This section elucidates the mechanisms through which various fungicides target specific aspects of pathogen biology. Each principal mode of action is categorized by a FRAC code, which indicates both the chemical class of the fungicide and the specific biochemical process it disrupts within the pathogen. It is important to note that multiple fungicides with similar chemical properties may be classified under a single FRAC code. All products mentioned herein are authorized for use in the United Kingdom, although not all are approved for application on ornamental plants.
Mode of Action: Nucleic Acid Metabolism
FRAC Codes 4, 8, 32
Certain fungicides impede the processes involved in the synthesis or replication of DNA and RNA, which are fundamental components of genetic material. By disrupting these processes, the growth, reproduction, and biological regulation of the pathogen are halted. These fungicides target four critical enzymes associated with nucleic acid metabolism. A significant number of these fungicides exhibit a medium to high risk of developing resistance, and some may exhibit cross-resistance with others that target the same enzymes.
Mode of Action: Cytoskeleton and Motor Proteins
FRAC Codes 1, 20, 22, 43, 50
Fungal cells possess a cytoskeleton, a network of protein filaments that provides structural integrity and assists in chromosome identification during cell division. Motor proteins facilitate the movement of substances within the cell. Given the essential role of these systems in growth and division, they represent effective targets for fungicides. Instances of resistance have been observed, particularly in powdery and downy mildews, with all fungicides in this category classified as having a medium to high risk of resistance according to FRAC guidelines.
Mode of Action: Respiration
FRAC Codes 7, 11, 21, 29, 38, 39, 45
Numerous fungicides inhibit various stages of the metabolic process that converts nutrients into energy, which is vital for the pathogen’s growth. These fungicides are categorized based on the specific enzyme sites they affect, some of which are complex and involve multiple targets. Notable examples include SDHI fungicides such as boscalid and strobilurins like azoxystrobin found in Amistar. Reports of resistance and cross-resistance have been documented, resulting in a medium to high resistance risk for these sites.
Mode of Action: Amino Acids and Protein Synthesis
FRAC Code 9
This category of fungicides inhibits the synthesis of enzymes and proteins that are crucial for the pathogen’s survival. Among five target sites, only one—code 9—features fungicides approved for use in the UK. This is particularly noteworthy as it includes cyprodinil, an active ingredient in Switch, which functions by obstructing the production of methionine, a vital component in numerous enzymes. There is a risk of resistance, thus it is advisable to apply Switch early. A maximum of three applications per year are permitted on ornamental plants, and these should be alternated with products from different groups.
Mode of Action: Signal Transduction
FRAC Codes 12, 13
Fungicides within this category inhibit the ability of fungal cells to respond to environmental stimuli such as temperature and moisture. They are classified based on the specific targets they affect, although currently, no products approved in the UK fall under these categories. Fludioxonil, present in Switch, is classified under code 12 and prevents spore germination while inhibiting the growth of the germ tube into plant tissue. This process typically initiates when spores sense a specific combination of moisture and temperature.
Mode of Action: Cell Membrane Targets
FRAC Codes 28, 46, 49
The cell membrane, primarily composed of lipids and sterols, serves to protect the cell’s internal components and maintain biochemical functions. Fungicides that compromise this membrane lead to cell desiccation and death. These fungicides target either the synthesis of the membrane’s lipid components or directly damage the membrane itself. This category includes carbamate fungicides and certain biological fungicides derived from plant extracts.
Mode of Action: Sterol Biosynthesis in Membranes
FRAC Codes 3, 5, 17
These fungicides disrupt the sterol components within cell membranes. Common examples include DMI fungicides such as propiconazole and tebuconazole (code 3). They are associated with a medium risk of resistance, and the FRAC group recommends anticipating some degree of cross-resistance. Codes 5 and 17 encompass products that present low to medium resistance risks.
Mode of Action: Cell Wall
FRAC Code 40
The outer layer of fungal cells, known as the cell wall, is predominantly composed of chitin, similar to the exoskeletons of insects. Oomycete pathogens, such as Pythium and Phytophthora, construct their cell walls from cellulose, akin to plant structures. Fungicides classified under code 40, such as mandipropamid in Syngenta’s Revus, inhibit the enzyme responsible for cellulose production in oomycetes. Other fungicides, classified under code 19, target the enzyme involved in chitin synthesis but are not approved for use in the UK.
Certain fungal cell walls contain melanin, which provides protection against environmental stresses and ultraviolet damage. In pathogens such as black spot in roses, melanin fortifies the walls that penetrate host tissues. The removal of melanin can weaken these fungi and diminish their pathogenicity. At present, no fungicides with this mode of action are authorized for use in the UK.
Mechanism of Action: Enhancing Host Plant Defenses
FRAC Codes P04, P06, P07
These agents are anticipated to play an increasingly significant role in the management of plant diseases and the enhancement of resistance. Rather than directly attacking the pathogen, they strengthen the plant’s inherent defense mechanisms. This category includes extracts derived from plants and microbial-based products, which present a minimal risk of developing resistance, if any at all.
Mechanism of Action: Multi-Target Activity
FRAC Codes M01-04
Fungicides classified under this group operate on multiple targets, thereby reducing the likelihood of resistance emergence. According to FRAC, no resistance has been reported to date, even for older fungicides such as copper and sulfur. Common fungicides like mancozeb and triazines are included in this category and continue to demonstrate effectiveness.
Mechanism of Action: Biological Agents with Multiple Mechanisms
FRAC Code BM02
A growing array of biological products now functions as fungicides. These products are derived from beneficial fungi, bacteria, and plant extracts. FRAC indicates that no resistance has been observed in any products within this category. This includes bacteria such as Bacillus subtilis and B. amyloliquefaciens, as well as fungi like Trichoderma asperellum and Gliocladium catenulatum. These agents operate through various mechanisms, including parasitism, membrane disruption, or competitive exclusion of pathogens.
Mechanism of Action: Unidentified
FRAC Codes 27, U06, U12
These substances lack a confirmed target or mechanism of action. In certain instances, the potential for resistance remains uncertain, while in others, resistance has been documented or is considered probable by FRAC.