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.

There is an increasing interest among Plant Pathologists to explore, more fully, the nature of parasitism. This interest lies in obtaining a clear understanding of the host-parasite relationship. The emphasis is placed on the physiological mechanisms involved which enables the plant pathogenic organisms to effect entry into the host body and to assimilate the available nutrients, to tolerate or overcome the resistance of the host and to induce disease in the host by their metabolic products or toxins. The host-parasite relationship of the common scab disease of potatoes, caused by Streptomyces scabies (Thaxt.) Waksman and Henrici, presents many features of parasitism which would be interesting to explore as far as the physiological mechanisms are concerned.

Disease resistance in plants is an attribute which ensures the survival of the species against the agents of disease. It is evolved in nature in the course of natural selection and is genetically controlled. Extensive use of this property is made in breeding resistance into susceptible but otherwise desirable varieties. The nature of resistance, however, is but little understood; although, on the basis of numerous morphological, anatomical, chemical and physiological studies, several theories of resistance have been offered.

Hoodia gordonii has been used by the San people for centuries as an appetite suppressant while they were on long hunting trips. These succulents are globally known as an important component in diet supplements and products which assist in weight-loss. Together with other plants these contribute towards a multimillion US dollar market, as many pharmaceutical and nutritional companies have made significant financial investments in the research and development for people who suffer from weight problems and obesity. In South Africa, H. gordonii is classified as being endangered, because so many pharmaceutical companies cashed-in on the dietary characteristics of these succulents. The plant is found in the South Western parts of Southern Africa mainly in the Karoo, Kalahari and Namib deserts. Plants were also grown in cultivated nurseries under favourable conditions for commercial use. However, diseases have dramatically hampered production in nurseries with almost total loss of crop. Although not many diseases have been documented on this plant, Fusarium wilt, a devastating fungal disease of H. gordonii, caused by F. oxysporum had been identified. This disease is not only responsible for economic losses, but also contaminates the soils with spores, which remain dormant until the next season as inoculum. This study concentrated on the morphological characterisation and molecular identification of the Fusarium wilt causing pathogen present in four areas (Kakamas, Klein Pella, Pofadder and Prieska) as well as the morphological evaluation of the host. Fusarium oxysporum was identified as the causal agent of Fusarium wilt on H.gordonii plants. The AFLP analysis and DNA sequences resulted in two distinct groups. Those that clustered in the AFLP cluster B were also grouped in a TEF cluster B, however some isolates from AFLP cluster A also grouped together with the TEF cluster B. The low genetic variation revealed by the AFLP analysis indicated that differences amongst the pathogen isolates occur, but the DNA sequences confirmed that these isolates share a common ancestor. DNA sequencing analysis was used to place 44 South African F. oxysporum isolates into the phylogenetic groups as described by OâDonnell and associates. South African F. oxysporum isolates clustered into two groups. The observed genetic variation amongst individual isolates was lower than the genetic variation between out-group isolates. Two clusters were identified; within each cluster isolates had a relatively high frequency of clones. These clones confirm that these pathogen isolates share similar allele frequencies. Results from sequencing data showed that the isolates fall within the FOSC, however there was no isolates identified that show 100% similarities when compared with all three genes sequences with recorded genetic sequences of F. oxysporum isolates in other parts of the world. Therefore, based on the current taxonomic system, of host specificity the fungus only infects H. gordonii, so the study has proved that a possible new formae specialis has been identified. New preventative measures must be applied to the host when planting in nurseries. Knowledge gained from analyses of the genetic fingerprinting, DNA sequencing of these isolates and the mporphological evaluation of the host might assist with the development of effective control strategies, i.e. resistance breeding against Fusarium wilt. This will provide an incentive to potential farmers to plant H. gordonii, thus improving production of this succulent for pharmaceutical companies and nurseries in South Africa.

