THE DETERMINATION OF PHYSIOLOGIC FORMS OF PUCCINI A TRITICINA ERIKSS. IN ENGLAND AND WALES

SUMMARY1 Forty‐six collections of Puccinia triticina have been tested for the determination of physiologic forms. The majority of these were studied as single‐spore cultures. With the exception of three from Portugal the collections were made in various parts of England and Wales. 2 Ten new physiologic forms of P. triticina (Forms 66–75 inclusive) are described for the first time. Forms 66–73 were obtained from collections in Britain, and Forms 74 and 75 from Portugal. 3 A mutation in pathogenicity is here recorded for P. triticina for the first time, in the origin in the greenhouse of a variant of Form 10 from a culture of Form 66. 4 An apparently inherent instability in the behaviour of Form 66, as exhibited by an abrupt change in reaction on the variety Hussar from a resistant to a susceptible type, is discussed. This change cannot satisfactorily be correlated with environmental conditions or mixture of cultures. Such instability was not observed in cultures of any other physiologic form encountered in this investigation. 5 Changes in environmental conditions affected the reactions of some of the differential varieties to certain physiologic forms. For example:(a).  Abnormal production of a type “x” reaction on certain of the differential varieties was found to be correlated in some forms with excessively high temperatures during incubation. Decrease, in light intensity, combined with decrease in temperature, was found to be responsible for the production of an abnormal type “” reaction by other forms. (b).  Increase in resistance in normally susceptible reactions was found in many forms to be associated with low light intensity and low temperature. Eesults, however, low light intensity seemed in general to be the more important factor in bringing about this in susceptibility. (c).  Increase in susceptibility in the normally resistant reactions of certain varieties to some physiologic forms was found to be correlated with increased temperature and light intensity. Increase towards susceptibility of normally resistant reactions was not usually of a very profound type. Resistant reactions were found on the whole to be less sensitive to environmental fluctuations than susceptible reactions.6 Infection of certain varieties, e.g. Malakof, Webster and Democrat, by Erysiphe graminis induced the development of pustules of a susceptible type by some physiologic forms of Puceinia triticina to which these varieties were normally resistant. 7 Form 66 appears to be the commonest and the most widely distributed form in Britain. This form was isolated fourteen times, Form 15 seven times, Form 67 five times, Form 68 three times, Form 70 twice, and Forms 10, 69, 71, 72 and 73 were each isolated once only. 8 Some variation in the annual distribution of physiologic forms was observed in the collections made near Cambridge, Newton Abbot, and Cardiff. 9 The fact that no previously described forms, except Forms 15 and 10, have been encountered is discussed. The apparent absence of an aecidial host for P. trUidna in this country presents a problem as to the origin of the forms found here. It is suggested that mutation may account for the occurrence of some of these forms in view of the origin during the course of this investigation of a variant of Form 10 from a culture of Form 66, presumably by mutation.