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Project summaries
Landbou Navorsingsraad
• Evaluasie van gevorderde teellyne - Arno Visser
Gevorderde teeellyne vanuit plaaslike programme sowel as teellyne van buitelandse programme word ge-evalueer met die oog op cultivar seleksie.
• Aartappel in vitro genebank - Andre Lezar
The potato breeding collection was maintained and consolidated. A total of 167 breeding lines (202 breeding lines were discontinued), 39 breeding lines that are virus-free, 22 breeding lines in the process of virus-elimination, 262 cultivars and 438 other important accessions utilised in the local potato-breeding programme were securely maintained in-vitro. 115 breeding lines were received for establishment in-vitro. Potato plantlets produced for the potato breeding programme totalled 29 000. A number of 101 potato germplasm accessions were prioritised for rejuvenation.
• Mass propagation of in vitro plants - Andre Lezar
Six commercially produced potato cultivars as well as 9 cultivars with sub-licences were safely maintained in-vitro. Disease-free potato mother material (106 490 plants) was supplied to 8 potato tissue culture facilities and greenhouses. The new 2007 clones 2, 3 and 4 from BP1, clone 4 from Van der Plank and all the Calibra clones all tested negative with rt-PCR. New tubers were received from potato certification services for rejuvenating Hertha in the in-vitro collection. The status of Aviva, Ronn and Ropedi must be decided upon as there have been no registered plantings for some time.
• Alternative control of soil borne diseases - Reinette Gouws
Common scab on potato is an ever increasing problem world-wide. It is one of the major factors limiting successful production of potatoes in South Africa because of its country wide distribution and cultivar susceptibility. The disease reduces the cosmetic value of ware potatoes, and with the growing consumer-demand for blemish-free produce, it is increasingly resulting in the downgrading of consignments on the market.
Streptomyces scabiei is regarded as the main causal agent of common scab and this assumption has been maintained in research conducted. However, results have shown that the disease can be caused by a complex of Streptomyces species, with the dominant species varying in different regions. One of the aims of this project was therefore to identify the main causal agent(s) of common scab on potatoes in South Africa by means of molecular techniques. In the characterization study conducted we investigated the incidence of the three most important Streptomyces species that cause common scab worldwide, Streptomyces scabiei / europascabiei, Streptomyces turgidiscabies and Streptomyces acidiscabies. It was found that S. scabiei / europascabiei is the main causal agent in South Africa with 56 % incidence on strains analyzed, followed with S. turgidiscabies (0.01%) and S. acidiscabies (0%). The occurrence of the most important pathogenicity factors involved in host infection was also determined. The nec1 virulence factor occurred in 18%, tomA in 62% and txtAB in 24% of isolates tested. The characterization study is on-going and has led to the development of an electronic database that includes the following information for each accession: digital image, location, cultivar, lesion type, coverage, morphological and molecular identity and pathogenicity factor analysis. This database is unique and will be a very valuable tool in managing common scab in South Africa in future.
Effective management strategies for common scab are limited and the chemicals currently registered for soil and tuber application are not adequate to control disease incidence. However, an integrated management strategy can be implemented and includes the use of tolerant cultivars, long-term crop rotations and cultural practices, including soil amendments. Rotation with brassicaceous crops and incorporation of Brassica residues into soil or ‘biofumigation’ has been shown to be effective in reducing common scab in greenhouse, tunnel and field trials. In the present study we are investigating the mechanism of suppression by means of tunnel trials and real time PCR analysis of microbial interactions. The first of two tunnel trials were planted, harvested and evaluated. The trial consisted of twelve treatments including three soil types (sand, loam, clay) and six repetitions. The tubers were evaluated for disease incidence (lesion type and tuber coverage) and yield. All three soil types showed a reduction of common scab incidence after biofumigation. Yield was not affected in any of the treatments. Real-time PCR analysis is currently being performed on Streptomyces spp. to determine microbial interactions in the root rhizosphere, on tuber surfaces and in amended and non-amended soil.
