Fungicide resistance management strategy for myrtle rust (Austropuccinia psidii) in New Zealand

Information relevant to plant nurseries, council-controlled areas and private land, including home gardens

Issued 20 April 2023

R. M. Beresford, Principal Scientist, The New Zealand Institute of Plant & Food Research Ltd

G.C. Hagerty, New Zealand Committee on Pesticide Resistance, Fungicides Task Group Coordinator

Disclaimer:

While every effort has been made to ensure the strategy outlined in this document for the prevention and management of fungicide resistance in Austropuccinia psidii (myrtle rust) has been developed according to scientific information and good agricultural practice, neither the authors nor any organisation with which they may be affiliated warrant that resistance to the fungicide groups mentioned will not develop or that the degree of resistance will decrease as a result of this strategy being followed.

Introduction

Myrtle rust is caused by the fungal pathogen Austropuccinia psidii. It only infects plants in the myrtle family (Myrtaceae), of which there are about 30 native species and at least that many exotic species in New Zealand. Species vary in genetic susceptibility to myrtle rust infection and those proving to be most susceptible include the natives, maire tawake (Syzygium maire; swamp maire), ramarama (Lophomyrtus bullata), rōhutu (Lophomyrtus obcordata) and pōhutukawa (Metrosideros excelsa), and the exotic species lilly pilly (Syzygium australe; brush cherry), which is a native of Australia. A full list of myrtle rust host species recorded in New Zealand can be found at: https://www.myrtlerust.org.nz/about-myrtle-rust/species-infected-with-myrtle-rust-in-new-zealand/.

Myrtle rust was first found in New Zealand in 2017 and is now widespread throughout the North Island and upper the South Island. It has also been found as far south as Canterbury and the West Coast. The impacts of myrtle rust are greatest in warmer northern areas.

Myrtle rust control with fungicides

See the glossary at the end for explanation of fungicide related terms used in this strategy.

Myrtle rust can be effectively controlled using fungicides. New Zealand Plant Producers Inc. (NZPPI) has suggested a list of fungicide products available in New Zealand that are suitable for controlling myrtle rust (Download.aspx (nzppi.co.nz)).

No fungicides registered in New Zealand have label claims specifically for myrtle rust control; however, fungicides registered for other uses can be applied under ‘off-label use’ (Guidelines (nzgap.co.nz)). The application rate for off-label use should be chosen from the label rate for the fungicide product on an appropriate equivalent crop (Ministry for Primary Industries ACVM Register (nzfsa.govt.nz)).

A Growsafe Approved Handler Certificate is generally required when purchasing and applying fungicides (Home (growsafe.co.nz)). Anyone can buy the fungicides that are available from garden supply shops without an Approved Handler Certificate, but Growsafe Basic training is still recommended. Some garden supply shop products contain fungicide active ingredients that are effective against myrtle rust (see tables below).

Most fungicides potentially affect human health and the environment. For the hazard classification system for fungicides see Risks of Agrichemicals (growsafe.co.nz). To find out about the hazards for individual products, search online for the safety data sheet (SDS) under the fungicide product name.

Fungicide resistance risk for myrtle rust

Nurseries in high-risk northern areas could require 20–30 fungicide sprays per year to control myrtle rust on vulnerable hosts. Although rust fungi are not generally considered at high risk from resistance development, such a large number of applications does pose a risk that needs to be managed.

Research overseas and in New Zealand has shown that modern single site inhibitor fungicides in FRAC mode of action Group 3 (DMI - demethylation inhibitors), Group 7 (SDHI - succinate dehydrogenase inhibitors) and Group 11 (QoI - quinone outside inhibitors) are highly effective against myrtle rust. Fungicides in each of these groups are at risk from resistance development in many fungal plant pathogens when they are applied repeatedly over a long period of time.

Some older broad spectrum fungicides have slight efficacy against myrtle rust (e.g. copper based fungicides, mancozeb and chlorothalonil). These are not at risk from resistance development but are also not very effective against myrtle rust in high disease pressure situations. They can be useful in mixture with the at-risk fungicides mentioned above to reduce resistance risk.

Fungicide resistance management guideline for myrtle rust

Fungicides in mode of action groups 3 (DMI), 7 (SDHI) and 11 (QoI) are suitable for controlling myrtle rust but are at risk from resistance. These should be used as follows:

Apply fungicide sprays preventatively at full label rate to new growth when disease risk is high but before disease appears or is still at low severity. If disease is present, preferably spray after infection has been removed. Where possible, spray other vulnerable myrtles in the vicinity.
A weather risk tool is available through NZPPI (https://nzppi.co.nz/DISEASE-MANAGEMENT/19881/) to help identify seasonal conditions favourable for myrtle rust infection and assist decision making on when sprays should be applied.
Make no more than five applications of each at-risk fungicide group per year (1 July – 30 June).
Make each application either in mixture with an effective dose of a multi-site inhibitor (FRAC MOA groups M1 [copper], M3 [mancozeb] and M5 [chlorothalonil]) or in strict alternation with either a single-site inhibitor in a different group or a multi-site inhibitor.
When choosing fungicides, make use of the group codes displayed on product labels to avoid mixed or consecutive applications of the same at-risk mode of action group.
The application rate of a fungicide used for myrtle rust control should be the recommended label rate for that fungicide on an appropriate other crop (Ministry for Primary Industries - ACVM Register (nzfsa.govt.nz)).
An application of a product containing a mixture of single site inhibitor fungicides in different MOA groups counts towards the maximum number for each group. An application of a product containing a mixture of single site inhibitor fungicides fungicides in the same MOA group counts as one application for that group.
The Environmental Protection Authority (EPA) may specify a maximum number of applications per year for particular fungicide products. This takes priority over maximum numbers indicated in this resistance management guideline for any product. (Controls for hazardous substances | EPA).

