No description available.

Thesis (PhD(Agric) (Horticulture))--University of Stellenbosch, 2005. / Bitter pit fruit in commercial consignments of apples still poses an economic threat to exporters from South Africa. Bitter pit develops pre-harvest, but gets progressively worse during storage and is only traceable once the lesions appear after storage. Accurate, early indications of bitter pit incidence will allow for remedial pre-harvest measures in the field, e.g. Ca foliar applications, to reduce the potential losses. Similarly, the automatic detection of a bitter pit fruit during packing will reduce financial losses by identifying unacceptable fruit before shipping. Fluorescence imaging is a fast, non-destructive technique, able to evaluate numerous fruits individually. Results of pre-harvest imaging on apples to identify fruit susceptible to bitter pit showed that pitted fruit were correctly classified, but misclassification of non-pitted fruit with fluorescence imaging was still too high. NIR-spectroscopy point meter readings could distinguish visible bitter pit lesions from healthy tissue. Important wavelengths associated with visible bitter pit were identified. This technique could also identify immature apples, more prone to bitter pit development. It could however not distinguish between bitter pit and non-pitted fruit when applied randomly on the calyx end of apples at harvest. Pre-harvest foliar applications to increase fruit Ca content and reduce bitter pit incidence, is a standard practice world wide. External Ca uptake by fruit was monitored to determine the efficacy of applications during different stages of fruit development. Two periods of efficient uptake of external Ca were identified, viz., cell division and the last few weeks before harvest. Foliar Ca applications from 40 days after full bloom were more effective in increasing fruit Ca content and reducing bitter pit incidence than at 80 days after full bloom, which was recommended previously. Mineral analysis of fruit has been used with variable success to predict bitter pit prior to harvest. The possibility of increasing the accuracy of existing predictive models by using analysis of individual fruit rather than pooled samples, was investigated. By improving the normality of different mineral distributions and decreasing the overlap between pitted and non-pitted fruit classes, it was attempted to improve the reliability of predictions based on variable threshold values. The Ca distribution showed a variation between pitted and nonpitted classes, but still a significant overlap between classes reduced the accuracy of the predictive capacity of this distribution. Even though our results produced a correct classification of 85% for non-pitted fruit, which can be useful, this was still below the required tolerance, of less that 2%, expected on the market. The effect of pruning and fruit bearing position on two-year-old wood on dry mass and Ca allocation of fruit was determined. ‘Golden Delicious’ fruit set was the lowest at the basal bearing position compared to the other positions evaluated and was contrary to expectations. Fruit in a terminal bearing position was superior to the basal position regarding total dry weight and fruit size. Distal wood possibly inhibited growth and set on the basal position via auxin distribution. Ca allocation differed between seasons and cultivars and could either be influenced by bearing position or presence or absence of re-growth.

Thesis (MSc)--University of Stellenbosch, 2001. / ENGLISH ABSTRACT: Phytophthora cactorum (Lebert & Cohn) Schrot. is the primary cause of crown, collar and root rot diseases of apple (Malus domestica Borkh.) trees worldwide. This pathogen is most destructive in commercial apple orchards under waterlogged soil conditions and has recently been identified as causing serious disease in some South African apple orchards. Crown, collar and root diseases are difficult to control because of their unpredictability and catastrophic nature. The use of resistant cultivars and rootstocks is economical and environmentally considerate. Therefore the need to develop screening techniques that will enable the selection of desirable disease resistant traits as part of an apple-breeding program in South Africa was identified. The work undertaken in this study was aimed at optimizing different techniques to test resistance. Using two direct inoculation techniques (excised stem and intact stem) the aggressiveness of lO isolates of P. cactorum on apple rootstocks was determined. The susceptibilities of five apple rootstocks were also compared. Results have shown isolate by rootstock interaction which means isolate aggressiveness was influenced by rootstocks tested. The selectivity of isolates suggests that there may be several strains of the pathogen. Population studies of the pathogen might contribute valuable information that could lead to better interpretation of results. Rootstock susceptibility was monitored in vitro throughout the season by inoculating at monthly intervals for 26-months. It was observed that during winter, rootstock susceptibility was low compared to high susceptibility during summer. These results have revealed new information regarding changes in the relative resistance of the different rootstocks over the growing season, e.g. the susceptibility pattern of rootstock MMl06 occurred 1 to -2 months later than that of other rootstocks. This finding has important implications on the way in which resistance test results are interpreted, and emphasizes the importance of not relying on point sampling. Furthermore, useful information has been acquired regarding the epidemiology of the disease with regard to "windows of susceptibility". The phenomenon of a phase shift in susceptibility of different rootstocks needs to be tested on a broader scale to assess whether it has any practical application on resistance testing. Although different inoculation techniques are applied in breeding programs, up to now there is no consensus on which technique works best for seedling selections. Since large numbers of individuals must be tested to improve the chances of detecting resistant genotypes, mass inoculations of young seedlings is a rapid way of identifying resistant individuals. Two different