Observations and experiments on cereal rusts in the neighbourhood of Cambridge, with special reference to their annual recurrence

Abstract

Part I. (a) Black rust (P. graminis). (1) Fresh uredo-sori of this rust are not found after the commencement of winter. Uredospores in old pustules exposed to winter cold soon lose their viability. (2) Observations and experiments show that this rust cannot overwinter in this locality even as mycelium inside the host plants. (3) Direct infection of wheat by sporidia is not possible. (4) There is no evidence to show that this rust can originate from "mycoplasm" or from intra-seminal sori. (5) The annual outbreak of this rust can be explained only by fresh infection through aecidiospores produced on barberry. (b) Brown and yellow rusts (P. triticina and P. glumarum). (6) Fresh uredo-sori of the brown rust of wheat and the dwarf rust of barley are found during the greater part of winter. Uredospores from the open during winter always show good germination. (7) Fresh uredo-sori of the yellow rust of wheat were found during the greater part of the winter of 1920-21. Uredospores from such pustules always germinated well. (c) Incubation period. (8) For the annual recurrence of rusts the factor of the greatest importance is the occurrence of plenty of uredospores on self-sown plants and tillers at the time when the autumn sown crop appears. The infection of young seedlings is followed by a comparatively long incubation, the exact duration of which is dependent upon conditions of weather-temperature being the chief regulating influence. (9) The length of the incubation period is not only not fixed at eight to ten days but is very variable. Although the last two seasons were comparatively mild the incubation period in some of the experiments was prolonged to five to six weeks. (10) These facts also suggest a satisfactory explanation of the phenomenon of spontaneous outbreaks (so characteristic of yellow rust) over large areas within the space of a few days. (11) Cultures of all the three rusts at comparatively low temperatures show the shortest incubation period for the yellow rust and the longest for the black one. A clear difference between the duration of the incubation periods of yellow and brown rusts suggests the cause of the usually earlier appearance of the former. (d) Influence of temperature on the viability of uredospores and the growth of the mycelium. (12) The uredospores of black rust retain their viability at comparatively high temperatures. Those of yellow rust suffer most from heat and are more viable when kept at lower temperatures. The uredospores of brown rust however combine the powers to resist cold as well as heat. (13) The uredospores of yellow rust are short lived and are impaired at temperatures which both black and brown rusts can withstand. (14) Higher temperatures are inhibitory to the growth of the mycelium of yellow rust, and therefore prolong the duration of the incubation period, and may even kill the mycelium. (15) Some of the most susceptible varieties of wheat and barley even when exposed in the open for over ten months do not become affected by yellow rust if the viability of the uredospores (the only source of infection) is impaired. (16) The observations and experiments recorded in this paper deny all possibility of an hereditary source of infection and offer an adequate explanation of the annual recurrence of these rusts. Part II. (1) The black rust of wheat can infect only wheat and barley, but does not infect rye and oats. (2) The black rust of Couch grass readily infects Couch grass, rye and barley, and contrary to what has been observed at other places can also infect one variety of wheat (Red Sudan). (3) The black rust of barley infected barley, rye and Couch grass but failed to infect wheat. This form is different from that on wheat and probably is identical with that on Couch grass. (4) As far as black rust is concerned specialization is not quite so fixed as has been recorded elsewhere. In this locality Couch grass infected with this rust may be a source of danger to barley. (5) The brown rust of rye is rigidly specialized to its host. (6) The brown rust of wheat casually infects rye. (7) The dwarf rust of barley and the yellow rust of wheat cannot infect other cereals.