Methomyl at seven concentrations, 0.051 to 0.204 kg ai/378.5 l water, was evaluated against eggs of pest and beneficial insects and mites associated with apples. The pest eggs studied were: Laspeyresia pomonella, Panonychus ulmi, Platynota spp., aphids, and Argyrotaenia velutinana. For A. velutinana, five rates of methomyl ranging from 0.013 to 0.204 kg ai/378.5 l water were used. Platynota spp., aphid, and A. velutinana eggs were highly susceptible to the material. P. ulmi eggs were not susceptible. Eggs of beneficial syrphid flies, Aphidoletes aphidimyza, Leptothrips mali, and Stethorus punctum were highly susceptible to methomyl. Eggs of Orius insidiosus were moderately susceptible. Chrysopa spp. eggs appeared to be unaffected by the material. LC₅₀ values for methomyl on eggs of Platynota spp. L. pomonella, O. insidiosus, Chrysopa spp., and S. punctum were generated. LC₅₀ values for P. ulmi eggs were unobtainable. Three rates of methomyl, used as an ovicide, were evaluated in the field for control of the Platynota spp. complex on apples compared to two formulations of FMC 45806, permethrin, and Bacillus thuringiensis. Methomyl provided the best control. Seasonal activity of P. flavedana and P. idaeusalis were monitored in reference to degree-day accumulations. Regression equations for male moth flights and egg deposition of the species are presented. Seasonal activity and prospects for development of a phenology model to predict seasonal activity of the species is discussed. / Master of Science

I. Scanning and transmission electron microscopy were used to investigate damage to apple leaves caused by the white apple leafhopper (WALH), Typhlocyba pomaria McAtee. Feeding holes created by WALH were found in the lower epidermis, with no visual exterior evidence of cell injury to the upper epidermis. The feeding holes were located in areas of the leaf with high stomatal density, and were therefore in close proximity to stomata. Groups of cells in the palisade layers were empty, or contained coagulated cell contents; adjacent palisade cells were apparently uninjured. These uninjured cells contained an abundance of starch granules, however, possibly indicating that they were not functioning properly. Spongy mesophyll cells dorsal to the feeding area were left intact as were the epidermal cells ventral to the feeding area. There was no evidence of cell wall collapse from the external view of either epidermis, or from internal leaf views. II. Scanning and transmission electron microscopy were used to investigate damage to apple leaves caused by the white apple leafhopper (WALH), Typhlocyba pomaria McAtee. Feeding holes created by WALH were found in the lower epidermis, with no visual exterior evidence of cell injury to the upper epidermis. The feeding holes were located in areas of the leaf with high stomatal density, and were therefore in close proximity to stomata. Groups of cells in the palisade layers were empty, or contained coagulated cell contents; adjacent palisade cells were apparently uninjured. These uninjured cells contained an abundance of starch granules, however, possibly indicating that they were not functioning properly. Spongy mesophyll cells dorsal to the feeding area were left intact as were the epidermal cells ventral to the feeding area. There was no evidence of cell wall collapse from the external view of either epidermis, or from internal leaf views. III. Two nine-year-old "Campbell Redchief Delicious’/MM.111 apple trees growing on the Virginia Tech Horticulture Research Farm were used to evaluate gas exchange characteristics of leaves injured by the white apple leafhopper (WALH). Two groups of leaves were used for gas exchange measurements. One group of 42 leaves had three leaves each with 0, 1, 2, 3, 5, 7, and 10 WALH per leaf on each of the two trees. This injury corresponded to 0, 12, 21, 28, 53, 70, and 100 cumulative WALH days, respectively. Gas exchange was measured both in the field and in the lab. A second set of leaves was selected which were visually rated as having zero, low, medium or high amounts of WALH damage, corresponding to approximately 0, 3, 7, and more than 10 WALH per leaf. Gas exchange measurements on the selected leaves were made in the field periodically through an entire day. When measured in the lab, photosynthesis (Pn) was reduced and transpiration (Tr) was increased by WALH feeding. Field gas exchange measurements indicated that WALH feeding reduced Pn and increased Tr before 1200 hrs, but no differences in Pn or Tr were measured after 1200 hrs. No explanation for the conflicting results is offered, and clear conclusions about feeding damage could not be made. / Master of Science

Master of Science

