A FORM OF LEPTOSPHAERIA AVENARIA ON WHEAT IN CANADA

In the late summer of 1942, a Septoria sp. with spores longer than those of Septoria nodorum Berk., and conforming rather closely to those of S. Avenae Frank, was found commonly in the Prairie Provinces of Canada on leaves of wheat and, in one instance, on leaves of barley. It was found again on wheat and more rarely on barley in both Eastern and Western Canada each year from 1943 to 1946. Occasionally, perithecia, attributable to the genus Leptosphaeria, were found on leaves that previously had borne numerous pycnidia of this organism. That the perithecia represented the perfect stage of this Septoria sp. was shown by the development of similar perithecia in many cultures established from pycnidiospores, and by the formation of pycnidia in certain cultures grown from ascospores of perithecia collected on wheat. The perithecia, asci, and ascospores conform closely though not identically with the description of Leptosphaeria avenaria Weber.The organism under discussion differs from S. nodorum not only in the size of its pycnidiospores but also in its inability to cause glume blotch, its longer incubation period, and lack of ability to attack the seedlings of certain wheat varieties susceptible to S. nodorum. It differs from S. Avenae in symptoms, host range, length of incubation period, and cultural characteristics. It differs from both these species by a lower parasitic vigour and a marked tendency to develop on fading leaves and sheaths. Owing to its morphological similarity to L. avenaria it is here described as a forma specialis of that species.

PHYSIOLOGIC RACES OF PUCCINIA GRAMINIS TRITICI IN CANADA, 1919 TO 1944

During the period 1919 to 1944, 65 physiologic races were identified from a total of 4543 isolates derived from uredial collections of Puccinia graminis Pers. var. Tritici Erikss. & Henn. Forty-nine races were obtained in the Prairie Provinces from a study of 3475 isolates; 40 in Eastern Canada from 1013 isolates, and 12 in British Columbia from 55 isolates. During this period, the predominant races have shown notable fluctuations in their prevalence. Races 36, 17, and 21 were the most common races until shortly after 1930 and were largely responsible for the severe rust losses suffered by Marquis and other common wheats during the decade preceding that year. Races 34 and 49 were collected frequently from 1927 to about 1935. All of these races diminished greatly in their prevalence between 1930 and 1936, whereas race 56, which was first collected in Canada in 1931, has become the predominant race since 1934. It was this race that played a major part in the stem rust epiphytotic of 1935. Another recent change in the racial population was a recrudescence in 1940 of race17, which for several previous years had been of minor importance. In 1941 this race challenged the pre-eminent position of race 56 but receded again in succeeding years to minor significance.The distribution of races is somewhat similar but not identical in different parts of Canada. Races 36 and 21 have been relatively more common in the Prairie Provinces than in Eastern Canada, while the contrary is true of race 38. Only about a dozen of the 65 races collected in Canada have thus far assumed much economic importance, a few others may be considered of minor significance, but at least two-thirds of the races have been found only occasionally and have, for reasons not fully understood, failed to gain even a limited distribution.A comparison of the number of physiologic races collected in Eastern Canada and the Prairie Provinces, respectively, has indicated a somewhat greater variety of physiologic races in the former region, a condition that may perhaps be explained by the presence of the common barberry in many localities in Eastern Canada.

THE EFFECT OF DDT ON THE STEM RUST REACTION OF KHAPLI WHEAT

Seedling leaves of Khapli wheat—a variety highly resistant to physiologic races of wheat stem rust prevalent in North America—became susceptible to stem rust a few days after they had been sprayed with DDT (1 oz. in 5 gal. water). The response to DDT was highly specific, as only one other of the resistant wheats tested (Arnautka) showed any indication of susceptibility consequent on spraying. Specificity of response was also indicated by the development of marked chlorosis on leaves of some varieties, whereas other varieties remained unaffected.

