Spores are key to the success of the liverwort
Fig 1. Dry liverwort spores are less than 3.5 microns in diameter, smaller than many fungal spores.
Fig 2. Liverwort spores quickly imbibe water and more than double in diameter (same scale as above)
Fig 3 (left) is a more magnified image of the previous image showing some internal structure of the spore.
The sporelings grow rapidly with mineral nutrients and a cool moist environment. In laboratory conditions new reproductive structures could begin to form in just 3 to 4 weeks, and the whole sexual life cycle, from spore to spore, takes about 3 months.
An experiment in 1925 and other more recent confirm that the sexual reproductive phase of liverworts is induced with long days (12 hours or longer photoperiod). Other similar experiments have shown that far-red wavelengths of light also promote the sexual stage and therefore spore formation. Far-red light is mostly invisible to our eye and is found in greater proportions to that of visible light in the shadows, under benches or under plant leaf canopies. This knowledge leads to interesting management options. Could greenhouse covers or supplementary films or sprays be manufactured that reduce or eliminate far red light for the purpose of eliminating liverwort spore production?
In any weed management strategy, look for strengths and potential weaknesses of a weed's biology and life cycle. Since liverwort spores are so important to the success of liverworts, we need to reduce or eliminate reproductive structures. The information on photoperiod tells us that reproductive structures could begin to be formed in the longer days of spring and summer. If you needed to target a critical time to rogue immature liverworts or apply herbicides, it would be in the period before reproductive structures could mature and produce spores.
Liverworts do not have the typical propagules associated with other common weeds. They don't have seeds or asexual propagules such as rhizomes and stolons that can spread and establish as new plants. However, growers wonder how liverworts can become so widely established in the nursery or greenhouse.
Probably the most important asexual spread in nurseries and greenhouses is a result of gemmae. On the upper surface of the liverwort there are 3 to 4 mm cup-like structures called gemma cups, and at the bottom of each gemma cup over a hundred tiny, disk-shaped gemmae can be produced. When water droplets from rain, sprinkler, or an irrigation wand hits the gemma cups, the gemmae are propelled out and can develop into separate plants. Each gemmae produces a clone of its mother plant.
When spores are released they can be spread by air currents. Each female structure produces spores by the thousands. They are very small (less than 3.5 microns in diameter when dry), on the smaller side of most aerially dispersed fungal spores and pollen (which range from 3 to 100 microns). See for yourself in this video below. I recorded in a calm, darkened office. (Be patient for about 20 seconds). When I opened the office door and the fresh air came in, whoosh, away flew the spores. In the office, not a big deal; when it happens in your nursery or greenhouse, it is a big deal. These spores might move significant distances even with the slightest of air movement!
Next week: what happens after the spores land in your nursery or greenhouse, and management options.
Spore Release video. Use full screen mode.
Unlike most weeds that growers deal with, liverworts are very primitive plants. Land plants arose from freshwater green algae around 500 million years ago. Bryophytes, consisting of liverworts, mosses, and hornworts, were some of the earliest groups of land plants. They have features distinct from those of other land plants: they lack a vascular system and lignified (hardened) cell walls, they have motile sperm that can swim in water, and their life cycle is dominated by a haploid stage. Liverworts are mostly composed of cells with nuclei that only contain one set of chromosomes (haploid). More evolved plants, the crops and weeds on land that we are familiar with, are primarily composed of cells that have two sets of chromosomes (diploid). Only their eggs and sperms are haploid.
It turns out that liverworts with their haploid nature, production of spores, ease of culture, and quick regeneration time-- the same characteristics that make them weeds-- are the same characteristics that help scientists study the most advanced forms of experimental molecular plant biology. Here is where liverworts shine. In 2015, Japanese molecular plant biologists created liverwort mutants by bombarding them with radiation. These mutants were studied with genetic markers to elucidate how land plants might use their phytochrome system and different wavelengths of light to regulate plant development. In addition, the liverwort's chromosomes have been analyzed. This has revealed that most of the genes that regulate growth and development in higher land plants are also found in liverworts. It has been suggested recently that many of the fundamental features of land plants appeared in bryophytes first and were then co-opted in vascular plants. So in a distant way, the crop plants you grow today have a little bit of liverwort genetic material in them. The next time you cuss out those tenacious liverworts growing in your nursery crops, think about how intrinsically important they are to the crops you grow!
For the next several Wednesday blogs, I will discuss liverwort biology and management. Don't forget to subscribe to receive these posts in a timely manner.
Generally, fungicides need to be applied preventatively, before infection occurs, to be most effective in controlling diseases. But what if Phytophthora fungicides are inadvertently applied after root infection? What can be done to improve their timing and effectiveness?
