No description available.

No description available.

No description available.

The sweet potato whitefly is an insect whose economic importance is continually expanding as it becomes a more efficient vector, increases its fecundity and broadens its host range. To keep pace with its ability to develop resistance to existing classes of pesticides, we have undertaken a program to examine the effectiveness of pesticides with novel chemistries and novel modes of action. Several have shown themselves to be very effective against the various life stages of the sweet potato whitefly. Once these are incorporated in to our arsenal of pesticides, we hope to be able to manage resistance by prudently using these new materials.

Populations of the sweet potato whitefly, Bemisia tabaci, have been shown to consist of both migratory and trivial flying morphs. The behavior of these forms as part of the process of short-range migration needed to be examined under field conditions. Insects were marked in a field of cantaloupes using fluorescent dust. During the first growing season traps, used to collect living whiteflies, were placed along 16 equally spaced transects (22.5° apart) radiating out from the field to a distance of up to 0.6 miles. Wind out of the northeast consistently carried migrating whiteflies to traps placed along transects in the southwestern quadrant because cold air drainages dictate wind direction during early morning hours. For this reason, during the second season traps were laid out in a rectangular grid extending 3 miles to the southwest of the marked field. If dispersal was entirely passive or wind directed patterns could be described using a diffusion model. Statistical examination of the data, however, demonstrate that the distribution on all days was patchy. Traps in the immediate vicinity of the marked field caught more whiteflies than the daily median. Large numbers were also collected from around the periphery of the grid. Whiteflies were far less prevalent in the grid's center. As a result, the distribution of captured whiteflies can be described as bimodal. These patterns confirm behavior observed in the laboratory, i.e., a portion of the population are trivial fliers that do not engage in migration and are consequently captured in traps near the field and a portion initially ignore vegetative cues and fly for a period of time before landing in distant traps. This second population comprises the second peak in the model that appeared 1.6 miles from the marked field. On a localized level, 1.6 miles seems to be how far whiteflies move in a day. Earlier studies indicate that whiteflies only fly one day.

No description available.

Phthorimaea operculella, commonly known as the Potato Tuber Moth, is an economically important agricultural pest worldwide. The baculovirus, Phthorimaea operculella granulovirus (PhoGV) has been considered as a means of control alternative to chemical control because of its host specificity and harmless impact on other organisms and ecosystems. An isolate of PhoGV obtained from a South African PTM population would be beneficial in the production of a biopesticide, which is not yet available. An efficient and cost-effective rearing method would be advantageous for potential commercial production. Commercial table and seed potato plantations and storage facilities located in Patensie, Bathurst, Howick and Ivanhoe were surveyed for PTM infestations. Patensie was the only site where milky discoloured larvae were found, a potential symptom of PhoGV infection. TEM analysis revealed no virus in these samples. Since no virus was found in the field-collected samples, PTM insects were collected to initiate rearing in the laboratory. PTM was raised by three different methods in the laboratory. A cost/benefit analysis, survival rate, fertility and sex ratio were recorded for each rearing method. Rearing method one was deemed unsuccessful for efficient commercial rearing, as survival percentage and fertility were low. Rearing methods two and three had high survival rates and high fertility, and were efficient and less labour intensive than rearing method one. Rearing method three was the most productive technique, but for commercial production rearing method two was considered the most manageable and efficient. The sex ratio was 1:1 for all three cultures. The cost analysis revealed that rearing methods two and three were less expensive than rearing method one because less labour was required to monitor insects. The success of rearing PTM for 19 months will enable these cultures to be up-scaled to a large production facility for mass rearing. Virus was not found in the field surveys or in laboratory cultures, therefore chemical, temperature, humidity and carbon dioxide stressors were used in an attempt to initiate a baculoviral infection. Symptoms were exhibited in larvae subjected to chemical, temperature and humidity treatments, but these were confirmed by TEM analysis not to be a result of PhoGV infection. The success of rearing PTM in the laboratory suggests that the method could be used in the commercial rearing of the insects in a large mass-rearing facility. The data obtained from induction protocols have allowed for better understanding for future induction for PhoGV and other baculoviruses in other insect species. The failure to isolate a South African PhoGV strain for developing a biopesticide against PTM has motivated further studies in obtaining a baculovirus in order for South Africa to develop a commercial product against this pest.