PHYSIOLOGIC SPECIALIZATION OF OAT STEM RUST IN CANADA

An account is given of the results of studies on the physiologic specialization of Puccinia graminis Avenae Erikss. and Henn. in Canada since the inception of the work. Twelve physiologic races were identified from 2586 isolates studied during the period 1921 to 1943. The annual surveys of the prevalence of physiologic races during this period show that each year races 1, 2, and 5 have comprised the bulk of the oat stem rust in all parts of Canada. The predominance of these races has been greatest in the three Prairie Provinces, where barberry is virtually non-existent. In regions where barberry is present, other races of greater range of pathogenicity have been found more frequently than in these three provinces. The occurrence of such races, however, was sporadic until 1943, in which year races 8, 10, and 11 attained a wide distribution, apparently traceable to wind-borne urediospores from the south. There is evidence that the strains of these races present in 1943 remain in the uredial stage for much longer periods than do strains of the same races collected in previous years—a fact that may have favoured their spread in this year.The role of barberry in the origination of generally virulent physiologic races is discussed. The possibility that such races may also originate by mutation is suggested by spontaneous pathogenic changes that occurred, in the greenhouse, in a culture of race 3 that gave rise to several cultures of race 7.The effect of environment on the identification of physiologic races is demonstrated by the seasonal influences on the reaction of the variety Sevnothree to races 1 and 11. On this variety the characteristic type 1 is frequently replaced, in summer, by type x.Experiments on the influence of temperature on the reaction of adult oat plants to stem rust showed that a constant temperature of about 80° F. does not appreciably affect the reaction of White Tartar and Richland, or of other varieties with the same type of resistance, but is capable of breaking down the resistance of certain varieties derived from the crosses Hajira × Joanette, Hajira × Banner, and Victoria × (Hajira × Banner).

MELANISM IN WHEAT INDUCED BY HIGH TEMPERATURE AND HUMIDITY

Results of experiments performed in the greenhouse have shown that, under conditions of a high temperature especially when combined with a high humidity, melanistic areas may develop on the glumes, lemmas, peduncles, and internodes of Apex and Renown wheat in the absence of any infection by pathogenic organisms.

THE INHERITANCE OF A MUTANT CHARACTER IN PUCCINIA GRAMINIS TRITICI

A study was made of the inheritance of an abnormal characteristic of Puccinia graminis Tritici Erikss. and Henn. race 21, namely, the production on barberry of white haploid pustules that developed few or no pycnia and rarely produced aecia but occasionally gave rise to uredia and telia. In this race, approximately 50% of the pustules were white, the remainder normal. By selfing studies and by crosses with another physiologic race, it was demonstrated that diploidisation of the mycelia of normal pustules by pycniospores from white pustules initiated physiologic races that produced white and normal pustules, on the barberry, in approximately equal numbers, whereas normal × normal matings produced normal rust and white × white matings were sterile. Uredia that occasionally arose in white pustules as a result of diploidisation by either pycniospores or mycelia of normal pustules gave rise to physiologic races producing white and normal pustules in about equal numbers. The capacity to develop white pustules is not confined to any one sex and is not limited to any particular physiologic race. It is assumed that a mutation affecting one of the conjugate nuclei took place in the original culture of race 21 and that, during meiotic divisions in the germinating teliospore, the mutant factor is segregated so that half of the sporidia give rise to white and half to normal pustules.

THE EFFECT OF HIGH TEMPERATURE ON THE STEM RUST RESISTANCE OF WHEAT VARIETIES

Eighteen stem rust resistant wheat varieties were tested, in the greenhouse, for their reaction to three physiologic races of Puccinia graminis Tritici Erikss. and Henn. at three different temperatures: a constant low temperature of about 60° F., a constant high temperature of about 80° F., and an intermediate temperature which fluctuated daily from 50° to 55° F. at night to 70° to 85° F. at midday.At the low and at the intermediate temperature some of the varieties proved immune while others proved highly or moderately resistant. At the high temperature five varieties (Bokveld, Iumillo, Gaza, Red Egyptian, and N.A. 95 Egypt) were immune or highly resistant; six varieties (Marquillo × Waratah, Hope, Hochzucht, Minor, Bobin Gaza Robin, and Federation × Acme) were moderately resistant; and seven varieties (Kenya, Syria, McMurachy, Sweden, Rhodesian, Talberg, and Eureka) were moderately or completely susceptible.