Our present knowledge, as indicated by the above data, proves that fire blight is due to a specific organism, a bacterium known in science as Bacillus amylovorus, which manifests itself in the following ways: 1. Blossom blight, due to the bacterial infecting the nectary of the flower and multiplying therein, later passing down into the stem by way of the ovary and pedicel. Apparent by the browning of the flowers, which later become blackened. 2. Twig blight, due to inoculation infections through wounds made by insects or other agencies on the young and tender shoots, blighting from the tips downward, the leaves turning brown and appearing as though scorched by fire. 3. Fruit blight, due to the progress of the bacteria up the pedicel into the pulp of the fruit; appearing as brownish or much darkened areas, later involving the entire fruit. 4. Canker blight, due to the entering of the larger branches by bacteria, conveyed by insect, bird, or mechanical agents, or by the bacillus working back from infected twigs, fruit-spurs, or water-sprouts. These cankers vary in size from barely visible areas to a girdling of the entire limb; they appear roughened and depressed with a distinct line separating the canker from the apparently healthy tissue. They are called "hold-over cankers." 5. Collar blight is nothing more than canker blight which attacks the base of the trunk usually through an infected water-sprout, borers, or mechanical injury. Trees afflicted with collar blight soon take on the sickly appearance of half starved trees, prematurely defoliating (partially or wholly), and finally dying. 6. Leaf blight, due largely to insect injury, the majority of infections the margin, either lateral or terminal, although central lesions are found. The blighted portion of the leaf being a light or yellowish brown with a faint purplish border at the advancing edge, which when active shows a narrow watery zone. During the warm sunny days soon after rains, when the trees are in blossom and the pollen gathering insects are busy visiting the flowers, and the hold-over cankers are exuding drops of the blight bacterial, fire blight begins its havoc. Insects come in contact with this gummy substance and later visit the blossoms, thereby inoculating the flowers with the active organism, blossom blight resulting. Bees are awarded first place in spreading the blight to the flowers but flies and all other insects which visit the bloom are also liable to spread the germ. Later on in the season, as long as the tree is in a vigorous stage of growth, the aphids, ants, leaf hoppers, borers, beetles, and in fact any of the biting or sucking insects which inhabit the apple tree, may spread the organism to the succulent tissue, causing twig blight, fruit blight, leaf blight, or body blight. It is a fact that the insect, which in one section of the country is most troublesome in disseminating blight-bacillus, may not be so important an agent in another section. Different varieties of apple trees may also vary in different sections as to their powers of resistance or susceptibility; the York Imperial being listed as a susceptible variety in Pennsylvania and Virginia while in West Virginia it seems to be quite resistant. We cannot control weather conditions which play so important a part in the spread of blight. The weather may put a tree in a responsive or susceptible condition to inoculation and may also control insect life to a great extent. And it is indeed difficult to try to "harden up" a tree by the application of phosphate and potash fertilizers and by the discontinuing of cultivation, especially when the soil is strong and rains are frequent. Hence the more practical methods must be followed: 1. Fight the insects which spread the disease. Spray with nicotine sulphate or some other tobacco extracts in combination with the early scab or codling moth sprays. Where "Black leaf 40" is used, the recommended strength is 1 to 800 parts of water. 2. Cut out the source of infection,- the hold over cankers--cutting back an inch or two into healthy bark, and disinfect the wound with bichloride of mercury (1-1000). Prune out all blighted branches and if possible cut out all blighted twigs, cutting back several inches into the healthy wood. A severe winter often kills the germs in the twigs. All the waste material should be hauled out of the orchard and burned. 3. It is well to discontinue cultivation and application of manure or nitrogenous fertilizers as soon as blight its appearance. 4. The trees are most susceptible to the attack when planted in low wet ground; hence drain the low spots in the orchard. Many bulletins have stated that winter pruning is inducive to wood and thus favors fire blight. Little attention should be paid to such recommendations for we need trees with good growth which will carry a crop of fruit and allow the sun to penetrate throughout the tree and color up the apples. Many fruit growers have bearing orchards and are not anticipating new ones, so to such,the talk of susceptible and resistant varieties means little. Then too, some of the susceptible varieties are the best commercial apples to raise, and when such is the case it means careful inspection for the hold-over cankers. It is however well to pay some attention to this fact when planting an orchard. What is needed badly is community spirit or cooperation in this difficult work of controlling fire blight. For if one's neighbor does not keep the disease out of his orchard, it is sure to spread to the adjoining orchard. So everyone should "pull together" and control blight which has and is now causing the apple grower heavy losses. / Master of Science