screening methods were tested during this study. Using the sand-bran technique, seedlings were transplanted onto inoculated soil and the root mass was used as a measure of resistance. In a second method zoospore inoculum was applied to seedlings growing in a sand:bark mixture at different concentrations and the seedlings were subjected either to water drenching or not. In both trials the aggressiveness of isolates differed significantly from each other and only higher inoculum concentrations were effective in causing disease. The age of seedlings used in tests emerged as an important factor. Seedlings under five-months-old should not be used. Drenching inoculated seedlings enhanced disease development but the production of sufficiently high numbers of zoospores was a laborious task. Thus, it is recommended that the sand-bran inoculum technique be tested with the drenching treatment for mass selection. In conclusion this study confirms the importance of both choice of isolate and choice of inoculation intervals in determining susceptibility of rootstocks to infection. In spite of the fact that stem inoculation bioassays have limited resemblance to natural disease situations, these bioassays are useful for obtaining an indication as to whether genotypes have a degree of resistance and merit further testing. For this reason refinement of the stem inoculation bioassay is worthwhile pursuing. With regard to seedling trials, both the sand-bran and the zoospore technique appear promising but refinement of these techniques is necessary in order to present a more practical way of testing large volumes of seedlings. / AFRIKAANSE OPSOMMING: Evaluering van inokulasietegnieke om weerstand teen Phytophthora cactorum in appels te evalueer: Phytophthora cactorum (Lebert & Cohn) Schrot. is die primêre oorsaak van kroon-, kraag en wortelvrot van appelbome (Malus domestica Borkh.). Dit is die mees verwoestende patogeen in kommersiële appelboorde waar daar versuipte toestande grond voorkom. P. cactorum is onlangs identifiseer as die patogeen wat ernstige kroon- en kraag-verotting in Suid Afrikaanse appelboorde veroorsaak. Kroon-, kraag- en wortelvrot is moeilik om te beheer as gevolg van die onvoorspelbaarheid en rampspoedige aard van die siekte. Die gebruik van kultivars en onderstamme wat weerstandbiedend is teen siektes en plae is omgewingsvriendelik en is ekonomies van belang, dus het die behoefte ontstaan om inokulasietegnieke te ontwikkelom weerstandige saailinge te identifiseer en te selekteer as deel van 'n appelteelprogram in Suid Afrika. Die doelwit van hierdie studie is om verskillende inokulasietegnieke te toets en te verfyn om weerstand in appelsaailinge te identifiseer. Deur gebruik te maak van twee inokulasietegnieke (die afgesnyde loot- en intakte loot tegniek), is die relatiewe aggressiwiteit van 10 isolate van P. cactorum en die vatbaarheid van vyf appelonderstamme ondersoek. Resultate het aangetoon dat die aggressiwiteit van die isolate gevarieer het na aanleiding van die onderstam wat getoets is. Die selektiwiteit van die isolate is 'n aanduiding dat daar moontlik verskeie rasse van die patogeen voorkom. Toekomstige studies op die populasiestruktuur van P. cactorum sal 'n belangrike bydrae maak tot die interpretasie van resultate oor weerstand en weerstandsteling. Die vatbaarheid van onderstamme was ook in in vitro proewe ondersoek deur maandelikse inokulasies toe te pas oor 'n tydperk van 26 maande. Dit is opgemerk dat die onderstamvatbaarheid gedurende die winter laag was in vergelyking met die somer. Nie al die onderstamme het dieselfe gereageer gedurende verskillende toetstye nie. Hierdie resultate toon aan dat die relatiewe weerstand van verskillende onderstamme oor die groeiseisoen verskil, byvoorbeeld die vatbare reaksie van die onderstam 'l\.1MI06' het een tot twee maande later voorgekom in vergelyking met ander onderstamme wat getoets is. Hierdie bevinding het belangrike implikasies op die interpretasie van weerstandstoetsing en beklemtoon die moontlike tekortkominge in enkelproefwaarnemings. Bruikbare inligting ten opsigte van die epidemiologie van die siekte is versamel wat beskryf kan word in terme van vensters van vatbaarheid wat verskil van onderstam tot onderstam. Verdere ondersoeke in die verband word aanbeveel. Hoewel verskeie inokulasietegnieke bestaan om jong saailinge vir weerstand te toets, is daar tot op hierdie stadium nog nie ooreenstemming oor die beste tegniek wat toegepas moet word om saailingseleksie te doen nie. Omdat groot getalle saailinge getoets moet tydens die seleksieproses sal massa-inokulasie van saailinge die aangewese metode wees. Twee verskillende inokulasie tegnieke is getoets in die studie. Deur gebruik te maak van die sandsemel tegniek, is saailinge geplant in geinfesteerde plantmedium, waartydens die wortelmassa van saailinge gebruik is om die reaksie op infeksie te kwantifiseer. Die soëspoor inokulasietegniek was toegepas op saailinge wat in 'n sand en basmengsel geplant is teen verskillende inokulurnkonsentrasies. 'n Waterverdrenkingsbehandeling is ook getoets. In albei hierdie proewe het die aggressiwiteit van die isolate van mekaar verskil. Slegs die hoër inokulumkonsentrasies was effektief in die ontwikkeling van die siekte. Die ouderdom van saailinge is ook uitgewys as 'n belangrike faktor wat 'n rol speel in weerstandstoetsing. Saailinge jonger as 5 maande word nie aanbeveel vir hierdie toetse nie. Verdrenking van saailinge het die voorkoms van die siekte verhoog, maar die produksie van groot getalle soëspore was 'n beperkende faktor in die uitvoering van die proef Dit word aanbeveel dat die sand-semel inokulasietegniek verder evalueer moet word onder verskeie toestande, onder andere deur dit met verdrenkinghte kombineer. Die belang van die keuse van isolaat en inokulasiedatum in bepaling van relatiewe weerstand van onderstamme teen P. cactorum is tydens die studie bevestig. Afgesien van die beperking van die staminokulasietegnieke in soverre dit verwyderd is van natuurlike infeksie, word die tegnieke aanbeveel om 'n indikasie te kry van die relatiewe weerstand van onderstamme. Beide die sand-semel en soëspoor tegnieke kan gebruik word om weerstandige saailinge te identifiseer, maar tegniese verfyning van hierdie tegnieke is nodig om saailinge in massa te evalueer.

