pesticide class is continuously applied for a given pest problem,
selection pressure for the resistant population will continue to
grow. The individuals that naturally carry the resistance will
reproduce increasing the resistant population over time.
This process may take a number of years to become evident
or it can happen very rapidly for pesticides that have highly
specific MoA and pests that can produce multiple generations
fairly quickly. Some highly specific pesticides have developed
resistance within 1 to 2 years of consistent use for a pest.
TERMINOLOGY
When a pest population is dominated by resistant individu-
als, this is termed practical resistance. Once a pest population
reaches practical resistance, the pesticide will no longer control
that pest for a long period of time. Additionally, many pesti-
cide resistant pests will exhibit cross resistance, which means
the population is resistant to other pesticides within that
class. There are also numerous turf pests that have developed
multiple resistances. Multiple resistances occur when a pest
population exhibits high levels of resistance to pesticides in two
or more pesticide classes with different MoA.
The specificity of the pesticide MoA can alter the significance
of resistance and reduce the time required for practical resistance
to occur. There are two general terms used to describe pesticide
resistance. Qualitative resistance occurs with pesticides that have
highly specific MoA, potentially targeting a single gene or amino
acid group within the susceptible pest. Think of qualitative
resistance as a simple "yes" or "no" answer. An individual within
the population will be resistant (Yes), and no amount of that
pesticide will control the pest any longer. If you were to apply two
times the rate of a product or shorten the interval between appli-
cations, there will be no injury on those resistant individuals.
Quantitative resistance occurs with pesticides that target
multiple locations within a target pest, but still have some
FIELD SCIENCE
22 SportsTurf | January 2016 www.sportsturfonline.com
A resistant (left) and sensitive (right) fungal isolate growing on increasing concentrations of a single-site mode of action fungicide. The resistant isolate's mycelial growth is not
impacted by higher fungicide concentrations, which indicates qualitative resistance. Image courtesy of Dr. Joey Young, Texas Tech.
