Fungicide resistance management strategy for barley ramularia leaf spot (Ramularia collo-cygni) in New Zealand

M. Christie 2022; updated 2023, R. Beresford

Disclaimer:

The fungicide and bactericide resistance information in the mode of action group table, the mode of action-based strategies and the crop-based strategies is intended to help manage the effects of pesticide resistance on plant disease control by agrochemicals in New Zealand. The New Zealand Committee on Pesticide Resistance (NZCPR) is a subcommittee of the New Zealand Plant Protection Society (NZPPS). The following parties: NZCPR, NZPPS, any people named in the strategies and any organisation the people may be affiliated with, do not guarantee that the resistance information is complete, consistent, accurate or up-to-date, or that resistance will not develop, or that the degree of resistance will decrease, as a result of the resistance information being used, and the parties accept no liability for loss or damage arising from the use of the resistance information contained in these strategies.

Introduction

Ramularia leaf spot (RLS), caused by the fungal pathogen Ramularia collo-cyngi is the major economic disease affecting commercial barley crops in New Zealand. (Foundation for Arable Research trial data). High disease pressure conditions include leaf surface wetness at GS 30 -31 and during the weeks following head emergence. The two main sources of disease spread are seed contamination (primary source) airborne spores (secondary source) from surrounding hosts. Early symptoms of RLS include small brown pepper spots, which develop quickly into typical RLS lesions on the upper side of the leaf.

Fungicides should be used as protectant treatments prior to appearance of disease symptoms when conditions are conducive to disease development.

Fungicide groups at risk from resistance in R. collo-cygni

Fungicide Resistance Action Committee (FRAC) mode of action (MOA) groups that are at risk from resistance and require resistance management for use in RLS control are as follows:

Group 3 Demethylation inhibitor (DMI) (medium to high risk)
Group 7 Succinate dehydrogenase inhibitor (SDHI) (medium to high risk)
Group 11 Quinone outside inhibitor (QoI) (high risk)

Table 1 lists the active ingredients available in these groups in New Zealand.

**Table 1:** Fungicides registered for ramularia leaf spot control in New Zealand by mode of action group
FRAC mode of action group		Active ingredient	Product trade names
3	DMI - triazole	Prothioconazole Mefentrifluconazole	Joust, Pilot, Procyon, Proline, Prothago, Thiazole, Vitalis Revylution
3 + 3	DMI + DMI	Epoxiconazole + prochloraz Prothioconazole + tebuconazole	Bolide Kestrel, Prosaro
3 + 7	DMI + SDHI	Epoxiconazole + Fluxapyroxad Prothioconazole + bixafen Prothioconazole + isoflucypram Mefentrifluconazole + fluxapyroxad	Adexar Aviator XPRO Caley IBLON Revystar
3 + 11	DMI + QoI	Epoxiconazole + azoxystrobin Prothioconazole + trifloxystrobin	Avior Gold, Delaro
7	SDHI (Succinate DeHydrogenase Inhibitors)	Fluxapyroxad Isopyrazam Isoflucypram	Imtrex, Systiva Seguris Flexi Vimey Iblon
11	QoI (Quinone outside inhibitors)	Trifloxystrobin	Provita

Resistance management recommendations by MOA group

DMI fungicides

Apply a maximum of 3 DMI-containing sprays per barley crop.
Repeated application of DMI or amine fungicides alone should not be used on the same crop in one season against risky pathogens (e.g. cereal powdery mildews, barley net blotch, scald) in areas of high disease pressure for that particular pathogen.
Reduced rates of DMIs can accelerate the shift to resistant pathogen populations. It is critical to use effective rates of DMIs in order to ensure robust disease control and effective resistance management. DMIs must provide effective disease control and be used at manufacturers' recommended rates.
When used in mixture, recommended effective rates of the SBI must be maintained. Split and reduced rate programmes, using multiple repeated applications at dose rates below manufacturer’s recommendations, provide continuous selection pressure and accelerate the development of resistant populations, and therefore must not be used.

SDHI fungicides

Apply a maximum of 2 SDHI containing sprays per barley crop.
When applying resistance management strategies for SDHI fungicides expect there to be cross resistance among all members of the SDHI group.
Apply SDHI-containing products at effective rates that will provide disease control and intervals according to manufacturers’ recommendations.
Always apply SDHI fungicides in mixture with a fungicide from a different mode of action group that provides effective control of the target disease when used alone. Refer to manufacturers' recommendations for application rates.
SDHIs used as a seed treatment in cereals providing foliar efficacy count against the total number of SDHI applications. (i.e. seed treatment counts as one application for season)
Apply the SDHI fungicide preventively or as early as possible in the disease cycle. Do not rely only on the curative potential of SDHI fungicides.
Reduced application rate programs including multiple applications must not be used. Refer to manufacturers’ recommendations for rates.

QoI fungicides

Apply a maximum of 2 QoI-containing sprays per barley crop.
Always apply QoI fungicides in mixture with a fungicide from a different mode of action group that provides effective control of the target disease when used alone. Refer to manufacturers' recommendations for application rates.
Apply QoI fungicides according to manufacturer’s recommendations for the target disease (or complex) at the specific crop growth stage indicated.
Apply the QoI fungicide preventively or as early as possible in the disease cycle. Do not rely only on the curative potential of QoI fungicides.
Split/reduced rate programmes using repeated applications accelerate the development of resistant populations and therefore must not be used.

Acknowledgements

FAR Cropping Strategies – Cereal Disease Management 2022
FRAC – (Fungicide Resistance Action Committee) Cereal guidelines
Novachem Agrichemical Manual