Fungicide Groups, Modes of Action & Effective Pathogen Control Cynthia Ocamb, Extension Plant Pathologist Botany and Plant Pathology Oregon State University

Fungicide resistance is a serious problem

•! resistance may occur gradually or suddenly
•! fungicides become less effective
- decreased sensitivity to fungicide
- growth unchecked at concentrations which inhibit wild-type population
•! can be a stable, inheritable adaptation
- Hop downy mildew populations resistant to Ridomil > 20 years

Risk factors ~ fungicide resistance

•! # of site(s) of action in the targeted microbe
•! fitness of resistant mutants
•! use of repetitive or sustained fungicide treatments
•! extensive areas of use
•! population size and reproductive rate of target pathogen
•! no use of other types of fungicides or cultural controls
•! cross-resistance with existing fungicides
(resistance to two or more fungicides mediated by the same genetic factor)

Resistance management strategies

As many strategies as possible should be used
•! avoid repetitive and sole use
•! tank mix or alternate with an appropriate fungicide
•! limit number of treatments
•! apply protective sprays early in the epidemic
•! avoid eradicant use
•! maintain recommended dose rate
•! integrate with non-chemical methods

Reduce humidity and high moisture periods within the plant canopy and field.

a. Provide adequate aeration within, and especially between, rows by
increasing plant and row spacing.
b. Plant cultivars, which are more erect and upright or that do not produce
excessive foliage near the ground level (open-base types).
c. Orient the bean rows in the direction of prevailing winds.
d. Time irrigations to allow drying of plant canopy before night fall.
e. Avoid excessive irrigation after petal fall.
f. Avoid excessive fertilization that can lead to dense, lush plant growth.

To reduce pathogen population within a field:

a. Rotate with non-hosts for 8 years.
b. Deep plowing buries sclerotia.
c. Field flooding during warm temperatures destroys sclerotia.

Take home message

* Fungicide resistance is a serious problem for crop production
* Pay attention to FRAC groupings for fungicide programs
* Utilize as many resistance management strategies as possible
•! practice non-chemical control methods (cultural control)
•! tank mix and/or alternate appropriate fungicides
•! limit number and timing of fungicide treatments
•! avoid eradicant use or too-late applications
•! maintain recommended dose rate and ensure even coverage
* Watch for and report any developing resistance problems