BSPP News Spring 2000 - Online Edition

The Newsletter of the British Society for Plant Pathology
Number 36, Spring 2000 

Conference and Travel Reports

14th Triennial Conference of the European Association for Potato Research
Sorrento, Italy : 2 - 7  May 1999

At first sight, the choice of venue for this Conference seemed strange for there was hardly a potato crop within 30 miles.  However, for climate, accommodation and ambience, it soon became apparent that it was an inspired choice.  The location of the Conference was the splendid Sorrento Palace Hotel, a self-contained congress hotel with accommodation for about 470 delegates, a large auditorium seating 1500 and several break-out rooms of various sizes.  This was set amongst the splendid scenery of the Amalfi peninsular, where if you were desperate for some exercise after a hard day of sessions, there were wonderful walks amidst the wild flowers.

At the pre-conference get-together on the Sunday evening the wine flowed freely and the food set the excellent standard that was to follow during the week. There were about 450 delegates, of which just over 30 came from the UK.  Two hundred lectures in six concurrent sessions, 160 posters and a choice of eight scientific and cultural visits, including Pompeii and Herculaneum, provided a very full programme. The President of the EAPR and Chairman of the conference organising committee was Professor Luigi Monti of the Department of Agronomy and Plant Genetics, Portici, Nr Naples.
The opening paper was given by Dr Walker from the International Potato Centre in Peru on 'World trends and patterns in the potato crop: an economic and geographic survey'.  Two points and one quote were of note.  The first point was that there was a decline in the consumption of 'starchy staples' per capita as economic growth increased and diets diversified.  Secondly, there has been a rise in the mean age of cultivars following the demand for Russet Burbank from restaurant chains such as MacDonalds.  His best quote must be that 'Economists have a way of making the obvious very painful!".

It was easy to tell that North America had rediscovered potato blight.  There was a substantial number of delegates from the USA and of all potato diseases, blight dominated the papers and posters by a long way.   Professor Fry from Cornell started the ball rolling by giving a presentation on 'The return of Phytophthora infestans, a potato pathogen that just won't quit'.  Professor Fry discussed the origin of blight in the Teluca Valley in Mexico now considered to be the centre of genetic diversity.

This introduction preceded a series of papers by Drs Flier and Turkensteen and others on the consequences of the new blight population and their characteristics.

Alternatives to agrochemicals are big at the moment. Dr Bång, from Sweden, reported that the new wonder control chemical for tuber blemish diseases is vapour from garlic.  In small-scale experiments significant disease control was obtained for silver scurf, black scurf and skin spot.  Yield from treated seed tubers was also increased.  Dr Lewosz from Poland reviewed physical and chemical control measures to inhibit sprouting and control diseases.  Of the chemicals he mentioned carbonates, bicarbonates and carboxylic acid.  His own work suggested proprionic acid vapour worked well against bacteria and silver scurf.

Thursday saw a programme of bacterial diseases and powdery scab.  Dr De Boer from Canada reported on laboratory testing by ELISA to index potatoes for ring rot.   No ring rot has been observed for 10 years but symptomless infection has been detected by ELISA.  Recent PCR has been used to validated the serological results.  The key question for the certifying authorities is the importance of symptomless presence.  The testing is now done by private accredited laboratories and results were presented on the auditing process.

Three papers from South Africa discussed brown rot (Ralstonia solanacearum). Dr Mienie presented a paper on biological control by Pseudomonas resinovorans.   As is usual for biocontrol agents, it was promising under glasshouse conditions but extensive field trials had not been undertaken. Dr Stander investigated the survival of Biovar II under different cropping systems and demonstrated that the pathogen is capable of surviving maize rotations, although there was less disease in comparison with bare fallow.  Soil survival exceed two years even under maize rotations.  Dr Theron discussed controls imposed as part of the SA certification scheme.  All fields registered for seed production are plotted using Global Positioning Systems (GPS) and all information relating to the field and provenance of the seed tubers kept on a database.  If brown rot is found a Bacterial Wilt Investigation Committee visits the farm to provide advice on the status of adjacent fields, crop rotation and sanitation measures.