Dissertation (PhD)--University of Stellenbosch, 2001. / ENGLISH ABSTRACT: The basic biology of Eriosoma lanigerum (Hausmann) and its natural enemy, Aphelinus mali (Haldeman), was investigated in the Western Cape Province of South Africa. The first instar nymph can be identified by the absence of cornicles and the adult female by the presence of the vulva. Body length and distance between cornicles can be used to distinguish between the 2nd , 3rd and 4th instars. The development of E. lanigerum was negatively influenced by temperatures above 27DC. The net replacement rate (Ro) and intrinsic rate of increase (rm) peaked at 20DC. The theoretical lower and upper threshold temperatures for development were estimated at 4.48DC and 28.07DC, respectively. Crawlers of E. lanigerum migrated from the roots up into the trees during spnng to start new infestations. Colonies became visible from December and maximum colony numbers were reached from the end of summer until autumn, at which time most of the colonies were parasitised by A. mali. Colony numbers declined at the end of autumn after high parasitism and the appearance of winged E. lanigerum. The aphid overwintered on the tree. Chemical sprays, rain during spring and high temperature influenced population numbers negatively. E. lanigerum can be monitored by counting the unparasitised colonies in leaf axils of half of each of 25 trees per 2 hectare plot with 5 unparasitised colonies as the economic threshold. Sampling error was high at 40% but increasing the number of trees did not reduce it. Presence-absence sampling, which will reduce the time required for monitoring, did not seriously compromise the reliability of decisions regarding the necessity for intervention. Chemicals containing nitrogen usually sprayed for bitterpit control stimulated the settling of E. lanigerum crawlers on Granny Smith trees, while fruit weevil barriers for the control of Phlyctinus callos us Boh. limited crawler movement into the trees but did not prevent colonisation. All the postembryonic developmental stages of E. lanigerum were parasitised by A. mali. Complete parasitism of the population was never reached as younger instars sheltered under other aphids and mummified aphids. Development of A. mali was not influenced negatively by high temperatures. The minimum developmental temperature and number of degree days needed for development of the larval stage and emergence of the adult from the mummy were .6.72°C and 172.41°D, and 10.27°C and 109.89°D respectively. Mummies collected during early winter survived long periods of cold storage in postdiapause. The rrummum threshold temperature for postdiapause development of A. mali was ·10.15°C. Most chemicals tested against A. mali adults were highly toxic to the wasp, except endosulfan and two growth regulators, flufenoxuron and fenoxycarb. The mortality of adults exposed to the fungicides tested was low within the first 24 hours. The percentage emergence from the mummies was high for all chemicals tested, but more than 60% of the adults died soon after emergence from mummies treated with chlorpyrifos. Nearly 30% of the adults died soon after they emerged from carbaryl (XLR-Plus) and fenthion treated mummies. The growth regulators, flufenoxuron and fenoxycarb, did not influence fecundity adversely. / AFRIKAANSE OPSOMMING: Die basiese biologie van Enosoma lanigerum (Hausmann) en sy natuurlike vyand, Aphelinus mali (Haldeman), in die Weskaap Provinsie van Suid-Afrika isondersoek. Die eerste instar nimf kan aan die afwesigheid van kornikels en die volwassenes aan die teenwoordigheid van die vulva uitgeken word. Die liggaamslengte en afstand tussen die kornikels kan gebruik word om tussen instar 2, 3 en 4 te onderskei. Die ontwikkeling van E. lanigerum word nadelig deur temperature bo 27°C beïnvloed. Die netto vervangingstempo (Ro) en intrinsieke tempo van toename (rm) was die hoogste by 20°C. Die teoretiese minimum en maksimum temperatuur drempelwaardes vir ontwikkeling was 4.48°C en 28.07°C onderskeidelik. In die lente beweeg E. lanigerum kruipers op vanaf die wortels tot in die bome om nuwe kolonies te begin. Kolonies is sigbaar vanaf Desember en die hoogste koloniegetalle word aan die einde van die somer tot die herfs bereik, wanneer die meeste van die kolonies dan ook deur A. mali geparasiteer word. Teen laat-herfs neem koloniegetalle af as gevolg van hoë parasitisme en die verskyning van gevleuelde E. ,lanigerum. E. lanigerum oorwinter op die appelboom. Chemiese behandelings, reën gedurende die lente en hoë temperatuur beïnvloed koloniegetalle nadelig. E. lanigerum kan deur die aantal ongeparasiteerde