BSPP News Autumn 2002 - Online Edition

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

GILB'02: Late Blight - Managing the Global threat and the 15th Triennial Conference of the European Association of Potato Research Hamburg, Germany, 11 - 19 July 2002

The GILB'02 (Global Initiative on Late Blight) conference was held immediately before the EAPR Triennial in Hamburg.  Very useful for those of us with a particular interest in Phytophthora infestans as we could attend two conferences for the price of one (no additional registration fee was charged for the GILB meeting).  GILB, formed in 1996, acts mainly as a facilitating organisation bringing scientists together and providing logistical support.  The previous GILB conference in Quito (March 1999) coincided with the declaration of a state of emergency in Ecuador; fortunately there was no such complication in Hamburg. Both conferences were held in the Congress Centre Hamburg, set in the extensive gardens of the 'Blumen und Pflanzen' (flowers and plants) park, which provided a useful escape for delegates between sessions. We were greeted by good blight weather - it rained stair rods.

The GILB Conference attracted 180 participants from more than 50 countries and nearly 100 posters were presented, indicating the level of interest in late blight. A tribute was paid to John Niederhauser for his contribution to knowledge and understanding of late blight. 

In his keynote address, Lod Turkensteen (Plant Research International, The Netherlands) presented a picture of increasing world-wide diversity of P. infestans with sexually reproducing populations of the pathogen replacing clonal ones everywhere and control becoming more and more problematic.  Oospores are apparently a source of primary infections in some years. Refrigerated storage increases the amount of latent infection in seed tubers, which then gets shipped elsewhere, so globalisation of the 'new blight' is very likely.  This is expected to give rise to the rapid development of genetic variation in the pathogen, and aggressive strains. Dr Turkensteen reiterated the threat posed by late blight to potato production worldwide, especially in areas where blight had not been a major constraint to potato production before.  The 'new blight' populations show epidemiological characteristics, which enable the pathogen to inflict more disease e.g., infection at a broader temperature range and noted that the window of effective protection by fungicides is narrowing.  He called upon breeders to breed for resistance that would be stable and durable. He challenged the International Potato Center (CIP) and GILB to take action to improve existing integrated late blight management strategies and initiate and facilitate efforts aimed to breed for potato cultivars featuring high levels of stable resistance to counterbalance increased levels of pathogenicity. 

Talks by Nicole Adler (International Potato Center [CIP], Ecuador) and by Dave Shaw (University of Wales, Bangor) suggested that factors such as host specificity and barriers to sexual recombination in P. infestans may be limiting diversity in ways that we are only just beginning to explore, and Jim Duncan (SCRI) asked if the danger of oospore-initiated infections might be more perceived than real.

Dr Eduardo Mizubuti spoke about two clonal lineages of P. infestans in Brazil designated BR-1 (A2 mating type) and US-1 (A1).  The BR-1 lineage is potato-adapted and US-1 is tomato-adapted.  He noted that the US-1 lineage caused disease at higher temperatures and the BR-1 at lower temperatures.

Progress has been made in breeding for host resistance, but Juan Landeo (CIP, Peru) emphasised that durability remains a major problem. Some varieties suitable for developing countries, which combine moderate and apparently stable field resistance with desirable agronomic traits, have proved successful in practice.  These allow resource-poor farmers to reduce fungicide inputs e.g. in Costa Rica, production has totally switched from susceptible cultivars to the resistant cv. Floresta, bred by CIP, and fungicide usage has been reduced by 50%.  In the developed world, however, breeding successful blight-resistant potato cultivars poses particular difficulties, since the market requirements for other characters are so specific and particular cultivars are strongly identified by end users.  New technologies can speed up selection: John Helgeson (University of Wisconsin, USA) described how molecular methods assist in breeding for late blight resistance, but there is still no substitute for careful disease resistance tests. He stressed the importance of validating glasshouse and molecular tests with field tests.  He said he had not had good results from detached leaflet tests.  This finding was endorsed in the discussion, and one poster provided a reminder that leaf position on the plant has a large effect. Unfortunately, getting varieties with improved blight resistance adopted commercially is tricky; Greg Forbes (CIP, Ecuador) commented that most developed countries have very little cultivar turnover, whereas the situation in developing countries is very different.

In discussions of breeding for host  resistance, diverse views were expressed as to whether it is desirable to use major-gene resistance. For some time the use of major R-genes has been avoided because the resistance breaks down, but some breeders favour their combination with field resistance, because of residual resistance after breakdown or because breakdown can sometimes be relatively slow (15 years in one case), and the level of resistance is higher while it lasts.  However, most breeders still favour the use of polygenic resistance rather than major genes, especially in view of current developments in the epidemiology and evolution of the pathogen (though the difference between the two types of resistance is less clear-cut than was once thought).  It was pointed out that resistance must be combined with high yield and quality in order to be accepted by farmers and supermarkets.