The effects of sterol-inhibiting fungicides (SIFs) on the free sterol and free fatty acid composition of apple leaves of Red Delicious and Jonathan cultivars were examiried over a 2 year period. Trees were treated in mornings vs evenings throughout each season and samples collected after 24 and 72 hours after each treatment. Generally, SIFs appeared to have an effect on the free sterol content of apple leaves after 24 hours, but the concentrations of free sterol returned to normal after 72 hours in the leaves of both cultivars. Morning versus evening application had little or no influence on leaf free sterol concentrations. There were increases in unsaturated and total fatty acid concentrations in Red Delicious leaves 24 hours following applications with the SIF, etaconazole, and the non-sterol-inhibiting fungicide (NSIF), metiram, early in the season. There were also increased concentrations of linoleic, linolenic and total free fatty acids in fenarimol and triadimefon-treated Jonathan leaves 72 hours after treatment. Early in the season, the SIF, fenarimol, caused an increase in linolenic acid in both Red Delicious and Jonathan leaves 72 hours after either morning or evening applications. Generally, both the Red Delicious and Jonathan leaves exhibited a decrease in saturated fatty acids following morning application whereas, an increase in saturation following evening application. Although SIFs may have had an effect primarily on the unsaturated fatty acids, particularly linolenic acid, early in the season, particularly linolenic acid, the fatty acid composition of the leaves appeared to return to normal later in the season. Ultrastructural observations were made of Red Delicious leaves 12, 24 and 72 hours after treatment with the SIF, bitertanol. Twelve hours after treatment thylakoids of chloroplasts appeared swollen and irregular resulting in loss of integrity of the organelles. However, after 24 and 72 hours, thylakoids of chloroplasts of treated leaves were similar to the controls. Infection of bitertanol-treated Red Delicious leaves by Spilocaea pomi was also examined at the fine structural level. Nuclear envelopes were not well defined and mitochondrial matrices appeared washed-out after 12 and 72 hours post treatment. There was dissolution of normally plate-like cristae of mitochondria, accompanied by the accumulation of minute electron dense bodies around their periphery. Invaginations and proliferations of the plasmalemma were observed as well as increased vacuolization of the cell. Further electron microscopic observations were made of the in vitro conidial state of Venturia inaequalis following the application of fenarimol. Conidia treated 2 hours with the fungicide for had necrotic areas throughout the cytoplasm. The plasmalemma was not well defined, and appeared to be degrading. Increased vacuolization was observed as were numerous lipid bodies and multivesicular complexes (MVCs) which contained vesicles of varying electron densities. Structural integrity of the organelles was such that they were difficult to discern. After 12 hours, the entire fungal cell was necrotic accompanied by the degradation of the cell wall. Detection of a selected number of SIFs in apple leaf tissue using bioassay procedures were also evaluated. It was found that the leaf disk and leaf extract bioassays examined in this study were ineffective in determining the presence of SIFs in apple leaves. / Ph. D.

Apple scab is the most important disease of apples in most of the world. The disease, caused by Venturia inaequalis, is controlled by numerous fungicide applications, regardless of the presence of inoculum in the orchard. Better timing of fungicide applications could be achieved if the airborne ascospore concentration (AAC) was considered in decision making. AAC can be measured in real time using spore traps. In this project, the relationship between AAC and lesions development was studied under controlled and natural conditions for five cultivars: Empire, McIntosh, Jonagold, Royal Gala, and Spartan. Potted trees were exposed to different airborne ascospore inoculum and the corresponding AAC were measured using spore traps. The spatial distribution of ascospores was studied in a commercial apple orchard plot. The potential ascospore dose (PAD) and the AAC were measured in 40 quadrats in the spring of 1999. (Abstract shortened by UMI.)

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.

No description available.

