Wilt, Rust, and Pasmo of Flax

H. H. Flor.

Growers of flax have to contend with wilt and rust nearly all the time. So do the plant breeders who try to develop better varieties of flax.

The early wilt-resistant selections, North Dakota Resistant No. 114, Frontier, Chippewa, Winona, and Linota, gave way to Redwing, Buda, and Bison, which had greater resistance to wilt and were less susceptible to rust. Bison became popular during the droughty 1930's but was so heavily damaged by rust in 1940, 1941, and 1942, when rainfall was heavier, that it was replaced by rust-resistant varieties.

About 65 percent of the flax acreage in southeastern North Dakota in 1943 was Viking, one of the rust-resistant varieties that replaced Bison. Viking is susceptible to the pasmo disease, however, and was so heavily damaged that in 1944 the acreage sown to flax was drastically curtailed and Viking comprised less than 5 percent of that sown.

Koto, developed as a rust-resistant replacement for Bison, was free from rust during several years of testing in nurseries throughout the flax-producing area of the North Central States, but was heavily attacked by Previously unknown races while it was being increased for distribution.

For more than 30 years Newland was resistant to rust in North America, but Dakota and Renew, varieties that carry the Newland gene for rust resistance, were attacked in 1948 by new races of rust in the Red River Valley of Minnesota and North Dakota.

The new rusts spread rapidly. They caused heavy damage in 1949 and 1950. Rust was widespread in 1951 and reduced the farm income from flax by approximately Lo million dollars in Minnesota, North Dakota, and South Dakota. Because of drought and the use of resistant varieties, losses from rust in 1952 were less than one-tenth those of 1951.

Probably because of wilt, flax has been considered a "new land" crop. Our center of production advanced with the frontier until it reached Minnesota, North Dakota, and South Dakota successively. Flax did better on newly broken land than other crops, but yields went down and often failed when flax was grown for more than a year on the same land.

H. L. Bolley, at the North Dakota Agricultural Experiment Station, determined in 1901 that the failure was caused by a parasitic fungus, which he described and named Fusarium lini. The fungus spreads through infected plants, soil, and seed. Once established in a soil, it may persist for 25 years or more.

WILT attacks flax plants at all stages of growth. The roots of seedlings may rot and the plants damp-off. More mature plants may quickly wilt and die. They may be stunted, their leaves may turn yellow and drop off, or they may merely ripen prematurely. The primary stem may die; new, apparently healthy, stems may develop at the first node. Nearly mature plants often have a brown infected streak that extends up one side of the stem and stands out from the healthy green part.

Resistance to flax wilt is not complete. On wilt-sick soil some wilt usually develops in all varieties, even the most resistant. The amount of wilt developing on the same plot varies greatly from year to year. A variety may wilt every year at one station and be consistently resistant at another. In some varieties, plants that show symptoms of wilt die quickly and the surviving plants continue vigorous growth and produce a good crop. In other varieties, few plants succumb, but nearly all are stunted, unthrifty, and produce little seed.

Bolley used the principle of survival of the fittest to select wilt-resistant varieties. On the thoroughly wilt-sick flax plot at the North Dakota Agricultural Experiment Station, he planted lots of flax from many sources and obtained wilt-resistant varieties from the surviving plants. The seed-flax industry in the United States undoubtedly was saved by the breeding of wilt-resistant varieties at the North Dakota and Minnesota experiment stations. Today most of the soils in the North Central States are so heavily infested that only wilt-resistant varieties can be grown successfully.

Flax wilt is most destructive in warm, dry seasons. The flax plant grows best in cool weather, but the wilt fungus thrives at high temperatures. W. H. Tisdale, working at the Wisconsin Agricultural Experiment Station, found that wilt did not develop at soil temperatures below 60 F. and that temperatures between 75 and 82 were most favorable for wilt.

At the Minnesota station, W. E. Broadfoot and N. E. Borlaug have shown that Fusarium lini is composed of many races. The races differ in cultural characteristics, pathogenicity on flax varieties, temperature requirements, and compatibility. A variety may be resistant to one race but susceptible to another race.

Antagonism exists between some races. Dr. Borlaug observed less wilt in a variety inoculated with a mixture of two races than when that variety was inoculated with either race alone. That raises the question as to the importance of antagonism between races under field conditions and the role of the other soil micro-organisms in development of wilt.

The "running out" of varieties often is due to a shift in the physiologic race population of the pathogen. The first wilt-resistant varieties, Frontier, North Dakota Resistant No. 114, Chippewa, Winona, and Linota, now wilt severely on the experimental wilt-sick soils at the Minnesota and North Dakota stations and show considerable wilt on some farm soils. Those varieties were highly resistant when they were introduced. When they were released, the races that attacked them probably comprised such a small portion of the population as to cause little damage. The races later built up on the congenial host until they became abundant enough to produce damage.