kolonies in die blaaroksels van die helfte van 25 bome per 2 hektaar blok te tel, met 5 ongeparasiteerde kolonies as die ekonomiese drempelwaarde, gemonitor word. Die steekproefnemingsfout was hoog (40%), maar kon nie verminder word deur die aantal bome wat gemonitor is te verhoog nie. Steekproefneming, vir aan- of afwesigheid van kolonies, wat monitortyd sal verminder, het min invloed op die betroubaarheid van besluitnemings oor die noodsaaklikheid van bespuitings gehad. Stikstofbevattende chemikalieë, wat vir die beheer van bitterpit gespuit word, stimuleer vestiging van E. lanigerum kruipers op Granny Smith bome, terwyl snuitkewerversperrings, vir die beheer van Phlyctinus callosus Boh., die opwaartse beweging van kruipers in die bome beperk, maar nie kolonievorming van E. lanigerum verhoed nie. Alle postembrioniese ontwikkelingstadiums van E. lanigerum is deur A. mali geparasiteer. Totale parasitisme is nooit bereik nie, omdat jonger instars onder ander bloedluise en gemummifiseerde bloedluise skuil. Die ontwikkeling van A. mali word nie deur hoë temperature benadeel nie. Die minimum ontwikkelingstemperatuur en graaddae, nodig vir ontwikkeling van die larwale stadium en die verskyning van die volwassene uit die mummie, was 6.72°C met 172.4loD en 10.27°C met 109.89°D, onderskeidelik. Mummies wat vroeg in die winter versamel is, het lang periodes van koelopberging oorleef. Die minimum temperatuur drempelwaarde vir A. mali ontwikkeling in postdiapouse was 10.15°C. Die meeste van die chemikalieë wat getoets is, was hoogs toksies vir die volwasse wesp, behalwe endosulfan en die twee groeireguleerders, flufenoxuron en fenoxycarb. Die mortaliteit van volwassenes wat aan swamdoders blootgestel is, was laag binne die eerste 24 uur na blootstelling. Die persentasie uitkoms vanuit mummies was hoog vir al die chemikalieë wat getoets is, maar met chlorpyrifos het 60% van die volwassenes net na uitkoms doodgegaan. Ongeveer 30% van die volwassenes is dood na verskyning vanuit mummies wat met carbaryl (XLR-Plus) en fenthion gespuit is. Die groeireguleerders, flufenoxuron en fenoxycarb, het nie die vrugbaarheid van die parasiet merkbaar beïnvloed nie.

Computer decision aids can help integrate and apply diverse sources of information and expertise to problems of integrated pest management (IPM) in agriculture and forestry. AppleMgr combines a rule-based expert system with databases and spreadsheets in a prototype decision aid intended to be expanded and modified for use by extension workers in the Northwest U.S. The program requires an IBMcompatible microcomputer with hard disk. AppleMgr concentrates on the two most important insect pests on apple in the Northwest--codling moth, Cydia, pomonella (L.), and San Jose scale, Quadraspidiotus perniciosus Comstock, and on phytophagous mites, whose control largely depends on predators. The primary goal of AppleMgr is to demonstrate an improved process of decision making in apple IPM. AppleMgr is composed of modules for diagnosis of pest injury, identification of pest and natural enemy specimens, and management. The first two modules arrive at conclusions through backward-chaining inference from user observations. The management module uses backward chaining supplemented with external calculation programs to find the net benefit of a pesticide application. A method is included to predict yield and fruit size from crop samples. Cullage from codling moth and San Jose scale, mite effect on fruit size, probability of biological mite control and pesticide efficacy are predicted from researchers' data and estimates. Selected relative beneficial and adverse side effects of pesticides are presented in spreadsheets. An analysis of packing house records for apple crops from eight orchards at three yields using 1987 and 1988 prices and packing charges showed that net crop value varied by up to $8000 per acre. The variability in crop value and the importance of adverse side effects of pesticides suggest that the commonly-used "action thresholds" for treatment are seriously inadequate. AppleMgr may point the way toward more dynamic and realistic methods of IPM decision making. / Graduation date: 1990