THE INFLUENCE OF LIGHT AND CERTAIN OTHER ENVIRONMENTAL FACTORS ON THE MATURE-PLANT RESISTANCE OF HOPE WHEAT TO STEM RUST

Experiments have been conducted to determine the degree to which the mature-plant reaction of Hope wheat to race 21 of P. graminis Tritici Erikss. and Henn. is affected by variation in light intensity, length of daily light period, temperature, soil moisture, and supply of mineral nutrients. All these factors gave evidence of influencing rust reaction. A 60% reduction in light intensity during the whole growing period of the plants tended to influence their rust reaction in the direction of increased susceptibility and caused a modification in the morphology of the plants in that they were softer, less rigid, and higher in moisture content than plants grown in full daylight. The shift towards susceptibility was, however, not very marked, and in only one experiment did the reaction of the plants approach complete susceptibility. Reduction in length of daily light period also influenced the rust reaction of the plants towards susceptibility. Plants receiving six hours of light daily were less resistant than plants receiving ten hours of light and these, in turn, were slightly less resistant than plants receiving full daylight for the period of the experiment (February to June). Experiments on the effect of temperature on the reaction of Hope wheat have shown that constant high temperature (75° to 80° F.) is capable of causing a partial or even complete breakdown of mature-plant resistance. Some evidence was secured that abundance of soil moisture and of mineral nutrients diminished rust resistance.

STRIPE RUST, PUCCINIA GLUMARUM, IN CANADA

A study was made of the distribution of Puccinia glumarum (Schm.) Erikss. and Henn. in Canada, its specialization, host range, and reaction to environmental conditions. Unlike Puccinia graminis Pers., this rust has a limited distribution, being confined to British Columbia, Alberta, and the western half of Saskatchewan. The natural hosts include a number of native grasses, particularly Hordeum jubatum L. and certain species of Agropyron, Elymus, and Bromus. Wheat and barley also become infected although to a rather limited extent: Stripe rust collected on the above-mentioned hosts has been studied in the greenhouse and has in all cases shown ability to attack wheat varieties. In all instances where identification of physiologic forms was carried out the rust strains were classified as either form 8 or form 13 of wheat stripe rust, the latter form being the more common. The fact that the present authors have collected known physiologic forms of wheat stripe rust on species of Hordeum, Elymus and Agropyron, and have shown that forms 4, 6, 8 and 13 can attack seedling plants of Hordeum, Agropyron, and Elymus species throw a doubt on the existence of the Hordei, Elymi, and Agropyri varieties created by Eriksson.Greenhouse studies showed that P glumarum is extremely sensitive to environmental conditions, particularly temperature. The optimum for uredospore germination is 10° to 12 °C., and for rust development 13° to 16 °C. Varieties susceptible at from 10° to 16 °C. developed resistance at higher temperatures, becoming extremely resistant at 25 °C. On account of the sensitiveness of this rust to high temperatures it seems improbable that it will ever become thoroughly established in Manitoba and Saskatchewan, as in these two provinces the summer temperature is probably too high to permit its development.

STUDIES ON THE NATURE OF DISEASE RESISTANCE IN CEREALS: II. THE RELATIONSHIP BETWEEN SUGAR CONTENT AND REACTION TO STEM RUST OF MATURE AND IMMATURE TISSUES OF THE WHEAT PLANT

In Part I of these studies it has been shown that the rapidly growing tissues of the wheat plant are more susceptible to stem rust than the older tissues. An attempt was made to discover if a physiological or chemical basis could be found for the difference in reaction of the young and older tissues.Analyses were made to determine the sugar content of young (susceptible) and older (resistant) tissues of four wheat varieties resistant in the adult stage and of the corresponding plant parts of three wheat varieties which in the adult stage showed little or no resistance to rust. The young tissues comprised the young leaves still enfolded by the uppermost sheaths and the young stems below the uppermost node; the older tissues were represented by the fully developed upper leaves and their adherent sheaths. The analyses showed a considerably higher content of sugars in the young than in the older tissues of the seven varieties tested. The difference was particularly great in the content of reducing sugars but rather slight in the disaccharide content (expressed as invert sugar). However, as all the varieties, irrespective of resistance or susceptibility to rust in the adult stage, showed much the same difference in the sugar content of their young and older tissues, it does not seem likely that there is any direct relation between sugar content and reaction to rust.