Application timing of recent fungicides used in winter barley disease control programmes

Abstract

A 3-year study was undertaken to determine the influence of recently-introduced fungicides on the traditional timings employed for winter barley disease control. Previous advice had been to apply a T1 spray at GS30-31, followed 3-4 weeks later by a T2 spray, when using programmes based on fungicides with good eradicant activity (triazoles and morpholines).

However, the first new products (strobilurins and cyprodinil) were most effective as protectants. The ability to apply these slightly earlier would increase flexibility, but control of established disease could be jeopardised if applied too late. There was also evidence that the T2 spray, previously seen as less critical than T1, might take on new significance with the greening effects of strobilurins, and that a T3 spray might even be justified.

Field trials were established, starting in autumn 2001, at four sites in England. Their geographic spread and chosen varieties were targeted to represent key foliar disease scenarios. These were rhynchosporium (Hants.), brown rust (Glos.), brown rust and rhynchosporium (Lincs.), and net blotch and rhynchosporium (Norfolk).
21 fungicide treatments were examined, involving one, two or three-spray programmes at a range of timings. Single or first (T1) sprays were based on Acanto (picoxystrobin). Second (T2) sprays were based on Amistar (azoxystrobin). For all treatments applied up to GS49, Opus (epoxiconazole) + Corbel (fenpropimorph), or Unix (cyprodinil), were evaluated as alternative partners. Foliar disease and green leaf area were assessed, and trials were taken to yield. Samples were analysed for physical grain quality, and margins were calculated.

Single sprays gave larger yield increases at T2 than at T1. Two-sprays always gave a benefit compared to T1 only, but were not always better than T2 only. Two-sprays were more effective at controlling rhynchosporium, but under varying disease pressure, T2 applications were the most crucial for protecting green leaf. However, the three seasons were not characterised by high disease pressure prior to T1. Within a 14 day window, timing of the T1 spray had no consistent effect on yield. It was also not critical whether it was leaf 3 or 4 emerging at the time of application. Similarly, timing of the T2 spray rarely had a significant impact on green leaf retention or yield. However, there was sometimes an advantage to spraying at GS39-49 where rhynchosporium was the main disease, or spraying at GS49-59 where brown rust was dominant.  Despite some improvements in disease control, there were no significant yield benefits or increases in margin as a result of applying a T3 spray.

Whilst the pattern of disease resulting from seasonal or regional weather will always determine which timings are most beneficial, fungicide sprays at T2 are at least as important as at T1 in winter barley. Linked to this, yield responses have been higher with brown rust or net blotch than for an equal incidence of rhynchosporium. With strobilurin fungicides, there is flexibility in the timing of the T1 and T2 sprays, and the interval between them, whether a protectant or eradicant partner is used. However, it was rarely of benefit to delay the T1 or T2 spray where a protectant partner was used, even though few trials had a high requirement for eradication.