Bill Fry (Cornell, USA) highlighted the risk of blight from non-traditional sources e.g. petunia in the US, and Dani Shtienberg (Volcani Center, Israel) described how blight in Israel is less severe in areas irrigated by re-cycled water.  This reduction in disease was linked with boron in the irrigation water, which it is suggested induces systemic resistance.  Could be useful in organic production?  However, the effective concentration range is narrow: trace amounts of boron are essential to the plant, but too much is phytotoxic.  Does mild phytotoxicity induce resistance to blight? Boron appeared to induce systemic acquired resistance against P. infestans, and also against early blight (Alternaria solani), a serious disease problem in hotter climates.

In the session on Integrated Pest Management (IPM), Oscar Ortiz spoke about the work of CIP in teaching farmers about blight and IPM in developing countries. This is clearly vital, in addition to research, if blight control is to be achieved.  It was pointed out in discussion that the use of fungicides is impractical or unsafe in some parts of the world, either because farmers cannot obtain or afford them or because protective clothing for application is impractical, and indeed ineffective, in hot conditions.  It had already been suggested that we should aim for a level of durable resistance that would achieve effective control with lower doses of fungicide than are needed at present, or ideally none.

The GILB conference ended with a presentation by Wanda Collins (former GILB co-ordinator, now USDA) on the GILB website (now including all GILB'02 abstracts, see www.cipotato.org/gilb) and a discussion of GILB's future role.  This may be limited by funding, as CIP has had to reduce its support due to its own financial problems, but it is hoped to hold another conference in the next 2-4 years.

A one day break between the conferences provided time to explore the fine Blumen und Pflanzen gardens more fully.  There's a lovely rose garden (with piped classical music!), water gardens and tropical houses which provided a welcome escape from showers.

The following week, the EAPR Triennial Conference, attended by just over 500 delegates, was enlivened by the dynamic presence of Professor Gerhard Wenzel, President of the EAPR and Chairman of the organising committee.  He called for younger people to focus on agriculture and on its queen: the potato. (We weren't told what was the king of agriculture!)

In the plenary session,  Dr Cortbaoui from CIP discussed the position of the potato in world food production. He highlighted the potential of potato production to feed people in developing countries where land may be limiting but human resources are large. However, this potential was not being fulfilled due to losses caused by pests and diseases, particularly late blight (Phytophthora infestans). He believed that losses could be reduced by improving varietal resistance in these areas. 

Hannelore Daniel (University of Munich, Germany), discussing the use of functional foods (foods that beside their general nutritional value, specifically affect certain bodily functions), commented on the need for rigorous safety evaluation when giving doses of compounds not achieved in the normal diet. 

In the first of two papers on GM potatoes, Dr Willmitzer examined the potential of genetic engineering to improve not just disease resistance but also to produce novel starch products. He discussed how work in his laboratory had examined starch metabolism by substituting and replacing genes to gain a better understanding of the processes involved. Klaus Amman (University of Bern, Switzerland) discussed safety assessment of transgenic plants: "remember the risk of non-adoption" was a memorable remark (he is part of Bio-Scope, whose website at www.bio-scope.org is worth visiting). He believed that in Europe gene flow between potato populations would not be an issue but that volunteer potatoes posed more of a problem. 

In the Pathology and Breeding sessions,  P. infestans again featured prominently, but other diseases did get an occasional look in.  Petra Müller (Germany), reporting on behalf of an EU consortium, gave a surprising report that Clavibacter michiganensis ssp. sepedonicus (Cms), the cause of potato ring rot, can survive for up to 260 days on iron, wood, rubber and in most soil types and, when inoculated onto "non-hosts", produced symptoms on rape. The role of machines in the spreading of Cms was demonstrated and transmission was strongly determined by the presence of wounds. 

Karima Bouchek-Mechiche (France) reported on molecular tools for identifying and detecting Streptomyces spp. causing potato scab. PCR primers were developed from the sequences of the 16S rRNA gene to detect S. scabies, S. europaeiscabiei and S. stelliscabiei (common scab), and S. reticulisacbiei (netted scab). Primers developed to detect the common scab producing species also amplified some saprophytic strains, whereas primers designed for S. stelliscabiei did not cross-react with any other species. However, primers derived from the nec1 gene (pathogenicity factor) could be coupled with the 16S rRNA gene primers to distinguish the pathogenic species that cause common scab. These molecular tools will be more useful and provide rapid results than traditional methods (morphological and biochemical characterisation). Jari Valkonen (Finland) talked about common scab pathogens in Northern Scandinavia. Potato scab was caused by two species, S. scabies and S. turgidiscabies, and thaxtomin (phytotoxin inducing scab symptoms) production greatly differed between the species and the pattern and amounts produced were responsible for the different types of scab symptoms. 

