Fungicide resistance management strategy for apple black spot (Venturia inaequalis) in New Zealand

Issued 22 September 2014

This version replaces all earlier versions

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

G.B. Follas Chair, New Zealand Committee on Pesticide Resistance (NZCPR)

G.C. Hagerty NZCPR Fungicide Resistance Task Group Co-ordinator

M.R. Butcher Technical Manager, Pipfruit NZ Inc. (PNZI)

Disclaimer:

While every effort has been made to ensure the strategy outlined in this document for the prevention and management of fungicide resistance in Venturia inaequalis (apple black spot or apple scab) 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.

Black spot (scab) of apple is caused by
the fungus Venturia inaequalis

Introduction

Following black spot control failures on some orchards during spring 2009 and 2010, testing of isolates of Venturia inaequalis (apple black spot or scab), in Europe and a preliminary survey by Plant and Food Research (PFR) suggested resistance may be developing to some fungicide groups. In response, PNZI commissioned Plant and Food Research to undertake extensive testing of resistance to four groups of fungicides; demethylation inhibitors (DMIs), Guanidine derivative (dodine), Quinone outside Inhibitors (QoIs or strobilurins) and Anilinopyrimidines (APs).

About 1,000 isolates of V. inaequalis were collected nationally (about 20 isolates per orchard) during the 2011-2012 and 2012-13 seasons from apple fruit lesions from 50 orchard blocks in Hawke's Bay (27), Nelson (12), Otago (6) and Waikato (5). Most of the samples came from ‘Royal Gala’ blocks. Data from the spray diaries indicated that for some fungicide groups, there was a lack of adherence to existing fungicide resistance management guidelines. Organic orchards with known histories, such as length of time organic and previous management, were included to help interpret the changes in sensitivity of V. inaequalis that are occurring in Integrated Fruit Production orchards over time.

The information to date from the study is disturbing and has major implications for the fungicide management of black spot in the New Zealand apple crop. There is a need to implement substantial changes to resistance management guidelines before the start of the 2013-14 growing season. There is also a strong need to modify fungicide use patterns by some members of the industry.

The industry will need to be VERY disciplined to work through this without compromising fruit quality at harvest and maintaining tree health.

The objectives of this strategy are to prolong the useful life of fungicide groups that are becoming affected by resistance, as well as those that are still effective but are at risk from resistance. It encourages protectant-based spray programmes and de-emphasises or discontinues use of fungicides that are most affected by resistance.

Although not yet part of the resistance testing programme, another site-specific group, the SDHIs, are also included in this strategy. These were relatively recently introduced into apple spray programmes, but are also known to be at risk from resistance development in several pathogens. Although SDHI resistance has not yet been reported for V. inaequalis, the declining effectiveness of other groups due to resistance means there is greater importance for effective resistance management for SDHIs.

Demethylation inhibitor (DMI) fungicides (FRAC Code 3)

Product perspective

Active ingredient	Trade names
Difenoconazole	Score®, Emerald®
Myclobutanil	Systhane*, Pipstar™ encaps™, Validus®
Penconazole	Topas®, Citara®, Pearl™
Flusilazole	Novall®, Megastar™

Resistance status

National sensitivity to DMIs is decreasing, although different DMI active ingredients are affected differently. Three seasons of orchard surveys showed sensitivity to myclobutanil and penconazole had decreased in all New Zealand apple-growing regions. Plant bioassays testing 24h curative ("reach-back") activity showed that myclobutanil, penconazole and fenbuconazole gave poor control of DMI-resistant strains of V. inaequalis, although they did control DMI-sensitive strains. Fenbuconazole is reported to have less curative activity than other DMIs, unless assisted by a penetrant. Flusilazole and difenoconazole gave good control of both resistant and sensitive strains. "Reach-back" activity of DMI fungicides was decreased for DMI-resistant strains.

DMI active ingredients showing reduced effectiveness against black spot should not be used in apple spray programmes. DMIs should not be used for "fire-fighting" when disease has become established. DMIs that are applied specifically for powdery mildew control also select for DMI-resistant strains of V. inaequalis.

Recommendations

Apply a maximum of two DMI-containing products per season and only in mixture with an effective dose of a broad spectrum protectant fungicide that is not at risk from resistance.
A maximum of four DMI-containing applications can be made in extremely wet seasons, but only where there is no history of black spot control failure following DMI applications or where the property has a history of low DMI use.
No DMIs should be used in orchards with suspected resistance (e.g. where disease control has been poor or previous DMI usage has been high).
DMI applications targeting powdery mildew control count towards the maximum number of DMIs for black spot resistance management.
Use DMIs at full label rate before disease has become established.
Myclobutanil and penconazole should be avoided in black spot control programmes because they have shown a marked reduction in effectiveness against DMI-resistant strains of V. inaequalis.

Guanidine derivative fungicides (FRAC Code U12)

Product perspective

Active ingredient	Trade names
Dodine	Zelam™ Dodine Gro-Chem™ Dodine Antagon®

Resistance status

Dodine sensitivity is holding up well and has improved since the 1990s as a result of the 2004 resistance management change that decreased the maximum number of applications from six to three per season. Since 2004 dodine use has been mostly pre-bloom period and its use before disease becomes established has probably helped improve V. inaequalis sensitivity. Survey results suggest sensitivity to dodine is slightly less in Central Otago and Nelson. Spray diary analysis shows reasonably good national compliance with the existing dodine resistance management guidelines.

Recommendations

Use a maximum of four dodine applications per season (previously three), but no more than two consecutive applications. Central Otago and Nelson growers should still use a maximum of three dodine applications before bloom.
Dodine should not be used in orchards with suspected resistance (e.g. where early-season disease control has been poor or previous dodine usage has been high).
Use dodine at full label rate before disease has become established.

