BSPP News Autumn 2002 - Online Edition

The Newsletter of the British Society for Plant Pathology
Number 43, Autumn 2002 

Conference Reports:

BSSP Presidential Conference: Plant Pathology and Global Food Security Imperial College, London : 8-10 July, 2002 

The aim of this meeting at Imperial College (part of the University of London) was to mobilise discussion of the key issues and publicise the role of plant pathology in addressing major global food security concerns. The meeting  was mainly used as a forum for addressing some of the important policy issues that are presently affecting those working in tropical agriculture, as well as those working on more fundamental aspects of plant pathology that may have an impact on developing country agriculture.

The three days of talks and poster presentations began with Professor Roger Plumb giving his presidential address. He mainly talked about his own work experience with viruses and he used viruses of the Poaceae as a case history in plant pathology. As an example, he cited Barley Yellow Dwarf and Barley Mosaic viruses to show how yields in the UK of wheat, barley and oats have increased from ~2 tonnes/ha to 8 tonnes/ha due to breeding, fertilisers and the fact that pathogens have been minimised. 

The next day began with session 1, 'Global Disease Concerns and Trade'. Megan Quinlan (CABI-Associate) spoke about 'pathogen introduction through aid and trade - plant quarantine and pest risk analysis needs'. The plant pathology community, Megan said, could assist in global trade and delivery of international aid by conducting framework Pest Risk Analyses and providing case studies and tools for risk management of key plant pathogens that are presently getting by plant quarantine systems in most of the world. 

'Sustainable Disease Management in Agricultural Systems' was the theme for session 2. Jules Pretty (University of Essex) set the scene with a talk on 'sustaining agricultural productivity through appropriate systems'. Adequate and appropriate food supply is a necessary condition for eliminating hunger, he said. But increased food supply does not automatically mean increased food security for all. 

Jill Lenné and David Wood (Agrobiodiversity International) spoke about 'Ecological approaches to sustainable disease management'. Different approaches to managing the interactions between food crops, pathogens, associated organisms and their physical environment to minimise losses were reviewed as well as the need to produce more food, more cost-effectively, less-labour- and knowledge-intensively and more sustainably to achieve global food security and conserve the natural resource base. They also looked at management of the crop and its relationship with other organisms in the field and the landscape and how improved management of soil, water, and climatic factors can contribute to sustainable disease management. They concluded that the reality is that farmers will make choices for disease management mainly based on sound socio-economic principles. 

'Constraints to research and barriers to uptake in disease management strategies' was introduced by R.J. Hillocks and S. Eden-Green, Natural Resources Institute, Chatham. Agriculture and rural development especially in developing countries have the potential to play a central role in poverty alleviation and sustainable development. The perception in the donor community is that the outputs of research have not been widely adopted by farming communities. Whilst this may be the case to some extent, there have been examples of successful adoption of agricultural research outputs. Strategies can be implemented at the stage of project design, to develop the linkages and pathways for adoption of the intended outputs. 

'Genetic resources, increased diversity, and disease resistance' was the topic introduced by S. Rajaram and Jesse Dubin from CIMMYT. Their talk revolved around the need for conserving genetic resources through gene banks for the benefit of all human kind. These resources have the potential of contributing highly useful genes for the improvement of crops, which should help bring about food security in resource-poor countries and in the future. Martin Wolfe (Elm Farm Research Centre, UK) looked at the benefits derived from inter-cropping systems which are not limited to control of diseases, pests and weeds but can also provide buffering against a wide range of other environmental variables. In terms of sustainability and food security, the diversity of produce from such systems can help to buffer the producer against unexpected variations in the market place.

The poster session attracted about 20 posters from scientists and students. This gave me an opportunity to present my own poster, which looked at the rice blast pathogen population structure in West Africa for improved disease management. We have generated baseline data on the genetic and pathotypic diversity of the rice blast pathogen populations in four West African countries: Côte d'Ivoire, Burkina Faso, Ghana and Nigeria. Key rice screening sites have been characterised and the dominant lineage/pathotype groups identified. This data is being used as a framework to identify resistance sources. Use of different strategies such as the 'lineage-exclusion', planting mixed stands of rice cultivars with a different spectrum of resistance to Magnaporthe grisea (causal agent of rice blast), coupled with appropriate cultural practices including seed hygiene and continued disease monitoring, could lead to sustainable control of rice blast in developing countries.

