Carboxylic acid amide (CAA) resistance prevention strategy

CAA spraying on potato
CAA fungicides are used against late blight in
potatoes and although the resistance risk is
considered to be low to medium, a
resistance prevention strategy is required.

M.S. Moore1, G.B. Follas2, G.C. Hagerty3 and R.M. Beresford4

1Bayer CropScience, 91 Treffers Road, Sockburn, Christchurch, New Zealand
2Syngenta Crop Protection Ltd., Tower 2, Level 7, 110 Symonds St, Auckland, New Zealand
3BASF New Zealand Ltd, 1/333 Harewood Rd, Christchurch, New Zealand
4Plant & Food Research, Private Bag 92169, Auckland 1142, New Zealand

(Revised August 2007)


to carboxylic acid amide (CAA) fungicides, which are currently represented in New Zealand by the three active ingredients, dimethomorph, iprovalicarb and mandipropamid. The CAA group was proposed in 2007 by the Fungicide Resistance Action Committee (FRAC) in Europe under their code No. 40 and includes compounds from the three different chemical groups, cinnamic acid amides (dimethomorph), valinamide carbamates (iprovalicarb) and mandelic acid amides (mandipropamid).

This strategy incorporates the strategy for dimethomorph which was previously listed separately in New Zealand (Beresford 2005, Beresford et al 1996). These guidelines have been developed in consultation with the New Zealand Association for Animal Health and Crop Protection (AGCARM) and are based on recommendations from FRAC.

Product perspective

Carboxylic acid amide derivates have been available internationally since the late 1980's and in New Zealand are currently available as dimethomorph, in Acrobat MZ690 registered in 1994, iprovalicarb, in Melody Duo registered in 2006 and mandipropamid, in Revus registered in 2007.

CAA fungicides exhibit translaminar movement and although their mode of action is not clear, it is proposed that they inhibit phospholipid biosynthesis and interfere with cell wall deposition. They are considered to have low to medium risk of resistance development and resistance management is required.

Table 1: Pathogens and crops targeted by carboxylic acid amides in New Zealand.
Plasmopara viticola Downy mildew Grape
Bremia lactucae Downy mildew Lettuce
Peronospora destructor Downy mildew Onion
Phytophthora infestans Late blight Potato

Current status of carboxylic acid amide (CAA) resistance

Dimethomorph, which causes cell wall lysis in oomycete fungi (Albert et al. 1988), has shown no cross-resistance to phenylamide fungicides (Jensen amp; Lundsgaard 1999). Mutants of Phytophthora capsici moderately resistant to dimethomorph were induced in the laboratory with chemical mutagenesis, but not through adaptation (Young et al. 2001). A small proportion of field isolates of P. infestans were found to be tolerant to dimethomorph in Russia, however resistant strains that were induced through repeated treatment of potato plots appeared to have reduced fitness (Dereviagina et al. 1999). Attempts to detect resistant strains in field populations of P. viticola in Germany were unsuccessful, as were attempts to induce resistant strains artifically (Bissbort & Schlosser 1991).

The FRAC website has reported that sensitivity monitoring studies over several years revealed that in the populations of the late blight pathogen, Phytophthora infestans, all isolates were fully sensitive to CAA fungicides (Cohen et al. 2007). However, in the populations of the grape downy mildew pathogen, Plasmopara viticola, isolates can be found in certain regions which are simultaneously resistant to all CAA fungicides. Therefore, in the case of Plasmopara viticola, a positive cross resistance among all CAA fungicides is obvious for the vast majority of isolates. The degree of sensitivity cover a wide range of EC50 values with isolates being sensitive, moderately resistant and fully resistant. Inheritance studies have shown that sexual crosses between sensitive and CAA resistant isolates of P. viticola lead to a co-segregation of resistance to dimethomorph, iprovalicarb and mandipropamid, but not to the phenylamide mefenoxam which was tested in parallel as an independent marker. Further, the inheritance studies showed that the gene(s) for resistance to CAA fungicides are inherited in a recessive manner. Therefore, the entire F1 generation of crosses between sensitive and CAA resistant isolates was sensitive, and only in the F2 progeny did CAA resistance reappear in a few isolates. These results suggest that the resistance risk can be classified as moderate (as compared to high for phenylamide and Qol fungicides) and that it can be managed by appropriate use strategies as proposed in this strategy.

Resistance prevention strategy

Observe manufacturer's recommendations for application rate and timing. Exceeding the maximum number of applications per season could increase the risk of resistance development. Apply preventatively when disease levels are low, but disease risk is high. Weather-based disease forecasting criteria are available to help effective timing of fungicides for onion downy mildew and potato late blight.


Apply a maximum of four applications per season. Apply CAA fungicides always in mixture with an effective fungicide that has an alternative mode of action.

Lettuce, onions and potatoes

Follow the manufacturers label directions. It is suggested that no more than 50% of the fungicide applications used against late blight or downy mildew should contain a CAA fungicide.

Implementation recommendations

Product labels should include a statement about resistance risk and a recommendation about the maximum numbers of carboxylic acid amide containing fungicides that should be applied.


Albert G, Curtze J, Drandarevski CA 1988. Dimethomorph (CME 151), a novel curative fungicide. Brighton Crop Protection Conference. Pests and Diseases Vol. 1: 17-24.

Beresford RM 2005. Dimethomorph management strategy. In: Martin NA, Beresford RM and Harrington KC eds. Pesticide resistance: prevention and management strategies 2005. New Zealand Plant Protection Society Inc. Hastings, New Zealand, 19-20.

Beresford R, Cromey M, Grbavac N, Smith B 1996. Morpholine resistance management strategy. In: Bourdôt GW, Suckling DM ed. Pesticide resistance: prevention and management. New Zealand Plant Protection Society, Rotorua, New Zealand. Pp. 142-143.

Bissbort S, Schlosser E 1991. Sensitivity of Plasmopara viticola to dimethomorph. Mededelingen Faculteit Landbouwwetenschappen, Rijksuniversiteit Gent, 56 (2b): 559-568.

Cohen Y, Rubin E, Hadad T, Gotlieb D, Sierotzki H, Gisi U 2007. Sensitivity of Phytophthora infestans to mandipropamid and the effect of enforced selection pressure in the field. Plant Pathology 56(5): 836-842.

Dereviagina MK, Elansky SN, Diakov YT 1999. Resistance of Phytophthora infestans to the dimethomorph fungicide. Mikologiya Fitopatologiya 33(3): 208-213.

Jensen CB, Lundsgaard J 1999. ACROBAT R WG - a new fungicide against downy mildew in onions and late blight in potatoes. 16th Danish Plant Protection Conference. Crop protection in organic farming. Pests and diseases. DJF Rapport, Markbrug 10: 213-225.

Young DH, Spiewak SL, Slawecki RA 2001. Laboratory studies to assess the risk of development of resistance to zoxamide. Pest Management Science 57(11): 1081-1087.