Pulsing with calcium sulfate was the most effective in reducing disease and, in the assays without inoculation, it significantly reduced AUDPC by 66% and maximum severity by 78%, and increased vase life of the flowers by 37%. The effects of calcium sulfate may be explained by considering that calcium ions increase cell wall resistance to degradation caused by the complex of polygalacturonases, and other enzymes involved in the process of pathogenesis of B. cinerea (Elad & Evensen, 1995). As the resistance of cellular structures against the enzymes was increased, the number of successful infections as well as the progress of disease was probably reduced.
One of the main responses of carnation (Dianthus caryophyllus L.) petals to exposure to ethylene is ion leakage due to alteration in the membrane integrity (Bo et al., 1997). This is probably true with rose petals as well. The calcium ion seems to inhibit ethylene production by maintaining plasma-membrane integrity (Nur et al., 1986). Consequently, there is a reduction in membrane permeability that leads to a reduction in electrolyte loss to the apoplast, which may affect conidium germination and penetration by the pathogen (Elad & Evensen, 1995). Roses grown on a high calcium concentration produced less ethylene, and had a higher resistance to B. cinerea than untreated ones (Volpin & Elad, 1991). These authors also pointed out the existence of marked differences among rose varieties in the response to treatment with calcium sulfate.
The other treatments did not reduce disease markedly, except in a few situations. Although in most situations AUDPC and maximum severity in the salicylic acid treatment did not differ from the treatments with sucrose and citric acid, the vase life of the flowers treated with this acid was considerably reduced. Probably the pulsing time with salicylic acid at 7.2 mM used in this work was too long. This combination may have injured xylem vessels and collapsed the water flux up to the petals, culminating with flower dehydration. This dehydration may have reduced both the vase life of the flowers and advancement of the disease due to lack of moisture in the invaded tissues. Despite the failure of treatment with salicylic acid in this study, some authors have shown its ability to induce resistance in plants (De et al., 1999), in addition to its direct effect on the pathogen (Gaur & Chenulu, 1982) and its effect on ethylene synthesis. Salicylic acid has been shown to have an inhibitory action toward auxin-induced ethylene biosynthesis when tested on mung bean (Vigna radiata L. Wilzeck) (Lee et al., 1999). Therefore, it is important to test different pulsing times with lower concentrations of this acid to evaluate its ability to control gray mold of roses.
The effects of sucrose and citric acid on the AUDPC, maximum severity, and vase life of the flowers were not statistically different for most of the situations. Apparently, sucrose provides energy for fundamental cellular processes, such as the maintenance of the structure and functions of mitochondria. It also regulates water balance by controlling transpiration (Nowak & Rudinicki, 1990). The latter authors emphasized the importance of knowing the concentrations tolerated by each species or even variety of flower. For example, roses, carnations, and gladiolus (Gladiolus liliaceus Houtt.) have different sugar concentration requirements in pulsing to extend their vase life (Halevy & Mayak, 1974). Citric acid is considered to maintain the water balance and to reduce bacterial proliferation in the vase solution, thus avoiding the obstruction of the xylem vessels. When combined with other substance, citric acid gave the best results in terms of longevity in bird-of-paradise (Strelitzia reginae Banks.) flowers compared to other combinations (Halevy et al., 1978b). It is possible that combining sucrose with citric acid or other substances may produce additive or synergistic effects, either by increasing the vase life or by reducing disease severity.
In the assay with inoculation, pulsing with calcium sulfate was the most efficient in reducing the AUDPC and in increasing vase life. However, in the repetition of the experiment, in which the volume of inoculum was doubled, there was a considerable reduction in the effect of the treatments. Here, the efficacy of calcium sulfate as well as of sucrose and citric acid was considerably reduced, probably because more inoculum available means more enzymes being produced to macerate the infected tissue.
In the experiment withSTS the treatments affected the progress of disease only the first time the assay with inoculation was conducted, but not in the repetition of the experiment. Reduction of the maximum severity was found in the assay without inoculation. Most studies using STS have been directed at improving the postharvest quality of many flowers without considering its effect on pathogens (Serek et al., 1994). The STS reduced the severity of gray mold on rose and carnation by inhibiting the ethylene action (Elad, 1988).
This inhibition reduced the senescence of the flowers, and consequently, the advance in pathogen colonization. The effect of STS on senescence is due to the action of the silver ion (Knee, 1992). This ion binds to specific sites in the cell membrane competitively blocking the ethylene action on it, culminating in a reduction of senescence. All treatments studied increased the vase life of the roses. These findings are similar to previous results (Serek et al., 1994; Han, 1998). Although STS has been shown to have good effect on reducing flower senescence, one should be aware that silver is a heavy metal, and, because of that, it has the potential to damage the environment and human health.
According to our study, treating rose buds with substances that reduce the rate of senescence of flowers is efficient to increase the vase life and to reduce the damage by pathogens such as B. cinerea that colonize senescent tissues. The most effective substances tested were calcium sulfate followed by sucrose and STS in that order. The other substances did not have a significant effect. Nevertheless, it will be necessary to combine the use of these substances with preharvest treatments to find additive or synergistic effects for satisfactory disease control.
We thank Dr. James R. Aist from the Department of Plant Pathology at Cornell University for critically reviewing this manuscript. We also thank the Brazil Flowers SA and its agronomist Mr. Joao Miranda for providing basic support and field facilities for the conduct of this work. Finally, we thank the Fundação de Amparo a Pesquisa de Minas Gerais (FAPEMIG) for financing this research work, and the Coordenadoria de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) of the Brazilian Education Ministry for supporting the first author with a scholarship during completion of his Master's Degree in Plant Pathology.