Vitamin A deficiency has been recognised as a widespread problem affecting about 750 million people, mostly in developing countries. Mortality due to vitamin A deficiency can be reduced by as much as 23% with improved vitamin A status. Vitamin A's immuneenhancing aspects strengthen the body's defence system against infectious diseases such as measles, malaria and diarrhoea, thus preventing death. Children beyond the weaning age (6 months to 6 years) are most at risk. In South Africa, one in three children has a low vitamin A status, with the rural areas being most affected. Orange-fleshed sweet potato (OFSP) has emerged as a promising plant source with a high beta-carotene content that can make a significant contribution to the vitamin A intake of individuals at risk of vitamin A deficiency. The purpose of this study was to determine the nutrient and sensory quality of OFSP. To this end, the nutrient content of different cultivars of OFSP was determined as well as the sensory characteristics and consumer acceptability was established. During the first phase, four different cultivars of OFSP i.e. Resisto, W119, Jewel and A 15, plus one composite sample, all cultivated by the Agricultural Research Council (ARC)-Roodeplaat, South Africa, were sampled for nutrient analysis. During the second phase (descriptive sensory analysis), four OFSP cultivars and one white-fleshed sweet potato (WFSP) cultivar i.e. Blesbok, were evaluated. A trained sensory panel was used to establish terminology for describing the sensory attributes of the different sweet potato cultivars in terms of its aroma, texture, flavour and aftertaste attributes. Consumer preference (n=180) for OFSP and WFSP was measured by means of a paired preference test. Focus group discussions were conducted to verify findings of the consumer preference test. The results obtained from the nutrient analysis confirmed that OFSP is an excellent source of beta-carotene. A 100 g portion of cooked OFSP can provide up to 6528 ~g beta-carotene, which is approximately 136% of the RDA for vitamin A for children four to eight years. High levels of other nutrients present in OFSP were identified namely vitamin C, calcium and zinc. OFSP further contributes 28% vitamin C, 13% calcium, 15% magnesium and 75.6% zinc of their daily requirements. This study confirms the valuable contribution that OFSP can make as a food-based approach to reduce vitamin A deficiency in individuals at risk. The sensory profiles indicated that OFSP differed from WFSP in colour, flavour and texture. OFSP is generally less moist and subsequently more dense and adhesive compared to WFSP. The latter being more fibrous and less firm than OFSP. Overall OFSP had and earthy aroma which was not typical of WFSP and had a sweeter flavour. The flavour of OFSP was described as similar to that of yellow vegetables such as butternut and pumpkin. Although the different OFSP cultivars had similar characteristics, differences were found in the moist, adhesive and grainy texture attributes as well as the vegetable sweet flavour. Resisto had the sweetest in flavour, with the most dense and pasty texture of the four OFSP cultivars. W119 had the grainiest texture. No significant differences were found in the earthy aroma, sweet potato and yellow vegetable flavour attributes. It can be concluded that in flavour, few differences were found among the OFSP, except in sweetness. OFSP differed primarily from each other in texture. The consumer preference test results showed that overall 85% of respondents preferred the taste of OFSP to that of WFSP, 53% liked the orange colour a lot while 24% liked the colour a little and the remaining 22 % disliked the colour (either a lot, a little, neither like nor dislike). The majority of the consumers (86%) indicated a willingness to buy OFSP. From these results it can be concluded that the taste and colour of OFSP are acceptable to consumers of sweet potato and that it has potential to be successful in the marketplace. Given the high level of consumer acceptability of OFSP, an opportunity exists to address vitamin A deficiency through commercially viable decentralised vine production centres. Such centres could be managed by commercial-, small-scale- and subsistence farmers in South Africa in key sweet potato production areas, with adequate water supply throughout the year. However, availability of commercially produced OFSP in retail sores in recent months, may change this prospect. / Dissertation (MConsSci)--University of Pretoria, 2008. / Consumer Science / unrestricted

