Science to the Grower: Are liverworts lost in the ozone?
by Richard Evans
The liverwort is unloved. It has the ugliest common name of any plant. Devoid of leaves, stems, or even a vascular system, it barely qualifies as a plant at all. It's considered primitive. It multiplies prolifically. Each archegoniophore, the structure that supports sexually-produced spores, releases up to 7 million spores, which remain viable for over a year (O’Hanlon 1926). (By the way, I’m glad I’m not O’Hanlon, who had to count 7 million liverwort spores.) Liverwort also reproduces asexually: cup-like structures generate a multitude of asexually-produced gemmae, each gemma giving rise to a new plant. Fragments of liverworts also produce new plants.
One liverwort species, Marchantia polymorpha, can wreak havoc in nurseries. A mat of liverworts on the surface of a potting substrate impedes water and nutrient entry, leaving the crop plant hungry and thirsty. In addition, liverworts can harbor pests and diseases, including fungus gnats, Fusarium and Pythium.
Control of liverworts in a nursery is difficult. Hand weeding, in addition to being tedious and expensive, may not be effective unless the top inch or so of substrate is removed. The registered chemicals for postemergent treatment have a spotty record of success against liverwort, depending on conditions during and after treatment. Quinoclamine, an algicide that has performed well in trials, as well as in extensive studies by a graduate student at Auburn University (Newby 2006; Newby and others 2006), will not be registered for use in the United States for human safety reasons.
Two scientists at the University of Guelph recently reported that use of ozone-treated irrigation water may reduce liverwort growth as a side benefit (Graham and Dixon 2012). Based on their previous investigations, which included assessment of ozone-related phytotoxicity in horticultural crops (Graham and others 2009), Graham and Dixon supposed that some of liverwort’s primitive characteristics would make it particularly sensitive to ozone. To test their ideas, they grew liverworts on rockwool and subjected it to a range of aqueous ozone concentrations and exposure times. They found that liverwort growth and development were controlled well by applying ozone-treated water five times per week. Although Graham and Dixon didn’t simultaneously treat a nursery crop, their previous work showed that the ozone concentrations and exposure times used in this study did not damage select woody nursery plants (Graham and others 2009).
Ozone treatment of irrigation water has its hazards and complications. Most operators use a batch treatment system, in which treated water is stored to allow ozone breakdown before use on crops. Graham and Dixon argue that carefully managed direct application of aqueous ozone in nurseries could provide the ancillary benefit of liverwort control, in addition to the pathogen control that is at the heart of ozone water treatment.
Graham T, Dixon MA. 2012. Liverwort control: An ancillary role for ozone-based irrigation water treatment systems? HortScience 47: 361-367.
Graham T, Zhang P, Zheng Y, Dixon MA. 2009. Phytotoxicity of aqueous ozone on five container-grown nursery species. HortScience 44: 774-780.
Newby AF. 2006. Liverwort control in container-grown nursery crops. Master of Science thesis, Auburn University, Auburn, AL.
Newby A, Altland JE, Gilliam CH, Wehtje G. 2006. Postemergence liverwort control in container-grown nursery crops. Journal of Environmental Horticulture 24: 230-236.
O’Hanlon ME. 1926. Germination of spores and early stages in development of gametophyte of Marchantia polymorpha. Botanical Gazette 82: 215-222.
Richard Evans is Cooperative Extension Environmental Horticulturist, Department of Plant Sciences, UC Davis.