Thesis (PhD(Agric))--Stellenbosch University, 2014. / ENGLISH ABSTRACT: The apple industry in South Africa currently relies entirely on chemical fungicides to control apple scab, caused by Venturia inaequalis. In this dissertation, alterative management strategies against V. inaequalis were tested for the first time in South Africa. New information on the behaviour of the sexual winter phase of V. inaequalis in different climatic conditions was found and sources of asexual inoculum overwintering in apple orchards were identified. The effect of leaf shredding on fruit and leaf scab incidence and severity was tested against a non-shredded, non-sprayed negative control, a positive control that followed a commercial fungicide programme and a combined treatment of a commercial fungicide programme with leaf shredding, from 2010 to 2013. Reductions in fruit and leaf scab incidence and severity in the leaf shredding treatment were significantly lower compared to the negative control. Quantitative real-time polymerase chain reaction (qPCR) of airborne ascospores trapped using volumetric spore traps was used to measure the reduction in airborne ascospores in the shredded plots, and confirmed the efficacy of shredding found by comparing scab incidence and severity on fruit and leaves. Shredding twice during leaf-drop increased the efficacy of the treatment. Results indicate that leaf shredding should be integrated into scab management strategies in future. However, practical considerations unique to South African orchards, e.g. timing of leaf shredding relative to leaf-drop and orchard layouts, need to be addressed. Pseudothecial densities (PD, number of pseudothecia per fertile lesion) and ascal densities (AD, number of asci per pseudothecium) were compared between in Koue Bokkeveld (KB), a cold winter region, and Elgin (EL), a warm winter region experiencing climate warming, in 2012 and 2013. Scabbed leaves were detached during leaf-drop and overwintered in their region of origin and in the other region. The PD in leaves collected in KB and overwintered in KB was significantly higher than for leaves collected in EL and overwintered in EL, and leaves collected in KB and overwintered in EL. These results agreed with what was expected, as temperature during pseudothecial formation (i.e. the first four weeks after leaf-drop) was significantly lower in KB than in EL. However, the PD for leaves collected in EL and overwintered in EL did not differ significantly from EL leaves overwintered in KB. AD values in all treatments did not differ significantly from one another. Results suggest that factors other than temperature may be involved in controlling PD, e.g. the EL population may include strains not present in the KB population, with higher optimal temperatures for pseudothecial formation. Apple buds and pygmy apples were collected and tested for presence, number and viability of conidia in 2010, 2011 and 2012. Pygmy apples are small, late season fruit that remain attached to the tree throughout winter, especially in regions with warmer winters where trees do not experience sufficient chilling to complete dormancy. High conidial numbers were found on outer bud tissue and low numbers on inner bud tissue, but viable conidia were only found on inner bud tissue, using microscopy, and generally in orchards with high scab levels in the previous season. Molecular methods using PCR-RFLP and qPCR confirmed the presence of high amounts of V. inaequalis DNA in outer bud tissues, although calculated conidial amounts were higher than data obtained when using microscopy, which could indicate presence of mycelia not detected during microscopic examination. Higher numbers of conidia with higher percentage viability were found on pygmy apples, which are a more likely source of asexual inoculum in South African apple orchards than the low number of viable conidia on inner bud tissue. / AFRIKAANSE OPSOMMING: Die Suid-Afrikaanse appelbedryf is tans afhanklik van chemiese swamdoders vir die beheer van die appelskurf patogeen, Venturia inaequalis. In hierdie proefskrif is alternatiewe bestuurstrategiëe vir die eerste keer in Suid-Afrika ondersoek. Nuwe inligting te opsigte van die gedrag van die geslagtelike winterfase van V. inaequalis, is onder verskillende klimaatstoestande ingewin en bronne van die oorwinterende ongeslagtelike inokulum in appelboorde, is identifiseer. Die invloed van blaarversnippering op die voorkoms en erns van appelskurf op vrugte en blare, is vanaf 2010 tot 2013 ondersoek en met ʼn negatiewe kontrole (onversnipperde blare sonder spuitprogram), ʼn positiewe kontrole (ʼn kommersiële swamdoderspuitprogram is gevolg) en gekombineerde behandelings (kommersiële swamdoderspuitprogram en blaarversnippering) vergelyk. Daar was ʼn betekenisvolle verskil in die voorkoms en erns van skurf op vrugte en blare met blaarversnippering teenoor die negatiewe kontrole. Kwantitatiewe intydse polimerase kettingvermeerderingsreaksie (kPKR) van luggedraagde askospore, vasgevang in volumetriese lokvalle, is gebruik om die afname van luggedraagde