The wilt-resistant varieties now grown may follow the same downward path. Many of them were developed from hybrids with Bison, however. Bison and hybrids in which Bison was the wilt-resistant parent have been grown on wilt-sick Plot 30 at the North Dakota Agricultural Experiment Station since 1925. Since 1930, races to which Bison is susceptible in pure culture tests have been obtained from plants grown on the plot; the wilt resistance of Bison apparently has not changed in that time.

The nature of wilt resistance has not been adequately explained. Dr. Tisdale found that the fungus, on entering resistant plants, stimulated cork formation in cells of adjacent tissues and so isolated the infection. The fungus may sometimes be isolated from above-ground parts of apparently healthy plants, however, so there may be other types of resistance.

Studies on the inheritance of resistance to wilt in flax suggest that multiple hereditary factors are involved. The hereditary behavior is hard to determine because we have no way of differentiating between normal wilting in resistant strains and in susceptible segregates.

Nearly all the flax in the North Central States consists of varieties that have been distributed since 1940. Extensive breeding of flax is being done at experiment stations at St. Paul, Minn.; Fargo, N. Dak.; Ottawa, Ontario; Saskatoon, Saskatchewan; and Winnipeg, Manitoba.

Hybrid material is tested on a wilt-sick plot that has been developed at each station. Wide variations exist in the amount of wilt and the vigor of growth of the variety on the different wilt-sick plots, but all tests seem rigorous enough to meet the farmers' requirements. Even Royal, Renew, Crystal, Rocket, and Victory varieties that wilt severely on the wilt-sick plots at St. Paul and Fargo are seldom damaged by wilt in commercial fields. On some farm soils cropped rather frequently to flax, considerable wilt develops in those varieties in hot, dry seasons, but loss due to wilt since 1940 has been small. Bison, Koto, Sheyenne, and Redwood have been highly wilt-resistant.

FLAX RUST occurs in nearly all important flax areas. It attacks only flax. In mild climates it may live over from one crop year to the next on wild or volunteer flax. In colder climates it overwinters on stubble and straw left in the field and on bits of straw in uncleaned seed.

The life history of the flax rust fungus, Melampsora lini, is complex. In the North Central States, infection starts each year with the germination of the overwintering teliospores. They are thick-walled spores formed in a palisade layer in the brownish-black crusts on vegetative parts of maturing flax plants. The germ tube of each teliospore usually bears four minute spores (sporidia), which infect the leaves and stems of flax and produce the rather inconspicuous pycnial stage.

The pycnium the sexual stage of the rust develops into an aecium bearing aeciospores if it is fertilized by pycniospores from a pycnium of opposite sex. Insects, water droplets, and wind may carry the pycniospores; the sperms. Aeciospore infection gives rise to a uredium. The aecial and uredial pustules look alike. They are slightly raised and covered with innumerable orange-colored spores. It is these pustules that give the name "rust" to the disease. Under favorable conditions the uredial generation is repeated about every 10 days, and there is a rapid build-up of inoculum.

Rust reduces the yield of seed and lowers the quality of linen and paper made from the fiber. Reduction in seed yield is roughly proportional to the area of plant tissue infected. Rusted plants use water inefficiently. The rust fungus uses food material that otherwise would be available for seed development. The pedicels, which bear the bolls, are frequently girdled in severe infections, and the bolls fail to develop or they break off or the plants may be killed prematurely. Fibers are weakened at the point where stem infections occur and break during processing. The black fungus crusts adhere to the fiber and cause flaws in the cloth and paper made from it.

The use of resistant varieties has been the best means of controlling rust. Varieties resistant in one region may be susceptible in another, however. All the varieties that are immune or resistant in North America are susceptible to races occurring in South America. Bison, susceptible to all races in Europe, North America, and South America, is immune in New South Wales, Australia. Bombay, highly susceptible in New South Wales, possesses the only gene known to condition immunity to all South American races.

The variable reaction of varieties in different regions and the succession of varieties developed for rust resistance that have succumbed to new races or to alterations in the prevalence of races emphasize the need for constant study of physiologic specialization in the fungus.

A series of differentials was developed by testing the reaction to rust of all varieties in the Flax Classification Nursery of the United States Department of Agriculture. The varieties in the nursery had been chosen for diverse type and origin from a worldwide collection of flax. Fourteen races of rust were differentiated in 1935 by the reaction of eight varieties. Three more varieties were added later in order to identify South American races, and five were added to identify hybrids between North American and South American races.