In total, 801 samples of potato leafroll disease were collected and tested for potato leafroll virus (PLRV) and beet western yellows virus (BWYV) in 1986, 1987, and 1988 using triple antibody sandwich enzyme-linked immunosorbent assay (TAS-ELISA) and virus-specific monoclonal antibodies. The samples represented 32 cultivars and originated in eight Canadian provinces and 12 American states. None of the samples tested positive for BWYV, whereas 772 (96.4%) tested positive for PLRV. Neither PLRV nor BWYV could be recovered, with aphid transfers to indicator hosts, from 28 of the 29 samples that tested negative for both viruses. PLRV was recovered from one sample that originally tested negative by TAS-ELISA; the indicator plant tested positive for PLRV by TAS-ELISA. Nucleic acid spot hybridization (NASH) using random primed and cloned cDNA probes was compared with double antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) and TAS-ELISA, and aphid transmission tests for detection and identification of PLRV and BWYV in 165 potato leafroll disease samples. All of the samples tested negative for BWYV with each of the assay procedures. PLRV was detected in all of the samples with TAS-ELISA, NASH with a cloned cDNA probe for PLRV, and with aphid transmission to ground cherry (Physalis pubescens). Both DAS-ELISA and NASH using random primed cDNA produced one false-negative result. Shepherd's purse (Capsella bursa-pastoris) was a host for 72% (119/165) of the PLRV isolates. The susceptibility of potato to BWYV was tested by inoculating Russet Burbank with three isolates of BWYV from Canada and four from the United States. Two of the isolates were in a mixed infection with PLRV. None of the isolates were transmitted by Myzus persicae to virus-free potato plants, either by themselves or in association with PLRV. Common weeds were surveyed in the potato-producing areas of British Columbia for PLRV and BWYV. In total, 10,098 weed samples, representing 98 species in 22 plant families, were collected and tested by TAS-ELISA from 1986 to 1989. BWYV was detected in 1% of the samples; the hosts were: chickweed, common groundsel, heart-podded hoary cress, hedge mustard, little western bittercress, prickly lettuce, shepherd's purse, and wild radish. PLRV was detected in three volunteer potato plants, two samples of shepherd's purse, and one black nightshade plant. The low incidence of PLRV in plants other than potato indicates that weeds are of minor importance in the epidemiology of potato leafroll disease in British Columbia. / Land and Food Systems, Faculty of / Graduate

This study relates market reforms to agricultural trade performance, in particular the export performance of South Africa’s potato industry. The market reform considered here is the deregulation of South Africa’s potato market. Changes in producer prices and volumes of exports and imports are the most important outflows of deregulation that the study focuses upon. In the first part, this study provides an overview of deregulation in the potato industry. The objective of this study is to analyse the impact of deregulation on the competitiveness and the level of integration of the South African potato industry in relation to potato markets in the SADC region. Competitiveness is perhaps the most fundamental idea in economics. Agricultural industries often fight to protect or increase their market share both domestically and internationally. The method used here to measure competitiveness is the Revealed Trade Advantage (RTA), a measure based on the share of a country’s net trade in a specific commodity relative to its total international trade. The impacts are investigated individually for the three sub-categories of potatoes: fresh/table, processed and seed. Comparisons are made between South Africa and selected countries in the Southern African region. Basic trend analysis illustrates that domestic potato production has increased significantly over the past decade. Exports as a percentage share of production have increased consistently from the late nineties to reach 8% of domestic production by 2003. However, from 2004 onwards exports have decreased as the domestic informal market for fresh potatoes expanded at a tremendous pace. The results of the Real Trade Advantage (RTA) analysis reveal that South Africa’s potato (fresh) exports are the most competitive in the SADC region. Yet, the competitiveness of the potato supply chain in South Africa was found to be marginal as far as regional competitiveness is concerned. Thus, the potato supply chain exhibits a negative trend in competitiveness when moving from the primary to the processed product. Another approach to gain a better understanding of the possible impact of deregulation on agricultural markets is to analyse the extent to which domestic commodity markets respond to changes in international prices. Hence, the level of price transmission between local and foreign markets can be analysed. The analysis consists of a set of econometric applications. Annual producer prices of various trading nations are analyzed by testing mostly for the existence of long run equilibrium between the price series of the various nations and the dynamics of the relation between the prices and their causality. The results of the price transmission analyses show that the South African potato market is not well integrated with other regional potato markets, despite some trade occurring. This can partly be explained by the fact that over the past decade on average only 6 percent of all potatoes in the local market were exported into the region. Further more one has to take the tradability of the good into consideration when analysing the level of price transmission and, therefore, the level of integration of markets, In other words; can the good be traded or not? Potatoes (fresh) are perishable and bulky and therefore not easy to transport. Export trade in fresh potatoes involves high transport and transaction costs which complicate the process of price transmission across markets. Even in the exceptional cases (Mozambique and Mauritius) where market integration was detected, price transmission was found to occur from South Africa to these countries and not vice versa. Hence, from the empirical evidence of this study it seems as if domestic prices are determined by domestic supply and demand dynamics and regional exports do not influence the formation of prices in the domestic market. Although the liberalization of the South African potato market has led to the lowering of tariffs and non-tariff trade barriers, only weak evidence was found that there is some level of market integration between South Africa and its main trading partners in the SADC region. Copyright / Dissertation (MScAgric)--University of Pretoria, 2009. / Agricultural Economics, Extension and Rural Development / unrestricted