26 April 1996

Different eyespot strains complicate control issue

By Andrew Blake

DEBATE over the best way to tackle eyespot has been given a fresh tweak with AgrEvos response to an article on diagnostic kits (Arable, 12 Apr) and talk of the importance of W and R strains.

The suggestion that growers can save money by opting for flusilazole (as in Punch C) instead of using the supposedly more eyespot-specific prochloraz (as in Sportak 45) are misleading, claims the firms Patrick Vernié.

"A prochloraz-based treatment is a more reliable and robust recommendation," he says.

Costings from independent market research (Farmstat) paint the true picture, he says. "A Sportak 45 treatment is more than £5/ha cheaper than a Punch C treatment when comparing like with like."

But AgrEvo does not recommend applying Sportak 45 on its own – it should always be used as a tank mix with other fungicides such as triazoles, making overall treatment costs similar, he maintains. "So it is not a valid comparison."

Varying responses

Other factors besides cost must be considered, stresses Mr Vernié. Plant pathologists increasingly believe the R and W strains of eyespot are separate species, giving vary-ing responses to different fungicides.

In the UK as a whole, the R type dominates. "The R strains represent about 70% of the total UK eyespot population."

Prochloraz has good activity on both strains, he says, whereas flusilazole is equally good against the W type but less active on the R type. Du Ponts diagnostic kit can detect the presence or absence of eyespot but does not distinguish between the two strains. "More importantly it cannot evaluate the future risk of development of the disease. So you are gambling if you go for flusilazole."

Scottish trials, examining treatments of Sportak 45 (prochloraz alone) and Sanction (flusilazole alone) on sites with mainly the R strain, found distinct differences in performance, he adds. "The yield benefits of prochloraz were up to 1t/ha compared with 0.6t/ha for flusilazole."

There have been ITCF reports of poorer than expected eyespot control by prochloraz in northern France, possibly due to resistance, he concedes. "But its yield benefits do not seem to have been affected."

Treatment must be based on individual farm assessments, he adds. But for growers who believe first wheats are generally risk-free, he has this warning. "ADAS work shows that first wheats drilled in late September/early October are at higher risk than second wheats sown in mid-October."

lDu Ponts Andy Selley says HGCA work shows that where eyespot is a risk there is little difference in the benefits provided by a full rate of either flusilazole or prochloraz. "But we have data to show that flusilazole is more active against the W strain." The significance of that, he maintains, is that the W type is more pathenogenic than the R type.

Recent research suggest the R type is becoming harder to find – in the south at least, he adds.

Bob Polley, pathologist at CSL, Harpenden, confirms that the 70% R-type dominance figure comes from surveys conducted in the late 1980s and may have changed. &#42