Thesis (MScAgric)--University of Stellenbosch, 2003. / ENGLISH ABSTRACT: Various aspects of the biology and ecology of woolly apple aphid, Eriosoma lanigerum, were investigated, including initial galling damage caused by E. lanigerum to the roots of apple trees, the possible relationship between E. lanigerum and Xiphinema and Pratylenchus nematodes and the effectiveness of Biostart 2000® and Furfural® as possible control agents of E. lanigerum in the orchard. Preliminary root damage by first instar E. lanigerum feeding was characterized by the mechanical injury of endodermis and parenchyma tissues. Damage by second, third and fourth instar E. lanigerum was similar, but the symptoms were more pronounced. Damage caused by adults included a pronounced swelling at infected areas of the root. Cell walls hardened until the root was radially strengthened with sclerenchyma tissue and nonconducting xylem vessels while the cuticle expanded greatly through the growth of corklike cambium tissue. There was no direct relationship between the population dynamics of E. lanigerum and those of Xiphinema and Pratylenchus nematodes. The occurrence of E. lanigerum appeared to be seasonal while P. penetrans and Xiphinema numbers fluctuated erratically. Undamaged root nitrogen levels seemed to correspond with the normal root growth cycle. Nitrogen levels from galled roots were significantly lower than those of undamaged roots, probably due to E. lanigerum feeding. Soils rich in fine sand and clay sustained higher populations of E. lanigerum and Xiphinema than sandy soils. The number of E. lanigerum found in soil samples correlated well with the damage index allocated to the samples. The numbers of Xiphinema found in soil samples also correlated well with the damage index allocated to the samples according to suspected Xiphinema damage symptoms. Both Biostart 2000® and Furfural® were effective as control agents of woolly apple aphid. Furfural'Ï, a chemical waste product of the sugarcane industry, was however not as effective as Biostart 2000®, a product that includes an activator and three bacterial species, Bacillus laterosporus, B. chitinosporus and B. licheniformis. The bacteria in the Biostart 2000® treated pots could replicate themselves under suitable conditions while Furfural® dilutes with each watering. Biostart 2000® is also easier to prepare than Furfural® since the components of Biostart 2000® readily mix to form a paste easily thinned by water, whereas Furfural® is an oily substance that does not easily disperse in water. Root damage was initiated soon after E. lanigerum started feeding, however there was no apparent relationship between E. lanigerum and the nematode species. The most promising, environmentally friendly control measure was Biostart 2000®. / AFRIKAANSE OPSOMMING: Verskeie aspekte van biologie en die ekologie van die appel bloedluis, Eriosoma lanigerum, was ondersoek insluitende aanvanklike galvorming veroorsaak deur E. lanigerum op wortels van appelbome, die moontlike verwantskap tussen E. lanigerum en Xiphinema en Pratylenchus nematodes en die effektiwiteit van Biostart 2000® en Furfural® as moontlike beheeragente van E. lanigerum in die boord. Aanvanklike wortelskade deur eerste ins tar E. lanigerum voeding was gekenmerk deur die meganiese beskadiging van endodermale en parenchiem weefsel. Skade veroorsaak deur tweede, derde en vierde instar E. lanigerum was soortgelyk alhoewel die simptome meer beklemtoond was. Skade deur volwassenes het 'n meer duidelike swelsel by geïnfekteerde wortelareas ingesluit. Selwande het verhard totdat die wortel radiaalsgewys versterk was met skierenchiem weefsel en nie-geleidende xileemvate terwyl die kutikula grootliks toegeneem het deur die groei van kurkagtige kambiumweefsel. Daar was geen direkte verwantskap tussen die bevolkingsdinamika van E. lanigerum en dié van Xiphinema en Pratylenchus nematodes nie. Die voorkoms van E. lanigerum was seisoenaal terwyl P. penetrans en Xiphinema se getalle onvoorspelbaar gefluktueer het. Onbeskadigde wortel stikstofvlakke het ooreengestem met die normale wortel groeisiklus. Stikstof vlakke van galwortels was noemenswaardig laer as dié van onbeskadigde wortels, heel waarskynlik as gevolg van voeding deur E. lanigerum. Grond ryk aan fyn sand en klei het groter bevolkings van E. lanigerum en Xiphinema onderhou as sanderige gronde. Die aantal E. lanigerum in grondmonsters het goed ooreengestem met die skade indeks wat aan die monsters toegeken was. Die aantal Xiphinema in grondmonsters het ook goed ooreengestem met die beskadigingsindeks wat aan die monsters toegeken is weens vermoedelike Xiphinema skade simptome. Beide Biostart 2000® en Furfural® was effektief as beheeragente van die appelbloedluis. Furfural'", 'n afvalproduk van die suikerriet industrie, was egter minder effektief as Biostart 2000®, 'n produk bestaande uit 'n aktiveerder en drie bakterie spesies, Bacillus laterosporus, B. chitinosporus en B. licheniformis. Die bakterië in die Biostart 2000® behandelde potte kon vermeerder onder gunstige toestande terwyl Furfural® na elke besproeiing verdun het. Biostart 2000® is ook makliker om aan te maak as Furfural® aangesien die bestanddele van Biostart 2000® geredelik meng tot 'n wateroplosbare pasta, terwyl Furfural® 'n olierige vloeistofis wat moeilik 'n waterige suspensie vorm. Wortelskade het plaasgevind kort nadat E. lanigerum begin voed het, alhoewel daar geen duidelike verwantskap tussen E. lanigerum en nematode spesies voorgekom het nie. Die mees belowende omgewingsvriendelike beheermaatreël was Biostart 2000®.

