Yellow rust will pose a far greater threat in 2010 than for many years, a situation which is compounded by the high proportion of susceptible winter wheat varieties in the ground, warns Paul Fenwick of Nickerson.

The resurgence of yellow rust in winter wheat last year included a new, aggressive race which affected several high-profile varieties and significantly increased the amount of inoculum in circulation. The heightened virulence of the disease is of great concern because, typically, it follows a three-year cycle. In year one, new races appear in patches, in year two the level begins to build and by year three it could be widespread. 2010 is the third year in the three-year cycle!

Yellow rust can survive even harsh Canadian winters so the UK’s worst weather for two decades will have done little to diminish its potential impact. Air temperatures below -4°C will not necessarily have eliminated lesions, because under snow cover the actual temperature experienced by plants may not have dipped much below 0°C. If the leaves remain green (where yellow rust infection remains at a sub-clinical level, with fungal growth contained within the leaf) the threat remains.

For the disease to have a serious impact there must be a source of yellow rust inoculum in the autumn, with early-sown crops and volunteers providing a ‘green bridge’ to the new crop. Months can pass between infection and sporulation, with the disease only becoming visible once temperatures rise. Growers with susceptible varieties must remain vigilant, monitor crops carefully and appreciate that whilst persisting cold conditions during March and April will slow the onset, a warmer spring with ‘dewey’ nights will encourage it to develop rapidly.

Seed certifications indicate that varieties with Recommended List resistance ratings of ‘4’ or less account for over 40% of the 2010 winter wheat area and others are ‘seedling-susceptible’. Prior to adult-plant resistance taking effect these seedlings may support the disease and transmit it to varieties which lack adult-plant resistance. As the weather warms up in early spring pustules appear and the disease re-generates, spreading from plant to plant and field to field.

A good fungicidal seed treatment such as Epona (fluquinconazole + prochloraz) on seedling-susceptible varieties will reduce the ‘green bridge’ effect and prevent early yellow rust, but plants can still be infected later in the season. Nickerson has purposely diversified its breeding programme to develop lines with robust resistance in order to reduce the risk of serious problems from disease. Consequently, our varieties demonstrate long-term durability in the field and we remain confident of their ability to maintain excellent resistance to any potential disease epidemic.

To maximise the efficiency and timing of fungicide programmes one has to appreciate the difference between early- and late-rusting varieties. In 2009 some varieties became severely infected during May and June, before dry weather prevented further infection. However, later in the season, while the crop was still green, yellow rust flared up again underlining the risk from late attacks.

Nine of the 15 varieties rated ‘highly resistant’ (8 or 9) to yellow rust were bred by Nickerson, including Claire (which after 10 years on the Recommended List is still highly resistant) together with Alchemy, Cassius, Istabraq, Panorama, Xi19, Zebedee and Edmunds. The high-yielding Group 3 Invicta, new to the Recommended List for 2010, also has an ‘8’ rating for yellow rust and an excellent agronomic profile.

To minimise the potential impact of yellow rust growers should select varieties with a high Recommended List rating against the disease. If they do grow varieties that are susceptible my advice would be not to drill more than can be sprayed in a day.