WILT, STEM ROT, AND DIEBACK OF THE PERPETUAL FLOWERING CARNATION

S ummary .1  An account is given of the occurrence of disease, known by growers generally as “stem rot” and “dieback”, among Perpetual Flowering Carnations under commercial culture in England. 2  By the plating out of collars of diseased plants, the following four fungi have been more or less frequently isolated: Verticillium cinerescens Wr.; Fusarium cvlmorum (W. G. Smith) Sacc.; F. herbarum (Corda) Fries.; and a fungus provisionally identified as F. Dianthi Prill, et Del. When isolations were made from the inner parts of stems, viz. from xylem and pith only, there was a great reduction in the frequency of isolation of Fusarium culmorum and F. herbarum . 3  These four fungi have been inoculated into plants at various stages of growth under normal cultural conditions. From the results, which are summarised on pp. 653–4, it is concluded that “stem rot” is a complex of three symptomatically and etiologically distinct diseases, as follow:(a)   Verticillium wilt, caused by V. cinerescens . (b)   Fusarium wilt, caused by F. Dianthi (provisional identification). (c)  Stem rot, caused by F. culmorum, F. herbarum , and possibly other Fusarium spp.A diagnostic key to these diseases is given on p. 654.4  Dieback, the symptoms of which are described on p. 660, appears to be caused mainly by F. culmorum . In one experiment inoculation by Verticillium cinerescens has also resulted in the development of a dieback type of disease. To what extent this may occur naturally is unknown. 5  The relative economic importance of the above diseases is discussed (pp. 662‐3). 6  Morphological and cultural features of the causal organisms are described, and experimental data on the influence of temperature on their growth are given. 7  In examination of the distribution of the wilt organisms in plants showing only very early signs of being diseased, V. cinerescens has been isolated from parts of shoots well beyond the limits of any macroscopically visible internal or external symptoms. The possible means by which these parts become infected are discussed. Fusarium Dianthi has also been isolated in similar circumstances, but has not as yet been found to occur beyond the limits of visible vascular alteration.8  The influence of soil moisture and temperature conditions on the incidence of Verticillium wilt is discussed. 9  It is shown that infection of a fresh crop by V. cinerescens may occur ( a ) as a result of the presence of contamination left by the preceding crop in the top soil and subsoil, and ( b ) by the introduction into the beds of a number of apparently healthy but actually infected plants, developed from cuttings which were themselves already infected before removal from the apparently healthy parent plants.It is pointed out that until this latter source of reoontamination of the beds can with certainty be avoided, control methods based upon isolation and sterilisation of the beds will fail to eradicate the disease. In the absence of any consistently resistant variety in general cultivation, it is recommended that a search for resistance be made amongst the numerous seedlings raised by specialists in the development of new varieties, and that the possibility of the occurrence of resistant sports in existing varieties be not overlooked. The above work was carried out during the author's tenure of an Agricultural Research Scholarship. Subsequently its continuation for a fourth year was made possible by means of a grant from a group of carnation growers. This latter scheme was organised by the British Flower Marketing Association, to which and in particular to its Secretary, Mr A. G. Forsyth, the author wishes to express his thanks. For the provision of facilities for carrying out experiments under commercial conditions, special thanks are due to Messrs Dudley Page of Hanworth and Messrs Dutton and Son of Iver. As regards the scientific aspect of the work, the author is indebted to Dr H. W. Wollenweber for assistance and advice during the course of three months spent in his laboratory and also for the identification of organisms which were sent to him from time to time. Finally thanks are due to Prof. W. Brown of the Imperial College of Science and Technology, South Kensington, who first brought the problem to the author's notice and under whose general direction the investigation was carried out.