Phil Jones (IACR Rothamsted, UK) and Mark Holderness (CAB International) looked at 'Meeting diagnostic needs in developing countries'. They highlighted massive challenges faced by developing countries in areas of diagnostic and advisory support systems. Even though new tools are available to simplify and expedite diagnosis, they are not appropriate or accessible to developing country users for reasons of capital cost, skills and infrastructure availability or inadequate maintenance budgets. Opportunities to overcome such challenges include use of low-cost diagnostic kits, greater access to appropriate electronic and other information resources, knowledge sharing systems, appropriate backstopping systems for local advisors, relevant training and new ways by which farmers can be more directly linked with supporting services. 

Peter Scott (CABI) talked about the impact of communications technology on knowledge transfer in developing countries. Peter's talk focussed on knowledge as the key tool for disease control as having knowledge gives confidence. The International Society for Plant Pathology, through its Task Force on Global Food Security, has defined an action programme with five activities all of which are concerned with knowledge and knowledge transfer. Examples were given of the extension of knowledge in plant pathology at an institutional level and at the level of the grower, focusing on the key role played by information and communications technology. 

Professor John Whipps (HRI) presented the third paper of the session on biotechnology and the development of biological disease control. He stated that biotechnology is gradually playing a more important role in the development of biological disease control. Areas involved include molecular biology and inoculum production, formulation and application procedures. There are now over 80 bacterial and fungal products on the market or undergoing registration with disease control capabilities and, for the majority, biotechnology has played an important role in their development. 

Margarita Escalar (ISAAA, South East Asia) presented the last paper of the final session, on implications of GM technologies for livelihoods in developing countries. She expounded on biotechnology as a research tool essential to develop new crop varieties needed to yield more with lower inputs, and to have greater tolerance to various pest types and to abiotic stresses. She said one product of the application of biotechnology was genetically modified (GM) crops expressing useful traits. Currently, GM crops are commercially grown in 14 countries, and are being field tested in twice that number. The benefits from agricultural biotechnology for the small-holder farmer in developing countries are many and may be expected to receive more recognition from the public in the future. According to Margarita, while the current GM crops contain many agronomic traits and are therefore of direct benefit to farmers, it is anticipated that in the near future, improved nutrition and health-enhancing traits in the public knowledge, and the regulatory systems to assure safety, GM crops are anticipated to play a key role in food security and economic development of many developing countries.

The P.H. Gregory competition for the best lecture by a student or other young pathologist was held in the afternoon of the first day. In all, we had ten talks ranging from biological control of late blight of potatoes to characterisation of a collection of flax-root-colonising fungi and their implication in root rots. The P.H. Gregory Prize was awarded to S. Surujdeo-Maharaj, of the University of the West Indies with his talk entitled, 'An optimised inoculation method to screen cacao (Theobroma cacao L.) for resistance to Witch's Broom Disease caused by Crinipellis perniciosa'. Three methods of inoculation (agar-drip, water-drip and spray) were tested. The optimised method (350,000 viable basidiospores/ml; 60h incubation; agar-drop technique) was able to produce 100% infection on both clonal and seedling plants of a susceptible genotype, on a repeatable basis.

I gratefully acknowledge financial support from BSPP to assist me to attend this conference. 

Jack Chipili 
Horticulture Research International, Wellesbourne

VIII International Plant Virus Epidemiology Symposium: First Steps into the New Millennium 

Aschersleben, Germany : 12 - 17 May 2002

This successful and stimulating international symposium was attended by 155 participants from 35 different countries in five continents. It was the eighth in the series of symposia held every three years under the auspices of the Plant Virus Epidemiology Committee of the International Society for Plant Pathology.

