A quantitative study of the course of fungal invasion of the apple fruit and its bearing on the nature of disease resistance.—Part I. A statistical method of studying fungal invasion

Abstract

In the course of investigations on the incidence of disease in apples stored at low temperatures, which have been in progress since 1920, it soon became evident that a regular series of fungal species appeared successively during the period of storage. It seemed desirable to investigate the problem more closely with a view to disentangling the factors concerned in the succession. The first method employed for the purpose consisted in taking a periodical census of the population of apples and noting the number of apples “invaded” and “not invaded.” The need for improvement on this method of studying fungal invasion became clear in the course of the investigation, and led to the development of the technique, described in the present paper. The method of the periodical census has been utilised by M. N. Kidd (1) who has constructed curves of the progress of fungal invasion and from them has derived “mortality curves.” The interpretation of these “mortality curves” is, however, somewhat difficult, representing, as they do, not only the effect of varying resistance in the apple, but also the interaction of this factor with the chances of casual infecticn, which includes the varying powers of penetration possessed by different species of fungi. The distribution of the two classes of apples “invaded” and “not invaded,” depends on the interaction of three factors: (1) the conditions of infection, (2) the progress of invasion after infection, and (3) the final stage of invasion. The bearing of these factors on the problem of invasion of apples will now be briefly considered.