Research has shown that the tomato's genetic resistance to the fungal disease powdery mildew is dependent on monogenic and polygenic resistance. Many resistance genes have been localised on tomato chromosome six. In addition, the resistance mechanisms and the specificity of the resistance genes in general have been shown to be very diverse. These findings are contained in the research carried out by Yuling Bai at Wageningen University for her thesis: 'The genetics and mechanisms of resistance to tomato powdery mildew in Lycopersicon species'. Her work provides important new information for growers in their fight against this major fungal disease.

Powdery mildew is a disease caused by the fungus Oidium neolycopersici, which was first identified in Dutch tomato cultivation in 1986. Since this date the fungus has spread worldwide and been the cause of major problems. Little is known about O. neolycopersici and, with the exception of a few varieties of tomato recently brought onto the market, virtually all tomato varieties are susceptible to the fungus. As a result, chemical pesticides are used to combat its effects. The research by Bai into the mechanisms and the genetic basis of resistance to O. neolycopersici offers fresh leads for the development of new tomato varieties with sustainable resistance.

Bai succeeded in crossing resistance genes identified in wild tomatospecies into nearly isogenic lines (NILs) from the - by nature susceptible - tomato cultivar "Money Maker". These plant lines are almost identical genetically, with the exception of the resistance genes in question.

Using molecular markers and resistance tests, Bai discovered five dominant monogenic resistant genes, all of which are located on chromosome six. Earlier research has shown that chromosome six is a hot spot of resistant genes, including resistances against the leaf mould disease, viruses, nematodes, lice and whitefly. In addition, Bai analysed the quantitative resistance in the wild tomato variety L. parviflorum and found that the resistance was defined by at least three genetic loci (Quantitative Trait Loci/QTL). One of these QTL was also localised on chromosome six, linked to the monogenic resistance genes.

The fact that many resistance genes are localised on chromosome 6 is interesting from an evolutionary perspective, since many resistances involve the gene-for-gene interaction. In such an interaction, a fungal avirulence factor is recognized by a plant resistance protein, which triggers the resistance response. When the fungal DNA encoding the avirulence factor is 'coincidentally' changed by small mistakes during the cell division, the plant no longer recognises (the avirulence factors of) the fungus and no resistance reaction takes place.

This means the plant is no longer resistant to that fungal isolate. Because the plant's resistance genes are arranged in tandem arrays, new varieties of the genes can arise via incorrect chromosome pairing and recombination during the sexual cell division. This can lead to renewed recognition of the fungal isolate.

Various isolates of the fungus are present in the different countries where powdery mildew appears. In order to find out whether certain resistance genes can specifically work against certain varieties of the fungus, seeds of the "resistant" tomato NILs were sent to Florida and various countries in Europe. These plants were then exposed to the local fungus. In the Czech Republic, a tomato line with a specific monogenic resistance was susceptible to the fungus, which shows that the resistance is race specific and can be overcome by the fungus.
Bai's findings have provided greater insight into the interaction between tomato plants and O. neolycopersici. As a consequence, sustainable resistance, which the fungus cannot overcome or if so only with difficulty, is now within reach.

Yuling Bai will defend her thesis: 'The genetics and mechanisms of resistance to tomato powdery mildew in Lycopersicon species' on Wednesday 29 September at 13.30 in the Wageningen University aula: Gen. Foulkesweg 1a, Wageningen.

For more information and illustrations of affected plants: Edwin Luijks, tel. +31 (0) 317 48 39 15, email: Edwin.Luijks@wur.nl.
Questions on the content should be addressed to Guusje A. Bonnema, email: Guusje.Bonnema@wur.nl or Y. Bai, email: Bai.Yuling@wur.nl

The Plant Sciences Group of Wageningen UR is a collaboration between
* Wageningen University, Plant Sciences
* Plant Research International
* Applied Plant Research (PPO)