Fungicide active ingredients at risk from resistance

The Group 3, 7 and 11 fungicides listed in the tables below are only a few of the fungicides included in these groups that could be used for myrtle rust control. A full list of fungicide active ingredients in these groups can be found at: frac-code-list-2022--final.pdf. The listed fungicides have either been suggested by NZPPI or otherwise tested against myrtle rust in New Zealand or overseas.

Demethylation inhibitors (DMIs; FRAC Group 3)

Active ingredient	Example products
Epoxiconazole	Accuro 125SC, AGPRO Epoxiconazole, Calibre®, Epoxi 125, Opus®, Radial® (with azoxystrobin), Stellar®
Myclobutanil	Validus® 200EW, Fungus Fighter, Yates Fungus Gun, Sentanil
Propiconazole	Aurora® Fungicide, Condor®, Pro-P™, Prozole™, Spotless, Tilt® EC
Tebuconazole	Capri™, Corona® 250WG, Folicur® SC, Hornet® 430SC, Orius® Fungicide, Prosaro® (with prothioconazole), Scorpio® (with trifloxystrobin)
Triadimenol	Citadel® Fungicide, Luna® Devotion (with fluopyram), Tribute® SC, Vandia® 250 EC, Whiteout®

Resistance status: Resistance in myrtle rust to Group 3 (DMIs) has not been recorded.

Succinate dehydrogenase inhibitors (SDHIs; FRAC Group 7)

Active ingredient	Example products
Benzovindiflupyr	Elatus® Plus
Fluopyram	Luna® Privilege, Luna® Devotion (with triadimenol), Luna® Sensation (with trifloxystrobin)
Fluxapyroxad	Imtrex®, Merivon® (with pyraclostrobin), Sercadis®
Isopyrazam	Seguris Flexi®

Resistance status: Resistance in myrtle rust to Group 7 (SDHIs) has not been recorded.

Quinone outside Inhibitors (QoIs or strobilurins; FRAC Group 11)

Active ingredient	Example products
Azoxystrobin	AGPRO Azoxystrobin 250 SC, Amistar®, Amistar® OPTI (with chlorothalonil), Avior Gold Fungicide (with epoxiconazole), Orbit™, Tazer®
Pyraclostrobin	Comet®, Convoy™, Pyrax®
Trifloxystrobin	Flint®, Provita®, Triplus™, TSTAR™, Twist®SC

Resistance status: Resistance in myrtle rust to Group 11 (QoIs) has not been recorded.

Glossary of fungicide related terms

Active ingredient (active constituent). The component(s) in a formulated fungicide product that specifically inhibit the target pathogen. Products also contain other chemicals to achieve effective delivery of the active ingredient to the plant. The active ingredient name is the common name of the fungicide (e.g., triadimenol).

Efficacy: The intrinsic ability of a fungicide to prevent infection or inhibit A. psidii, and thereby control myrtle rust, determined under controlled conditions.

FRAC: Fungicide Resistance Action Committee (Europe)

Mode of action (MOA): The biochemical pathway(s) within fungal cells inhibited by a particular fungicide. FRAC assigns a code number to each MOA Group (frac-code-list-2022--final.pdf). The product label displays all the active ingredient groups contained in the product, as well as the group code numbers. When fungicide resistance develops in a pathogen to a particular fungicide, then all the active ingredients within the same MOA group are expected to be affected by resistance. However, in practice different active ingredients within a group are often affected by resistance slightly differently.

Mode of action Group 3 (Demethylation inhibitor; DMI): Single-site inhibitors with a mode of action that blocks the demethylation step in sterol biosynthesis necessary for chitin cell wall formation in fungi. These are also referred to as azole or triazole fungicides, based on their common chemistry.

Mode of action Group 7 (Succinate dehydrogenase inhibitor; SDHI): Single-site inhibitors with a mode of action that blocks mitochondrial respiration in fungal cells by inhibiting the succinate dehydrogenase enzyme that catalyses the oxidation of succinate into fumarate in the Krebs cycle.

Mode of action Group 11 (Quinone outside inhibitor; QoI [strobilurin]): Single-site inhibitors with a mode of action that blocks mitochondrial respiration in fungal cells at the quinone outside binding site of the cytochrome bc₁ complex.

Multi-site inhibitors: Groups M1, M3 and M4. Older fungicides that inhibit many metabolic pathways in the target pathogen (also known as broad spectrum fungicides). These are generally not at risk from resistance development in the pathogen.

Protectant. A fungicide that is only active against the pathogen on the plant surface where it prevents infection.

Single-site inhibitors. Modern synthetic fungicides that inhibit a specific metabolic pathway in the target pathogen. These are often at risk from development of fungicide resistance in target pathogens.