b"However, the United Sorghum Checkoff Program and others in the industry call these varieties tolerant rather than resistant, because they aren't resistant in the way growers usually under-stand the word.These hybrids are not completely resistant in the way that Bt corn and other crops like that are, when you plant the seed and essentially dont have to worry about it, Bean says.Bean explains that pest tolerance occurs when the plant simply tolerates more aphids without a loss of health and yield. Conversely, true resistance can come from either antixenosis, (or non-preference), which occurs when the pest no longer desires the plant due to odor, texture or color; or antibiosis, when the plant itself causes injury, reduced longevity or reduced repro-duction of the pest.Brent Bean is the agronomy director for the United SorghumThe current primary resistance is caused by antibiosis, mean-Checkoff Program.ing it physically affects the aphid as opposed to just changes their behavior, such as altering their preference for the plant, Felderhoff says.Research shows that this antibiosis reduces the aphid popu-lation growth rate by around a half.Because this reduction is not complete enough to remove all risk to the plant, Bean explains the intentional shift in wording.We used the term tolerant so growers would know that resistance, in this case, doesn't mean they won't need an insec-ticide, Bean says. They may very well have to still use one, but hopefully only once, and they will have more time before they reach that threshold. Genetic GainsResearchers have been diligently working to breed resistant varieties. Multiple research programs have identified that gene to be RMES1 on chromosome 6, Hayes says. Thats the gene for the Chad Hayes is a sorghum breeder with the Agricultural Researchsource of resistance that the U.S. sorghum industry is using and Service of the U.S. Department of Agriculture. its working very well. Breeders use molecular markers to track resistance. The marker is a KASP (kompetitive allele-specific polymerase chain Terry Felderhoff, Kansas State University research assistantreaction) marker, which allows breeders to add resistance to professor, says the primary reason for identifying the resistanceexisting varieties. gene was the exchange of germplasm and knowledge.Not only can we use that marker to find resistance in breed-The initial source of the resistance came from sorghum germ- ing programs, but if a company has a product on the market plasm exchange from Ethiopia, along with knowledge exchangenow with high yield that does not have aphid resistance, with from Haiti, China, and southern Africa, he explains. Sharing ofthis marker and the technology we have available to us, they can these resources enabled everyone to leverage and aid each otherintegrate that resistance into current products, Hayes says. in identifying a primary source of genetic resistance.This reduces the risk of yield drag in resistant offerings. Typically, anytime you incorporate resistance to a pest, it Resistance vs Tolerance often comes from a source that is not as high yielding or has Texas A&M AgriLife Extension recommends a threshold of 20 toother agronomic problems, Hayes explains. But, using this 30% of plants infested with 50 or more aphids per leaf. marker to introgress resistance into their current products Non-resistant varieties were hitting those thresholds in asreduces that risk. little as two or three days, Bean says.Scientists first identified resistant varieties in 2015 that wereManagement Mattersable to withstand aphid attack for much longer, preventing aphidWhile research gains are important, so too are growers efforts populations from as quickly reaching those thresholds. Thisto manage the risks. allowed growers more time to scout fields and get insecticidesWhen the sorghum matures around soft dough, the resist-applied when aphids were present. ance to sorghum aphid isnt as good as earlier and we can see 32/ SEEDWORLD.COMDECEMBER 2023"