unit (Gelman Sciences, Ann Arbor, Mich.). Noninoculated controls, sprayed with deionized water, were included to monitor background disease levels (those infections not resulting from laboratory inoculation). Three cultivars of cut roses—‘Royalty’, ‘Sonia’, and ‘Gold Rush’ —were obtained from a commercial grower. The stems were recut 30 cm below the receptacles and all leaves, except the top two or three, were removed. An experimental unit consisted of three flowers in a 0.5-liter bottle containing 200 ml of preservative solution (Hammer and Marois, 1988). After inoculation, the roses were stored in 16-liter glass chambers at 2.5 ± 1C. Test atmospheres were introduced at the top of each chamber above the flowers and exhausted from the bottom. The atmospheres were humidified and condensation was present on the petals throughout storage. The roses were removed from cold storage 7 days after inoculation and disease severity was quantified as the number of lesions on each flower. Subsequently, opening and vase life were evaluated for 7 days at 21 ± 1C, as described by Hammer and Marois (1988, 1989). Fresh weight was recorded daily. Experiment I. ‘Royalty’ and ‘Sonia’ roses were inoculated and stored at 2.5C in air mixed with 0%, 5%, 10%, or 20% (v/ v) CO2. The flow rate was 10 Iiter·hr -1 through each chamber. Carbon dioxide treatments were applied for 3, 5, or 7 days, and J. Amer. Soc. Hort. Sci. 115(1):102-107. 1990. then the roses were stored in air for 4,2, or 0 days, respectively, for a total cold storage period of 7 days. Inoculated control roses were stored in air for the entire 7 days. Seven days after removal from storage, the treatments were ranked subjectively for CO2induced leaf damage. This experiment was a complete factorial with four CO2 concentrations and three treatment times. There were three flowers of each cultivar for each factor-level combination. A regression model was fit using reciprocal transformation of CO2 concentration to linearize the function. Carbon dioxide treatment durations were included in the model using indicator variables, and partial F tests were used to compare the estimated regression functions for 3and 5-day treatment with that for 7-day treatment. Experiment II. Roses were stored in air mixed with 0%, 5%, 10%, or 15% CO2 for the first 5 days of storage at 2.5C, then in air for the final 2 days of cold storage. The flow rate was 10 liter·hr -1 through each chamber and CO2 concentrations were confirmed by gas chromatography. Carbon dioxide-induced leaf damage was scored 7 days after removal from storage using the following hedonic scale: 0 = no visible damage; 1 = leaves darker green than controls, no more than five small flecks of brown discoloration; 2 = brown discoloration in a mosaic pattern, <30% of the leaf area involved; 3 = obvious brown discoloration, 30% to 60% of the leaf area discolored; and 4 = severe dark brown discoloration, > 60% of the leaf area discolored. The rating for each rose was based on the most severely discolored leaf. A rating of 3 or greater was considered commercially unacceptable. This experiment was a complete factorial with four CO2 concentrations, two inoculation levels (inoculated and noninoculated), and three cultivars in a split-plot design. Carbon dioxide concentration and inoculation were the main-plot factors and cultivar was the subplot factor. A main plot consisted of a 15liter storage chamber and contained three subplots (0.5-liter bottles). There were two replicates (bottles) of each factor–level combination, with three observations (roses) per replicate. The experiment was repeated and the data were pooled for analysis. Analysis of variance and F tests ( = 0.05) were used for J. Amer. Soc. Hort. Sci. 115(1):102-107. 1990. all data to identify significant main effects and interactions. In all cases, the three-way interactions were found to be nonsignificant and were dropped from the models. Where significant two-way interactions were found, the data were grouped by cultivar and/or by inoculation for further analysis. Quadratic regression models were fit to the disease severity data. For the fresh weight, leaf damage, and vase life data, no meaningful regression models were found. Thus, pairwise and multiple comparison procedures were used to separate means. Experiment III. Sulfur dioxide treatments were applied for the full 7 days of storage at 2.5C. The flow rate was 15 liters·hr -1 through each chamber and SO2 concentrations were monitored using a fluorescence detector (Model 8850, Monitor Laboratories, San Diego, Calif.). The inlet SO2 concentrations were 0, 0.5, 1.0, 2.0, and 4.0 μl·liter, and the steady-state SO2 concentrations at the exhausts were 0, 0.03, 0.10, 0.25 and 0.75 μl·liter, respectively. Vase solutions were replaced after storage to minimize possible poststorage effects of bisulfite