1. Steam sterilization of the tobacco plant bed did not control ringspot. 2. The virus was not transmitted from diseased to healthy plants by the common tobacco flea beetle (Epitrix parvula) although they lived and multiplied on tobacco plants within the cages. 3. The cucumber flea beetle (Epitrix cucumeris), leaf hopper (Empoasa fabea), Aphis (Macrosiphum solanifolii) and the lightning bug (Photinus scintillans) did not survive when caged on tobacco, and no infection was obtained with them. The tobacco worm (Phlegethontius quinquemoculata) survived but did not transmit ringspot. 4. Stick weed, sometimes called yellow crown beard, (Verbisina alternifolia) and sweet clover (Melilotus alba) were found naturally affected with ringspot, infection was readily obtained on tobacco with the expressed juice from these plants. 5. Twenty-five other species of weeds were tested for ringspot with negative results. 6. The rate of spread of the disease was not definitely determined. 7. The percentage of ringspot infection in ten counties in Virginia in 1927 was 2.5. In Washington County in 1928 it was 3 per cent, and in 1929, 7.6 per cent. 8. There was an average injury of 20 per cent to the affected plants 9. It is estimated that ringspot caused a total loss of $12,768.00 in Washington County in 1929. / M.S.

The tobacco flea beetle is one of the major insect pests of tobacco in Virginia. The adult injures the plants by eating small irregular holes in the leaves and the larva by feeding on the roots of the plants. The tobacco flea beetle hibernates in the adult stage in leaves and trash around the edge of tobacco fields and also under trash in the tobacco field, especially around the old tobacco stumps of the previous season. More beetles survived the winter in the soil around the base of the tobacco stumps than in the organic material around these stumps. During the spring of 1938 the tobacco flea beetles emerged from hibernation at Chatham, Virginia, over a period of 70 days. The eggs of the tobacco flea beetle are normally deposited in cracks or small depressions in the soil near the base of its food plant. A female of the overwintering brood deposited 154 eggs during her oviposition period of 53 days. The number of eggs deposited by the later broods were much less and the oviposition period shorter. In the early spring at Chatham, Virginia, the average incubation period for eggs was about 10.5 days, whereas in midsummer eggs generally hatched in from 4 to 7 days. The eggs require a certain amount of moisture for incubation. The larva molted its skin three times. The larva is subterranean in habit and feeds on the roots of the tobacco plant and other plants of the family Solanaceae. The stems of young succulent food plants are often injured by larvae in the second and third instars. Larval records indicate that the larval period may vary from a maximum of 44 days in the early spring to a minimum of 15 days in midsummer. The length of the prepupal period ranged from 1 to 6 days. The pupal stage is passed within an oval shaped earthen cell which is formed by the nature larva. It is usually found about an inch below the surface of the soil. The duration of the pupal period ranged from a maximum of 15 days in the spring to a minimum of 3 days in midsummer. In the fall the pupal period ranged from a minimum of 10 days to a maximum of 16 days. Newly emerged tobacco flea beetles, which are lighter in color than the beetles of the older breed, confine their feeding to the lower leaves which are near the ground. After a few days they spread to other parts of the plant. The preoviposition period for the seasonal broods were as follows: Overwintering brood, 14 days; first brood, 7.5 days; second brood, 7 days; third brood, 9 to 11 days. No eggs were deposited by females of the fourth brood. The tobacco flea beetle deposits very few eggs on air-dry soil as compared with egg deposition on moist soil. The average length of life of individuals of the overwintering brood under caged conditions was approximately 57 days and for the first brood about 53 days. Tobacco is the favorite food plant of the tobacco flea beetle, put other plants of the family Solanaceae are attacked also. The overwintering generation and two later generations of the tobacco flea beetle attacked the tobacco in the field in Pittsylvania County during the season of 1938. There were probably two later broods which were not well defined on suckers during the latter part of the summer and early fall. The tobacco flea beetle was found to be attacked in the adult stage by a hymenopterous parasite; family Braconidae, species Microstonus epitricis (Vier.) The following cultural practices are important in preventing the spread of the tobacco flea beetle: Location of the plant bed, destruction of the plant bed after the plants are set, destruction of suckers, and fall and winter plowing. / Master of Science