askospore in versnipperde behandelings te meet. Die doeltreffendheid van versnippering as behandeling, is bevestig deur die voorkoms van appelskurf te vergelyk met die ernstigheidsgraad daarvan op vrugte en blare. Die uitvoer van blaarversnippering twee keer gedurende die blaarvalperiode het die effektiwiteit van hierdie behandeling verhoog. Hiervan kan dus afgelei word dat blaarversnippering voordelig sal wees vir die bestuur van appelskurf en in toekomstige bestuurspraktyke ingesluit moet word. Praktiese oorwegings, uniek aan Suid-Afrikaanse boorde, soos boorduitleg en die tydsberekening van blaarversnippering teenoor blaarval, moet egter in ag geneem word. Pseudothesiale digtheid (PD; die aantal pseudothesia per vrugbare letsel) en askale digtheid (AD; die aantal aski per pseudothesium) is gedurende 2012 en 2013 vir die Koue Bokkeveld (KB), 'n koue winterstreek, en warm winterstreek Elgin (EL), 'n winterstreek wat klimaatsverwarming ervaar, vergelyk. Blare, met skurf, is gedurende blaarval gepluk en oorwinter in hul gebied van oorsprong, asook in die ander klimaatstreek. Blare wat in KB versamel is en in KB oorwinter het, se PD was aansienlik hoër as dié wat in EL versamel is en in EL oorwinter het, sowel as dié wat in KB versamel is en in EL oorwinter het. Hierdie resultate stem ooreen met wat verwag is, om rede die temperatuur gedurende pseudothesiale vorming, d.w.s. die eerste vier weke na blaarval, aansienlik laer in KB as in EL was. Die PD van blare wat in EL versamel en daar oorwinter het, het egter nie betekenisvol verskil van blare wat in KB oorwinter het nie. Die AD-waardes tussen behandelings verskil nie noemenswaardig nie en word as onbeduidend beskou. Die verkrygde resultate dui aan dat daar ander faktore as temperatuur betrokke is by die beheer van PD, bv. die EL-skurfpopulasie, waar die warmer klimaat meer optimaal is vir pseudothesiale vorming, rasse wat nie in die KB-bevolking teenwoordig is nie, mag insluit. Appelknoppe en dwerg-appels is gedurende 2010, 2011 en 2012 versamel en vir die teenwoordigheid, aantal en lewensvatbaarheid van konidiospore getoets. Dwergappels is klein laatseisoen appeltjies wat reg deur die winter aan die boom bly hang; veral in die streke met warmer winters waar die bome nie die nodige koue ervaar om dormansie te voltooi nie. Met behulp van mikroskopie is ʼn hoë aantal spore op die buitenste knopweefsel en lae getalle in die binneweefsel bespeur; maar lewensvatbare spore is net in die binneweefsel van knoppe waargeneem, wat hoofsaaklik afkomstig is van boorde wat hoë vlakke van appelskurf in die vorige seisoen ervaar het. Molekulêre tegnieke, PKR-RFLP en kPKR, is gebruik vir bepaling van V. inequalis DNA hoeveelhede op die buitenste knopweefsel. Hoër getalle konidiospore is met die molekulêre analise gevind, as dié verkry met mikroskopiese ondersoek en dui op die moontlike teenwoordigheid van miselium wat nie met visuele waarneming sigbaar was nie. Meer konidiospore met 'n hoër vlak van lewensvatbaarheid is op dwerg-apples gevind en dit is moontlik 'n meer waarskynlike bron van ongeslagtelike inokulum in Suid-Afrikaanse appelboorde, as die lae getalle van lewensvatbare konidiospore op die binneweefsel van die appelknoppe.

Thesis (MSc)--Stellenbosch University, 2003. / ENGLISH ABSTRACT: Apple (Malus x domestica Borkh.) is an important horticultural crop worldwide and in the Western Cape. The income generated from apple and other deciduous fruit production amounts to approximately 25% of the gross total value of horticultural production in the Western Cape. Unfortunately diseases and pests adversely affect fruit production in this region. Woolly apple aphids (Eriosoma lanigerum L. (Hausmann» have a significant effect on the apple industry in the Western Cape. Damage caused is two-fold, occurring aerially and terrestrially. Insects colonise the plants, feeding off the phloem sap. Aphid infestation around the root system results in repeated infestation of the foliage as it serves as a reservoir of aphids. In extreme cases, the apple cores are also infested, thus affecting the sale of apples. In 1962, Northern Spy was identified as a woolly apple aphid resistant rootstock and has since then formed the basis for traditional rootstock breeding programmes. The Er1 gene in Northern Spy confers resistance. According to one report, the natural resistance of Northern Spy was overcome in South Africa in 1968, but this was not confirmed in an independent study. The main objectives of this study was to firstly identify molecular markers more closely linked to the woolly apple aphid resistance gene, Er1, than existing markers, by applying