Paul Gans - Plant Health Management

Blight from oospore infection: could it happen here?

For many years it has been known that the blight fungus, Phytopthora infestans, has the potential to form oospores. These are the hard-walled resting spores, which are the result of the two mating strains of the fungus completing the sexual cycle. To date there has been no evidence that outbreaks in the UK are the result of oospores surviving and causing new outbreaks. During the last two years the BPC "Fight against Blight" survey has found a dramatic increase in the frequency of the second "A2" mating strain of P. infestans, increasing the chances of sexual mating in UK populations. It is therefore important to be aware of the risks.

A recently published report in the Netherlands contains interesting information from surveys of outbreaks over the period 1998 to 2005 and may give us some important clues. A total of 184 outbreaks were inspected and analysed. Out of these, 36% were associated with seed and 17% with oospores. Another 43% were attributed to an external source (dumps, volunteers in non potato crops other potato crops). A small number (5%) remained unclassified. The survey figures were broken down into 4 regions, North West, North East, South West and South East. More than three quarters of the oospore outbreaks were in the North East, which includes the district were starch potatoes are grown. This agrees very well with earlier Dutch research, which estimated the survival of oospores to be up to 4 years on sand and up to 3 years on clay soils. In the Western regions, where no oospore outbreaks were found, rotations are usually 1 in 4 or 1 in 5 and clay soils predominate. In the South East potatoes are often grown on sandy soils and rotations are in the order of 1 in 4. Starch potatoes in the North East are grown on sandy soils and rotations of 1 in 2 are not uncommon. Recently published data from Finland also demonstrate that outbreaks have occurred as a result from over-wintering oospores and potatoes immediately after potatoes were found to increase risk.

These results spell out a very clear message about rotations. Many growers in the UK already appreciate the importance of wide rotations for potatoes to produce healthy crops for other reasons (PCN, Rhizoctonia). Simple good practice is likely to significantly reduce risk. Volunteers and solanaceous weeds can also contribute to risk. Dutch researchers also found that oospores can be generated on Black night shade (Solanum nigrum), bittersweet night shade (Solanum dulcamara) and on the PCN trap crop Solanum sisymbriifolium. It is important to remain vigilant because although data from other countries may provide pointers, we are witnessing a changing situation.

Evenhuis B, Turkensteen LJ, Raatjes P, Flier WG (2007) Overzichts reportage 1998-2005 van primaire harden en eerste aantasting door Phytophthora infestans. Plant Research International BV, Wageningen Nota 435.