A study of the host plants of the tobacco flea beetle was made in Pittsylvania County from April 1 to September 23, 1939. The work was divided into field observations, rearing experiments end population studies. From field observations made throughout the season it was found that this insect fed mostly on the solanaceous plants. especially tobacco. In the spring before the tobacco was transplanted and in the late summer after the harvesting of the crop, the flea beetle fed on a wide variety or plants. Rearing experiments were made using 29 plant species representing 11 families most of which were known as hosts of the adult beetle. These were planted in a small plot of land of a common tobacco soi1. Beetles were caged over these for 4 days and records were kept as to the number of adults that developed from the eggs laid. No beetles emerged from any of the non-solanaceous plants, whereas at least a few developed on all the Solanaceae tested. Many more beetles developed on tobacco, potato and jimson weed than on any of the others. For the population studies tobacco flea beetle and solanaoeous weed counts were made in 14 tobacco fields selected for this purpose. Beetle counts were made at weekly intervals from May 26 until the middle of August and were computed to show the average number of beetles per tobacco plant. Weed counts were made in June and September and were computed to show the average number of weeds per 100 square yards. The data indicated that only tobacco in the beds and potato in the field were of importance in building up the early flee beetle infestation of tobacco. / Master of Science

Thielaviopsis basicola (Berk. and Br.) is a widespread, soil-borne, pathogenic fungus which causes a root rot on tobacco (Nicotiana tabacum L.). Presently, control is achieved primarily through the use of resistant cultivars of tobacco or chemicals. In order to develop a disease prediction program, disease-inoculum density relationships must be determined. It was the purpose of this study to develop an improved procedure for estimating the populations of T. basicola in naturally infested soil. A second objective was to determine the horizontal inoculum pattern of T. basicola in tobacco field soil, and the third objective was to determine the relationship between black root rot of tobacco and inoculum density of T. basicola in naturally infested soil. Two fields in southwestern Virginia were randomly and systematically sampled to assay the population density of T. basicola in the soil. Populations of T. basicola were isolated using a modified carrot-disc baiting technique and calculated using a colonization correction factor. Frequencies of calculated population densities of T. basicola in soil samples were compared, using a Chi-square test, to the expected frequencies indicated by the Poisson, Neyman Type A, and negative binomial distributions. Temperature-tank studies were conducted to investigate inoculum density-disease relationships. Two cultivars of tobacco were grown in soil naturally infested with T. basicola. The soil was thoroughly mixed before use to preclude inoculum clumping. The results obtained with either systematic or random sampling indicated that propagules of T. basicola are clumped or aggregated in tobacco field soil, as there was a good fit of the data to the negative binomial distribution. Also, the variance to mean ratios were significantly (P = 0.05) greater than 1.0 which is indicative of a clumped pattern. Other indexes of aggregation indicated a low to moderate degree of inoculum clumping in soil. For soil temperature-tank tests, root rot incidence was 96-100% at all natural population densities for both slightly resistant cv. Burley 21 and susceptible cv. Judy's Pride plants, and T. basicola was consistently associated with rotted roots. Low R² values were obtained in regression analyses of T. basicola inoculum density versus mean percent root rot per Burley 21 plant and per Judy's Pride plant. / Master of Science

