Types of Rot, Rate of Rotting, and Analysis of Pectic Substances in Apples Rotted by Fungi

Abstract

The rate at which the fungi grew through apples was determined in various ways and used to estimate their rate of linear advance. Five fungi were studied;they were Sclerotinia fructigena (firm-brown coloured rot, rapid growth through apples), Botrytis cinerea (soft, light-brown coloured rot, rapid growth through apples), Psyrenochaeta furfuracea (firm to soft rot, variable in colour but generaly dark, slow growth through apples), Penicillium expansion A (soft, white rot, slow growth through apples) and Penicillium expansum B (soft, white rot, medium rate of growth through apples). S. fructigena which had the highest rate of linear advance which was about three times that of P. furfuracea which had the lowest. Methods for extracting different types of pectic substances from sound and rotted tissues are described, and details are given of a rapid and reasonably accurate colorimetric method of determining the anhydrogalacturonic acid content of these extracts. The firm-rot fungi reduced the water-insoluble pectic substances by 10–20 per cent., but the soft-rot fungi caused much larger changes, up to 70 per cent. being degraded, The firm-rot and soft-rot fungi had different effects on the pectic substances insoluble in dilute acid but soluble in dilute alkali. The soft-rot fungi had little effect on these substances, or reduced their concentration, whereas the firm-rot fungi caused substantial increases compared with sound tissue. These results are considered in terms of pectic enzyme activity. Analysis of extracts by paper chromatography showed that galacturonic acid, absent from sound tissue, was present in each type of rotted tissue. Di- and tri-galacturonic acids were present in rots caused by P. expansum, and these rots probably also contained products from the break-down of other polysaccharides.