AFLP technology to selected seedlings, identified to be resistant by conventional phenotyping. If identified, these markers can be incorporated into existing breeding programmes. Secondly, previously identified RAPD and SCAR markers were tested to determine their applicability in local populations for use in breeding programmes. Ultimately the segregation of the Er1 gene in South African populations can be determined if tightly linked markers are identified. Three families were derived from crosses of each of three resistant genotypes, namely Northern Spy, Rootstock 5 and Russian Seedling and a susceptible cultivar, Braeburn. For the three successive years of the study, each resistant genotype was allowed to cross-pollinate in isolation with the susceptible parent. Two hand-pollinated families, Russian Seedling x Liberty and Russian Seedling x Northern Spy, were also included in the study. The amplified fragment length polymorphism (AFLP) technique was used in an attempt to identify markers in the resistant and susceptible seedlings. No markers were identified using this technique. Known sequence characterised amplified regions (SCAR) and random amplified polymorphic DNA (RAPD) markers were used due to their suitability in marker-assisted selection for woolly apple aphid resistance. Varying results were obtained with these markers and no conclusive information was acquired with regard to the segregation of the Er] gene in any of these rootstocks and crosses. This underlines the need for the development of markers that can readily be applied in local breeding programmes. The identification and integration of such markers will greatly benefit the local and world wide apple industries. / AFRIKAANSE OPSOMMING: Appels (Malus x domestica Borkh.) is wêreldwyd en in die Wes-Kaap 'n belangrike landbougewas. Inkomste gegenereer deur appels en ander sagtevrugte vorm bykans 25% van die bruto inkomste uit vrugte in die Wes-Kaap. Siektes en insekpeste verlaag egter die produksie van vrugte in hierdie streek. Appelbloedluise (Eriosoma lanigerum L. (Hausmann» het 'n groot invloed op appelproduksie in die Wes-Kaap. Skade word bogronds en ondergronds aangerig. Insekte koloniseer die plant en leef op floeëmsap. Besmetting van die wortels lei tot herhaalde besmetting van bogrondse dele aangesien die insekte aanteelop die wortels. In uiterste gevalle word die vrugte geaffekteer, wat vrug-verkope beïnvloed. 'Northern Spy' is in 1962 geïdentifiseer as 'n onderstam met natuurlike weerstand teen appelbloedluis en het vir lank die basis gevorm vir tradisionele telingsprogramme. Weerstand word verleen deur die Erf geen. Volgens een verslag is die natuurlike weerstand van Northern Spy egter in 1968 in Suid-Afrika oorkom, maar dit is nog nie in 'n onafhanklike studie bevestig word nie. Die hoof doelstellings van hierdie studie was om eerstens deur middel van die AFLP tegniek molekulêre merkers te identifiseer wat nouer gekoppel is aan die appelbloedluis weerstandsgeen, En, as bestaande merkers. Hierdie tegniek is toegepas op saailinge wat deur konvensionele fenotipering geselekteer is. Indien merkers suksesvol geïdentifiseer is, kan dit in bestaande telingsprogramme geïntegreer word. Tweedens is bestaande RAPD en SCAR merkers ook getoets om hul toepaslikheid te bepaal vir gebruik in plaaslike teelprogramme. Oplaas sal die segregasie van die Erf geen in Suid- Afrikaanse populasies ook deur middel van nou gekoppelde merkers bepaal kan word. Kruisings van elk van die drie weerstandbiedende genotipes, naamlik 'Northern Spy', 'Rootstock 5' en 'Russian Seedling', en die vatbare kultivar, 'Braeburn' , het drie families daargestel. Elke weerstandbiedende genotipe is toegelaat om in isolasie te kruisbestuif met die vatbare ouer. Twee hand-bestuifde families, 'Russian Seedling' x 'Liberty' en 'Russian Seedling' x 'Northern Spy', is in 'n latere stadium van die studie ingesluit. Die AFLP tegniek is gebruik vir die identifikasie van polimorfiese merkers tussen vatbare en weerstandbiedende populasies. Geen merkers is egter geïdentifiseer nie. Bestaande SCAR en RAPD merkers is vervolgens gebruik om te bepaal of hulle geskik is vir gebruik in merker-bemiddelde seleksie vir appelbloedluis weerstand. Wisselende resultate is verkry ten opsigte van amplifikasie, herhaalbaarheid van resultate was swak en geen onweerlegbare bewyse oor die segregasie van die Erfgeen is bekom nie. Dit beklemtoon die noodsaaklikheid om merkers wat geredelik in plaaslike teelprogramme toegepas kan word, te ontwikkel. Die identifikasie en integrasie van sulke merkers sal die plaaslike en wêreld-wye appel industrieë aansienlik bevoordeel.