This study is concerned with the resistance of tobacco seedlings to a second attack of downy mildew caused by Peronospora tabacina Adam. Observations were made on the development and duration of resistance following recovery of tobacco seedlings from downy mildew. The duration of the period of resistance is variable and the observations indicate that plants in the greenhouse recovering during the summer remain resistant longer than do plants recovering from an initial attack during the winter. Samples of leaf tissue from recovered (resistant), infected and previously uninfected (susceptible) plants grown in soil in the greenhouse were analyzed for ammonia-nitrogen, nitrate-nitrogen, reducing sugars and total sugars. Similar analyses were made of samples of leaf tissue from tobacco plants grown in crocks of quartz sand and irrigated with nutrient solutions and from plants grown in crocks of sand-peat mixture to which various mixed fertilizers were added. There was no correlation between the ammonia-nitrogen content of tobacco plants, as determined in these experiments, and the response of the tobacco plant and its susceptibility to attack by P. tabacina, but there was a strong indication that such a relationship exists. Recovered plants contained a higher ratio of total sugar to nitrate-nitrogen than did comparable plants which had never been infected. Furthermore the ratio of total sugar to nitrate-nitrogen was lower in plants infected for a second time than it was in plants from the same lot shortly after they had recovered from the initial attack. It seems probable however that the changes in the nitrate-nitrogen and sugar content in recovered tobacco leaves are an indication of recovery and are not responsible for the resistance possessed by such plants. A series of experiments were performed to test the immediate effect of several nutrient salts and of sugar upon the sporulation of the fungus on the leaves of diseased tobacco plants. There was a very definite response in sporulation to some of these treatments, especially to potassium nitrate, calcium nitrate, and sucrose, but it is not known whether the response of sporulation of the fungus to such treatment is related to the type of resistance possessed by recovered plants. Sodium chloride and calcium chloride were dissolved in water and applied to the soil in which young tobacco plants were growing in the greenhouse. Although increased chloride in one of the sand cultural experiments seemed to protect the plants somewhat against downy mildew, application of solutions of these salts to the soil at the rates used (100 lbs. And 138 lbs. Per acre respectively) produced no noticeable effect on the severity of the disease or subsequent recovery of the plants. The leaves of recovered (resistant) tobacco plants are usually reduced in number, the lower ones having been killed by the initial attack of the fungus. Removal of the leaves from healthy tobacco plants in imitation of this condition did not produce resistance to an attack by P. tabacina. The artificial production of necrotic areas on healthy tobacco leaves in simulation of those usually following sporulation of P. tabacina on diseased leaves had no effect either on the susceptibility of the plants to infection or upon sporulation of the fungus on leaves already infected at the time the necrotic areas were induced. Results of attempts to confer immunity from downy mildew upon an entire plant by infection of a single leaf of the plant were inconclusive, since under the conditions of the experiment it was impossible to be sure that the single leaf was infected. However, the indication is that infection of a single leaf will not render the entire plant immune from a second attack by the fungus. / Master of Science

A new selective medium (TB-CEN) was developed for isolating Thielaviopsis basicola, cause of black root rot of tobacco, from soil. TB-CEN medium contains etridiazol and nystatin to inhibit the growth of undesired fungi, and unautoclaved extract from carrot to selectively enhance for the growth of T. basicola. Inoculum and/or population densities of T. basicola in five burley tobacco fields were 74-166 propagules per g of soil, and 0-12 propagules per g of soil in three other burley fields. Inoculum and/or population densities of T. basicola in 12 flue-cured and 2 sun-cured tobacco fields were 0-26 propagules per g of soil, and 101 and 402 propagules per g of soil in two other flue-cured fields. Environmental factors apparently had a strong effect on black root rot development, since root rot and plant stunting were severe in two burley