An evening welcoming reception in the town hall was hosted by the Mayor of Aschersleben. The opening session on the following morning included brief presentations by Thomas Kuehne, the principal symposium organiser, the Mayor of Aschersleben, and Manfred Lueckemeyer representing the German Federal Ministry of Consumer Protection, Food and Agriculture. Aschersleben's Agricultural Research Station was founded in 1920 and rebuilt after the second world war by Professor Maximillian Klinkowski. Since 1992, the two institutes in Aschersleben (Institute for Resistance Research and Pathogen Diagnostics and the Institute for Epidemiology and Resistance) have belonged to the newly founded Federal Centre for Breeding Research on Cultivated Plants (www.bafz. de). Their current main emphasis is on disease resistance and epidemiology of plant pathogens.

In the Chairman's address, Roger Jones (Australia) spoke on "Using epidemiological information to develop effective integrated virus disease management strategies". He stressed the dangers of deploying single control measures against virus diseases and the synergistic benefits that result from combining control measures with different modes of action. He described the types of epidemiological information needed for effective integrated management strategies to be developed for specific pathosystems. Integrated virus disease management strategies need to be tailor-made by selecting the mix of host resistance, cultural, chemical or biological control measures most suited to each individual pathosystem. He gave examples of integrated management strategies designed for viral pathogens transmitted in different ways.

The first major session, on Virus resistance in plants, began with a contribution on "Durable virus resistance through conventional approaches" by Herve Lecoq (France). He comprehensively reviewed how different types of host resistance to viruses operate, how virus evolution can combat host resistance, and the tendency of resistance-breaking strains to be poorly competitive in mixed infection with other strains. The numbers of nucleotide changes in the virus genome required to overcome host resistance contribute to its durability. Combining cultural control measures with virus resistance improves resistance durability.

 In a talk on "Molecular-biological aspects of virus resistance", Jari Valconnen (Sweden) described how single dominant and recessive resistance genes operate, and the way that characterisation of resistance genes can provide novel insights into virus resistance mechanisms, viral synergism and the 'recovery' phenomenon. During the last two years, genes controlling RNA silencing (encoding polymerases and helicase) and a universal RNA surveillance system (encoding calmodulin-related protein) have been found in plants. The helper protein of potyviruses (HC-pro) and several other viral proteins can act as silencing suppressors.
Other papers in this session were on: control of mite-transmitted pigeon pea sterility mosaic disease in India through viral resistance (Jones, Scotland); ability of barley yellow mosaic virus 2 to overcome the ym4 resistance gene in barley and the pivotal role played by viral RNA1 (Kuehne, Germany); the outstanding performance of transgenic resistance to cucumber fruit mottle virus in cucumbers carrying viral coat protein or replicase genes (Gal-on, Israel); the behaviour of pathogen-derived resistance in transgenic plums challenged with aphid inoculated plum pox virus and exposed for 5 years in the field (Ravelandro, France); and the instability in field conditions of pathogen-derived resistance to potato virus Y (Schubert, Germany).

The next day was devoted to Virus-vector interactions, which started with a presentation from Edgar Schliephake (Germany) on "Aphid behaviour and virus transmission". He reviewed the features of aphids that make them well suited to transmitting viruses, surviving adverse environmental conditions and rapidly exploiting opportunities to colonise plants. The relative advantages of using different types of insect traps were discussed along with use of electrical penetration graph data to provide information about probing and virus transmission by aphids.

There followed papers on: detecting tristeza virus by squash capture-PCR in aphids visiting citrus trees (Cambra, Spain); using chi-maeric viruses to determine molecular mechanisms of transmission of beet western yellows and cucurbit aphid-borne yellows viruses by aphids (Herrbach, France); the specificity of the potyvirus helper component in aphid transmission (Huet, Israel); using aphid vector resistance to control groundnut rosette virus disease in sub-saharan Africa (Willekens, UK); whitefly transmission of sweet potato viruses (Valverde, USA); the relationship between cucumber vein yellowing virus and its whitefly vector Bemisia tabaci (Caciagli, France); genotypic variation in B. tabaci in Africa (Abdullahi, (Germany), and transmission of wheat dwarf virus by the leafhopper Psammotettix alienus (Manurung, Germany).

