Fungicide resistance management strategy for potato late blight (Phytophthora infestans) 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

Late blight, or potato blight, caused by the oomycete pathogen Phytophthora infestans, causes a devastating disease in potatoes that was a major contributor in the 1840’s to the potato famines that affected Europe, Ireland (1845–1852), and the Scottish Highlands (1846). It is estimated that it still destroys around 5% of the world’s potato crops annually. It is also a serious disease of other solanaceous plants, including tomatoes, capsicums, chillies, eggplants, pepinos and tamarillos. Conditions favouring late blight development are long periods of rainy weather causing prolonged leaf wetness during early crop development.

Of the fungicides used to control late blight in New Zealand potato crops are listed by mode of action group and their general resistance risk profile (FRAC) is as follows:

Fungicide groups at risk from resistance in P. infestans

Of the fungicides used to control late blight in New Zealand potato crops, the Fungicide Resistance Action Committee (FRAC) classifies the following mode of action (MOA) groups as at risk from resistance development and they require resistance management:

Group 4 Phenyl amide (PA) High risk
Group 11 Quinone outside inhibitor (QoI) High risk
Group 21 Quinone inside inhibitor (QiI) Medium to high risk
Group 27 Cyanoacetamide-oxime (CAO) Low to medium risk
Group 40 Carboxylic acid amides (CAA) Low to medium risk
Group 45 Quinone outside Inhibitor,
stigmatellin binding type (QoSI) Medium to high risk

The Group 29 active ingredient, fluazinam, is considered low risk for resistance development. Also classified as low risk are Groups M1, M3, M5, (multi-site inhibitors), Group P07 (plant defence inducers) and Group BM02 (biological control agents). These are all important for use in mixing or alternating with medium to high risk single-site inhibitor fungicides to delay resistance development. Table 1 lists the fungicides registered for controlling late blight, grouped by FRAC mode of action group code.

Note that the product Cannon® contains the Group 3 DMI fungicide difenoconazole which does not have efficacy against P. infestans but does have efficacy against sensitive strains of potato early blight (Alternaria solani). The chlorothalonil in the Cannon® co-formulation is a muti-site inhibitor with activity against P. infestans.

Note also that there is no cross resistance among the QoI, QiI and QoSI fungicide groups.

**Table 1:** Fungicides registered for control of late blight in potato crops in New Zealand.
FRAC mode of action group		Active ingredient	Product trade names	Max. Number of applications per season
3 + M5	DMI + dithio-carbamate	Difenoconazole + Chlorothalonil	Cannon®	No more than 3 treatments per season
4	Phenylamide (PA -fungicides)	Metalaxyl + mancozeb Metalaxyl	Ridomil® Gold MZ, Speartek® Ventura®	No more than 3 treatments per season
11	QoI fungicides Quinone outside inhibitors	azoxystrobin	Amistar®, Atlantis, Avior, Azoxystar, Inspire, Roxy, Tazer	Max. of 4 applications of QoI fungicide per season or a max. of 33% of the total applications per season
		Fenamidone	Reason®
		Mandestrobin	Intuity™
21	QiI Quinone inside inhibitors	cyazofamid	Ranman®	Max 4, high risk max 3 applications
27	Cyanoacetamide – oxime	cymoxanil	Curzate® Concord® Curfew®	Max 3 times in succession and 5 times in season.
29	unspecified	Fluazinam	Curalan®, Flick, Gem®, Nando®, Pinnacle®	Max of 6 applications or 50% of total applications per season
40	CAA (Carboxylic Acid Amides) CAA + M3 CAA + M5 CAA	Dimethomorph Dimethomorph + mancozeb Dimethomorph + Chlorothalonil Mandipropamid	Sovrin® Flo, Sphinx Acrobat®MZ690 Cobra® Revus®	Max. of 4 applications per season. Under high disease pressure make 2 close interval applications
45 + 40	QoSI + CAA	Ametoctradin + dimethomorph	Zampro®	Max 3 consecutive and 4 for season
M1	Inorganic	Coppers - various	Various	Mixing partner
M3	Dithiocarbamates	Mancozeb Mancozeb + copper	Various Mankocide®	Mixing partner
M5	Mixing partner	Chlorothalonil	Various	Mixing partner

General resistance management requirements

Observe maximum number of applications and mixing/alternation recommendations for medium and high risk MOA groups and use those groups only when disease levels are low and risk of disease is high, as indicated by available downy mildew weather risk prediction tools.
Use at-risk products preventatively and not curatively and do not use at reduced application rates.
Mixing partners are important for resistance management to reduce pathogen to exposure the at-risk fungicide groups. They must have adequate efficacy against P. infestans and be applied a rate that would provide satisfactory disease control if used alone.
Mixing partners suitable for potato late blight include some multi-site products (Groups M1, M3, M5), some biological control agents (Group BM02) and some plant defence inducers (Group P 07) (Table 1).
Use non-chemical management approaches to minimise disease risk, such as avoiding overhead irrigation late in the day and at night, maintaining good soil drainage, destroying volunteer potato plants and crop debris as soon as the crop is harvested. Use crop rotation to minimise risk of carry-over of fungicide resistant pathogen strains.
Some product labels have inadequate resistance management statements and chemical proprietors need to ensure labels are updated to provide adequate and accurate information on resistance risk and resistance management.

Resistance management recommendations by MOA group

Group 4 PA Fungicides

Apply a maximum of 3 PA-containing applications per season preferably in mixture with a fungicide from a different MOA group that is not at risk from resistance.
Apply a maximum of 2 consecutive applications and application intervals should not exceed 14 days and may be shorter when disease risk is high.

Group 7 SDHI fungicides

Apply a maximum of 2 SDHI-containing applications per season preferably in mixture with a fungicide from a different MOA group that is not at risk from resistance.

Group 9 AP fungicides

Apply a maximum of 2 AP-containing applications per season preferably in mixture with a fungicide from a different MOA group that is not at risk from resistance. Pre-mixed formulations with fluazinam are suitable for this purpose.

Group 11 QoI fungicides

Apply a maximum of 4 QoI-containing applications per season or up to 1/3 of the total number of fungicide applications targeting late blight, preferably in mixture with a fungicide from a different MOA group that is not at risk from resistance.

Group 21 QiI fungicides

Apply a maximum of 6 QiI-containing applications per season and a maximum of 3 consecutive applications, preferably in mixture with a fungicide from a different MOA group that is not at risk from resistance.

Group 27 Cyanoacetamide-oxime fungicides

Apply a maximum of 5 Cyanoacetamide-oxime-containing applications per season and a maximum of 4 consecutive applications, preferably in mixture with a fungicide from a different MOA group that is not at risk from resistance.

Group 40 CAA fungicides

Apply a maximum of 4 CAA-containing fungicide applications per season preferably in mixture with a fungicide from a different MOA group that is not at risk from resistance.

Group 45 QoSI fungicides

Apply a maximum of 4 QoSI-containing fungicide applications per season preferably in mixture with a fungicide from a different MOA group that is not at risk from resistance.
Apply a maximum of 3 consecutive applications.