Powdery scab still represents a challenge to researchers.  Dr Theron also popped up again to report on attempts to control the disease with zinc products and indicated that there was some reduction in severity.  He also reported on the spread of powdery scab from symptomless tubers to uncontaminated land and viewed this as a cause for concern when certifying potatoes.  Dr Merz from Switzerland posed the question 'Why should we care about powdery scab?'.  He answered it by indicating that the disease is on the increase due to the growing of more susceptible cultivars, more widespread use of irrigation and the ban on mercury as a tuber treatment.  He proposed an international initiative to co-ordinate efforts to develop an integrated control strategy.

Dr Tsror from Israel presented a paper on the control of tuber blemish diseases on imported seed.  He indicated that black dot was not only responsible for reducing tuber quality but also yield.  Fenpiclonil reduced the incidence of black dot on daughter tubers.

In the evening Dr Turkinsteen organised an impromptu meeting on potato late blight for interested parties.  Discussion centred around GILB (Global Initiative on Late Blight).  This is now six years old but is just taking off.  The meeting also considered a European dimension to tackle the problem.  Important areas for consideration were resistance breeding, more information on the pathogen, particular new strains, A2s and the role of oospores.  Dr Shaw (Bangor) is co-ordinating a European project on population studies. Dr Fleir proposed a Web site for interested parties.  He can be contacted at

GM potatoes featured, as would be expected, with a fascinating talk presented by Dr Mason from the Boyce Thomson Institute in New York on the opportunities for edible vaccines from potatoes.  It is hard to believe after hearing this talk that transgenic technology can be so easily dismissed by the public in Europe.

Friday saw a reduced programme as some presenters left early either without presenting or had given their paper on the previous day (look at notice boards more frequently!).  Dr Bus presented a paper on  control of powdery scab in the Netherlands by looking at the use of 'trap' crops.  Oilseed rape, white mustard and Italian ryegrass were used but they had no significant effect on the incidence and severity of powdery scab on the tubers of the following potato crop.  Due to a low incidence of the disease there were no significant differences between chemical treatments used for control (zinc, fluazinam and mancozeb).

Posters were supposed to be discussed in the relevant sessions.  How effective this was depended on the chairman of the session and the presence of the authors at those sessions - not often was our impression.  That was disappointing.  It was also rare to find an author present at his/her poster at the relevant time.

On the Wednesday we had the scientific and site-seeing excursions.  Two visits included the University of Naples Botanical Garden, based at Portici, laid out on the classical lines of Linnaeus.  Their speciality was cycads and succulents.  At the same time was a visit to the genetics department of the university.  This was housed in a castle and the laboratories appeared to consist of a series of chambers no wider than the corridors at CSL.  In the other half of the day there were visits to either the ruins at Pompeii or Heracleanum.  Nothing prepares you for their size.  Pompeii had in 79 AD, the time of the volcanic eruption, a population of 20000.  Only a quarter of Heracleanum has been excavated.  The remainder is under the present town, which was built on the ash which covered the original town.

Melvyn Askew was elected on to the Council of the EAPR.  Ulla Bång was elected for a further three-year term as Chairman of the Pathology Section.  A decision was taken to hold the next pathology section meeting in Poznan, Poland in July 2001 on the subject of diseases of the growing season.  It was also agreed to invite contributions on viruses and nematodes.  The next triennial meeting of the EAPR will be in Hamburg in 2002.

Many thanks were due to our host for organising an excellent conference in every respect.  The organisers of the next one in Hamburg will have a hard act to follow.  What did we learn?  That there is still an awful lot to find out about the 'humble' potato since its introduction to Europe in the late 16th Century and there is still a place for both fundamental and practical plant pathologists in potato pathology!

Nigel  Hardwick
Central Science Laboratory, Sand Hutton
and Stuart Wale
Scottish Agricultural College, Aberdeen


7th International Plant Virus Epidemiology Symposium
Almeria, Spain :  April 11 - 16, 1999

The seventh International Symposium on Plant Virus Epidemiology was held in Almeria on the south east mediterranean coast of Spain.  The Almeria district was chosen as the location because it is the centre of Spain's lucrative intensive plastic plant house based vegetable industry, and this 'out-of-season' industry is threatened by virus diseases particularly those transmitted by whiteflies.  The chief conference organiser was Alberto Fereres of the Consejo Superior de Investigaciones Scientificas in Madrid.