A series of experiments evaluated the efficacy of 8 commercially available biological and conventional fungicides for the management of crown rot on Gerbera caused by Phytophthora cryptogea. The fungicides were either applied before the pathogen was introduced to the soil (preventatively) or after the pathogen was introduced to the soil and root infection had already occurred. The new conventional fungicides oxathiapiprolin (Segovis®) and mandipropamid (Micora®) provided effective preventative control of crown rot. None of the other 6 tested fungicides effectively prevented disease in this particularly rigorous evaluation. These ineffective fungicides included mefenoxam (Subdue Maxx®), cyazofamid (Segway®), Aluminum phosphonate (Areca®), etridiazole (Terrazole®) and Trichoderma species (Root Shield Plus®), and Bacillus amyloliquefaciens (Triathlon®).
When mefenoxam (Subdue Maxx ®), was applied to the soil after the soil was infested with P. cryptogea, Gerbera were mostly free of any above-ground and root symptoms. At the end of the experiment, the pathogen was recovered from healthy looking root balls when the roots were cultured in the laboratory (See previous blog post regarding the problem of masking symptoms). The other 7 fungicides were ineffective when applied after the soil was inoculated; the treated plants eventually died although more slowly than the untreated plants.
It is important to time the application of Phytophthora fungicides to prevent infection. Applications can be made prior to environmental conditions that are conducive to propagule spread and infection, such as before rainy periods. If disease is detected in the nursery crop, it is important that diseased plants should be rogued from the crop early, before fungicides are applied. A scout should key in on above ground symptoms and confirm that roots or root crowns are rotted. If diseased plants are found, nearby plants that aren't necessarily showing above ground symptoms need to be checked for root rot too. All plants with root symptoms should be rogued, and fungicides could be applied to nearby plants to prevent infection from outlying infectious propagules.
See Phytophthora above-ground and root symptoms:
For a complete research report: please contact me at firstname.lastname@example.org
This fungicide evaluation was partially funded by the California Cut Flower Commission (KKRF Endowment) and California Association of Nurseries and Garden Centers (CANERS Endowment).
Question: Which of the four Gerbera plants above are infected with a deadly plant pathogen?
Answer: Most observers would say the badly wilted plants, that is, the second and fourth from the left. However, the correct answer is the second, fourth, and the healthy-looking third plant from the left!
An astonishing observation was made recently in a series of experiments to evaluate the efficacy of several conventional and biological fungicides to control disease caused by the deadly plant pathogen, Phytophthora cryptogea. This soil- inhabiting pathogen begins its deadly course by infecting susceptible roots. It develops up to the root crown near the soil line, the plant wilts, and eventually dies. However, in the experiments, Gerbera were mostly free of any above-ground and root symptoms when a conventional fungicide, mefenoxam (Subdue Maxx ®), was applied to the soil several days after the soil was infested with P. cryptogea. At the end of the experiment, however, the pathogen was recovered from healthy looking root balls when the roots were cultured in the laboratory (Yes, the third plant above and others like it!). Mefenoxam was stopping or slowing the development of the disease once it was initiated but did not kill the pathogen. Of the other 7 fungicides tested, no other fungicide masked the existence of root infection in this way.
Normally fungicides are applied before disease occurs to prevent infection. But in another twist to the story, when mefenoxam was applied before P. cryptogea was applied to the soil, it just did not work well, and plants died at the end of the experiments. As expected, other fungicides controlled disease, some of them very well, when applied before the pathogen was applied to the soil.
This provides some new information about the unique mechanism of action of mefenoxam. Also, since mefenoxam treatment of plants with infected roots could still produce plants of high quality, they could easily be inadvertently harvested and shipped in the trade. This “masking of symptoms” is thought to occur in the trade but until now has not been experimentally demonstrated for Phytophthora root infections. Still to be determined: does the mefenoxam suppressive activity wear off and, if so, could disease symptoms begin to develop (perhaps as a potted flowering plant on the dining room table or planted in the landscape). This also has significant implications for our nurseries that produce native plants for restoration purposes.
Could these symptomless carriers of Phytophthora be planted in sensitive natural areas and be sources of inoculum for new disease in native plants? The potential ecological problem of releasing new Phytophthora species, disease symptoms, and their management is described in several published newsletter articles. http://ucnfanews.ucanr.edu/newsletters/Download_UCNFA_News_as_PDF70423.pdf
This is the first post of the new Nursery and Flower Grower blog. It will be a weekly post covering subjects about horticulture and pest management for the grower and associated industry. My goal is to produce concise, interesting, and useful articles that cover a single subject, idea, or news. You will see lots of images that I have taken over my 38-year career as a Farm Advisor, and some new video clips too. Please feel free to subscribe to this blog by clicking the “subscribe” link in the upper right-hand portion of this web page.
Next week: More on the Evaluation of Phytophthora fungicides and biological control.