• Investigation into insecticidal efficacy against potato tuber moth occurring on potatoes - Dr Diedrich Visser
Bioassays with insecticides against different potato tuber moth populations have been completed. Fourteen tuber moth populations have been collected across South Africa and reared in an insectarium at ARC-Roodeplaat. A detached-leaf bioassay technique was used to determine the survival of first instar larvae on treated leaves. The project involved three phases, i.e. a) using diagnostic dosages, b) using cumulative half dosages, and c) comparing results with a susceptible reference population. The latter was done in the place of LD50 tests (explained in detail in the project proposal). No insecticide showed any resistance in the potato tuber moth when using field recommended (diagnostic) dosages - all insecticides gave 100% mortality of larvae after feeding on treated leaves for 9–12 days. Cumulative half dosages also did not show any severe resistance, except at the lowest dosages of around 10% of the diagnostic dosage. However, when comparing the results using this low dosage with that of the susceptible reference population, no conclusion could be made that resistance was present in any of the populations against any insecticide. Our conclusion is that none of the insecticides showed resistance relating to tuber moth control when evaluating several populations collected across the country.
• Investigation into insecticidal efficacy against aphids occurring on potatoes - Dr Diedrich Visser
During the past season all three aphid species i.e. Myzus persicae, Macrosiphum euphorbiae and Aphis gossypii were reared in greenhouses and evaluated against leaf applied aphicides in the laboratory (bio-assays). None of the three species survived when using field recommended dosages. Cumulative half-dosages of each insecticide showed that most insecticides were still effective even at 25% of the recommended dosage when evaluating Myzus persicae and Macrosiphum euphorbiae. Aphis gossypii, however, showed better survival in relation to the other two species at dosages of 25% and lower, when evaluating acephate, methamidophos and dimethoate. However, when using the normal field recommended dosages, all insecticides tested showed between 97 and 100% mortality for all three species. With lowering dosages is can be deduced that Macrosiphum euphorbiae is the most susceptible aphid with Aphis gossypii the most difficult species to control. Initial tests with soil-applied insecticides in the greenhouse showed varying results. A field trial with foliage-applied insecticides showed that all insecticides are effective against Macrosiphum euphorbiae with control figures of 86% or higher. During the last phase of the project the focus will be on systemic insecticides. A second field trial will planted and evaluations in the greenhouse will continue.
UNIVERSITY OF STELLENBOSCH
• Variation in South African isolates of PVY with an emphasis on tuber necrotic strains - Prof Dirk Bellstedt and Chris Visser
The outbreak of potato necrotic ringspot disease or tuber necrosis caused by the PVYNTN strain in 2005, has highlighted the change in the risk of potato virus Y (PVY) infection for the South Africa potato producer. Since 2004, we have assessed strain variation of PVY that are present in South Africa using coat protein gene sequencing and PCR. Our results show that the older PVYO and PVYN strains have largely been replaced by new strains which are hybrids or recombinants of these older strains. These new strains are responsible for tuber necrosis, which can lead to crop rejection, and also severe yield reduction. The appearance of these new PVY strains, of which PVYNTN is the most severe, poses an increased risk to the potato farmer. Control of these strains can be achieved through the planting of uninfected tuber material. This highlights the importance of stringent testing for PVY in seed stocks, stricter aphid control and the planting of PVY resistant potato varieties.
• Investigations into the variation of South African isolates of PVY using whole genome sequencing - Chris Visser and Prof Dirk Bellstedt
An assessment of the variation in South African isolates of PVY was previously undertaken using coat protein sequencing and PCR for the detection of strains. However, the biggest amount of the genetic material of the virus has not been investigated and this contains a great deal of additional information which can be informative to assess strain variation and to predict the formation of tuber necrosis. As a result, the variation in a number of isolates will be assessed by determination of the whole genome sequences. This research has progressed very well and the results are very interesting as a result of which these preliminary results will be presented at the symposium.
UNIVERSITY OF PRETORIA
• Management of Potato virus Y (PVY) in seed potatoes – Dr Kerstin Kruger
Aphid monitoring
To predict the risk of spread of aphid-transmitted viruses and to provide support for making decisions it is essential to reliably estimate aphid numbers and the spatial distribution of aphid vectors and their virus sources. In general the field trials will be used to calculate a vector pressure index based on the number of species and the number of individuals per species caught in traps, data on virus transmission efficiency of aphid vector species, cultivar susceptibility and virus inoculum. To develop such a vector pressure index a long-term monitoring trial commenced in the Christiana region in December 2006 and continued until the end of March 2009. Weekly suction trap samples in addition to weekly yellow bucket trap samples have been sampled and are being processed. Six yellow bucket traps were placed evenly around circular potato fields at each of six sites. Samples are being sorted and aphids counted and identified. Pearson correlations were calculated between the weekly aphid counts of yellow bucket and suction trap samples. Initial results suggest that, overall, the total number of aphids (abundance) from yellow bucket traps in the field (pooled across six sites) are highly correlated with suction trap abundance. Greater variation was observed in the correlation between suction trap abundance and the abundance at individual field sites. The results suggest that in the study region suction trap aphid abundance can be a good predictor of aphid abundance in the field at most sites. Initial analysis of abundance of individual species recorded during four weeks shows that week-to-week variation can be considerable. Due to this variation in abundance, a longer time series is required to draw firm conclusions. Therefore, the monitoring trial was extended from December 2008 to March 2009 to obtain a larger data set.