In the opening keynote presentation Mike Thresh (UK) summarised the past 100 years of plant virus epidemiology, tracing the build up in the knowledge base on the subject, from the discovery of viruses to the current position at the end of the present century. Mike emphasised how the ecological tradition in plant virology has recently recovered despite having reached a low ebb not too long ago.  He welcomed the increasing application of new techniques and innovations to help solve epidemiological problems and emphasised the continuing role of epidemiology in addressing the viral challenges we face as we move towards the next millennium.  Mike also traced the history of the Plant Virus Epidemiology Committee since its foundation in 1978 through the series of international meetings it has organised. The main aim of the group is to bring together researchers from all over the world working on epidemiology and control of plant virus diseases (and other interested parties) to discuss current research interests. A well deserved tribute to Mike was given in the conference dinner at the close of the symposium for the way he has so effectively 'fathered' the epidemiology group as its founding chairman over the last 21 years.
The first oral session dealt with 'plant virus transmission mechanisms' and began with Alberto Fereres who discussed electrical monitoring of insect probing and feeding behaviour during non-persistent virus transmission by aphids. Tom Pirone (USA), Benny Raccah (Israel) and Stephane Blanc (France) described virion-helper component-stylet interactions in non-persistent aphid transmission and methods of purifying helper components. Henryk Czosnek (Israel) described increased transmission of tomato yellow leaf curl virus through sexual contact between whiteflies, with more transmission occurring from male to female than female to male.  There were two presentations from the John Innes Centre (UK) on molecular aspects of the mechanisms involved in transmission of geminiviruses.  Finally, Dick Peters (the Netherlands) reported infection of transmitting and non-transmitting western flower and onion thrips by tomato spotted wilt virus.

The second session was entitled 'current approaches to plant virus epidemiology'.  Olga Esteban (Spain) described a 'squash capture PCR' for direct detection of non-persistently transmitted viruses in single aphids.  By selection of appropriate primers, nested PCR can be used to differentiate transmissible and non transmissible strains of the same virus within an aphid.  Chuck Nibblet (USA) reported differentiation of citrus tristeza virus strains in citrus samples based on minor sequence variations in their capsid proteins detected by RT-PCR.  Peter Markham (UK) used PCR and dot-blot hybridisation to study the geographical distribution of cotton leaf curl disease in Pakistan and the requirement for an additional nanovirus-like component for expression of typical leaf curl symptoms in cotton plants.  Ossmat Azzam (IRRI) used RT-PCR to show that a single field site can contain isolates of rice tungro spherical virus with different evolutionary histories.  Fernando Garcia-Arenal (Spain) and Donato Gallitelli (Italy) used RNA sequence information to study biodiversity and population changes in isolates of cucumber mosaic virus. Daniele Bourdin (France) and Trefor Woodford (UK) discussed the taxonomic status of the Myzus persicae vector group including M. persicae, M. antirrhinii and M. nicotianae, and the relative efficiencies of clones of each as potato leaf roll virus vectors.

The third session concerned 'whitefly-associated problems of vegetable crops'.  Emilio Rodriguez-Cerezo (Spain) traced the history of whitefly transmitted virus problems afflicting the plastic house cucurbit industry in Almeria.  In the early days, beet pseudo yellow virus transmitted by the glasshouse whitefly was the main concern but in 1990 Bemisia tabaci arrived and displaced the glasshouse whitefly bringing with it serious problems with cucurbit yellow stunting disorder.  Both viruses cause identical yellowing symptoms.  Two B. tabaci biotypes (B and Q) are present in Spain and both transmit the virus readily.  Very frequent application of insecticides is usually needed to keep the yellowing symptoms the virus causes at bay.  Avoidance of overlapping host crops and good weed removal are key control measures to decrease the virus source.  Rodrigo Valverde (USA) discussed three viruses commonly found together in the field in sweet potatoes in Louisiana: sweet potato feathery mottle, sweet potato chlorotic stunt and sweet potato leafcurl. Gail Wisler (USA) discussed the spread of three tomato-infecting viruses transmitted by whiteflies into new areas: tomato infectious chlorosis, tomato chlorosis and an unnamed virus.  Confusion of their symptoms with those of nutritional problems leads to underestimation of their occurrence. Enrique Moriones (Spain) reported two different types of tomato yellow leaf curl in Spain, the Sr and Is types.  The virus arrived in Spain in 1992 and was represented by the Sr biotype.  Starting in 1996, the more severe Is type, which is more readily transmitted by B. tabaci type B, displaced the Sr type.  Philip Stansly (USA) described management practices for geminivirus epidemics in field-grown tomatoes in Florida and neighbouring states. John Colvin (UK) described the spread and management of tomato leaf curl virus in tomato in southern India where insecticides are sprayed as often as every second day to control its whitefly vectors. Henryk Czosnek (Israel) discussed breeding tomato for resistance to tomato yellow leaf curl using resistance from Lycopersicum hirsutum.  This resistance is broad spectrum as it is also effective against certain other geminiviruses.  Moshe Lapidot (Israel) reported breeding for tolerance to tomato yellow leaf curl virus from tomato breeding lines TY172 and TY197.

