Not everything you see on a tree's bark is disease, although that's usually the first thought of a plant pathologist. Below are bacterial canker and black streak which are both significant problems in some California avocado orchards. Their cause is normally an issue of water/salt management.
But sometimes these patches are innocuous. In a recent farm call, and the “thing” on the bark was a hybrid organism called a lichen. This is a classic odd ball of nature. It consists of a blue-green algae called a cyanobacteria – photosynthetic single cells making sugar – and a fungus that serves to protect the algae from the elements. The fungus is farming the algae, in a way. Neither, though, does well on its own. In fact, it's been difficult to recreate the lichen relationship in the lab, starting with the separate, independent partners.
These are tough “beasts” surviving on most continents, in some very harsh environments. When the going gets rough, they just close up on themselves and survive until the right moisture conditions occur. And, within an hour they can spring forth full of color. The rest of the time, they might just hang on a rock.
Some of the algae also fix nitrogen from the air, so lichen really are self-sustaining. They just need somewhere to grow, a tree, a rock, a roof. They take amazing shapes and colors and they are also known for producing some interesting toxins that could be used as medicines.
These life forms have been studied for over 150 years and have been recognized as kind of the standard for a "mutualism", a symbiosis where the two participants benefit from each other. Only in the last few years has it been recognized that there is often a third component to this symbiosis - various yeasts that are the likely producers of the toxins. Toxic for some life forms like other bacteria, algae or animals, but not others. For some animals, lichens can be an important food source, such as for caribou, reindeer and musk ox. Animals and lichen that are all growing in a harsh climate.
The California state lichen is Spanish Moss or Lace Lichen, neither moss nor lace nor Spanish, but Ramalina menziesii. It is not to be confused with that other Spanish Moss found around the world, in the genus Usnea. It's all worth reading about, because it can get so confusing
Snails and Slugs (May 22, 2019 from 3-4pm)
Presenters: (!) Dr. Cheryl Wilen (UC IPM), (2) Dr. Rory Mc Donnell and (3) Dr. Dee Denver (Oregon State University), (4) Dr. Adler Dillman and (5) Dr. Irma De Ley (UC Riverside). The webinar will cover an overview of snail and slug biology, damage and management with emphasis on brown snail and Italian white snail, and current research on slug biocontrol using nematodes. One DPR CE unit (other) and one CCA CE unit (IPM) are approved.
And What Else Are the
UC Ag Experts
|UC Ag Experts Talk: Snails and slugs||5/22/2019|
|Uc Ag Experts Talk: Management of Weeds in Citrus Orchards||6/19/2019|
|UC Ag Experts Talk: Citrus Dry Root Rot||7/24/2019|
What is involved in the webinars?
A series of 1 hour webinars, designed for growers and Pest Control Advisors, will highlight various pest management and horticultural topics for citrus and avocados. During each session, a UC Expert on the subject will make a presentation and entertain write-in questions via chat during and/or after the presentation. As we develop this program, we may expand to other crops.
Topics: pests and diseases of citrus, avocado and other crops
And Next up is:
Management of Weeds in Citrus Orchards (June 19, 2019 from 3-4pm)
Dr. Travis Bean, assistant weed science specialist in UCCE, will discuss the importance of weed management in citrus, tree age and variety considerations, scouting and weed identification, cultural and mechanical practices, and pre- and post-emergence herbicides. One DPR CE unit (other) and one CCA CE unit (IPM) are pending.
Register in advance for the webinars by clicking on the event links above.
Are there Continuing Education units?
When the subject discusses pest or disease management, continuing education units will be requested from DPR (1 unit per session). Participants will pre-register, participate in the webinar and be awarded the unit. The sessions will be recorded and hosted on this web site for future study. However, continuing education units will be awarded only to the participants who attend the live version of the webinar.
Who is involved?
This webinar series is brought to you by Ben Faber (UC ANR Ventura Advisor) and Dr. Beth Grafton-Cardwell (Depart of Entomology UC Riverside Extension Specialist) with the technical support of Petr Kosina (UC IPM Contect Development Supervisor) and Cheryl Reynolds (UC IPM Interactive Learning Developer).
- Author: Steven A. Tjosvold
Plant diseases– their occurrence and severity– result from the impact of three factors: the host plant, the pathogen, and the environmental conditions. This is represented with the disease triangle.
If any one of the three factors is missing, the triangle is not complete, no disease will occur. Simply, plant disease will not occur if there is no viable pathogen, or no susceptible host plant, or the environmental conditions are not favorable. The severity of disease depends on the favorable level of each factor. How susceptible is the plant? How virulent is the pathogen? How conducive are the existing environmental conditions in supporting disease and pathogen spread?
The triangle also helps illustrate that the 3 factors are interacting with each other. The clearest example of this is how the environment factor interacts with the pathogen and host factors. Previous blogs illustrate the importance of leaf wetness on pathogen infection and disease severity. Long durations of free water on a susceptible plant can increase pathogen infection and disease severity. At the same time, the low- sunlight conditions, when these wet periods could occur (e.g. winter), could also be stressful to the plant, and the plant is less likely to mount defensive reactions to fend off infection.
Learn about the biology for any disease you are managing. Consider the disease triangle and the three interacting factors, and how management practices might help weaken or break the triangle's bond. For example, could you grow non-susceptible plant varieties or species? Could you eliminate the pathogen through judicious sanitation practices? Could you manage leaf wetness and relative humidity to create unfavorable environmental conditions for disease?
Could you grow non-susceptible plant varieties or species?
Although these three plant hosts all have rust diseases, each host is infected with a unique plant pathogen that attacks its specific host. In managing these diseases, a strategy of host resistance might be employed, where crops of non-susceptible hosts are rotated into the infested area. Conceivably these three hosts could be rotated.
Could you eliminate the pathogen through judicious sanitation practices?
Sanitation is a key management practice that employs the strategy of eliminating the pathogen from the growing area. Here, sanitizing shoes before entering the greenhouse. Cleaning soil from tools and sanitizing them before use. Properly covering and disposing of rogued plants or cut flowers (not shown here!).
Could you manage leaf wetness and relative humidity to create unfavorable environmental conditions for disease?
AND Then, can you add the 4th factor for Disease? That of TIME. It takes a certain amount of time for a given disease to show up. Some strike rapidly and others are chronic and persistent. Different time frames allow for different urgency for treatment. It's always best to remove one of the three Triangle factors before dealing with the Disease Pyramid which includes Time:
The Disease Quadrangle
UC Ag Expert talks about avocado diseases
Following the recent webinar on Citrus Thrips presented by Beth Grafton-Cardwell, a second webinar on Avocado Diseases will be offered in November. Check out the Thrips webinar and if interested enroll in the avocado one.
or on YouTube
Date: November 14, 2018
Time: 3:00 PM - 4:00 PM
Contact: Petr Kosina firstname.lastname@example.org
Sponsor: UC Ag Experts Talk
Register in advance for this webinar:
Ben Faber, UCCE Ventura Farm Advisor, will talk about avocado diseases. He will cover the identification and biology of root rot, crown rot, bacterial canker, leaf/stem blight, black streak and sun blotch. Then he will discuss the management management practices to prevent these diseases and the chemicals available to treat them. Resistance management tactics will be discussed.