Virulence has been recessive in all crosses thus far studied, except on one variety, Williston Brown. Thus the first-generation cross of two races, which between them attacked 15 of the 16 varieties used to differentiate races, attacked only the three that were susceptible to both parent races.

From 133 second-generation cultures of this cross 64 races were identified, 62 of which had not been isolated previously. None of the progeny attacked all the varieties that were susceptible to both parent races, but we got no indication of any inherent relation between the genes for virulence that would prevent the occurrence of such a race.

Resistance to rust in flax has been inherited as a dominant character. Most of the rust-resistant genes lie in three chromosomes. The genes in two of the chromosomes seem to be allelic. Some crossing-over has been observed between some genes in the third. The number of genes in each differential varies from one to four.

Most of the genes for rust resistance in flax are sharply differential, conditioning either high resistance or susceptibility. Some are intermediate, however, with considerable variability both to different races and to environmental changes. Twenty-two different rust-conditioning genes have been isolated from the 16 differentials, and eight additional genes were isolated from other flax varieties. Other varieties are being tested to secure additional sources of rust resistance.

There appears to be also a gene for gene relationship between virulence in the pathogen and resistance in the host. Races attacking a large number of varieties are able to do so because they are homozygous for a large number of recessive genes. A variety is resistant to all races not possessing the specific gene for virulence on it. Varieties with two or more resistant genes are resistant to all races not possessing the two or more specific genes for virulence on them. A differential with two resistant genes cannot differentiate between races attacking neither or either of its resistant genes.

Koto, a variety developed as a rust-resistant replacement for Bison, derived its rust resistance from Morye, one of the rust differential-host varieties. Koto was free from rust during several years of nursery tests in several localities in the North Central States. When released for field increase, however, it rusted heavily, indicating that races attacking it were present but had escaped detection. This was confirmed in subsequent investigations, which showed that (besides Morye) another differential, Tammes' Pale Blue, also carried the Koto gene but that races attacking Koto could not be detected because of a second gene in Tammes' Pale Blue and the three genes in Morye that condition resistance to North American races.

The primary aim of such studies is to facilitate the development and maintenance of disease-resistant varieties. To promote the objective, the flax rust differential-host list of varieties has been revised. Each of the new differentials possesses a single gene for rust reaction to North American races and shows sharp differences in reaction. The 18 new differential lines include seven that have been highly resistant or immune from all North American races, three that possess genes conditioning resistance to some North American races in varieties now in production, and eight that have differentiated races of flax rust throughout the world most satisfactorily. Thus, physiologic race tests with the new differentials will give the flax breeder a more accurate picture of the virulence of the races with which he must contend and of their distribution and prevalence. If the genes in the differentials serve as sources of resistance, the occurrence of single genes in each promotes an early discovery of changes in race pathogenicity and avoids the masking of virulent races by multiple rust-conditioning genes.

Each line possessing a different rust-conditioning gene has been back-crossed to Bison with the goal of getting lines that combine the agronomic type, wilt resistance, and pasmo tolerance of Bison with each source of rust resistance. Such lines will serve as testers for determining rust races present and as a reservoir of desirable breeding material. They also should facilitate the development of new rust-resistant varieties if the varieties now being grown are attacked by new races.

PASMO, caused by Septoria linicola, was first identified in Argentina in 1911. It was found in North America in 1916. Now it occurs in nearly all important flax-growing areas.

Pasmo, primarily a disease of mature tissues, attacks all vegetative parts of the flax plant. On the leaves the lesions are more or less circular, greenish yellow at first, and dark brown in later stages. On the stems the lesions are somewhat elongated. Later they spread up and down and around the stem. As the disease progresses, the stem lesions coalesce, bolls and pedicels become infected, and the entire plant turns dark brown and ripens prematurely. Leaf and stem lesions are dotted with small, dark, flasklike pycnidia, which bear the spores. During wet periods the spores ooze through a pore at the tip of the ripe pycnidium and are spread by wind, rain, and animals.

The disease may be carried from one region to another by spores adhering to the seed or on pieces of infected straw in unclean seed. In regions where it is established, it overwinters chiefly on infected straw and stubble of the previous year.

Pasmo usually does not become severe until the flax crop is approaching maturity. Then the yield of seed may not be reduced seriously.

Pasmo may be very destructive when conditions are exceptionally favorable for its development. Long warm and humid periods, especially when storms cause the flax to lodge, favor the development of pasmo. In southeastern North Dakota in 1943, Viking flax was killed prematurely on thousands of acres. Flax has been heavily damaged by pasmo in Texas in wet seasons.

Although no variety is classed as resistant to pasmo, appreciable differences in susceptibility exist. Viking (B. Golden) and most of the Argentine-type flaxes are highly susceptible. Redwing, Rocket, Royal, and Victory are susceptible. B5128, Bison, Buda, Dakota, Koto, Redwood, and Sheyenne are moderately susceptible. Crystal, Marine, and Minerva show considerable tolerance under field conditions.

No control measures assure against loss from pasmo. Losses may be kept at a minimum by growing the less susceptible varieties, rotating crops, destroying overwintering inoculum on the straw and stubble of the preceding year's crop, and treating seed with a suitable fungicide.

H. H. FLOR is a plant pathologist with the division of cereal crops and diseases, Bureau of Plant Industry, Soils, and Agricultural Engineering. Since 1931 he has been engaged in the study of flax diseases in cooperation with the North Dakota Agricultural Experiment Station at Fargo. He is a graduate of the University of Minnesota.