4.5.2 “Fungicides”

The term “fungicide” refers, as its name suggests, to agents that control fungi. However, the same substances may also be active against Oomycetes (or water moulds), the important group of organisms that contain Phytophthora spp., but have now been assigned to a completely different kingdom (the Chromalveolata).

Perhaps the most widely-used fungicides are various copper compounds, which are active against a wide spectrum of plant diseases. Copper is more likely to be a soil/environmental issue, and since these compounds are essentially contact fungicides, it would be difficult to distinguish exogenously applied sprays from background levels in residue tests. 

The MRL set for copper ions is 50 mg/kg. Organic producers are still permitted to use copper, albeit on a restricted basis (see section 3.4.2). The MoA of copper compounds is described as multi-site (FRAC group M1), therefore the risk of fungicide resistance is considered to be low.

Phenylamide compounds (FRAC group A1) have protective, curative and systemic action against Phytophthora, disrupting the unique nuclear RNA synthesis pathways in Oomycetes. Metalaxyl was discovered by Ciba Geigy (now Syngenta) in 1977. 

It consists of a number of isomers (compounds with the same formula but a different arrangement of atoms in the molecule and different properties) and it was later discovered that one in particular, metalaxyl-M, showed greatest biological activity. In 1996, the company re-patented the latter as mefenoxam (marketed as ‘Ridomil-gold’) thus doubling the patent life. Residue studies and submissions for registration in the EU refer strictly to this isomer, which was included on EU/91/414 Annex 1 effectively as a new substance (confirmed under legislation 02/64/EC). 

Supervised GAP residue trials for the latter were carried out by Syngenta on fermented dry beans and using the local processing methods, in order to obtain MRLs. 

Residue trials included rates of 90 g mefenoxam/ha (2 x normal rate). Under EU legislation, the status of (chemically) unresolved metalaxyl has now been clarified and the MRL includes mixtures of all constituent isomers including metalaxyl-M (i.e. the sum of isomers).

Residue analysis has recently focused on metalaxyl and benalaxyl, especially since farmers might spray within its one-month pre-harvest interval (PHI: one of the principal means of mitigating high residue levels). 

Extension efforts should therefore focus on timely application (regular monitoring) and only applying copper fungicides near to harvest. It is also thought that there is a high risk of resistance to these AI by Phytophthora spp. and agrochemical companies have introduced alternative MoA. Carboxylic Acid Amide (CAA) fungicides (FRAC group H5, previously placed in F5) disrupt cell wall deposition (the cell walls of Oomycetes differ from the fungi, and contain glucan-cellulose rather than chitin). 

Two AI, dimethomorph (DMM) and mandipropamid, have now been registered for use against Phytophthora in cocoa and provide much needed MoA diversity for better resistance management (note that dimethomorph is no longer approved for use in the EU May 2024).

Table 4.2 Properties of some systemic black pod ‘fungicides’ in current use for cocoa

The ‘newer chemistries’ MoA groups active against Oomycetes, including FRAC F5 and C8 (QxI: Quinone x Inhibitor) compounds, are now being registered (see Appendices 3 and 4) in many cocoa-growing countries. 

The market remains dominated by copper-based control agents, with mixed AI products including metalaxyl and newer chemistries. 

Late season (near to harvest) applications of copper-only products probably present a lower risk of residue violations than mixtures, but the latter are important for resistance management.