The study further highlights that, of the 54 species recorded in the study region, non-colonizing aphid species (i.e., species that do not reproduce on potato) are more important regarding transmission of PVY than colonizing species (those that reproduce on potatoes) in this region. This suggests that PVY management strategies, such as planting of crop borders, should focus largely on non-colonizing species.
Crop borders
Plant borders have been suggested as a management tool to reduce incidence of non-persistently aphid-transmitted viruses in crops, the rationale being that aphids lose their transmission ability when feeding on a non-virus host plant before moving into potato fields. To determine whether border crops have potential in reducing Potato virus Y (PVY) incidence in large fields in South Africa, aphid landing patterns within 20- to 40-ha fields were examined and the hypothesis tested that aphid landing rates in large potato fields are higher at the edges compared to the interior. In the largest field, aphid landing rates tended to be higher at edges of fields for up to 7 (12) m compared to the interior.
Although edge effects were apparent, patterns within and among sites were not always consistent and depended on factors such as aphid abundance, aphid species and trap position. Importantly, the study has shown that roads leading through fields and centre pivots create edge effects in addition to field borders. This could lead to an increase in PVY incidence because of higher aphid numbers.
To identify a suitable non-virus host plant as crop border, a semi-field trial was undertaken to determine the attractiveness of lucerne, maize, wheat and soybean compared to potato. The aim was to identify crop plants that not only serve as a crop border but also attract potato colonizing and, importantly, non-colonizing aphid species to enhance the edge effect, i.e. increase the number alighting in crop borders instead of field interiors. Based on aphid abundance and species composition, including both colonizing and non-colonizing species, maize and wheat were the most suitable crops of those tested. However different cultivars differ in their attractiveness to aphids. Although it has been determined that aphids tend to land more frequently in the edges of potato fields (edge effect), the effect varied. Therefore, crops/cultivars that are strongly attractive should be selected to strengthen the edge effect.
Another aim of this study was to compare aphid species assemblages between the Pretoria trial with a heterogeneous planting of maize, lucerne, potato, wheat and soybean (semi-field trial) and the Christiana trial with homogeneous fields consisting of potato. Similarity of aphid assemblages between Pretoria and Christiana, based on species composition and abundance, was relatively low. However, comparing presence/absence data, species overlap between the two regions was relatively high, suggesting most of the variation between the two regions was mainly due to differences in aphid abundance. Therefore, maize and wheat could be planted as trap border crops in Christiana. In addition, most aphids landing in potato fields in Christiana do not colonize potato. The most typifying species in this region colonize Poaceae and are vectors of PVY.
Alternate hosts of virus and vectors
The spread of PVY is influenced by the amount of virus inoculum available. Many weed species serve as a PVY reservoir and host plants of aphid vectors. Lists of host plants of aphid species occurring in the study region and alternate hosts of PVY have been compiled. Samples from 309 plants belonging to 10 families and 25 species collected in the vicinity of potato fields were analysed for PVY. Of these, plants from 12 species tested positive for PVY. This highlights the importance of weed management within potato fields to reduce the spread of PVY because not only PVY-infected potato plants but also weeds can serve as virus inoculum.
• Brown spot (Malroes) of potatoes in South Africa - Dr Jacquie van der Waals
In recent growing seasons, a new leaf blight was observed in potato fields in various production regions in South Africa. Symptoms were observed before early blight, from about 60 days after emergence of the potato plants. Typical leaf symptoms were small circular brown lesions, first visible on the abaxial sides of leaves. Lesions resembled those of early blight, but were smaller and did not show concentric rings. During favourable environmental conditions, severe infections were seen as coalesced lesions, blighted leaves and stems. Such severe infections occurred in seasons when high humidity, leaf wetness and warm temperatures were present. Yield losses were reported due to this leaf blight, as conventional fungicidal spray programmes did not adequately control the disease.