Thesis (MScConsEcol (Conservation Ecology and Entomology))--University of Stellenbosch, 2009. / ENGLISH ABSTRACT: Pseudococcus viburni (Signoret) (Hemiptera: Pseudococcidae) (obscure mealybug), is a common and serious pest of apples and pears in South Africa. Consumer and regulatory pressure to produce commodities under sustainable and ecologically compatible conditions has rendered chemical control options increasingly limited. Information on the seasonal occurrence of pests is but one of the vital components of an effective and sustainable integrated pest management system needed for planning the initiation of monitoring and determining when damage can be expected. It is also important to identify which orchards are at risk of developing mealybug infestations while development of effective and early monitoring tools for mealybug populations will help growers in making decisions with regards to pest management and crop suitability for various markets. It is also essential to determine the presence and efficacy of naturally occurring biological control agents in orchards so as to ascertain the potential of biological control as a viable alternative in orchards. However, under the current integrated pest management protocol, it has been difficult to determine this, due to the sporadic and relatively low incidence of mealybug infestations in some orchards, or by simply relying on naturally occurring field populations of biocontrol agents. Knowledge of the environmental conditions under which P. viburni population levels may become destructive is also essential for timing the release of insectary reared natural enemies as well as understanding the population ecology of this pest and its natural enemies. Information was gathered regarding the seasonal phenology of P. viburni and its natural enemies in pome fruit orchards in the Western Cape Province during the 2007/08 and 2008/09 growing seasons. Seasonal population studies showed that P. viburni has multiple overlapping generations with all life stages present throughout the year. The highest orchard infestations occurred during the summer period until early winter (January to early June). This was followed by a decrease in population from late June to November, before another increase in December. Presence-absence sampling of mealybugs on the host plant revealed that woody parts of the tree, such as the trunk and old stems were the most preferred sites for mealybug habitation, due to the availability of protected refuge sites. Migration of mealybug populations to newer growth and the upper sections of the tree crown, such as the new stems, leaves and eventually the fruit, was observed from December throughout the summer period until the early winter in June. Fruit colonization in both apples and pears commenced in January, when the fruit had developed a size sufficient for P. viburni to penetrate and occupy spaces such as the fruit core, calyx and stem end. There was no evidence of P. viburni occurring beneath the soil surface or on the roots of host trees. Two natural enemies of mealybugs, namely Pseudaphycus maculipennis (Mercet) and Coccidoxenoides perminutus (Girault), were found to be active in apple and pear orchards in the Western Cape. However, the status of C. perminutus as a parasite of P. viburni still needs to be verified despite evidence of emergence from P. viburni mummies, which was not sufficient enough to suggest that it is a useful biological control agent. Seasonal abundance trends of the two natural enemies revealed that their lifecycle is synchronized with that of the host. However, there was no evidence of P. maculipennis activity in Ceres. No predators were found during the course of this study. The rate of P. viburni parasitism at harvest was 46.52%, with P. maculipennis and C. perminutus constituting 98.966% and 1.034% of the parasitoids recovered from mealybug mummies, respectively. Studies on the use of pheromone traps as early monitoring tools for P. viburni showed that there was a positive and significant relationship between the fruit infestation and number of P. viburni adult males caught in pheromone-baited traps (r2 = 0.454). The action threshold level was estimated to be 2.5 male P. viburni caught per trap per fortnight at an economic threshold of 2% fruit infestation. / AFRIKAANSE OPSOMMING: Pseudococcus viburni (Signoret) (Hemiptera: Pseudococcidae) (ligrooswitluis), is ‘n algemene en ernstige plaag van appels en pere in Suid-Afrika. Druk deur verbruikers en regulasies om kommoditeite onder volhoubare en ekologies verenigbare toestande te produseer het chemiese beheeropsies toenemend beperk. Inligting oor die seisoenale voorkoms van plae is een van die essensiële komponente van ‘n effektiewe en volhoubare geïntegreerde plaagbestuurprogram. Dit is in die aanvanklike beplanning van monitering en om te bepaal wanneer skade verwag kan word. Dit is ook belangrik om boorde vroegtydig te identifiseer wat die risiko het om witluisbesmettings te ontwikkel. Die ontwikkeling van effektiewe en vroeë moniteringstegnieke vir witluisbevolkings sal produsente help met besluitneming rakende plaagbestuur en die geskiktheid van gewasse vir verskeie markte. Dit is ook noodsaaklik om die teenwoordigheid en effektiwiteit van biologiese beheer agente wat natuurlik in boorde voorkom te bepaal ten einde die potensiaal van biologiese beheer as ‘n lewensvatbare alternatief vas te stel. Onder die huidige geïntegreerde plaagbestuurprotokol was dit egter moeilik om laasgenoemde te bepaal weens die sporadiese en relatiewe lae voorkoms van witluisbesmettings in sommige boorde of deur bloot staat te maak op die veldpopulasies van biologiese beheer agente wat natuurlik voorkom. Kennis van die omgewingstoestande waaronder P. viburni bevolkingsvlakke skadelik raak is ook noodsaaklik vir die beplanning van vrylating van biologiese beheer agente, asook om die bevolkingsekologie van hierdie plaag en sy natuurlike vyande te verstaan. Inligting oor die seisoenale fenologie van P. viburni en sy natuurlike vyande in sagtevrugte boorde in die Westelike Kaapprovinsie is gedurende die 2007/08 en 2008/09 groeiseisoene versamel. Seisoenale bevolkingstudies het getoon dat P. viburni verskeie oorvleuelende generasies het met alle stadia teenwoordig regdeur die jaar.