A study of certain aspects of leaf spot disease of celery

Abstract

The Lincoln form of Septoria apii is shown to be pathogenic towards celery (Apium graveolens L.).

The biologic and morphologic characters of the organism are described. Significant differences are found to exist between the mean diameters of pycnidia from different spots on the upper surface of leaves, and between pycnidia from the upper and lower surfaces of the same spots. The diameters of pycnidia measured, range between 66.5µ and 209µ; mean 128µ. Spore lengths range between 30µ and 67.5µ, with a mean of 46µ.

The biologic and morphologic characters of the Lincoln form and forms of other workers are compared, and the effectiveness of these characters in the comparison of the forms is discussed. The Lincoln organism evidently resembles the form S. apii-graveolentis more closely than S. apii.

The growth of the organism in culture is described, and a method for distinguishing between the forms of Septoria on celery is suggested. Use of this method indicates that the Lincoln Septoria may be distinct from both those of Cochran (1932), and may perhaps be regarded as a third form.

The occurrence of saltation in culture is reported, thus supporting the varietal concept of Laibach (1921), regarding the forms of Septoria on celery. Factors favouring the production of saltants are considered. The cultural characteristics of several saltant forms are compared with those of the normal form. Possible origins of saltation are considered, and further experimental work is suggested. Incoculum form 3 month old saltant colonies failed to infect the host.

Three types of spore germination (two apparently not previously reported), and the fusion of germinating spores, are described briefly and discussed.

The preparation of stained, mounted sections of the diseased host tissue, is briefly described.

The selection of a technique to quantitatively measure the amount of viable Septoria apii inoculums on seed of the host, is described. Examination of seedlings for the presence of disease lesions would appear to be the most successful method.