No abstract

A mutation for loss of wall pigment in urediospores and teliospores of Puccinia helianthi Schw, is described. The mechanism of inheritance of spore colour could not be established precisely. Wall pigment appeared to be determined by one or a small number of genes, whereas pigment of the cytoplasm was unstable and influenced by environmental factors. Evidence for multiple fertilizations in single aecial cups supported earlier reports that the aeciospore should be considered the unit of fertilization in the rusts. Somatic segregation for colour was observed in F, cultures. IntroduotionInheritance of urediospore colour in Puccinia graminis f. sp. tritici Erikss. & Henn. has been shown to be controlled by two genes: one determining carotenoid pigment in the cytoplasm and the other, brown pigment in the spore wall (Johnson, Newton and Brown, 1934;Green, 1964). A mutation for spore colour in P. helianthi Schw. was described by Brown (1940). It affected pycnia and aecia as well as uredia and telia. The genetic basis of the colour change was not established. Spore size, shape and echinulation, viability, host range and temperature tolerance were also affected and Brown suggested that more than one gene was probably involved.Colour mutants have been extensively used as genetic markers in the cereal rusts. Studies on P. helianthi have lagged behind those on other species in this respect. A spontaneous mutation for loss of wall pigment in a culture of sunflower rust at Macdonald College afforded an opportunity to extend genetic studies on P. helianthi which have so far met with little success (Miah and Sackston, 1970). Besides determining the mechanism of spore colour inheritance in P. helianthi, it was hoped that this study would help to resolve the controversy over the unit of fertilization in the rusts (Dinoor, Khair and Fleischmann, 1968) which has been a fundamental difficulty in studies on the inheritance of pathogenicity in the rusts. M'aterials and MethodsA yellow-spored mutant of P. helianthi arose spontaneously from a culture of "Canadian" race 2 on a single leaf in an otherwise normal detached leaf culture growing at 10°C. Two of the five pustules found were cultured (as Y1 and Y2) for further study. Urediospores from the others, when transferred to susceptible leaves, failed to induce infections.Single spore isolates of these cultures were maintained on detached sunflower leaves on moist filter paper in Petri plates in controlled environment growth chambers at 22 + 2°C day and 20 -+ 2°C night temperature, daylength of 16 hr and light intensity at plant level of 13200 -+ 2200 lux (1200 2 200 ft-c) from cool white VHO fluorescent tubes supplemented with incandescent bulbs, giving approximately 4400 lux at leaf surface within the plates. Cultures were also grown on plants in the greenhouse (22 to 25°C) isolated beneath plastic covers.

Detached leaves of sunflowers on various substrates in petri plates retained some green color for 36 days at day and night temperatures of 22 and 20 °C respectively, and for 55 days at a continuous temperature of 10 °C. Water agar proved best of the substrates tried. Uredia developed in 9 to 10 days after inoculation with urediospores at 22−20 °C, and in 14 to 20 days at 10 °C. Telia developed in 14 to 20 days after inoculation at 22−20 °C, and in 22 days at 10 °C. Teliospores produced at 22−20 °C could not be induced to germinate. Those produced at 10 °C began to germinate 15 days after formation. Several complete cycles from teliospore to teliospore were produced on detached leaves. The time required per cycle averaged about 80 days.

No abstract

Sunflower rust (Puccinia helianthi) was collected in Texas on wild Helianthus spp. believed to be ancestors of the cultivated sunflower (Helianthus annuus). Race 1, the least pathogenic on the standard "Canadian" differentials, was the most common. Races 1 and 2 predominated on H. annuus and H. debilis, race 2 on H. praecox, and race 4 on H. petiolaris. Most host species proved to be heterozygous for rust resistance; cross-infectivity tests between them were inconclusive. Puccinia helianthi shows a high degree of adaptive variability, which suggests that local specialization may be relatively unimportant.