fields that had 148 and 158 propagules per g of soil, but were not severe in the two flue-cured fields that had 101 and 402 propagules per g of soil. All of the cultivars planted in the four fields were susceptible. Black root rot was the major disease associated with the stunting of tobacco plants in the burley region of Virginia, but not in the flue- and sun-cured regions. No evidence was found to indicate that endomycorrhizae were involved in tobacco stunting in Virginia. T basicola inoculum density-disease severity studies were conducted both in soil-temperature tanks and in the field. Tobacco seedlings were grown in temperature tanks (20-23 C) for 30-31 days in naturally infested field soil (pH 6.5). For all cultivars tested (Burley 21, NC95, and Va Gold), the mean percent of roots that were rotted increased significantly (P=0.001) as inoculum density increased (R² range for regressions=0.93-0.97). Severe levels of root rot occurred at inoculum densities of 50-200 propagules per g of soil. Significant (P=0.01) reductions in plant growth occurred at inoculum densities as low as 5-10 propagules per g of soil. In a study conducted on a commercial burley tobacco (cv. B21-Ky10) field, inoculum densities of 150 and 683 propagules per g of soil were associated with moderate and severe levels of black root rot, respectively. Differences between soil-temperature tank and field studies appeared to be due to variations in environmental- and host-related factors. In another burley field study, the fungicide imazalil, which completely inhibited the growth of T. basicola when amended into agar media at a concentration of 1.0 μg a.i./ml, failed to control black root rot when it was added to transplanting water (50 ml/plant) at concentrations as high as 1,500 μg a.i./ml. / Ph. D. / incomplete_metadata

Thesis (MSc)--University of Stellenbosch, 2002. / ENGLISH ABSTRACT: Seed germination is the most vulnerable time in a plant's life cycle, since the thick protective seed coat ruptures and the moist and humid soil environment not only favours seed germination, but also the growth and development of plant pathogens. Infection of plant seeds during germination, however, is the exception rather than the rule. Plant seeds have - - -developed a--cemplex preformed defense mechanism that includes anttfungal agents thatdiffuse into the surrounding environment to form a protective layer around the seed. This protective layer prevents fungal and bacterial pathogens from infecting the young seedling. Over the last decade, scientists have studied the defense mechanisms of different seeds in an effort to understand and ultimately to introduce and/or manipulate these mechanisms in plants as part of the plant's endogenous disease resistance to pathogens. Various chemical compounds, peptides and proteins that showed strong in vitro activities against various fungi were isolated in these efforts. The mere demonstration of in vitro activity alone, however, is not sufficient to assign a defense role to these antifungal agents. Typically, mutant plants that have lost the ability to produce the antifungal agent, or mutants that are overproducing the agent, have been used to correlate the mutant phenotype to either a decline or increase in disease resistance respectively. Genetic transformation and the subsequent development of transgenic plants have made an unprecedented impact in this regard, specifically in understanding the role of specific defense-related proteins and their interaction with plant pathogens. In this study, the antifungal peptide, Hs-AFP1, from Heuchera sanguinea, a plant defensin, was evaluated in a heterologous in planta environment as a defense protein with potential for engineering disease resistant crops. The in vitro assays performed with Hs-AFP1 against Botrytis cinerea showed antifungal activities of 88% growth inhibition at a concentration of 8 J,lg/ml of the purified peptide, while inducing a characteristic hyperbranching effect on the Botrytis hyphae. Tobacco was subsequently transformed with a construct, pFAJ3068, expressing Hs-AFP1 under the strong constitutive 35S promoter. The peptide was targeted to the apoplastic region with the signal peptide from Mj-AMP2, an antimicrobial peptide from Mirabilis jalapa. Due to reports of peptide instability in transgenic plant systems, two additional constructs were prepared and transformed into tobacco to anticipate possible Hs-AFP1 instability in the heterologous tobacco environment. A putative peptide stabilization