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.  

Liverworts can spread and propagate themselves with tiny sexually-produced  spores and asexual propagules.  The flat green leaf-like body of the liverwort spreads horizontally across the media surface by dividing into two branches at apical notches. Each apical notch contains a meristem (here circled in white), basically a growing-point, that can carry on growth laterally if there is media to grow on. Growing-point fragments that are dislodged or torn away from the mother plant have the potential to start new plants. Rhizoids, similar to root hairs, can establish new connections to the media to absorb water and mineral nutrients. 

 

 

 

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.

 

Liverworts also spread through sexual reproduction. Each individual plant is either male or female. Male plants make reproductive structures topped by a flattened, lobed disc that produce sperm. Female plants make umbrella-like reproductive structures that produce eggs. In the presence of water, swimming sperm from the male plant can be splashed up on a nearby female structure. The fertilized ovum develops and remains attached under the lobes of the female structure. Male and female spores are produced on hair-like structures that move to facilitate spore release and dispersal.

 

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.

Liverwort (Marchantia polymorpha) is a tenacious weed that is well known to Central Coast nursery growers.  They form dense colonies on the soil surface that can impede water infiltration into the pot, use available plant nutrients, and can choke the growth of slow growing plants,  vegetative- cuttings and seedlings. They are especially a nuisance in moist environments, shade houses, and greenhouses. And these weeds are tough. No conventional or biorational chemicals have proven to be very effective in managing them well.

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:

http://ucnfanews.ucanr.edu/Articles/Feature_Stories/Phytophthora_Crown_and_Root_Rot_Symptoms_and_Detection/

 

For a complete research report:   please contact me at satjosvold@ucanr.edu

 

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.