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TWENTY-NINTH ANNUAL SYMPOSIUM OF THE SOILBORNE PLANT DISEASES INTEREST GROUP OF SOUTH AFRICA

PROPAGATION MATERIAL AND SOILBORNE PLANT DISEASES

The Soilborne Plant Diseases Interest Group of South Africa hosted the 29th Interdisciplinary Symposium on Soilborne Plant Diseases on 18 and 19 September 2019 at the Vredenburg Research Centre of the ARC-PHP in Stellenbosch.  The topic for this year’s symposium was Propagation material and Soilborne Plant Diseases.  The event was attended by representatives of Research Councils, National and Provincial Departments of Agriculture, private companies, universities and farmers and participants represented a wide range of disciplines.

The following aspects were introduced and discussed in depth:

  1. State intervention in South African agriculture.
  2. Scientific method and habeas corpus: assessing the significance and management of seedborne pathogens causing soilborne plant diseases.
  3. A bird’s eye view of vegetable seed.
  4. Wheat seed: South African cultivars from cross to commercial seed.
  5. Quality of grain and oilseed crops in South Africa.
  6. Grafted vegetable transplants: A tool to overcome soilborne pathogens in Israel.
  7. Quality propagation material: the foundation of a sustainable and successful citrus industry.
  8. The science of crop protection.
  9. Host-mediated microbiome engineering for the control of soilborne plant disease.
  10. Regulatory aspects and nematode-free propagation material.
  11. Pod-nematode – a serious seedborne threat to the groundnut industry.
  12. Influence of bacterial contamination on propagation material quality.
  13. Seed quality and soilborne plant diseases.
  14. Protection of grapevine propagation material against black foot disease.

The opening address was delivered by Prof Nick Vink (Stellenbosch University, Stellenbosch) and the keynote address by Prof Lindsey Du Toit (Washington State University, Mount Vernon, USA). 

Conclusions reached by the speakers at this symposium can be summarized as follows:

  1. South Africa has a long history of support to agriculture, but this support was discriminatory, as it was focussed on white farmers. Nevertheless, there are lessons to be learnt from this history. These lessons are important, because it is common cause that the failure of land reform in the country is largely the result of the failure to establish a comprehensive farmer support system to help new farmers to become established, whether in the communal or commercial farming areas.
  2. In this era of increased global seed trade, the field of seed pathology is becoming increasingly important to try and mitigate in a robust, scientifically-based manner, the potential risks associated with movement of important seedborne pathogens that can also be soilborne. Expertise is needed that can account for the very complex nuances associated with seed pathology, including seed health testing and associated phytosanitary regulations and policies, in order to balance the need to facilitate seed trade while reducing risks associated with seed movement. Unfortunately, this is undermined by the extremely limited number of seed pathologists in public, private, and regulatory positions, and the scarce opportunities for education and training in seed pathology. A tremendous opportunity exists for plant pathology professional associations, university departments, private industry, and government agencies to seek out opportunities and resources to develop this badly needed area of expertise.
  3. Although the vegetable seed industry is much smaller than that of grain crops, it is much more complicated. The risk of seedborne plant pathogens is higher, compounded by the international movement of seed.  Seed companies strive to provide vegetable seed of high quality to growers, increase disease-resistance profiles of these nutritionally important crops, and to maintain sustainable seed production.
  4. The process from making the first cross until the producer can plant the cultivar can take up to 14 years. Sensako currently has a range of commercial cultivars and there are several pre-commercial cultivars from the Cape, irrigation and winter breeding programs. With continuous innovation our vision is to optimize cereal production and profitability for the benefit of cereal producers and processors alike through the development of adapted wheat cultivars for South African growing conditions.
  5. Grain and oilseed commodities are processed into food and feed products or exported unprocessed. South Africa produce a variety of grain and oilseed commodities and has a huge advantage as grain and oilseeds produced locally are of a generally good quality when compared with similar commodities produced elsewhere in the world.  Through Annual Crop Quality Surveys conducted, we have a database of quality data generated over different seasons and production regions.  This data can be used to advise the Regulatory Authorities on proposed regulations, to improve the efficiency in the agricultural industry, to identify trends with regards to changing environmental conditions, to ensure food safety and food security for the local population as well as information required by seed breeding companies to develop cultivars/hybrids best suited for production under South African conditions.  
  6. Grafting vegetable seedlings enables to overcome various soilborne pathogens, but rootstock genetic resistance should be expanded and stabilized. Since the latest findings suggest a synergistic effect of co-infection of viruses and other soilborne pathogens, it should be taken into account when we investigate epidemic or unusual disease severity in the field.
  7. The citrus industry of southern Africa has shown remarkable growth recently with export volumes reaching a record number of more than 130 million, 15 Kg, cartons. Since successfully implementing the citrus Improvement Scheme, the southern African citrus industry is free of many devastating exotic pests and diseases as well as contributing to an increase in sustainable yields and very high quality export fruit. Effective soilborne disease control can only be successful if all factors having an effect on the root system of the plant are taken into consideration and if it formed part of an Integrated Pest Management approach. Soilborne research assist the Citrus Improvement Scheme by ensuring that the plant material that is available for new plantings are free from soilborne pathogens. Quality propagation material is non-negotiable for a sustainable export driven southern African citrus industry.
  8. As Agricultural Scientists we have the responsibility to produce more good quality food on less arable land, with the additional challenge that grower margins are shrinking due to increasing input costs. To ensure that food remains affordable, science is key in finding innovative methods to farm more efficiently. This includes optimising plant performance and management of pests and diseases. The development of a crop protection solution is becoming complex in a highly regulated environment. This requires a significant investment in optimising efficacy of new crop protection solutions together with detailed environmental and formulation studies. Optimising crop performance requires the integration of traditional crop protection methods including biological solutions, seeds, traits and agronomic practices through digital technologies. Its success is dependent on access to data to ensure educated decisions.
  9. Classic plant breeding focuses on genes as the main source of variation. More recently there is an increasing realisation of the importance of phenotypic plasticity imparted by epigenetics and the plant microbiome which impact plant adaptability and survival. In addition, interplay between these two phenomena is being uncovered. Because epigenetics and the plant-associated microbiome are both key sources of phenotypic variation it can be argued that both together should be considered by plant breeders in that the holobiont (plant + microbiome) should be the unit under selection. These ‘non-conventional’ mediators of phenotypic variation can be seen as a toolbox for plant adaptation to the environment, the epigenetic effects of which can be inherited over generations (Vannier et al. 2015, Front Plant Sci. 6: 1159).
  10. Propagation material in the form of rooted plants, seed and tubers can be infested by the endoparasitic, semi-endoparasitic and migratory endoparasitic nematodes. The seed and gall nematodes found in seed and bulbs are also a risk to producers and it is essential that these risks are reduced by the implementation of quarantine and regulatory measures. In some instances, these measures exist in South Africa but often the wording fails the producers. This is particularly true where the words “visually free” are used and the nematode has no known external root symptoms. Producers should be made aware of the short comings of the system where it is pertinent e.g. in the case of pome fruit. In other cases, like groundnuts, a certification scheme must be put in place to ensure the current spread of infested seed. An example of a success is the Citrus Certification Scheme.
  11. The main effect of pod nematode on groundnut is qualitative, causing downgrading of yields to crushing grade. Control of this nematode is almost impossible. Nematicides is not sufficient in keeping this nematode under control and its short life cycle and high reproduction potential renders the use of crops other than groundnut in a crop rotation system unsuitable. The use of groundnut cultivars resistant to this nematode may be the only other alternative. Although resistant genotypes were identified, there are currently no resistant cultivars available on the market. The resistant sources should be included in a groundnut breeding program to develop resistant cultivars.
  12. The devastating effect that bacterial phytopathogens can have on agricultural production cannot be underestimated. Of the top ten most important bacterial pathogens in the world, seven have been reported in South Africa, of which most are soilborne and can spread through infected propagation material. Not only does this pose a serious threat to our respective industries, it also elicits immediate consideration from the research community to continue investigating possible interactions to broaden their understanding of the mechanisms at hand and to utilize that in developing innovative solutions to the problem.
  13. Poor variety purity, low germination percentages and low vigour all are an indication of poor seed quality leading to poor stand establishment caused by, among others, disease, insect, and weed problems, which will incur treatment costs. Often poor quality seed results from storage under sub-optimum conditions particularly in the case of farm-saved seeds.  Once planted this seed will in turn result in reduced yield and poor seed/grain quality. Krishibid Afzal Husain makes a profound statement regarding the importance of seed quality in the Daily Star of May 12, 2016: “The input like fertilizers, irrigation and plant protection measures and suitable agronomic practices contribute greatly towards enhancing yield and quality of the produce. If good quality seed is not used, the full benefit of such inputs and agronomic practices cannot be realised. The cost of seed represents a mere fraction (2-3%) of the total cost of production, but the quality of seed plays a decisive role in influencing the growers’ profitability. Quality seeds, therefore, is the basic critical input upon which all other inputs will depend for their full effectiveness”.
  14. Black foot disease of grapevine is one of the most important diseases, especially in grapevine nurseries and newly established vineyards. Although the disease occurs in various countries where grapevines are cultivated, very little is known about the disease especially with regards to control.  In this presentation recent advances that have been made to protect grapevine propagation material against BFD were discussed.  Although treatment of propagation material /grafted vines with the biological control agent Trichoderma before, during and / or after planting in nurseries delivered promising results it is still not as effective as hot water treatment of dormant nursery vines before establishment in new vineyards.
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