construct, pHs-EXG1, consisted of a fusion between Hs-AFP1 and the antifungal exo-glucanase (encoded by EXG1) from Saccharomyces cerevisiae. A control construct, pMj-EXG1, expressing EXG1 targeted to the apoplastic region with the Mj-AMP2 signal peptide, was also prepared and transformed into tobacco to normalize the background antifungal activity as a result of the exoglucanase in the fusion construct lines. Tobacco was successfully transformed with pFAJ3068, pHs-EXG1 and pMj-EXG1, resulting in transgenic tobacco lines designated THs, THE and TME respectively. Transgene expression was confirmed for the THs and THE transgenic lines. The translation of these transcripts into proteins was also confirmed with Western blot analysis. Moreover, the heterologous production of Hs-AFP1 in tobacco led to an increase in disease resistance to B. cinerea in the THs lines in comparison with the untransformed tobacco controls. An increase of up to 42% in disease resistance was observed in an in planta detached leaf assay. Crude protein extracts from the THs lines were also analyzed in an in vitro quantitative fungal growth assay. This assay confirmed the results obtained with the disease resistance assay, with crude protein extracts exhibiting up to 40% fungal growth inhibition. The incubation of B. cinerea in the presence of crude protein extracts from THs lines resulted in hyperbranching of the fungal hyphae, which is characteristic of Hs-AFP1 activity. From these analyses it was clear that the heterologously expressed Hs-AFP1 was quite stable in the transgenic environment. The fusion between Hs-AFP1 and EXG1 did not increase the stability of Hs-AFP1, but rather led to a loss of the Hs-AFP1 activity. All the analyses performed showed the THE lines to be reduced in their ability to inhibit fungal infection in comparison to the THs line. Also, microscopic analysis of the effects of the crude THE extracts on B. cinerea growth showed no hyperbranching activity, again confirming the loss of peptide activity due to the fusion to EXG1. This is in agreement with previous work, in which sarcotoxin 1A was fused to a reporter gene and also lost activity. Although integration of the Mj-EXG1 expression cassette was confirmed, no mRNA levels could be detected with Northern blot or RT-PCR analysis of the TME lines. These lines also did not show any in vitro antifungal activities, probably indicating post-transcriptional gene silencing. This silencing was overcome in the fusion constructs that were expressed in the THE plant lines. These lines also showed EXG1 protein activity, as measured by ~-glucosidase assays. Although the THE lines did not serve the functions originally envisaged, they fortuitously showed that a fusion strategy might stabilize glucanase expression in a transgenic environment. A variety of glucanases have been shown to be prone to gene silencing when overexpressed in a plant environment and the yeast glucanase can now be added to that list if it is not present as a fusion protein. Overall, this study confirmed that Hs-AFP1 is involved in plant defense systems and provided valuable information on the stability of small peptides in a heterologous environment. The positive results obtained with overexpressed Hs-AFP1 on fungal inhibition in this study merits further investigations into the use of this peptide in the engineering of disease-resistant crops. / AFRIKAANSE OPSOMMING: Saadontkieming is die mees vatbare tyd vir siekteontwikkeling gedurende 'n plant se lewenssiklus. Die saadhuid bars en die vogtige grondkondisies bevoordeel nie net saadontkieming nie, maar ook die groei en ontwikkeling van plantpatogene. Infeksie van plantsade tydens ontkieming is egter die uitsondering eerder as die reël. Plantsade besit komplekse -veraeaigingsfueganfsmes-reen moontlike - patoqeeninteksies. Die meqanismes sluit die produksie van antifungiese agense, wat tydens saadontkieming na die omliggende omgewing diffundeer om 'n beskermende sone om die ontkiemende saad te vorm, in. Die gevolglike antifungiese sone beskerm die saad teen infeksie deur bakterieë en swamme. Gedurende die laaste dekade het navorsers baie aandag aan die bestudering van plantsaadverdedigingsmeganismes gegee. Dié kennis word gebruik om die verdedigingsmeganismes beter te verstaan, asook om dié meganismes te manipuleer en/of oor te dra aan plantspesies met inherente swak weerstandsmeganismes wat gereeld aan plantpatogeeninfeksies onderhewig is. Navorsing op plantsade het tot die isolasie van