Thesis (MScAgric)--University of Stellenbosch, 2001. / ENGLISH ABSTRACT: Apple replant disease (ARD) is one of the major impediments to the establishment of an economically viable apple orchard on sites previously planted to apple. In spite of extensive research on ARD, the etiology remains to be fully elucidated. A possible biological origin of ARD etiology in South Africa was investigated by the dilution of replant field soil with sterilised soil. Commercial orchards with ARD were selected for use in pot trials and disease severity evaluated after three months, by measuring shoot length, dry mass of plants as well as root discolouration. Although diluting replant soil to 25 and 50% (v/v) significantly reduced the effects of ARD, symptoms were only absent in 0% replant soil. It was clear that seedlings planted in any mixture containing replant soil, even only 25% replant soil, consistently exhibited symptoms of stunted growth and root discolouration similar to those seedlings grown in 100% replant soil. This indicates that ARD in South Africa is primarily of a biological nature. As an initial step in formulating sustainable disease control alternatives to replace methyl bromide, pot trials were conducted to assess the impact of compost treatments as well as biological control products on ARD. Compost as well as sterilised and unsterilised compost teas (compost extract) significantly increased seedling growth even under optimum nutrient conditions when compared to the control, suggesting that they negate the effects of ARD. Results also indicated that applying high concentrations of compost does not necessarily provide additional growth benefits compared to lower concentrations. Results with biocontrol formulations were less favourable. Only one of the biocontrol formulations, a combination of Bacillus spp. (Biostart®) improved growth significantly compared to the control. There was, however, some inconsistency with results for the different trials conducted using this product. Fungal as well as nematode populations associated with ARD soils were characterised to the generic level to get a clearer understanding of the etiology of ARD in South Africa. Pythium and Cylindrocarpon spp. were consistently isolated from all six replant soils in all trials that formed part of this study, indicating that these fungi may have a role in ARD etiology in South Africa. Nematodes implicated in ARD development were inconsistently associated with ARD soils used in these studies. This suggests that nematodes do not have a primary causal role in ARD etiology in South Africa. Field trials were conducted in commercial orchards to assess the impact of organic amendments and promising biological control products, as indicated by the pot trials, on ARO severity under field conditions. These biological soil amendments were also compared with the standard chemical control methods for ARO, methyl bromide and chloropicrin. In all three trials established, compost and mulch as well as manure and mulch, consistently increased growth to the same extent as the standard chemical treatments and by combining these chemical treatments with organic amendments a significant, additional growth increase could be attained. Biocontrol formulations evaluated in field studies gave variable results. Biostart® improved growth when applied on its own, but not in combination with the chemical Herbifume (metham-sodium). Inoculating soil with effective microorganisms (EM), consisting primarily of photosynthetic bacteria, had no significant effect on growth. Results from this study indicate that application of organic amendments could possibly substitute for soil fumigation in replanted apple orchards. However, compost quality standards need to be implemented and because few types of compost are universally effective, different types of composts should be compared in specific soil environments before recommendations can be made. Oue to variable results with biocontrol products, ARO management with these biological soil amendments cannot be guaranteed at this stage and further studies are recommended. / AFRIKAANSE OPSOMMING: EVALUERING VAN DIE SPESIFIEKE APPELHERVESTIGING-PROBLEEM IN BOORDE IN DIE WES-KAAP Appelhervestiging-siekte (AHS) skep 'n groot probleem in die vestiging van jong appelbome op grond waar daar reeds voorheen appels verbou is. Ten spyte van omvangryke navorsing is die oorsaak van die probleem nog hoofsaaklik onbekend. 