Genetic map of ‘pot’ may aid hops researchers
Marijuana’s genetic “map” is expected to help researchers chart the disease defenses of the hop plant.
Hops are in the same family as cannabis sativa, an herbaceous plant known for its psychoactive properties.
In 2011, Canadian researchers mapped the “genome” — or hereditary information — of cannabis sativa to study its biomedical properties.
That map is now providing a blueprint for the hop plant’s genetic data, such as the layout of chromosomes, said John Henning, a USDA hop breeder, at the 2014 American Hop Convention in Portland, Ore.
The cannabis sativa map will help researchers get a general sense of where hop chromosomes and partial chromosomes are located, he said.
Aside from piecing together genetic data, the cannabis sativa map will validate existing information about the hop genome, Henning said.
Such research is involved in finding genetic “markers” associated with resistance to powdery mildew and downy mildew, common fungal diseases, he said.
These markers are useful for “targeted breeding” — researchers can identify markers for resistance, which makes them less dependent on observable plant traits in the field, he said.
Markers provide a “bird’s eye view of what is going on,” allowing breeders to understand the basic location of certain genetic traits without isolating the exact genes involved, he said.
While mapping is good enough for breeding purposes, “deep sequencing” of hereditary data will explain precisely which genes are responsible for certain traits and which regulators turn them on and off, he said.
“It’s like a zoom-in on a lens,” Henning said.
Aside from genetic resistance, researchers are studying how to manage disease with better field practices.
While post-harvest control measures are still being evaluated, it appears late-season pruning can impede powdery mildew, said David Gent, a USDA plant pathologist.
Researchers have made progress in understanding when buds are susceptible to the fungus and have narrowed the target window for pruning.
Mechanically or chemically killing hop “flag shoots” infected with the fungal pathogen in late summer can prevent broader re-infection the following year, he said.
In the Northwest, powdery mildew has to survive on living hop tissue, he said. While such flag shoots are relatively rare, even a small number can cause outbreaks of the disease.
Some flag shoots can survive chemical pruning and mechanized pruning is more effective at reducing powdery mildew, but the latter method is more expensive, he said.
Knowing precisely when to prune hops without harming yields will depend on the variety and require experimentation, Gent said.
In Europe, controlling powdery mildew is complicated by the disease’s ability to survive on dead tissue.
In the Northwest, only one “mating type” of the fungus lives on hop plants and dies without a living host.
In Europe, there is another mating type present and the two strains can reproduce sexually, forming a reproductive structure known as a “cleistothecia.”
A cleistothecia can survive on dead tissue and re-infect new plant growth with the disease, said Gent.
This dynamic has obvious disease management implications, which is why brewers and breeders must carefully follow plant quarantine regulations when importing hops from overseas, he said.