verskeie chemiese agense, peptiede en proteïene, wat sterk in vitro aktiwiteite teen 'n wye reeks swampatogene vertoon, gelei. Die vermoë van dié agense om swamme in 'n in vitro omgewing te inhibeer, is alleen egter nie 'n bewys dat hulle 'n rol in plantverdeging speel nie. Studies waar mutante gebruik word, is gewens om addisionele bewys te lewer dat die substanse 'n rol in plantverdediging vervul. Sodanige mutante sluit plantlyne, waarin die geen van belang gemuteer is of ooruitgedruk word om so die rol van die geen in 'n in planta omgewing te bepaal in. In hierdie toepassings het genetiese transformasie en die daarstelling van transgeniese plante 'n ongeëwenaarde bydrae gelewer. In dié studie is die antifungiese peptied, Hs-AFP1, wat aan die peptiedgroep van plant- "defensins" behoort en van Heuchera sanguine a afkomstig is, in 'n heteroloë in planta omgewing geëvalueer as 'n verdedigingspeptied met die potensiaal om in die generering van transgeniese siektebestande gewasse gebruik te word. Die antifungiese aktiwiteit van Hs-AFP1 is teen Botrytis cinerea in 'n in vitro reaksie geëvalueer, waar die toediening van 8 ,",g/mlgesuiwerde Hs-AFP1 peptied aanleiding gegee het tot 'n 88% afname in hifegroei van B. cinerea. Hipervertakkings van swamhifes, 'n kenmerkende eienskap van Hs-AFP1 aktiwiteit, kon duidelik waargeneem word. Tabakplante is voorts getransformeer met 'n konstruk, pFAJ3068, wat die koderende geen van Hs-AFP1 onder die sterk konstitutiewe CaMV 35S promotor bevat het. Die peptied is met behulp van die seinpeptied wat afkomstig is van die Mirabilis jalapa antimikrobiese peptied, Mj-AMP2, na die apoplastiese omgewing geteiken. Voorheen is gerapporteer dat transgeniese peptiede in die heteroloë omgewing soms onstabiel is. Dit het gelei tot die generering van twee addisionele konstrukte om die moontlikheid van peptiedonstabiliteit te ondervang. 'n Stabiliseringskonstruk, pHs-EXG1, bestaande uit In versmelting tussen Hs-AFP1 en In antifungiese eksoglukanase van Saccharomyces cerevisiae, gekodeer deur EXG1, is in tabakplante getransformeer. In Kontrolekonstruk, pMj-EXG1, met die EXG1-geen saam met die Mj-AMP2-seinpeptied, is ook voorberei en in tabakplante getransformeer. Dit is gebruik om die antifungiese aktiwiteit van die eksoglukanase in die antifungiese aktiwiteitstoetse van die stabiliseringskonstruk te kwantifiseer en te normaliseer. Tabak is suksesvol met pFAJ3068, pHs-EXG1 en pMj-EXG1 getransformeer, wat onderskeidelik gelei het tot die sogenaamde THs, THE en TME transgeniese tabaklyne. Transgeentranskripsie en -translasie in die THs en THE tabaklyne is onderskeidelik deur Noordelike- en Westelike-kladanalises bevestig. Die aktiewe uitdrukking van Hs-AFP1 het die vermoë van tabakplante om B. cinerea infeksies te weerstaan, met tot 42% verhoog in vergelyking met ongetransformeerde kontrole tabakplante tydens 'n in planta siekteweerstandstoets. Totale proteïenekstrakte van THs tabaklyne is voorts ook in In in vitro inhibisietoets geëvalueer, wat gelei het tot resultate wat goed met dié van die in planta toetse ooreenstem. Die totale proteïenekstrakte het swamgroei met 40% geïnhibeer en die kenmerkende hipervertakking van Hs-AFP1-aktiwiteit is ook mikroskopies waargeneem. Resultate wat verkry is vanaf al die analises wat op die transgeniese THs tabaklyne uitgevoer is, het aangedui dat Hs-AFP1 baie stabiel in die heteroloë tabakomgewing is en peptiedstabiliteit was dus nie In probleem, soos verwag is nie. Die fusie tussen Hs-AFP1 en EXG1 het dus nie die stabiliteit van die reeds stabiele Hs-AFP1 peptied verder verbeter nie, maar het wel tot die verlies van Hs-AFP1 aktiwiteit gelei. Die antifungiese analises van die THE tabaklyne het verder bevestig dat dié lyne selfs swakker inhibisie van B. cinereainfeksies tot gevolg gehad het, as ongetransformeerde tabakplante. Mikroskopiese analises van totale THE proteïenekstrakte het voorts ook geen kenmerkende hipervertakkings in die swamhifes vertoon nie, wat alles daarop dui dat die Hs-AFP1-deel van die fusieproteïen as gevolg van die fusie met EXG1 geïnaktiveer is. Dié resultaat is in lyn met vorige navorsing, wat getoon het dat In ander peptied, sarcotoxin 1A, sy antifungiese aktiwiteit verloor indien dit met In verklikkergeen versmelt word. Alhoewel integrasie van die pMj-EXG1-konstruk in die TME-tabaklyne bevestig is, kon geen mRNA met Noordelike-klad- of trutranskriptase-PKR (RT-PKR)-analises waargeneem word nie. Die TME plant het ook geen antifungiese aktiwiteit in in vitro toetse getoon