Browsing by Author "Martin, Michael Menne."

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An epidemiological analysis of the Phytophthora and Alternaria blight pathosystem in the Natal Midlands.
(1980) Putter, Christoffel Antonie Johannes.; Martin, Michael Menne.; Rijkenberg, Fredericus Hermanus Johannes.
The history of the development in Natal of a forecasting service to warn of outbreaks of late blight disease caused by Phytophthora infestans is presented. The late blight pathogen and Alternaria solani, the causal organism of early blight disease, interact on potatoes and tomatoes to form a blight disease complex. Evidence is presented to show that it is expedient to manage this blight complex as a whole rather than to direct control at only one of the components in ignorance of the consequential enhancement of the potential of the other. In a search for an improved blight complex management strategy, factors concerning the possible existence of an annual migration of Phytophthora infestans inoculum, first postulated in the 1960's, along an east-west route across Natal, are collected and collated. Corroboration of the existence of the Phytophthora-pathway is given, inasmuch as it represents a serial outbreak of late blight along a temporal gradient. The possibility that the pathway is a manifestation of disease resulting from the erruption of pre-existing inoculum along an environmental gradient, can not specifically be excluded. However, the peculiar pattern of anabatic and katabatic winds along a river-valley network, superimposed on a continuous cropping pattern and its concomitant opportunity for blight to be endemic in the province, supports the postulated Phytophthora-inoculum pathway A fungicide spray trial was conducted in order to investigate the possibility of us i ng the pathway phenomenon as the framework for an improved blight control strategy and to explore the nature and level of the competitive interaction between Phytophthora infestans and Alternaria solani. This trial revealed that the interaction between the components of the blight complex was differentially altered by weather patterns and fungicide combinations. Treatments in which metalaxyl (Ridomil) alone was used for the control of late blight, gave a yield similar to those with propineb (Antracol), which inhibits A. solani primarily but also hus some negative effect on P. infestans. The yields from both these treatments were siguificant ly (p < 0,05) better than the yields recorded in the unsprayed control plots. A treatment in which Ridomil and Antracol were combined such that each was applied according to its recommended concentration, gave yield increases of 32,3% over the unsprayed control, although the yield from the Ridomil/Antracol treatment was not significantly greater (p < 0,05) than the yields recorded where either Ridomil or Antracol were used. A computer simulator, named GAUSE, was developed to simUlate the consequences of the competition between various combinations of P. infestans and A. solani. Results simulated by GAUSE corroborated those obtained from the field trial and support the conclusion that diseases of complex etiology require more than simplistic, univariate analysis of single cause-and-effect pathways. The competition quotient CQ is developed as a new parameter of competitive interactions. It is calculated as the ratio of the amount of disease in the absence of competition, to the amount of disease when the causal pathogen is competing with another pathogen in the same niche. The CQ may be calculated from various standard epidemiology statistics and it is used to demonstrate that the competitive displacement phenomenon places constraints on the interpretation and application of Vanderplank's basic epidemiology equations. A new pathosystems management concept namely the pathotope (pathos = suffering; topos = place0 concept, is introduced, having developed from the notion that epidemics have spatial as well as temporal attributes. Accordingly, an area in which individual farms are at the same level of probability at risk to disease, delimits the pathotope. The concept can be described at many integration lsvels and is presented as an important quantitative unit of comparative epidemiology. The pathotope concept accomodates such notions as are contained in the postulated Phytopnthora-pathway and is especially suited to integration with disease forecasting methods. An example of the application of the pathotope approach is presented and a strategy is proposed by which fungicide spraying is initiated and applied synchronously as determined by the degree of communal risk to attack and epidemic increase of disease. Within a pathotope, several common factors collectively determine the vulnerability of the group to disease. If a coherent, uniform strategy is to be developed and implemented by pathotope members, it is necessary that all members have access to the relevant information and that it be collected and disseminated conveniently and rapidly. A computer-based disease monitoring and mapping system which achieves these objectives is presented.
Purification and electron microscopy of the tomato spotted wilt virus.
(1976) Martin, Michael Menne.; Thung, T. H.; Noordam, D.
No abstract available.
A study on avocado sunblotch disease.
(1980) Da Graca, John Vincent.; Martin, Michael Menne.
