Golf Course Management

AUG 2018

Golf Course Management magazine is dedicated to advancing the golf course superintendent profession and helping GCSAA members achieve career success.

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64 GOLF COURSE MANAGEMENT 08.18 resistant populations naturally exist, and sus- ceptible biotypes are removed with repetitive use of herbicides. is leaves little competition for the normally low populations of resistant biotypes, and allows them to spread rapidly (Figure 4). Predictive models and history suggest con - tinued use of a product with a single mode of action usually results in resistance develop - ing in approximately 10 years, although as few as seven years has been observed. How - ever, by rotating or mixing products with dif- ferent modes of action, one can dramatically extend the time before resistance occurs. For example, it is predicted that it takes 45 years to produce resistance if a herbicide with a par - ticular mode of action is used for two consecu- tive years and then rotated to another product for the third year. Sixty years are predicted for resistance to develop if herbicides are rotated annually, and resistance can be delayed for up to 90 years if a product with a particular mode of action is used in only one of three years, and products with other modes of action are used in rotation in the other two years. Resistance most often occurs when an amino acid associated with the plant's DNA strand is replaced by other amino acids. is substitution then causes plants to overpro - duce enzymes the herbicide normally blocks. Alternatively, extrachromosomal DNA mol - ecules are produced that overproduce various enzymes that are normally blocked by certain herbicides. From a scientific point of view, it is important to know the specific avenues by which resistance occurs, but end users are more interested in managing its occurrence. Herbicide use strategies We believe that adjusting the regular ap- plication timing is just as important to de- laying resistance as alternating or combining herbicides that have different modes of ac - tion. For example, on fairways and roughs, instead of using a traditional pre-emergence herbicide in late summer/early fall, it is bet - ter to wait until mid-fall and combine tra- ditional pre-emergence herbicides with her- bicides with early post-emergence activity. About 75% of yearly Poa annua germination occurs in late September through October when daytime temperatures are consistently in the mid-70s F (~24 C). Postponing treat - ments until mid-fall (e.g., November), when most of the yearly crop of P. annua has ger - minated but plants are still relatively small Top-10 herbicide-resistant weed families Figure 3. Top-10 weed families that have evolved resistance to herbicides. Poaceae has nearly twice as many species as any other plant family, likely because of the wide genetic variability in this family. Illustration by Ian Heap, www. weedscience.org, 2018 Figure 4. A classic example of a mixed Poa annua population, in which green, healthy plants are resistant to herbicides, while adjacent yellow, dying ones are susceptible to herbicides. 0 20 40 60 80 100 Poaceae Asteraceae Brassicaceae Cyperaceae Ameranthaceae Scrophulariaceae Polygonaceae Alismataceae Chenopodiaceae Caryophyllaceae No. of Species Weed family 81 42 22 12 11 9 8 7 7 6

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