'n Moontlike biologiese oorsaakleer is in Suid-Afrika ondersoek deur die hervestigings-effek te probeer verminder deur die vermenging van hervestigingsgrond met gesteriliseerde grond. Kommersiële boorde met 'n appelhervestigingsprobleem is geselekteer en gebruik in potproewe. Die ernstigheidsgraad van die siekte is na drie maande se groei geevalueer deur lootlengte, droë massa en wortelverkleuring te meet. Alhoewel verdunning van die hervestigingsgrond tot 50 en 25% (vlv) die effek van AHS op groei betekenisvol verminder het, kon die skadelike effek van die veroorsakende faktor slegs uitgeskakel word deur saailinge in 100% gesteriliseerde grond te plant. Dit was duidelik dat saailinge wat in enige grondmengsel geplant is waarin hervestigingsgrond voorgekom het, selfs al was dit net 25%, konsekwent simptome van vertraagde groei en wortelverkleuring getoon het. Dit is 'n aanduiding dat AHS in Suid-Afrika hoofsaaklik biologies van aard is. Potproewe is uitgevoer as 'n eerste stap in die formulering van volhoubare siektebeheer-strategieë, om die impak van kompos-behandelings en biologiese beheer produkte op AHS te ondersoek. Kompos sowel as gesteriliseerde en ongesteriliseerde kompos-tee (kompos-water) het, selfs onder optimale voedingsomstandighede, die groei van saailinge betekenisvol verbeter. Dit dui aan dat hierdie behandelings die effek van AHS kan teenwerk. Resultate het ook daarop gedui dat hoër kompos konsentrasies nie noodwendig enige addisionele voordele vir groei inhou in vergelyking met laer konsentrasies nie. Resultate met biologiese beheer produkte was minder gunstig. Slegs een van die produkte wat geëvalueer is, 'n kombinasie van Bacillus spp. (Biostart®), het groei betekenisvol verbeter in vergelyking met die kontrole. Resultate was egter inkonsekwent vir die verskillende proewe waarin hierdie produk gebruik is. Swampopulasies sowel as aalwurmpopulasies wat met hervestigingsgrond geassosieer word, is geïdentifiseer tot op generiese vlak om vas te stel waardeur AHS in Suid-Afrika veroorsaak word. Pythium en Cylindrocarpon spp. is konsekwent van al ses hervestigingsgronde geïsoleer wat daarop dui dat hierdie twee swamgenera 'n beduidende rol in AHS ontwikkeling in Suid-Afrika mag hê. Aalwurms wat aangedui is in die literatuur om 'n moontlike rol in AHS te hê, was slegs in enkele gevalle geassosieer met hervestigingsgronde waarvan in hierdie studie gebruik gemaak is. Die gevolg-trekking is dus gemaak dat aalwurms nie 'n betekenisvolle rol speel as hoof-veroorsakende organisme onder Suid-Afrikaanse toestande nie. Veldproewe is uitgevoer in kommersiële appelboorde om vas te stel wat die effek van organiese materiaal, asook belowende biologiese beheermiddels, soos aangedui deur potproewe, op AHS onder veldtoestande is. Die biologiese grondtoedienings is ook vergelyk met die standaard chemiese beheermiddels (metielbromied en chloorpikrien). In al drie proewe wat gevestig is, het kompos met 'n deklaag, sowel as kraalmis met 'n deklaag, groei betekenisvol verbeter tot dieselfde mate as chemiese middels. Daar kon ook 'n beduidende, addisionele groeitoename gemeet word in gevalle waar chemiese middels met organiese materiaal gekombineer is. Resultate met biologiese beheer formulasies wat onder veldtoestande geëvalueer is, het gevarieer. Biostart® het groei verbeter wanneer dit alleen toegedien is, maar in kombinasie met die chemiese middel Herbifume (metham-sodium) het dit geen effek gehad nie. Die inokulering van grond met 'n oplossing van effektiewe mikroorganismes (EM) wat hoofsaaklik uit fotosinterende baterieë bestaan, het ook geen betekenisvolle effek op groei gehad nie. Die gevolgtrekking is gemaak dat toediening van organiese materiaal moonlik as plaasvervanger vir metielbromied-beroking kan dien in die beheer van AHS. Die nodige komposkwaliteit-standaarde moet egter eers geïmplimenteer word. Omdat feitlik geen kompos universeel effektief kan wees nie, is dit ook nodig dat verskillende tipes kompos met mekaar vergelyk moet word in spesifieke grondtoestande voordat verdere aanbevelings gemaak kan word. As gevolg van variërende resultate met biologiese beheer produkte kan AHS beheer met hierdie middels nie gewaarborg word op hierdie stadium nie en verdere studies word aanbeveel.