Avocado sunblotch disease is a graft-transmissible disorder known for over 60 years and has now been recorded in at least eight countries around the world. Affected trees develop yellow, depressed streaks on young stems and fruit, marked rectangular cracking of the mature bark and a decumbent style of growth. Often a tree with symptoms produces completely symptomless shoots, termed 'recovery' growth, which are latently infected. There is a reported 95 to 100% transmission of sunblotch through the seed of such branches, and "the resultant seedlings are themselves symptomless. Indexing for sunblotch to ensure that scion and, in view of seed transmission, especially rootstock material is free of the disease is very important . The standard method used for many years has been to graft tissue onto healthy indicator seedlings and observe for symptom development for 18 months to two years. One aim of the study presented in this thesis was to develop more rapid methods for detecting the sunblotch agent. By conducting the standard indexing method in a glasshouse at controlled high temperatures of 30/28º C (day/ night) and by cutting back the indicator plants every three months, the time was reduced from two years to eight months. While this represents a considerable saving in time, the ideal must be to develop a laboratory diagnostic test that requires no more than a few days, at most, to complete. A comparative study was therefore initiated on the phenol metabolism of healthy and sunblotch-infected avocados to determine whether infection causes any major change that may reliably serve as a marker for diagnostic purposes. Significantly increased peroxidase (PO) and phenylalanine ammonia-lyase (PAL) activities, decreased indoleacetic acid (IAA) oxidase activity and higher sinapic acid levels were detected in bark tissue showing sunblotch symptoms, but not in symptomless 'recovery' growth. In contrast, increased polyphenoloxidase (PPO) activity and isoenzymes, total soluble protein levels, water soluble phenols and reduced ferulic acid levels were found in the bark of all infected trees tested, both with symptoms and symptomless. However, these latter changes have been associated with other plant-virus systems and are therefore not necessarily specific for sunblotch. Neither is any sufficiently large to be definitive as a diagnostic test. Two unidentified phenols were detected in infected, mature bark, but not in infected young bark and leaves. introduced the possibility of rapid disease detection by polyacrylamide gel electrophoresis (PAGE) of extracted RNA's as used for known viroids. In this study the presence of previously reported small molecular weight sunblotchassociated RNA's was confirmed using PAGE methods requiring two to four days to complete. This thesis presents as a further development a more rapid method of PAGE detection of RNA's enabling indexing for sunblotch to be completed in under six hours. Whilst the biochemical studies did not reveal diagnostically meaningful differences between healthy and infected avocados, there were tendencies towards differences between healthy and symptomless carrier tissues, further investigation of which may lead to a future understanding of symptom development and the symptomless condition. These include apparent higher PO and lower PAL activities in symptomless carrier tissue, as well as higher PO isoenzyme a[1] and lower IAA oxidase isoenzyme a[1] activities. General studies on sunblotch-infected avocados showed that fruit from symptomless 'recovery' growth branches are significantly larger and have a higher oil content than those from healthy or diseased branches, the latter finding possibly indicating a more advanced state of maturity of 'recovery' growth fruit due to earlier flowering. The avocado sunblotch agent was shown to have an in vivo thermal inactivation point of 55º C, a temperature higher than the avocado tissue can withstand thereby eliminating the possibility of thermotherapy of infected twigs. In a host range study four lauraceous plant species, Persea Schiedeana, Cinnamomum zeylanicum, C. camphora and Ocotea bullata, were successfully infected with sunblotch by grafting from infected avocado. This is the first demonstration of any host other than avocado. A phanerogametic member of the same family, Cassytha filiformis, was shown to be able to transmit the disease from avocado to avocado. No hosts from other families were found. During an electron microscope study of sunblotch-infected avocado leaf tissue, gross alterations of the chloroplasts in the yellow areas were observed. These changes included organelle swelling, loss of grana and stroma lamellae, rearrangement of remaining membranes and presence of vesicles. Also in the yellow areas paramural bodies were encountered in higher numbers and displaying altered structure than in healthy and symptomless infected leaf tissue. This study on avocado sunblotch disease was successful in both of its aims. Firstly with regard to quicker indexing techniques, the standard method using indicator plants was shortened from two years to eight months, while a rapid, six-hour test based on PAGE analysis, was developed. Secondly, more light has been shed on the biochemical and ultrastructural effects of sunblotch on its host, the avocado, as well as providing information regarding the thermal sensitivity and the host range of the agent.