CAA Fungicides

As shown in the table, the group name Carboxylic Acid Amides (CAA) has been chosen. This name best represents compounds from the three different chemical groups.

The mode of action of CAA compounds is directly linked to the inhibition of cellulose synthesis in the Oomycete plant pathogen (Blum et al. 2010).

Uptake studies with 14C labelled Mandipropamid (MPD) showed that this Oomycete control agent acts on the cell wall and does not enter the cell. Furthermore, 14C glucose incorporation into cellulose was perturbed in the presence of MPD. Gene sequence analysis of cellulose synthase genes in laboratory mutants, insensitive to MPD, revealed two point mutations in the PiCesA3 gene.  This gene is known to be involved in cellulose synthesis.  Both mutations in the PiCesA3 gene result in a change to the same amino acid (Glycine-1105) in the protein.

Sensitivity monitoring studies over several years revealed that in populations of the late blight pathogen, Phytophthora infestans, all isolates were fully sensitive to CAA fungicides. However, in populations of the grape downy mildew pathogen, Plasmopara viticola, isolates can be found in certain regions, which are resistant to all CAA fungicides.

Inheritance studies (Gisi et al. 2007) showed that sexual crosses between sensitive and CAA resistant isolates of Plasmopara viticola lead to a co-segregation of resistance to dimethomorph, iprovalicarb, benthiavalicarb 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 some isolates. These results suggest that the resistance risk can be classified as moderate (as compared to high for phenylamide and QoI fungicides) and that it can be managed by appropriate product use strategies.

Blum M, Waldner M, Gisi U. A single point mutation in the novel PvCesA3 gene confers resistance to the carboxylic acid amide fungicide mandipropamid in Plasmopara viticola. Fungal Genetics and Biology 2010; 47(6): 499-510.

Gisi U, Waldner M, Kraus N, Dubuis PH, Sierotzki H.  Inheritance of resistance to carboxylic acid amide (CAA) fungicides in Plasmopara viticola. Plant Pathology 2007; 56(2): 199-208.

(Reviewed January 2024)

Plasmopara viticola is classified by FRAC as a high risk pathogen. Long-term experience with CAA fungicides demonstrates that the resistance risk of Plasmopara viticola to this fungicide group is moderate and can be managed through appropriate use strategies.

• Apply CAA fungicides preferably in a preventive manner
• Apply a maximum of 50% of the total number of intended applications for disease control not exceeding a total of 4 CAA fungicide sprays during one crop cycle. In areas of high resistance, the total number should not exceed a maximum of 3 applications during one crop cycle
• Always apply CAA fungicides in mixture with effective partners such as multi-site or other non-cross resistant fungicides
• An effective partner for a CAA fungicide is one that provides satisfactory disease control when used alone at the mixture rate
• Alternation with fungicides having other modes of action is recommended in spray programs

For more detailed product recommendations refer to the use guidelines published by the respective CAA manufacturers.

(Reviewed January 2024)

Phytophthora infestans is classified by FRAC as a medium risk pathogen. Long-term experience with CAA fungicides demonstrates that the resistance risk of Phytophthora infestans to this fungicide group is low to moderate. For effective resistance management a precautionary strategy has to be implemented.

• Apply CAA fungicides preferably in a preventive manner.
• Apply a maximum of 50% of the total number of intended applications for late blight control. 
• Apply CAA fungicides always at recommended dose rates.
• Apply CAA fungicides using not more than 2 consecutive applications.
• Apply CAA fungicides preferably in mixtures with effective partners belonging to different modes of action.
• Alternation with fungicides having other modes of action is recommended in spray programs.
• Good agricultural practices must be considered to reduce source of inoculum, disease pressure and resistance risk, e.g. consider to plant resistant varieties and refer to disease prediction models.

In regions with reported cases of resistance:

• CAA fungicides must be used in mixtures, with not more than 2 consecutive applications.
• In case mixtures cannot be applied for regulatory reasons, apply CAA fungicide in strict alternation.

For more detailed product recommendations refer to the use guidelines published by the respective CAA manufacturers.

(Reviewed January 2024)

Pseudoperonospora cubensis is classified by FRAC as a high risk pathogen.

• Apply CAA fungicides preferably in a preventive manner
• Apply a maximum of 50% of the total number of intended applications for disease control not exceeding a total of 4 CAA fungicide sprays during one crop cycle. In areas of high resistance, the total number should not exceed a maximum of 3 applications during one crop cycle 
• Always apply CAA fungicides in mixture with effective partners such as multi-site or other non-cross resistant fungicides 
• An effective partner for a CAA fungicide is one that provides satisfactory disease control when used alone at the mixture rate
• Alternation with fungicides having other modes of action is recommended in spray programs

For product recommendations refer to the use guidelines published by the respective CAA manufacturers.

(Reviewed January 2024)

Some of the downy mildew pathogens are classified by FRAC as moderate risk pathogens (e.g. Bremia lactucae). In spite of the use of CAA fungicides for more than 20 years against a range of such Oomycete pathogens, no reports on the occurrence of less sensitive field populations are available.

For effective resistance management, a precautionary strategy has to be implemented. 

• Apply CAA fungicides preferably in a preventive manner 
• Apply a maximum of 50% of the total number of intended applications for disease control 
• Alternation with fungicides having other modes of action is recommended in spray programs 

For more detailed product recommendations refer to the use guidelines published by the respective CAA manufacturers.