Corynebacterium sepedonicum (Spiekermann and Kothoff), Skaptason and Burkholder, is the causal agent or bacterial ring rot of potatoes. Histological observations have invariably revealed that it primarily inhabits the water conducting vessels of the potato. [...]

Venturia inaequalis (Cke.) Wint. is the incitant of apple scab, a disease of great economic importance throughout the world. This pathogen, in the early stages of infection, is confined to the area between the cuticle and the epidermis of the host and thrives there in intimate contact with these layers. This rather peculiar growth habit seems to indicate that lipids might have an important role in determining the host specificity and the pathogenicity of V. inaequalis. [...]

Cercospora beticola Sacc. is a necrogenic pathogene which produces brown small nearly circular spots on the foliage of table beets, sugar beets, mangolds and Swiss chard. On beet leaves the spots have a reddish-purple border, while on mangolds there is usually a pale green border around each spot. The center of the spot is grayish due to long septate conidia produced on conidiophores which come out through the stomata.

The so-called age of antibiotics is generally considered to be of recent origin but the foundation of its beginning goes back to the last century. The phenomenon of antibiosis, the production by one organism of a specific chemical substance or substances which have an injurious effect upon another organism, was noticed by many of the early microbiologists. Although Tyndall, Pasteur, and DeBary worked in different fields of biology, each mentioned this phenomenon of antibiosis in his writings.

Red clover (Trifolium pratense L.) is one of the most important and most widely grown cultivated legumes in Canada. The early flowering varieties are the most popular in the clover growing areas of Quebec, Ontario and the Maritime provinces because they are able to make a rapid recovery after the first cutting. Red clover is subject to numerous diseases and insects, some of which cause severe injury to the plant, while others inhibit plant growth and thus reduce the yield or quality of the bay.

Wheat, an important food crop of the world, suffers from a number of diseases which cause a heavy loss to the growers every year. Stem rust (Puccinia graminis tritici Eriks. and Henn.) is one of these serious diseases and it often occurs in an epiphytotic form. It is a common observation that some wheat varieties are very susceptible to a particular rust race and are sometimes completely destroyed while other varieties prove to be resistant. The knowledge of these facts led plant pathologists to realise the importance of resistant varieties and to study the nature of resistance to this disease.

Disease resistance in a plant is a manifestation of a complex phenomenon which involves among other things, the inherent and acquired physiologic properties of not only the host but also those of the pathogen influenced by ever-changing conditions of both the macro- and micro-environment.

The study of various aspects of the wilt diseases of plants which are caused by fungi inhabiting the vascular systems of the host plants has engaged the attention of a number of research workers for over half a century. Various theories have been proposed to explain the wilt symptoms in several species of plants, yet none can be considered to fully explain how the fungus in the vascular elements causes the plant to become diseased and die. The wilt disease caused by Fusarium oxysporum f. lycopersici (Sacc.) Snyder & Hansen is specific to the tomato, Lycopersicon app.

Autoradiograms of plants which had one of their leaves (presentation leaf) photosynthesizing in C14O2 show high concentration of radioactivity in the growing point and roots and low concentration in leaves below the presentation leaf. Infection on two of these lower leaves increases the concentration of radioactivity in them, especially around lesions, but has no marked and consistent effect on the concentration in other organs. Infection on the presentation leaf reduces photosynthesis and export of C14 to other parts. Heat girdling of petioles of healthy and infected leaves show that in both cases, C14 export takes place through phloem and its import through xylem. Radioactivity in detached leaves exposed to C14O2 , either in light or in darkness, is uniformly distributed in healthy leaves, but accumulates around lesions in diseased ones. Starch has similar distribution. Pathological increase in transpiration and metabolic activity are probably responsible for the altered cl4 distribution in diseased leaves.

The possibility that toxins produced by bacterial and fungal pathogens may play a role in the etiology of the diseases which these microorganisms bring about has received a good deal of attention and the so-called toxin theory of disease causation has gained more and more prominence in recent years. According to this theory, many of the symptom expressions of parasitic diseases especially those occurring in the non-invaded parts of the host are due partly or entirely to those secretions of the pathogen formed while it is growing within the host.