Potential disease control tools most likely to be effective against myrtle rust - desktop review

We need a range of different control options for myrtle rust in New Zealand because the disease affects many plant species, both native and exotic, in many different environments, including natural, urban and commercial.

This research undertook a comprehensive review of literature on the best control options, e.g. cultural, chemical, and biological controls, that have been used to manage myrtle rust and other rust diseases.

Researchers identified some control practices and options that can be implemented immediately for short-term control of myrtle rust in New Zealand.

They also identified further control options that will require more research to determine how specific, effective and feasible they are. Engagement with Māori to incorporate kaupapa and mātauranga to co-develop control solutions is critical. There is no single solution to controlling myrtle rust. Ideally, the most effective way is to use a combination of control methods, known as integrated disease management.

Download the report: Potential disease controls most likely to be effective against Austropuccinia psidii - desktop review

Download: List of overseas researchers


Pilot trials for control of myrtle rust

Controlling myrtle rust with fungicides provides a way to prevent infection of valued (native or exotic) trees in the short term, while long-term management strategies are developed.

An extensive review of the treatments used globally to control rust diseases indicated that some fungicides were more effective than others (strobilurins and triazoles).

Researchers trialled products available in New Zealand that contained these types of fungicides, to see if they could be used as preventative treatments against myrtle rust on iconic New Zealand species. The focus of this trial was pōhutukawa.

They determined that combining the fungicide with other substances that maximise spray coverage significantly improved coverage of the fungicide sprays on upper and lower leaf surfaces, particularly Actiwett (an alcohol ethoxylate).

They then tested the preventative effects of three different fungicides (Timorex Gold, Vandia and Radial) in controlled studies.

Of the three fungicides tested for preventative control in these preliminary studies, Radial was the most effective, while the natural product extract or “biological”, Timorex Gold, was not effective.

Download the report: Pilot trials for control of myrtle rust using fungicide


Chemical control – review of control methods and fungicides

Development of in-depth knowledge on the fungicides likely to control myrtle rust in New Zealand is a high priority since the rust is going to require ongoing management.

This review highlights numerous information gaps and research needs with respect to chemical control of myrtle rust in New Zealand. Researchers say that if addressed, these would improve options for chemical control of the disease. Research needs identified include:

  1. Targeting the most highly susceptible iconic and native myrtles in New Zealand for chemical control trials with selected fungicides.

  2. An evaluation of the efficacy of different fungicides and mixes applied at different times/seasons and under both natural and controlled conditions to assess their potency against myrtle rust on New Zealand host species.

  3. Creating a database that has all used/identified fungicides, their efficacy against myrtle rust, availability status in New Zealand, and if possible, making that database accessible for all stakeholders that play a key role in monitoring myrtle rust in New Zealand.

The report notes there is a question around who will pay for the cost involved in spraying native tree species.

Download the report: Chemical control - review of control methods and fungicides


Optimising fungicide use against myrtle rust in plant nurseries

Myrtle rust affects the production of some vulnerable native and exotic species in the Myrtaceae (myrtle family) in plant nurseries. To date, the only method available to manage myrtle rust is the use of agrichemical products (fungicides). However, it is important to use these products rationally to prevent the development of fungicide resistance.

This project sought to help New Zealand Plant Producers Incorporated (NZPPI) develop a structured approach for optimising use of fungicides against myrtle rust in nurseries and guidelines to prevent fungicide resistance.

The research was conducted in outdoor trials at the Plant and Food Research (PFR) facilities in Pukehoe, Auckland, and used an online weather-risk tool previously developed by AgHort and PFR, in collaboration with NZPPI. Researchers compared the efficacy of seven commercial products (with different active ingredients and modes of action), under different application regimes (calendar-based vs using the weather-risk tool) during a high-risk (summer) and lower-risk (autumn) season.

Commercial fungicides differed in their efficacy against myrtle rust. Rust control and growth flush protection were greatest for the demethylation inhibitors (DMI) triadimenol (Vandia®) and myclobutanil (Validus®). The use of the weather risk tool resulted in fewer applications during autumn (4 vs 6 using calendar spray), but not in summer, when calendar-based spray was best. The authors propose an application regime alternating products with different modes of action, avoiding more than five applications of each at-risk group per year, to prevent fungicide resistance.

Disclaimer: This report is intended to support myrtle rust control in nurseries only, following product use guidelines for safe application by trained professionals. It does not seek to promote fungicide use in natural estate, home gardens or other environments where the risks of fungicide application have not been properly assessed, and inadequate product use could threaten human or animal health and the environment.

Download the report: Risk based fungicide management for myrtle rust in nurseries