Science - February 10, 2010

Virus versus virus in tomatoes

Market gardeners can protect tomatoes against the aggressive Pepino mosaic virus by employing a mild form of this virus. They do, however, have to select the right virus strain, as shown in the research by PhD student Inge Hanssen.

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Many variants of the CH2 strain of the virus originating from Chile are showing up currently in the horticulture sector. 'This strain is spreading very fast, without facing any resistance whatsoever', says Hanssen's supervisor Bart Thomma of the chair group Phytopathology. The aggressive variants can cause much damage to plants. The virus affects the leaves and the fruits of the tomato plant. Tomatoes with resulting yellow patches are not marketable.
Therefore, some growers pull out an old trick of the trade: they use mild virus strains to keep the aggressive virus at arm's length. 'The tomato plant would then be full of the mild virus, giving the aggressive strains no chance to infect it; the tomatoes will then have none of the symptoms', says Thomma. This form of 'cross protection' was regularly used in the past by the horticulture sector. Later on, preference was given to cultivating resistance in the plant.
Kinship
Hanssen, who works at the Flemish research station Scientia Terrae, went in search of virus strains lurking in tomato greenhouses. It appears that the European virus strain (EU) is on the comeback, and the Chilean strain is showing up increasingly. She also tried to find out if cross protection can be used to prevent problems caused by aggressive variants of the Chilean strain. She discovered that the mild version of the Chilean virus protected the plant from its aggressive brother. The European mild variant and the Peruvian strain LP originating from a nephew of the tomato did not offer enough protection. 'There must be sufficient kinship for the mild virus to be effective', concludes Thomma.
Some growers decide to infect the tomato with a mild variant of the virus early in the growing season. 'It is a rat race; the grower needs to ensure that the mild Chilean virus gets there before the aggressive virus.'
This strategy only works if the mild virus belongs to this Chilean strain, explains Thomma. Many Dutch growers use the mild variant of the widespread EU virus strain. This EU variant does not offer any protection against the aggressive CH2 virus. What's more, the damage can escalate if both strains come together in a merger infection, as Hanssen's research pointed out.
Risks
Flemish and Dutch tomato growers are not the only ones who face problems from the Pepino mosaic virus; this problem occurs worldwide, says Thomma.
The best strategy is to build resistance to the mosaic virus in the tomato. However, this takes time. Cross protection can be carried out in the meantime, but there are also risks. 'Because you can't control the viruses completely. The mild Chilean virus could adapt itself and become more aggressive. In addition, that mild Chilean virus may be safe for the tomato, but could cause problems, for example, for other plants. Although the virus can't do this itself, the risk does arise in cross protection.'

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