Species in the Lolium genus, particularly annual and perennial ryegrass, are important seed crops in Oregon that are marketed in other parts of the country and world for cover crops, turf, and pasture. However, de-domesticated, weedy populations are troublesome in many cropping systems (I will refer to these weedy populations as Lolium spp.). It is astounding the variety of climates and different cropping systems that Lolium spp. are problematic. Their control is drastically complicated when populations evolve herbicide resistance. In fact, there are 125 reports of herbicide-resistant Lolium spp. around the world (as of December 2020), some evolved resistance to as many as seven mechanisms of action in a single population. No other species has shown such adaptability. So why is Lolium spp. so adaptable? My colleagues Maor Matzrafi (Agricultural Research Organization, Israel) and Christopher Preston (The University of Adelaide, Australia), and myself tried to answer that very question in a review paper published this week in Pest Management Science (Matzrafi et al., in press).
We came up with quite a few thoughts, but I would like to focus on four that I think are key to make Lolium spp. unique and adaptable. The first is the natural genetic variability that populations exhibit, increasing the chances that at least a few individuals in a population are adapted to a new challenge, for example, a new herbicide or other management practice. The second is a mating system that forces plants to cross with other individuals and avoid self-pollination, forcing the exchange of genetic material and give chance to a new combination of traits (technically called gametophytic self-incompatibility). Interestingly, this system may break down and plants may be able to self-fertilize under some extreme environmental conditions (pseudo-self-compatibility). The third is the human-driven movement of propagules that aid the mixture of populations from distant locations, enhancing the exchange of genetic material and introduction of individuals with new, adapted traits. Finally, the last characteristic is the capacity of different species to hybridize (L. multiflorum can hybridize with L. perenne, and both hybridize with L. rigidum). We went a bit further and suggest (although these hypotheses have yet to be tested) the involvement of endophytes and epigenetics in the rapid adaptation (I will save these for a future blog article).
Because of the large diversity among populations and within populations, Lolium spp. is a species to watch out for and take preventive and reactive measures to reduce their infestation as much as possible. Climate change, for example, is happening at a fast pace, and Lolium spp. may continue to adapt and move to new regions. Lolium spp. will also adapt to changes in management practices. This all means that diversity in management practices is key to a successful control program for Lolium spp.
Citation
Matzrafi, M., Preston, C., Brunharo, C. Evolutionary drivers of agricultural adaptation in Lolium spp. Pest Management Science (in press). DOI: https://doi.org/10.1002/ps.6219