Researches on two diseases of economic plants caused by phytophthora species

Abstract

I. A Disease of Atropa belladonna L. causing a Wilt and Root Rot.II. The WHITE TIP DISEASE of LEEKS caused by Phytophthora Porri nov.spec.Two diseases of economic plants which cause considerable less and the fungi which cause them are described.Atropa belladonna L. is attacked by a species of Phytophthora which causes a wilt and root rot. It is known only in Great Britain and Holland. The fungus was grown in artificial culture and its life history and vegetative characters are described. It differs from Phytophthora erythroseptioa in the pro- -duction of paragynous antheridia, in mixed fertilisa -tion, and in slight mycelial characters. The measurements are almost identical. It is considered to be a variety of Phytophthora erythroseptica and it full diagnosis, based on the original of Pethybridge is given.It is proved that it is really responsible for the disease by inoculation experiments. The best method of control was found to be Mercuric Chloride solution (1:1000). It was found to be capable of infecting potato plants and also capable of causing a rot of potato tubers, the pink flush being produced. The pink flush of infected potatoes is proved not to be a diagnostic characteristic of Phytophthora erythroseptica and a list is given of those species of Phytophthora that can also give a pink flush to infected tubers. The spread of the disease is discussed and it is considered that the conidia which are produced on the surface of the soil help in distributing the fungus by the emission of zoospores which swim in the water present, being carried further distances by draining water. The spread is slow and is connected with the absence of aerial fructifications on the shoot system which would be distributed by wind currents. The fungus has been proved to live saprophytically in the soil and forms its oospores there, which are able to resist the winter conditions. The spread is also considered to be helped by the distribution of infected debris and by contaminated soil on cultural implements and boots. Ho other natural host plant has been found.Climate is shown to have a relation to the disease both by its relation to the over-wintering, temperatures being too low for the survival of the fungus except by the oospores and by its relation to the progress of the disease, there being a connection between the summer temperatures and the temperatures at which the fungus grows best. For the fungus in culture the minimum, optimum, maximum and death point temperatures are 4.5°C., 23°C.,30°C. and below 40 C.A "White Tip" disease of leeks is described and the economic effect discussed. It is shown not to be a new disease but an old one which had been considered to be an abnormal manifestation of quite another disease, namely, Mildew caused by Beronspora Schleideni. It is not known except in Great Britain and the fungus is stated to be a new record of a Phytophthora species. The fungus has been isolated in pure culture, the procedure being fully described, as great difficulty was experienced. Its life history and cultural characters are described and its characteristics compared with other species. It is decided that it is a new species and is named Phytophthora Porri n. s p . , an English diagnosis being given. Repetitional diplanetism has been demonstrated in this fungus.The importance of the type of sexual organs is discussed and it is decided, for both diseases, that they are not a main diagnostic feature, but that they may be used as confirming evidence in decisions regarding the specificity or variety of a Phytophthora.The fungus was proved capable of living saprophytically in the soil, where oospores and conidia were produced. The progress and spread of the disease are discussed and infection from the soil via the roots or stems is not supported, but evidence is orought forward to support the theory of leaf infection as the first stage of the disease. Wind is put forward as a contributory agent in the spread of the disease. Climate is also related to the seasonal appearance and the earliest dates at which the disease appears seems to be connected with the difference in the incubation period caused by the different temperatures.Infection may be connected with natural mean temperatures approximating the temperature optimum for conidial production.Control measures have not been tested owing to the fungus having only been recently isolated and used in controlled experiments.