In the third paper of this series (6) the literature bearing on the chemistry of plant diseases, especially the effect of disease on the composition of plants and the chemical differences between resistant and non-resistant varieties of the same species of plant, was reviewed in some detail. Since the present work is a continuation of the former, it will not be necessary to make reference again to the results of other investigations, except incidentally during the discussion. In the previous work on the brown rot of plums, caused by the fungus Sclerotinia cinerea, it was found that during the progress of the rotting the H-ion concentration of the sap increases markedly; that oxalic acid is produced, but hardly in sufficient quantity to account for the increased acidity; that tannin decreases during the rotting; that protein nitrogen increases, due probably to the protein formed in the fungus mycelium. Resistant varieties did not differ conspicuously from non-resistant varieties, so far as the analyses showed, except that the resistant varieties usually had a more acid sap, and that more oxalic acid was produced in them. The tannin content of green plums usually increases after picking from the tree, but infection by Sclerotinia entirely inhibits this increase. Experimentation In the present work the ordinary proximate analyses, together with the determination of calcium, were made, using four varieties of plums at three stages of maturity, grown at the University Fruit I Published with the approval of the Director as Paper no. 272, Journal Series, Minnesota Agricultural Experiment Station. Presented at the meeting of the American Chemical Society, September 9, 1921.