In a presentation on the relationship between thrips development and tospovirus transmission, Moritz (Germany) reported that the separation of salivary gland from midgut during larval development is the reason why only first and early second instar thrips larvae can acquire tospoviruses. The final two papers were on: festuca leaf streak virus reaching the salivary gland being responsible for its transmission by its plant hopper vector Javesella pellucida (Lundsgaard, Denmark), and an entertaining contribution on contact transmission of rice yellow mottle virus in rice crops by rats and cattle (Sarra, Mali).

On Wednesday, we had a scientific excursion to three sites:
1. In an impressive field experiment at the Institute of Horticultural Crops at Quedlinburg, many brassica species infected with turnip mosaic virus (TuMV) showed a diverse array of viral symptoms. TuMV-resistant brassica germ-plasm was demonstrated along with new resistance sources in Chinese cabbage, horseradish, primitive forms of cabbage and Raphano-brassica hybrids and breeding lines of cauliflower. Breeding different brassicas for TuMV resistance was described by the researcher involved (Reiner Kraemer).

2. At Morgenrot we saw extensive field trials plots belonging to the plant breeding company Saatzucht Joseph Breun, where large-scale breeding of barley for resistance to barley yellow mosaic and barley mild mosaic viruses is underway on land heavily infested with both. Susceptible and resistant responses to challenge with soil-borne virus inoculum were evident.

3. At Gatersleben, we visited the genebank of the Institute of Plant Genetics and Crop Plant Research. This is one of the biggest in the world, with more than 10,000 accessions. It is important as a genetic resource for breeders and in preventing genetic erosion in crop plants and their wild relatives.

The topic for the next morning was Molecular and general virus epidemiology, beginning with a presentation from Rene van der Vlught (Netherlands) on "Plant virus epidemiology: facts and future". He emphasised how understanding the disease triangle and interactions between its constituents (virus-host-environment) is essential to assess the risk of epidemic development. Modern techniques for virus detection assist enormously in obtaining information needed to develop this understanding. The recent European outbreaks of potato virus Y-NTNstrain in potato and pepino mosaic virus in tomato are examples of successful deployment of the latest molecular and immunological methods for this purpose.

Papers with a molecular emphasis included: variation of two strains of wheat dwarf virus differing in host preference and nucleic acid sequence, from wheat and barley (Kvarnheden, Sweden); variation in the coat proteins of field populations of maize dwarf mosaic virus from corn (Salomon, Israel); and characterisation of Zanzibar cassava mosaic virus, a new cassava virus (Maruthi, UK). Harper (UK) described how banana streak virus disease can arise in banana plants from release stimulated by 'stressful conditions' of integrated copies of the virus sequence incorporated within the host B genome. He reported that such incorporation into the viral genome also occurs widely with other badnaviruses in several different crop plants. 

More traditional virus epidemiology papers described: western flower thrips-vectored outbreaks of tomato spotted wilt virus in potato (Thompson, South Africa); aphid vector behaviour and epidemiology of viruses infecting lettuce and broccoli crops (Fereres, Spain); biotic factors driving epidemics of tomato leaf curl virus in India (Colvin, UK); epidemiology and control of carrot virus Y (Latham, Australia); the current continent-wide position of the African pandemic of cassava mosaic virus (Legg, Uganda); and spread of little cherry disease (Eppler, Germany).

In the session on Dynamics of virus spread, Mike Thresh (UK) provided a comprehensive review of spatial patterns of virus spread. He described the influence on spatial virus pattern of wind direction, landscape features and planting schemes. He provided examples of random, clumped and systematic patterns in the distribution of diseased plants for scenarios with soil-borne and aerial vectors, and with internal or external virus infection sources. He also discussed different types of virus infection gradients. These spatial patterns and gradients influence the effectiveness of isolation and other approaches to virus control by an appropriate disposition of plantings to prevent or delay virus spread.