nie en dit het geblyk dat die pMj-EXG1-konstruk aan geenafskakeling in die heteroloë tabakomgewing onderworpe was. Dié afskakelingseffek is egter in die THE plante oorkom, aangesien laasgenoemde sterk EXG1 proteïenaktiwiteit met J3-glukosidase aktiwiteitstoetse vertoon het. Alhoewel die THE plante nie die stabiliteit van Hs-AFP1 verbeter het nie, het dit onwerwags tot die stabilisering van EXG1 in In heteroloë omgewing gelei. Versmeltingstegnologie kan dus moontlik gebruik word as 'n strategie om ander glukanases, wat bekend is vir geenafskakeling in transgeniese omgewings, heteroloog uit te druk. In die geheel gesien, het dié studie getoon dat Hs-AFP1 'n onbetwiste rol in plantverdedigingsmeganismes speel en daar is voorts ook meer kennis oor die stabiliteit van peptiede in 'n heteraloë plantomgewing ingewin. Die positiewe resultate t.o.v. die verhoogde siekteweerstand in die transgeniese THs plantlyne regverdig ook die verdere bestudering van dié peptied om transgeniese siekteweerstand in gewasse te bewerkstellig.

D.Sc. (Botany) / Please refer to full text to view abstract

[Summary]: The larvae of Anomala opacicollis (Pér), of Melolonthid and of other Rutelid species, attack tobacco in Southern Rhodesia, and are more commonly called "whitegrubs". Whitegrubs are widespread in the sandveld areas where tobacco is grown and, at the Trelawney Station of the Tobacco Research Board of Southern Rhodesia, where this work was done, A. opacicollis was the predominant species. An account o£ the one year life cycle is given. The adults eat the leaves of various indigenous trees and an account of an experiment on the food preferences of A. opacicollis adults is given, and the main food sources in the Trelawney area are listed. An experiment is described which shows that the beetles prefer to lay their eggs in the veld or in manured broken land rather than in normal ploughed lands. The larvae are erratically distributed in lands and experiments on larval movement suggest that this is mainly due to concentration of the larvae at discrete concentrations of organic matter in the soil. The temperature and moisture condition of the top 3" of soil are found to be those which attact the larvae. It is suggested that tho larvae prefer and move to soil which contains the lowest amount of moisture which keeps the soil air spaces saturated. lt appears that the preferred temperature decreases with increasing soil moisture content and it is suggested that this is due to the respiratory requirements of the larvae. Soil pH, compaction and fertiliser content and the presence of plants do not appear to influence larval movements. A theory is developed concerning the mechanism of movements in the soil and it is suggested that, in the absence of a continuous gradient, the speed but not the direction of movement is influenced by conditions to which the larvae are sensitive. Evidence in support of thie theory is given. Studies on the survival of larvae at different soil moisture contents and temperatures show that conditions in the top 3" of soil are not likely to be lethal in spite of the high temperatures and low moisture contents found there. The relation of whitegrub behaviour to agricultural problems is discussed. Other workers published evidence indicating that early ploughed lands were attractive to whitegrubs; this is shown not to be the case and the previous evidence is re-interpreted. Such lands tend to contain concentrations of whitegrubs round the borders. The reason for this is discussed and it is suggested that lands should be ploughed early to confine whitegrub infestations in this manner. Soils low in fertility through repeated cultivation are commonly believed to contain more whitegrubs than virgin soil. However, behaviour studies suggest that it is the activity and not the size of the population which is affected by soil fertility. The time at which tobacco is planted is known to determine the extent of subsequent whitegrub damage. The reason for this is to be found in the life cycle of whitegrubs and the results of an experiment on time of planting in relation to whitegrub damage are given. Some insecticides are shown to be more repellant than others to A. opacicollis larvae and the influence of this fact on the assessment of soil insecticide effectiveness in the field is discussed. Reference is made to a method developed by the writer for the determination of insecticide effectiveness under the conditions of erratic whitegrub distribution which normally occur.