Journal Info: Weed Science, 53:30-36, 2005;

Background and Objectives:

Since the onset of agricultural production, control of weeds has been a major issue. In the last decade, there have been numerous weed biotypes that have developed multiple herbicide resistance causing trouble for the
producers to control such weeds. Particularly in the Midwest, crops such as corn and soybean struggle with a weed known as Waterhemp. This waterhemp weed out of Adams County, Illinois, just recently proved resistance to three
modes of action. It was not controlled after application of atrazine (PSII inhibitor), lactofen (protoporphyrinogen oxidase inhibitor), and cloransulam, chlorimuron, and thifensulfuron (all acetolactate synthase inhibitors). A study was conducted to quantify waterhemp's resistance to
herbicides covering the three modes of action and confirm reduction in the number of options producers have in controlling waterhemp.

Materials and Methods:

From two different population sources in Illinois, (ACR) known resistance to Lactofen and three ALS- inhibiting herbicides), and (WCS) susceptible female waterhemp plant were taken and grown for seed production.
Waterhemp plants from these two seed sources were then grown in the greenhouse and were treated with one of each herbicide from various groups; (6) PPO inhibitors, (2) AlS inhibitors, and either atrazine(PS2 inhibitor), paraquat(PS1 inhibitor), or glyphosate(5-enol.3-phosphate synthatase inbitor). Treatments of six replications per herbicide and a control were randomized and the experiment was carried through two times. Plant tissue above the soil was collected 14 days after treatment and analyzed using a process to calculate GR50 which is growth reduction by 50%.

Results and Discussion

As indicated by the title of the article, this waterhemp (Amaranthus tuberculatus) biotype was ultimately found to resist herbicide control that act on three different sites of action. Specifically, plants were recognized to resist application of three herbicides mixed (lactofen, imazamox and atrazine) creating a unique find in the U.S. in terms of cross resistance by the alteration of host target site. Directly, this impacts soybean growers in the middle of the country, but imparts a valuable lesson to all growers (turfgrass managers included). This means that in a relative short period of time we can see significant change in plant biology, where, a plant reproduces generations successively year to year in the environment of chemical control agents, Turf managers may heed this example and create weed control programs that incorporate constantly changing herbicide modes of action, and still need to scout for signs of resistance. (depending on the levels of selection pressure)

Mark, Ryan, Jeff

Background and Objectives:
Our journal article was about rigid ryegrass var. WALR 50 being resistant
to glyphosate or otherwise known as round-up. The mode of action for
glyphosate was ESPS synthase inhibiting and other herbicides that were
involved in the study were diclofop (Illoxan) and chlorsulfuron (Corsair).
“The study was conducted because rigid ryegrass is the most widespread and
severe weed in Australian grain production systems and at the same time is
the world’s most herbicide resistant prone weed.” There are very few
plants that are resistant to glyphosate and resistance to the herbicide is
becoming a more popular topic. Rigid ryegrass was also the first weed
documented to become resistant to glyphosate.

Materials and Methods:
In a plot of rigid ryegrass any plants that survived an application of isopropylamine salt of glyphosate were collected. These plants were planted again and sprayed with isopropylamine salt of glyphosate again. The surviving plants were cross-pollinated and these seeds (WALR 50) were used the next year in six experiments to test the resistance of these plants to glyphosate. Known species (VLR 1) and (NLR 70) were used as controls in the experiments.

Results and Discussion:
Two varieties of rigid Ryegrass were compared for herbicide resistance to glyphosate. NLR 70 a confirmed glyphosate resistant population and WALR 50
with unconfirmed resistant populations were analyzed using the dose
response curves. Little mortality existed in either population at the
recommended glyphosate rate (490g ha). WALR 50 was found to be 3X more
resistant than a known susceptible variety (VLR 1) confirming WALR was
resistance to glyphosate.
Although rigid ryegrass is an uncommon weed in turfgrass systems, this is
a good example of resistance to a regularly used herbicide, glyphosate
(Roundup). Other varieties of ryegrass, annual and perennial, are found
regularly in turfgrass systems making it likely that this resistance could
be found if glyphosate is abused. The article also indirectly contributes
to the known fact that repeated use of one chemical on any plants will
result in resistance. Turfgrass managers should follow labeled rates of
glyphosate, mix modes of action, and follow other methods to avoid
resistant plants.

Background and Objectives:
Dinitroaniline herbicide has been very successful at controlling populations of annual bluegrass in bermudagrass. Dinitroaniline controls by interrupting the DNA processes of the plant. This mode of action makes it a good controller of annual bluegrass, because it persists in the soil and it is not mobile. After using dinitroaniline for period of eight years, a golf course in North Carolina may have developed a resistance to this herbicide. Over the years resistance to simazine in annual bluegrass has been recorded, therefore resistance is possible in annual bluegrass. The objective of this research is to determine if the annual bluegrass population on this golf course has resistance to dinitroaniline. It is important to know if the annual bluegrass is resistant, so they can begin other control methods to eliminate and reduce this resistant strain.
Materials and Methods:
Annual Bluegrass was treated with herbicides in fall and spring. After all the herbicide applications there were plants left alive. These living plants prompted an experiment to see if the bluegrass was resistant. The bluegrass plants from two fairways were placed in flats and treated with six rates of herbicides containing oxadiazon, pendimethalin, prodiamine, and pronamide. Seeds from the known susceptible and suspected resistant plants were placed into petri dishes with herbicide and were placed into a growth chamber for 3 weeks.
Results and Disscussion:
Resistant biotypes of annual bluegrass were present at a North Carolina golf course after eight years of extreme selection pressure. The selection pressure came from lack of herbicide site of action rotation and residual herbicide that persisted in the soil for six months. The superintendent used herbicides that inhibited the formation of microtubules in the spring for eight straight years.
Two types of annual bluegrass were used in this experiment; susceptible annual bluegrass and the annual bluegrass from the golf course were the selection pressure had occurred. The resistant annual bluegrass was 6 to 105 times more resistant to the herbicide prodiamine (a microtubule inhibitor) compared to the susceptible annua bluegrass. The other three herbicides used in this experiment (pedimethalin, pronamide and oxadiazon) which have a different site of action proved to be effective in controlling both the resistant and susceptible annual bluegrass biotypes.

Adam, Aaron, Nick

Background and Objectives:
Golf course in New Jersey having trouble controlling smooth crabgrass on their tees with fenoxaprop (tees had received one or more apps. of the herbicide per year for eight years). Objectives of study were to: verify resistance, document tolerance to increasing rates of the herbicide, evaluate sensitivity of the biotype to other herbicides. and determine if cross-resistance was present.

Materials and Methods:
Smooth crabgrass (R) - resistant to herbicide; taken from tees.
Smooth crabgrass (S) - susceptible/non-treated; taken from surrounding rough.
Large crabgrass - Used strictly for comparison.
Plants were placed in pots (two plants per pot). Four replications. Fenoxaprop was applied with carbon dioxide backpack sprayer.

Results and Disscussion:
Verification of resistance: Herbicide worked on smooth (S) and large crabgrass from rough. Did not work on smooth (R) crabgrass from tees.

Extent of resistance: Biotype considered to have high degree of resistance (could not be controlled even when treated with up to as much as eight times the recommended maximum rate of the herbicide).

Response to other postemergence crabgrass herbicides: Specific strain stull susceptible to other herbicides used for postemergence crabgrass control.

Testing for cross-resistance: Cross-tolerance to other ACCase herbicides exists in this specific biotype in some degree. Suggests that resistance mechanism could be caused by an altered site of action.

Mitch Savage
Adric Ryan
Chad Nowak