|
 Rust on dianthus. Photos by Ann Chase, Chase horticultural resourcesFungicides are classified in a variety of ways, including biochemical mode of action, fungicide resistance action committee (FRAC) code or their mobility.
Mobility describes fungicide movement after it is applied to a plant. To understand differences in mobility, it’s important to know the difference between absorption and adsorption.
Fungicides that can be taken up by the plant are absorbed. Fungicides that adhere in an extremely thin layer to plant surfaces are adsorbed. Because fungicides are either adsorbed or absorbed, they have two basic forms of mobility: contact and penetrant.
Regardless of the type of mobility that a fungicide possesses, no fungicide is effective after the development of visible disease symptoms. For that reason, timely fungicide application before establishment of the disease is important for optimal disease management.
Contact fungicides
Contact fungicides are adsorbed. They are susceptible to being washed away by rain or irrigation, and most (but not all) do not protect parts that grow and develop after the product is applied.
Most older, multi-site fungicides (such as captan, chlorothalonil, mancozeb, and copper) are contact fungicides. Contact fungicides:
- Must be applied before spores land on and infect leaves.
- Prevent spore germination, so they are preventive treatments.
- Have no effect once the infection is established.
Penetrant fungicides
Penetrant fungicides are absorbed, so they move into plant tissues, and penetrate beyond the cuticle and into the treated leaf tissue itself.
There are various kinds of penetrants, characterized by their ability to spread when absorbed by the plant. Localized penetrants remain in the area of initial plant contact and undergo very little movement within the plant (a process called translocation).
All penetrant fungicides are systemic, because they are absorbed by the plant and translocated to other plant tissues — in other words, they move through the plant’s system.
Systemic fungicides can be further subdivided based on the direction and degree of movement once they have been absorbed and translocated inside the plant:
- Xylem-mobile fungicides (also called acropetal penetrants) move upward from the point of entry through the plant’s xylem.
- Amphimobile fungicides (also called true systemic penetrants) move throughout the plant through its xylem and phloem.
- Locally systemic fungicides (essentially synonymous with localized penetrant) have limited translocation from the application site.
- Translaminar fungicides are absorbed by leaves and can move through the leaf to the opposite surface they contact, but are not truly systemic and do not move throughout the plant.
Systemic fungicides:
- Can stop or slow infections within 72 hours of exposure.
- Must be applied soon after initial infection.
- Are ineffective once the fungus begins producing spores.
- Have limited curative activity.
Click here for the Mobility and Classification of Commonly Used Fungicides chart.
Janna L. Beckerman is associate professor, Department of Botany and Plant Pathology, Purdue University, jbeckerm@purdue.edu.
|
