Leaves of Better Times rose infested with the two-spotted spider mite, Tetranychus bimaculatus, produced considerably more ethylene than comparable healthy leaves. Leaves of Physalis floridana and of Nicotiana tabacum, with necrotic lesions induced by copper sulfate also produced ethylene, but in slightly smaller amounts than did comparable leaves with lesions induced by viruses. Healthy leaves shredded with a sharp knife just before testing generally produced more ethylene than did uninjured leaves, but the increased quantity varied with the species of plant. These results indicate that ethylene is a product of injured or dying cells rather than the cause of the necrosis that occurs.

The fungus diseases investigated appear to belong in three categories. With some diseases, such as those caused by the obligate parasites, relatively little ethylene is produced; with certain others, such as black spot of rose and shot hole of cherry, caused by facultative saprophytes, relatively large amounts of ethylene are produced. Between those extremes is a large group of diseases with which intermediate quantities of ethylene are produced. Observations of diseases in this category indicate that in some cases slow yellowing and eventual abscission may occur. In each of the five diseases of roses. that were studied, there was a positive correlation between the degree of yellowing and defoliation and the amount of ethylene produced. Only with black spot was there rapid defoliation and production of large amounts of ethylene. The anthracnose and the brown canker diseases may result in some yellowing and possibly abscission and production of a moderate amount of ethylene. With the rust and the powdery mildew diseases there was little or no yellowing or abscission and only negligible amounts of ethylene were produced. It appears, then, that the large amount of ethylene produced with certain diseases is the cause of the rapid yellowing and early abscission of infected leaves.

Flowers as well as foliage may be affected by ethylene from diseased tissues. In experiments reported by Dimock and Baker in 1950, it was shown that flower drop ("shelling") of snapdragons and calceolarias and closing of the blooms ("sleepiness") of carnations could be caused by the enclosure of diseased tissues with normal healthy snapdragon, calceolaria, or carnation flowers. In these tests, chrysanthemum flowers infected with the chrysanthemum ray blight fungus, Mycosphaerella ligulicola, or with Botrytis cinerea were used as ethylene sources.

In other experiments, conducted by C. W. Fischer, Jr., and J. R. Keller in 1951, brominated activated charcoal enclosed in sealed containers with flowers was highly effective both in controlling growth of molds and in preventing ethylene damage to the blossoms. In those tests, chrysanthemum flowers infected with M. ligulicola and carnation blooms infected with B. cinerea were used as Sources of ethylene. The brominated activated charcoal was not effective unless it was in close proximity to the blooms, but not touching them.

SINCE HEALTHY CELLS normally produce a small amount of ethylene, there would seem to be a minimum threshold concentration which must be exceeded if the toxic effects of ethylene are to occur. Investigations at Cornell University of storage of flowers have demonstrated that if certain ethylene-sensitive healthy flowers are stored in airtight containers enough ethylene is produced to cause self-injury. With most healthy tissues, ethylene production is slow, and, unless confined, dissipation into the atmosphere is sufficiently rapid to prevent accumulation of ethylene within plant tissues in toxic quantities.

Stimulated ethylene production appears to be associated with aging, diseased, or dying cells. In such cells the normal respiratory cycle may be partially disrupted to produce ethylene in abnormal quantities. Necrosis or death of cells does not seem to be the entire answer, although in experiments where phytotoxic chemicals were used there was a positive correlation between the degree of necrosis and the quantity of ethylene produced. Where injury to the surface layer of plant cells occurs, as by feeding of spider mites, a large number of cells are affected but necrosis is not readily evident.

The observed relationship between temperature and ethylene production, and the decrease in ethylene production to near-zero under anaerobic conditions have direct application to storage of cut flowers. Recent work at Cornell University on low-temperature storage of flowers demonstrated that for most flowers the length of the storage period and the quality of the flower after removal from storage was directly related to temperature and to type of storage pack. The best results were obtained with temperatures near 0 C. (32 F.) and with a nearly airtight pack or container.

For prepackaged flowers or for flowers stored in water in a moderately tight cold room, the potential damage that can be done if ethylene-producing diseased material is included is serious. Observations have indicated that once a blossom is injured by ethylene it immediately becomes more liable to attack by Botrytis. Thus a chain-reaction type of response is initiated that will lead to more ethylene production and thus to more injury.

The observed effects of ethylene produced by diseased plant tissues emphasize the desirability of either near-perfect control or complete elimination of plant disease. If adequate disease control is maintained in field or greenhouse and in the storage room, one of the important factors in successful long-term storage of cut flowers is reduced to negligible proportions.

C. E. WILLIAMSON, assistant professor of plant pathology in Cornell University, is a native of Indiana. His work with diseases of ornamental plants began at Cornell in 1937. His studies on ethylene effects began with the demonstration of stimulated ethylene production by rose leaves affected with black spot. He is a graduate of Wabash College and Cornell University.

A. W. DIMOCK, professor of plant pathology in Cornell University, has specialized in diseases affecting ornamental crops for many years. His interest in ethylene effects was a natural consequence of his close association with Dr. Williamson during his studies on ethylene production by diseased plant tissues and with Dr. C. W. Fischer, Jr., during his studies on effects of ethylene on cut flowers in storage.