What’s so important about MOA groups?

A.R. ChaseAlthough I’ve never been particularly interested in the actual way a fungicide kills a fungus, I always try to learn the mode of action grouping (MOA) for new products as they are developed and enter the ornamental market place.

MOA Code
Different letters (A to I, with added numbers) are used to distinguish fungicide groups according to their biochemical mode of action in the biosynthetic pathways of plant pathogens. MOA codes are assigned a letter and/or number
 
One of the more important codes for fungicides is M which denotes multi-site inhibitors and thus denotes a low risk of resistance development. Fungicides with a single site MOA are more likely to result in resistance development. Also, certain fungi are more likely to develop resistance due to the nature of their reproduction and other factors. Some of the pathogens most likely to develop resistance include Botrytis, powdery mildew fungi and Pythium.
 
Keep in mind that many of the active ingredients in ornamental fungicides are marketed for many other crops. If field resistance to a fungicide has been shown in other crops for one fungicide member of a MOA Group, it is most likely, but not always true, that cross resistance to other fungicides in the group can occur.
 
Cross-resistance describes the ability of a fungus resistant to one fungicide to be resistant to closely related fungicides even though it has never been exposed to those fungicides previously. Further, the level of cross resistance can differ between fungicides in one group or even with different fungal species or biotypes of the same fungus.

Fungicide resistance
Being aware of MOA groups can help you to avoid fungicide resistance development. Most cases of resistance occur when a single product or group of similar products is used repeatedly. This is what has happened in parts of the ornamentals industry with some Pythium spp. and their resistance to mefenoxam (Subdue MAXX). Resistance management is based on the fact that the mode of action  for one chemical class usually differs enough from another chemical class that the fungus or bacterium cannot become resistant to both MOAs simultaneously.  
 
There are a number of important fungicide/bactericides that have been reported to be ineffective in certain crops due to development of resistance (Table 1). The final column in the table shows the Fungicide Resistance Action Committee comment on the possibility of resistance development for a MOA group.
 
The risk level does not always match the reported resistance development. For instance, copper is listed as low risk to resistance development based on the fact that these fungicide/bactericides have multiple sites of action that are thought to confer a low level of risk. However, there are many reports of resistance in bacteria such as Xanthomonas leaf spot and blight on vegetables and ornamentals.

MOA Group specifics
There has been a number of new fungicides released recently with many of them falling into a few MOA groups, i.e., sterol inhibitors (DMI) and strobilurins (Table 2). Four of the most commonly used fungicides belong to one of the following chemical classes: sterol inhibitors, strobilurins, coppers and phosphonates.
Knowing product differences

Sometimes fungicides sharing a MOA can be substituted with equal results. At other times a substitution can lead to unacceptable results. In some cases, the chemical properties are distinct despite having the same MOA and giving one product an advantage over the others.
 
The strobilurins are a good example of products that have a range of efficacy based on whether or not the exact chemical is systemic and/or redistributes across the leaf surface. Compass O and Cygnus each redistribute in the air while Heritage has a high degree of systemic action in the xylem. Insignia is not systemic and is not redistributed by the air. Thus, the best strobilurin for a particular fungus is not the same.
 
This year for many growers raising prices was not an option so they looked at reducing their costs in order to remain viable. Lowering night temperatures and applying post-patent (generic) fungicides are considered ways to reduce costs. Post-patent fungicides are often less expensive and usually have similar labels compared to brand name fungicides. 
 
Ann Chase is president, Chase Horticultural Research, (530) 620-1624; archase@chaseresearch.net; www.chasehorticulturalresearch.com.