Quinone outside Inhibitor (QoI or strobilurin) fungicides (FRAC Code 11)

Product perspective

Active ingredient	Trade names
Kresoxim-methyl	Stroby™, VistaFlo®, Ardent®
Pyraclostrobin (+ boscalid)	Pristine®
Trifloxystrobin	Flint®, Provita®
Trifloxystrobin (+ pyrimethanil)	Flint®Star (no longer sold)

Resistance status

The 2011-12 orchard survey showed the G143A gene mutation giving resistance to QoI fungicides occurred in 54% of isolates, nationally. Plant bioassays have shown that V. inaequalis isolates carrying the G143A mutation are not controlled by QoI applications and a substantial loss of QoI efficacy can be expected in orchards with greater than 50% resistance frequency. QoI fungicides should not be used if control failure following QoI use has occurred in previous seasons.

Spray diaries show resistance has developed despite apparent compliance with existing resistance management guidelines. This suggests an urgent need to tighten the guidelines to preserve remaining effectiveness of QoI fungicides where resistance frequency is still low.

Recommendations

Apply a maximum of one QoI-containing product per season and only in mixture with an effective dose of a broad spectrum protectant fungicide that is not at risk from resistance.
No QoIs should be used in orchards with suspected resistance (e.g. where disease control has been poor or previous QoI usage has been high).
QoI applications targeting powdery mildew control count towards the maximum number of QoIs for black spot resistance management.
Use QoIs at full label rate before disease has become established.

Anilinopyrimidine (AP) fungicides (FRAC Code 9)

Product perspective

Active ingredient	Trade names
Cyprodinil	Chorus®, Mirano®
Pyrimethanil	Scala®, Diva™
Pyrimethanil (+trifloxystrobin)	Flint®Star (no longer sold)

AP fungicides are most effective at cool temperatures and are more effective against black spot leaf lesions than fruit lesions. They should be used early in the season, prior to fruitlet formation.

Resistance status

National testing for AP resistance has been completed and indications are that significant resistance causing loss of efficacy is present in Nelson and Waikato orchards, to a lesser extent on Central Otago but increasing and somewhat mixed in Hawkes Bay .

Recommendations

Do not use APs in Nelson or Waikato regions, in all other regions….
Apply a maximum of two AP-containing products per season, apply only in mixture with an effective dose of a broad spectrum protectant fungicide that is not at risk from resistance and, where possible, avoid consecutive applications.
No APs should be used in orchards with suspected resistance (e.g. where disease control following AP applications has been poor or previous AP usage has been high).
Use APs at full label rate before disease has become established.

Succinate dehydrogenase inhibitor (SDHI) fungicides (FRAC Code 7)

Product perspective

Active ingredient	Trade names
isopyrazam	Seguris Flexi®
penthiopyrad	Fontelis®
boscalid	in Pristine®
fluxapyroxad	Sercadis®

Resistance status

SDHI fungicides have a site-specific mode of action and are at high risk of resistance development in some pathogens, although resistance has not yet been reported for Venturia spp. SDHIs have not yet been tested for V. inaequalis resistance in New Zealand, but because of resistance to other groups, there is greater importance for effective resistance management for SDHIs.

Recommendations

Apply a maximum of four SDHI-containing products per season, with not more than two consecutive applications, and apply only in mixture with an effective dose of a broad spectrum protectant fungicide that is not at risk from resistance.
Use SDHIs at full label rate before disease has become established.

Mixtures of at-risk fungicides

Mixing an at-risk fungicide with an effective dose of a fungicide from a different group is an important method to delay resistance development. However, products that contain mixtures of at-risk fungicides (e.g. QoI+AP or QoI+SDHI) are undesirable because they reduce flexibility in spray programmes and compromise resistance management strategies when resistance starts to develop to one component.

The QoI+SDHI mixture, Pristine® (pyraclostrobin + boscalid) must be considered as a stand-alone SDHI product for resistance management, given the widespread QoI resistance in New Zealand. It should therefore be used in tank mixture with a broad spectrum protectant, as per the SDHI resistance management strategy. The QoI+AP mixture, Flint®Star (trifloxystrobin + pyrimethanil), should not be used as resistance to the QoI component is widespread and resistance to the AP component may also be widespread.

Broad spectrum protectants for mixing with at-risk fungicides

Biological, copper-based and sulphur-based fungicides are not suitable as mixing partners for fungicide resistance management because they do not provide effective fungicidal control of black spot, either because their performance is too variable or the rate required to avoid plant phytotoxicity is too low.

Active ingredient	Trade names	Preferred use period
captan	various captan containing products	Green tip to pink, then from end of November to PHI¹ cut off
dithianon	Alaris®, Delan®	Green tip to PHI cut off
mancozeb	various mancozeb products	Green tip to PHI cut off
metiram	Polyram®	Green tip to PHI cut off
thiram	Thiram	Green tip to PHI cut off
ziram	Mizar® Granuflo	Green tip to PHI cut off

¹PHI = pre-harvest interval

Resistance management strategy implementation

Implementation of this strategy to manage fungicide resistance for black spot in pipfruit requires a resistance statement on the product label for every at-risk fungicide, like the example below:

"Resistance prevention or management
Seguris Flexi® contains the GROUP 7 fungicide, isopyrazam. Do not use another product containing a GROUP 7 fungicide in conjunction with Seguris Flexi® as detailed in the resistance management guidelines below."
[Details of the resistance guidelines for this product then follow].