Isolations from leaf lesions were made on Potato Dextrose Agar (PDA) under aseptic conditions. The fungus Alternaria alternata (Fr.) Kreissler was consistently isolated and identified based on morphological characteristics. Koch’s postulates were conducted by inoculation of healthy potato leaves. Spores at a concentration of 106 spores/ml were suspended in an oil / surfactant mixture and sprayed onto leaves until run-off, while control plants were sprayed with a sterile oil / surfactant mixture until run-off. Plants were covered by polyethylene bags for 2 days to achieve high humidity levels and maintained in a greenhouse at 25 ± 2 ˚C. Three days after inoculation, plants were exposed to a moisture regime simulating that of in-field irrigation. Plants were placed in a fogging chamber twice a week for one hour at a time.
Leaf blight symptoms similar to those observed on diseased potato plants in the field began to develop three weeks after inoculation. Isolations made from these lesions once again consistently yielded Alternaria alternata. Control plants did not develop any symptoms. This is the first report of A. alternata causing leaf blight on potatoes in South Africa.
Future research will focus on evaluating fungicides for the control of this disease, determining the genetic diversity of the pathogen in South Africa and investigating the infection process of A. alternata on potatoes.
• Monitoring irrigation systems and management practices on potato farms in the Gamtoos Valley – Dr Martin Steyn
The extent and sources of irrigation water losses on the Gamtoos irrigation scheme are investigated in this study. The project runs concurrently with a wider WRC project, which investigates irrigation losses from dam wall release to root zone application for a range of crops on various irrigation schemes. The current PSA study however specifically focused on potato producers. The first objective was to establish the current efficiency levels of centre pivots used for the irrigation of potatoes and, secondly, to establish the status of irrigation management practices on the scheme. This was done by following irrigation management throughout the growing season for a number of selected potato fields. Observed irrigation practices were then compared to estimated crop water requirements, using a crop modelling approach. This helped us to establish whether irrigation water was applied inefficiently due to over or under irrigation at certain crop stages.
Irrigation system efficiencies were evaluated in cooperation with the ARC-IAE. Results indicated that system performances were often below minimum requirements. In cases where efficiencies were low, it could mostly be related to pressure problems and worn nozzles (poor maintenance).
Soil moisture probes were installed on a number of potato fields for hourly soil water content monitoring. Rain gauges (manual and electronic) were used to collect rainfall and irrigation data. Automatic weather stations were used for collection of hourly weather data. Preliminary results provided some valuable insight into current scheduling practices followed by farmers. Serious cases of over irrigation were limited, but cases of under irrigation were evident. Currently many farmers do not use scientific scheduling methods to manage their irrigation. It is believed that the use of valuable resources, such as irrigation water, can only be optimized if proper irrigation management strategies are implemented.
A final report will contain recommendations for benchmarks and improved irrigation management. These findings and guidelines will also be communicated to farmers at a farmers’ day or working group meeting. Farmer needs and possible training opportunities will also be identified.
• Promotion of sound irrigation management practices amongst potato producers in the Sandveld - Dr Martin Steyn
The objective of this new project is to compare different scientific irrigation scheduling tools on farms in the Sandveld. The project will link up with and support the objectives of the diversity project. It will firstly guide producers with regard to the suitability, accuracy, and ease of use of such tools. Secondly, the project will demonstrate to producers the advantages of scientific irrigation scheduling tools and try to motivate them to adopt and apply some of these methods.
Some of the tools commercially available for irrigation management are installed on potato farms in the Sandveld and its performance will be monitored and compared over a three year period. These methods include three soil measurement tools (capacitance probes) and two computer models. This includes the SWB and SEBAL models. The latter model uses satellite images to estimate daily crop water usage. The Western Cape Department of Agriculture is a partner in the project and will be responsible for this part of the research.
The soil measurement equipment has just been installed on five farms in the Sandveld. As this will be the first season, some teething problems with data collection, telemetry and data transfer can be expected. These will need to be ironed out timely in order to ensure collection of good quality data for the remainder of the project period.
At the end of each cropping season, the recommendations of the different soil and atmospheric scheduling methods will be compared in order to establish their performance, accuracy and acceptability. This project will eventually enable us to report on the cost, data quality, convenience (ease of use) and value for money of each scheduling method. It is hoped that this will enable producers to make informed decisions on the most suitable scheduling methods for their conditions, and to convince them to adopt irrigation scheduling as a sustainable production practice in the long term.