Papers followed on: spread of iris yellow spot tospovirus by Thrips tabaci in onion (Gera, Israel); decreasing spread of tomato spotted wilt virus by thrips in thrips-resistant pepper (Maris, Netherlands); modelling tomato spotted wilt virus epidemics in potatoes (Jericho, Australia); and the epidemiology of several viruses in different crops in the Ukraine (Polischuk, Ukraine). Thackray (Australia) described a well-validated simulation model that uses pre-growing season rainfall to forecast aphid outbreaks and barley yellow dwarf virus epidemics in a typical Mediterranean-type climate. Predictions for date of aphid arrival, amount of virus spread, yield losses and the need for early insecticide sprays are provided. 

At the conference dinner that followed, participants were treated to a delicious multiple course meal plus a choice of wines, followed by typical folksinging by singers from the Harz mountains complete with yodelling and audience participation. Presentations were made to Thomas Kuehne to thank him and the Aschersleben team for all the hard work in organising such a successful conference and to Gerhard Proeseler for his role in the conference organisation and to wish him well in his retirement.

Francisco Morales (Colombia) began the final session of the conference, on Strategies for virus control, with a presentation on "ecology and epidemiology of whitefly-transmitted viruses in Latin America". This was a "big-picture" paper that started by outlining which virus diseases are currently most important on major crops grown from Mexico and the Caribbean islands in the north right through to the southern cone of South America. He emphasised the widespread damage caused by whitefly-transmitted begomo-viruses and discussed modelling studies using the FloraMap program with data from 304 geo-referenced points that help explain their distribution. The key to occurrence is climatic conditions that favour their vector Bemisia tabaci, in particular a dry season of at least 4 months and a mean temperature above 21oC for the hottest month of the year.

Presentations on control strategies included: extension and uptake by farmers of single control measures versus integrated control approaches for managing cassava mosaic virus (Sseruwagi, Uganda); using cross-protection with mild cassava mosaic strains to prevent infection by severe ones in a post-epidemic situation (Owor, Uganda); using resistant cultivars and phytosanitary measures to control sweet potato virus disease caused by mixed infection with sweet potato chlorotic stunt and sweet potato feathery mottle viruses (Gibson, UK); breeding of tomato resistant to tomato leaf curl virus (Muniyappa, India); hypersensitive and susceptible responses of rootstocks to infection with prunus necrotic ringspot and prune dwarf viruses (Lankes, Germany), and use of physical barriers, neem oil and Bion to control spread of potato virus Y in seed potatoes (Legor-buru, Spain). Addressing the important issue of how to control viruses in organic crops without herbicides or pesticides, Doring (Germany) demonstrated good suppression of spread of potato virus Y using straw mulch groundcover to decrease aphid vector landing rates in organic seed potato production.

An important feature of the symposium was the large number of interesting posters, more than 100 in total on a very diverse array of topics, confirming that plant virus epidemiology is alive and well in the new millennium. In particular, those on "Molecular and general virus epidemiology" were most impressive in both number and content.

The conference ended with an open discussion with contributions from invited speakers and the audience on four topics: likely virological consequences of further spread of the B type of Bemisia tabaci within Africa; future prospects for deployment of plants with pathogen-derived transgenic virus resistance in different parts of the world; how to use limited financial resources to best effect when funding epidemiology research; and how best to foster a partnership of mutual benefit between researchers involved in molecular laboratory research and those involved in practical field epidemiology studies.

This symposium was not only scientifically stimulating but also very efficiently organised, maintained the high standards set by past meetings of this group. The weekend Spring Festival that greeted participants on arrival at Aschers-leben, with street entertainment and a throng of happy people, the warm spring weather and sunshine every day, the spring flowers and trees out in blossom, and the many attractive traditional-style buildings in the town all helped to provide an ideal backdrop to the event. The Mayor of Aschersleben who graciously provided the reception and facilities, the German organising committee, and especially Thomas Kuehne and the staff of the two Aschersleben Institutes are to be congratulated warmly over a job well done.

 I am grateful to the BSPP for providing part of the funding that helped me to attend this very worthwhile conference.

Roger Jones
Agriculture Western Australia