After the coffee break, James Legg (IITA), Peter Markham and John Colvin (UK) described different facets of the current pandemic of cassava mosaic viruses in East Africa.  The severe form of the disease found mainly in Uganda is caused by a hybrid between east African cassava mosaic and African cassava mosaic.  The whitefly vector B. tabaci favours cassava as a host and multiplies better on infected than healthy cassava, especially in susceptible and sensitive cultivars, and increase in whitefly numbers is one of the main factors driving the pandemic.  Intensive rouging of symptom-affected cassava plants is advocated to prevent whitefly build up on infected plants.  Marcia Roye (Jamaica) described genetic diversity among geminiviruses infecting crops and weeds in Jamaica.  Fransisco Morales (CIAT) reviewed the current position with whitefly-transmitted viruses in Latin America.  At least 40 such viruses are known and the advent of the B biotype of B. tabaci in Latin America has greatly exacerbated the situation.  Thomas Henneberry (USA) described management strategies for dealing with Bemisia.  Avoidance of whitefly susceptible cultivars is important along with good irrigation and fertilisation, as stressed plants favour their build up and promoting healthy growth to avoid stress decreases their numbers.

After the poster session on Tuesday, there was an introduction to the technical field trip. Details of how the local plastic house vegetable industry at Almeria operates was provided along with information on the agronomic practices employed, virological and insect vector issues and a discussion of integrated disease management strategies.  The Almeria region produces 35% of Spain's horticultural export earnings with 50% of its production exported.  It has 27,000 hectares of plastic houses.  This was followed by a meeting on the activities of the Plant Virus Epidemiology Committee of the International Society of Plant Pathology.

On Wednesday, the technical field trip involved visits to plastic houses where tomatoes, capsicums and various cucurbitaceous crops were being grown.  Among the virus diseases commonly found in the plastic houses are tomato yellow leaf curl and cucurbit yellow stunting disorder (whitefly-borne), tomato spotted wilt (thrips-borne), cucumber mosaic, watermelon mosaic and squash mosaic (aphid-borne), and melon necrotic spot (Olpidium vectored). Integrated disease management strategies have been developed for each individual crop and are being gradually adopted.  These greatly decrease the requirement for multiple insecticide sprays.  There were also visits to a vegetable packing plant and to a local horticultural research station where experiments involving vegetable crops in plastic houses and fruit trees of a range of types were demonstrated.

On Thursday, the fourth session concerned 'modelling plant virus epidemics'.  Larry Madden (USA) set the ball rolling with a theoretical assessment of the impacts of different virus-vector transmission mechanisms on plant virus disease epidemics. This was not an easy talk for the less mathematically minded!  Johnson Holt (UK) described a new general model of plant virus disease spread that incorporates vector aggregation.  With cassava mosaic virus spatial aggregation of vectors on cassava plants is an inevitable consequence of infection which promotes their reproduction.  Vector aggregation decreases the effective contact rate and therefore the predicted abundance of infected hosts.  His model takes this into account.  Eventually overcrowding leads to emigration of vectors and dispersal of inoculum to other fields.  In contrast, Mike Jeger (the Netherlands) discussed modelling virus source effects in tree nurseries rather than vector populations.  Sarah Pethybridge (Australia) described using spatial analyses of spread patterns of three viruses in hop plantations to hypothesise on the means of spread. Merrit Nelson (USA) then gave a 'big picture' account of analysis of regional virus epidemics and vector incidence using GIS and geostatics to guide management decisions. Pamela Anderson (CIAT) described the ongoing development of a mathematical model as an analytical tool to prioritise integrated virus disease management research on whitefly-transmitted viruses in Latin America.  This was another 'big picture' talk covering the situation, particularly in tomatoes, in Latin America and the Caribbean. Forest Nutter (USA) reported on temporal and spatial analysis of data on spread of two phytoplasma diseases infecting papaya plantations in the Northern Territory of Australia.  Debbie Thackray (Australia) described a simulation model forecasting aphid outbreaks and cucumber mosaic virus epidemics in lupin crops in the mediterranean-type climate of Western Australia.