• Development of a web-based advisory service for potato producers -
Dr Martin Steyn
This project aims at developing a computer software package to make production guidelines and the latest research results more readily available to farmers. In addition to production guidelines, this will also include useful calculators that could be applied to make fertilizer and irrigation recommendations on-line.
An initial pilot study on the feasibility of this project was conducted during the past year. In this study the different approaches followed by similar web-based products were investigated and the most appropriate was selected. Initial information to be included in the package was collated and a basic framework for the multimedia package was developed with the help of a computer programmer. Initially the multimedia package will be run from a CD-ROM, but ultimately it could be hosted on a web site for easy access by producers. Progress to date has been encouraging the basic framework of the interactive multimedia package is currently being finalised.
Once the system development is completed, it will be placed on CD-ROM and given to a sample of potato producers for testing and evaluation. The final product will be improved, based on feedback received during the evaluation period. It is foreseen that this tool will make latest potato production information more readily available to producers. The advantage of an electronic on-line system, as opposed to printed media, is that information can be easily and rapidly updated as new information becomes available. This will ensure that information reach end-users more timely.
• Risk assessment, diagnosis and sampling of soil-borne diseases on potatoes in South Africa - Dr Jacquie van der Waals
Potatoes are subject to a number of bacterial and fungal diseases, due to their high starch and sugar content. Of particular importance in commercial potato production are those soil-borne diseases that affect the marketability or processing potential of tubers. For many years no techniques were available to measure the amount of inoculum in soil in order to give growers an indication of disease risk in specific fields. Recent research has focused on developing DNA-based tests, to provide growers with a method of assessing disease risk of soil-borne pathogens. Disease risk assessment is the probability that a certain intensity of incidence or severity of the disease will be reached. Numerous host, pathogen, environmental and man-related factors must be taken into account in assessing the risk of disease. Since many of these factors remain fairly constant from year to year, emphasis must be placed on estimating (or determining with reasonable accuracy) initial inoculum and, subsequently, on closely monitoring environmental factors that will affect disease development.
The occurrence of soil-borne diseases, however, is difficult to predict. Quantitative information about the inoculum density in soil and its relationship with disease development is required for disease risk assessments based on pre-plant inoculum levels. The primary aim of this project is to develop risk categories for DNA tests and to provide interpretation and applications thereof to growers. This research is being conducted in collaboration with researchers from the Scottish Crops Research Institute (SCRI), Scottish Agricultural College (SAC), Central Science Laboratory (CSL), University of Tasmania, Department of Primary Industries, Australia (DPI), New Zealand Plant and Food and the South Australia Research and Development Institute (SARDI). Research programmes are run in parallel with one another to develop and validate DNA tests for major soil-borne pathogens of potatoes.
The proposed project focuses on four important potato tuber pathogens, namely Spongospora subterranea, (powdery scab), Streptomyces scabiei (common scab), Colletotrichum coccodes (black dot) and Rhizoctonia solani (black scurf). The project will run over four years.
During the 2008 / 9 growing seasons, samples of the four diseases under consideration were collected. PCR detection techniques were optimized for South African pathogens, using published species-specific primers. In addition to this, field trials have been conducted in the Sandveld and Limpopo. Results are pending. Pot trials are currently underway to confirm quantification of inoculum from soil using Real-time PCR methods. Alternative host species for the various pathogens (both weeds and other crops) are being investigated.
This project will aid growers in making informed decisions before planting, in order to reduce risks associated with soil-borne diseases.
DEPARTMENT OF AGRICULTURE WESTERN CAPE
• Investigation into the influence of climate on the activity and intensity of potato viral vectors in the winter rainfall region – Dr Kobus Laubscher
The western Cape experienced another good raining season during 2008. During the first part of the year, little rain was experienced compared to the raining pattern during 2007 for the same period. Because of this phenomenon, we started to concentrate on certain months of the year during which low peach aphid numbers will have positive influences on the production of clean potato seed. The months that are concentrated on are March, April and May.
Experience with seed plantings during these months has over the past number of years delivered the best results regarding the production of clean seed. The low level of aphid activities during this period has resulted in a significant reduction of the non-certification of seed potatoes because of low levels of virus infections of the seed crop. One must acknowledge the fact that climate will have an influence on production practices for potatoes and that these
results may be an early indication of what problems may lie ahead in managing clean seed potato production for the Sandveld.
Some producers concentrate now on this part of the year for planting their seed crop to produce clean seed potatoes.
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