Venturia inaequalis (Cke.) Wint., the apple scab fungus, grows during the summer on the living leaf, producing a mycelium which develops only between the cuticle and the epidermis. But when the infected leaf dies naturally, this mycelium then grows as a saprophyte and sends its hyphae all through the tissues of the dead leaf. It is during this saprophytic part of its life cycle that the fungus passes through its sexual stage. Ascogonial hyphae arise in the mesophyll of the dead leaves and, after fertilization by antheridia from oppositely sexed thalli, initiate the perithecia. These fruiting structures then are the result of the fertilization of an ascogonium from a mycelium of one sex by an antheridium from a colony of opposite sex. The perithecia do not enlarge greatly and show little internal differentiation during the winter. As soon as snow has melted, however, the asci in the perithecia become differentiated, the perithecia enlarge rapidly and by the time the apple trees start to grow, the twocelled ascospores are formed and, if the weather is suitable, are discharged into the air, from the dead leaves on the ground. These ascospores cause primary scab infections in the orchard. The dead leaves in which apple scab is living and fruiting, are certain to harbour other saprophytic organisms. It has been a common observation that dead leaves support growth of many saprophytes. No doubt these saprophytes, in their struggle for existence, must influence [...]

Two theories have been offered to explain the wilting symptoms displayed by plants attacked with vascular invading micro-organisms. These are the bundle plugging theory and the toxin theory. According to the former the movement of water through the water conducting vessels of the stem is hindered or even prevented by the clogging of the vessels with the pathogenic bacteria or fungi and other obstruction materials such as tyloses and gums formed under the stimulation of the pathogen. The proponents of the toxin theory claim that the wilting symptoms are due to toxins formed by the invading organisms or resulting from the interaction between host and parasite. Each of these theories is supported and weakened by experimental evidences. One approach to the solution of this problem is to study the effects of the disease upon certain aspects of the physiology of the host, such as transpiration, stomatal movement and photosynthesis. Many investigators working on diseases caused by wilt organisms have measured the transpiration course of the wilting plant. The methods used for these measurements vary a great deal and are very often specially adapted to the type of plant under observation, although the weighing method seems to have been most generally adopted. Weights are recorded daily or at different intervals during the day. To obtain more accurate frequent measurements of the water uptake is not necessary.[...]

Acronecrosis, or top-necrosis, is a systemic virus disease which manifests itself in certain potato varieties following graft introduction of certain specific plant viruses. Acronecrosis is externally characterized by a necrotic spotting of the uppermost leaves followed by a dying of the plant from the top, downward, in the absence of any mottling. Necrotic areas are invariably found in the stem. These necroses, which are most severe in the distal portions of the plant, originate usually but not always, In the internal phloem and spread into the surrounding tissues. Necroses, similar in their origin and appearance to those in the stem, are found in the petioles, frequently being so severe as to lead to the death of the leaf blades. Axillary buds also become infected and are ultimately killed. In the tubers the necroses develop in the same manner as those in the stem, and spread rapidly to the storage parenchyma. Cork layers abstricting the necrotic areas are always found in affected tubers and are produced occasionally in stems grown at high temperatures. Usually the tuber necrosis spreads by way of the vascular tissues to one or more of the eyes and kills the buds, causing an irregular, sunken depression on the surface of the tuber. Often, buds of some eyes survive to produce secondary plants which soon develop necrotic spots similar to those in the parent plant. [...]

Helminthosporium sativum P.K. & B. causes a disease of barley affecting the seedlings as well as the roots, leaves and heads of mature plants. This disease results in an annual reduction in barley yield and is at least partially responsible for fluctuations in yield from year to year and for the restriction of the present barley growing area. The exact amount of loss, however, is difficult to estimate, and often is attributed to factors other than this disease. Machacek (1943) stated that the common root rot of cereals and grasses was widespread in Manitoba.

Venturia inaequalis (Cke.) Wint., the apple s cab fungus, grows during the summer on the living leaf, producing a mycelium which develops only between the cuticle and the epidermis. But when the infected leaf dies naturally, this mycelium then grows as a saprophyte and sends its hyphae all through the tissues of the dead leaf. It is during this saprophytic part of its life cycle that the fungus passes through it s sexual stage. Ascogonial hyphae arise in the mesophyll of the dead leaves and after fertilization by antheridia from oppositely sexed thalli, initiate the perithecia.

Birdsfoot trefoil, Lotus corniculatus L., and closely related species are long-lived perennial legumes, which have a potential in agriculture for replacing clovers and alfalfas in locations which have proven to be generally unsuitable for the growth of most other legumes. However, the life of birdsfoot trefoil has been only two or three years in Quebec, instead of at least ten which is expected in the southeastern area of the United States (Drake 1958). The reduction in the life span of this plant in Quebec is generally attributed to "winter killing," but the cause of winter killing could be due to a number of factors such as: frost; Sclerotinia trifoliorum Erikss. (crown and root rot or red clover); or other factors.