The fifth oral session was entitled 'epidemiology of arthropod-borne viruses'.  Anna Maria Pereira (Portugal) reported on the occurrence of tospoviruses since Western Flower Thrips was first found in 1989 in Portugal.  Tomato spotted wilt virus was first found in the following year and impatiens necrotic spot virus in 1994.  The former is now widespread.  Mariano Camba (Spain) reported on citrus tristeza virus in Valencia. Dick Peters (the Netherlands) described different patterns of spread of rice yellow mottle virus in irrigated rice in Africa and concluded that most were consistent with contact rather than beetle transmission.  Pablo Vercruysse (Belgium) reported on facets of the epidemiology of the carrot motley dwarf complex in parsley.  Mats Lindblad (Sweden) described the epidemiology and control of the leafhopper-borne wheat dwarf virus in winter wheat.  Abdullah Gera (Israel) described transmission of iris yellow spot tospovirus by Thrips tabaci. Angeles Achon (Spain) reported on the occurrence of maize dwarf mosaic and sugar cane mosaic viruses infecting maize.  The former was by far the most abundant in Spain where Sorghum halepense was the key reservoir host.

The final oral session dealt with 'management and control strategies'. Fernando Ponz (Spain) described a way of classifying isolates of potato virus Y from pepper distinguishing four groups based on their reactions to pepper virus resistance alleles.  Peter Thomas (USA) gave a paper on pathogen derived transgenic resistance. Michel Ravelandro (France) reported on the performance of a transgenic plum line (C5) transformed with a coat protein construct of plum pox virus. Amit Gal-On (Israel) described production of a full length infectious clone (AG1) of attenuated zuchini yellow mosaic virus that harbours a point mutation which abolishes aphid transmission.  Field experiments in squash and watermelon demonstrated a protective effect of the clone when used for cross protection.  Tefion Jones (UK) reported that, in assessments made over five years in the field, blackcurrant gene Ce conferred effective resistance to blackcurrant reversion disease and to its gall mite vector.  A second gene P conferred only partial resistance to both.  Yeheskel Antignus (Israel) provided an update on the use of UV-absorbing polythene in plastic houses to protect against infestation with B. tabaci type B and western flower thrips vectors.  Dramatic reductions in insect vector numbers were recorded where UV light is removed by making plant houses of this plastic. The final two papers dealt with integrated disease management strategies for virus disease control, the first from a virologist's perspective and the second from an entomologist's standpoint.  Roger Jones (Australia) described the development of effective integrated disease management strategies that have been widely adopted for control of cucumber mosaic and bean yellow mosaic viruses in lupins. Mike Irwin (USA) stressed the problems of using traditional IPM approaches for controlling insect feeding damage when dealing with insect vectors and attempts to minimise spread of the viruses they transmit.  He also emphasised the additive effect in terms of the amount of control obtained when different types of control measures are combined and the importance of considering interactions.

The symposium was very intensive with many contributions in each session.  There was also a very extensive set of poster presentations complementing the oral sessions, but covering a much broader range of issues.  The conference dinner on the last day was most enjoyable complete with delicious Spanish food, a fine array of local wines and "flamenco" dancing.
This symposium was well organised, productive and informative.  There was a lot of stimulating discussion both in the formal sessions and, especially, outside them.  I have returned home in an enthused frame of mind with a lot of new information and useful contacts. The organisers of the symposium, Alberto Fereres and his helpers from Madrid and Almeria, should be heartily congratulated on a job well done.  I thank the BSPP for the travel award that enabled me to participate in and present research at this memorable meeting.

Roger Jones
Co-operative Research Centre For Legumes In Mediterranean Agriculture And Agriculture Western Australia