Some Ornamentals

Rust and Other Disorders of Snapdragon

W. D. McClellan.

Of the numerous diseases of snapdragons, rust probably causes the most concern.

Rust itself seldom kills the plant, although in dry weather the affected leaves may dry up, and in humid regions the rust-infected spots are entrances for the secondary fungi that may kill leaves and stems.

Rust can be told by its reddish-brown pustules on the under sides of the leaves. The powdery chocolate-brown masses in the mature pustules are the spores of the rust fungus (Puccinia antirrhini).

Rust affects snapdragons at all stages of growth. The leaves are most heavily infected, but often stems, petioles, and calyx structures may be attacked severely. The first indications of the disease are small yellow flecks, less than one-sixteenth inch in diameter, just under the epidermis on the under side of the leaves. If conditions are favorable, the brown. powdery spore masses may appear 48 hours later. Leaves that are severely attacked droop as though they needed water.

Snapdragon rust has an interesting history. It is one of the few plant diseases whose spread can be traced from its origin. Rust was first collected on snapdragons at Santa Cruz, Calif., in 1879 by C. L. Anderson. Next it was found at Berkeley, Calif., in 1895 by W. C. Blasdale, a chemist at the University of California, who reported his studies on rust in 1903. The fungus soon spread throughout California and was found in Portland, Oreg., in 1909. Rust appeared in Lake Forest, Ill., in 1912, in the New England States in 1915, and in Canada in 1916. It reached Bermuda in 1922 and was first found in Europe at Grignon, France, in 1931. By 1933 rust had spread throughout northwestern France and to England. Three years later it had been reported throughout Europe and in Egypt. It was observed in Russia in 1937 and in southern Rhodesia in 1941.

Wind-borne spores account for the local spread of snapdragon rust. Probably the long-distance spread of the disease began (50 years ago or so) when snapdragons were propagated by cuttings that were shipped widely to florists, nurserymen, and private gardeners. It is also possible that rust may have spread in infected seed or in rust-infected plant materials associated with the seed.

Snapdragon rust is confined to members of the genus Antirrhinum and a few other genera of the Scrophulariaeae, such as Linaria and Cordylanthus. C. O. Blodgett and G. A. L. Mehlquist, working at the California Agricultural Experiment Station, tested a number of species and varieties of Antirrhinum and found that seven species, A. asarina, A. chrysothales, A. glandulosum, A. mourandioides, A. orontium, A. ibanjezii, and A. siculum, were resistant to two races of rust. Those species, however, are so far removed from Antirrhinum majus, the common snapdragon, that I doubt whether they would be of much value in a breeding program.

Harold E. White, of Massachusetts State College, tested 56 wild species and strains of Antirrhinum and found A. charidemi, A. calycinum, A. ibanjezii, A. siculum, and four strains of A. glutinosum to be resistant. Other strains of A. glutinosum, as well as A. molle, vary in susceptibility to rust. Other species of Antirrhinum said to be susceptible include A. nuttallianum, A. virga, A. vagans, A. assurgens, A. torluosum, A. coulterianum, A. glandulosum, and A. vexillo-calyculatum.

Many rusts have life cycles that involve alternate hosts. No such alternate host is known for snapdragon rust. Only the uredial and telial stages are known.

E. B. Mains, working at Purdue University, obtained no infection with germinating teliospores on young snapdragons. He deduced that this rust is heteroecious and has pycnia and aecia on an alternate host. The rust probably lives over on parts of infected plants that survive the winter in greenhouse or out-of-doors. Such infections provide inoculum for infection the following spring.

The best temperature for uredospore germination and infection is 50 to 55 F. The optimum temperature for the completion of the life cycle of the fungus is 70 to 75 . Still higher temperatures may result in the abortion of some of the infections previously established at lower temperatures, however, for only a low percentage of incipient infections will survive temperatures of 90 F. Uredospores are killed by prolonged exposure to temperatures above 94 .

In cooperative tests at Beltsville, Md., Urbana, Ill., Wooster, Ohio, Los Angeles, and Ithaca, N. Y., A. W. Dimock and Kenneth F. Baker discovered a close correlation between the observed intensity of rust and that expected on the basis of temperature and moisture records. At the first three locations, rust development was prevented or greatly restricted by high temperature or insufficient moisture or both. At Los Angeles and Ithaca, temperature and moisture conditions were more favorable and severe rust developed.

In the drier weather that prevails in Los Angeles, host injury resulted almost exclusively from drying of the rust-infected tissues. Under conditions of high average relative humidity and frequent rainfall, as at Ithaca in summer, injury was due primarily to the action of secondary fungi (principally Fusarium species), which invaded through the rust pustules, destroyed the rust, and advanced into the previously healthy host tissues, killing the leaves and stems. Where secondary fungi were controlled at Ithaca by. the use of selective fungicides or by the prevalence of unfavorable low temperatures, however, there was relatively little host injury even though rust infection was severe.

On the basis of their results, the severity of snapdragon rust can be predicted from the climatic characteristics of a given area. For instance, if temperatures remain at 45 to 65 F. when plants are wet with rain or dew for 6 to 8 hours, infection by air- or water-borne uredospores is certain. If, in addition, the day temperatures reach 70 to 75 but do not often exceed 80 or 90 , the development and sporulation of the fungus are rapid, and the complete cycle from infection through pustule formation and sporulation to infection may be completed every 12 days.

An area or season in which those conditions commonly occur is favorable for severe rust development. The Pacific coast is such an area. In other districts that have such high temperature as to prevent either infection or sporulation or both, rust would be expected to be unimportant except when snapdragons are grown in greenhouses in winter probably much of the Mississippi Valley, the Atlantic Coastal Plain, and the South.

Control measures include selection of resistant varieties and healthy cuttings, the use of fungicides, and the avoidance of syringing and excessive humidity.

S. L. Emsweller and H. A. Jones, after extensive research in California, reported that resistance is controlled by a single dominant gene. That makes for easy development of resistant varieties, providing good flower types can be obtained. They also found that there are modifying genes that permit the selection of immune plants from highly resistant parents. Later, in 1937, C. E. Yarwood, of the California Agricultural Experiment Station, found a race of snapdragon rust that would affect the so-called resistant strains in the California seed fields. Apparently that new race had not been prevalent before that time in the district where the earlier work had been done.