Ulla Bang (Sweden) reported that natural pesticides can control potato diseases, and that garlic vapours have great potential. The contamination of progeny tubers by Helminthosporium solani (silver scurf), Rhizoctonia solani (black scurf), and Polyscytalum pustulans (skin spot) was consistently reduced after treatment of seed with garlic vapours, whilst eucalyptus oil and sage showed some fungicidal activity. As a contrast to natural products, Leah Tsror (Israel) reported that fungicide treatments (Diabolo, Lirotect, Fungazil, Celest, Octav) of seed tubers significantly reduced the disease index for silver scurf compared to non-treated controls during storage and on daughter tubers in field trials. She believed that seed potatoes from Europe were one of the main sources of inoculum for these diseases. Seed tubers contaminated with these diseases were treated with a selection of fungicides in the Netherlands before being exported to Israel.
Andreas Keiser reported on quality assurance in potato production in Switzerland. He surveyed problems associated with potato production on farms using conventional, organic and intergrated farming methods. On organic farms, a reduction in quality associated with Rhizoctonia was observed which was partly associated with an increase in the number of potato crops that followed two years of grass and the lack of seed tuber treatment. A high occurrence of powdery scab was only observed in the variety Agria (the most commonly grown variety in mainland Europe). 

Gunter Adam (Germany) talked about the development of array technology and its potential to be at the leading edge of breeding, plant production and consumer protection stages. For example, immediate screening of imported material for all known quarantine pathogens with one chip, the immediate ability to identify the breeding line or variety at every stage of development, and the possibility of monitoring wheat or barley for toxin-producing fungi are a few applications.

Back to blight!  Lod Turken-steen reported alternative hosts for Phytophthora infestans. This was the first report of P. infestans causing late blight symptoms on the indigenous weed species black nightshade and bittersweet in The Netherlands. These alternate host species are able to attenuate disease pressure through spore production on infected foliage and production of oopsores. Reinhold Bassler (Germany) talked about tuber treatments with metalaxyl variants against P. infestans. Tuber treatment delayed the outbreak of P. infestans, and it was possible to slow down the spread of the epidemic, but the substances used could not guarantee protection against late blight for the whole vegetation period. It was clear that further research with respect to dose, formulation and possible fungicide mixtures are necessary. Geerd Kessel (The Netherlands) reported that oospore formation was significantly inhibited by all fungicide applications (eg., Acrobat, Penncozeb, Shirlan, Curzate M) after spraying eight days after inoculation of plants with A1 and A2 strains of P. infestans. Jozefa Kapsa (Poland)  reported that the first late blight symptoms on stems were noticed mainly in the middle parts of plants of crops in Poland in 1997-2001, and occurred least frequently on the lower parts.  Laboratory and field trials also showed that effective control of stem late blight in potato crops could be achieved by spraying with systemic or trans-laminar fungicides and should be provided during three months or longer. 

In the molecular breeding section, C. Wegner reported on how the activation of defence responses in potatoes by an endogenous pectate lyase had improved resistance in potatoes to Erwinia caratovora subsp. caratovora the cause of soft rot. Pectate lyase (PL) enzymes are major virulence factors of E. caratovora subsp. caratovora, causing degradation of cell wall pectin into unsaturated oligo-galacturonates, which are known to elicit plant defence responses. These workers inserted DNA sequences specific for PL into the cultivar Desiree. Tissue tuber in PL-expressing showed improved resistance compared with the control cultivar. 

Although many workers discussed scientific developments that may in future have an impact on crop production other workers showed how simple improvements in cultural practice can prevent problems. A. Hanafi discussed methods of control of potato tuber moth (Phthorimea operculella) in North Africa. Although the moths do not cause damage to the foliage the larvae can cause considerable loss of market value to the tubers. Insecticides can be used to control moths on the foliage, but, as they have no impact on the larvae and moths are abundant on indigenous plants, this method of control was not recommended.  One of the most effective methods of preventing damage by the larvae was to prevent exposure of tubers by regularly ridging soil onto the potatoes as they developed. 

One of the oddest but most practical papers came from the EU Blight-Mop project. It has proved difficult to reduce blight infection using agronomic strategies alone, but a fascinating new method for dealing with volunteer potatoes was reported: applying pigs to potato crops!  Apparently they consumed all the volunteers in 3 days without damage to soil structure!

EAPR delegates not only had the opportunity to view over 100 posters offered for their own conference, but also most of the GILB posters as well.  Both poster sessions were well-attended.   The conference dinner was held in the evening on a Mississippi style paddle-steamer with plenty of food and entertainment as we sailed down the river. 

Edited from reports by:
Louise Cooke, Department of Agriculture & Rural Development, Northern Ireland
Danny Cullen,  Scottish Crop Research Institute
Diana Earnshaw, University of Wales, Bangor
Alex Hilton, SAC Aberdeen
and Ruth Solomon-Blackurn,  Scottish Crop Research Institute.