- Author: Chris M. Webb
Ventura County UCCE Farm Advisor, Ben Faber shares his knowledge of avocados. Today’s topic:
There are over 900 named varieties of avocado…how do you pick one? Well, first, the selection at retail nurseries is somewhat limited, compared to what is offered by nurseries that supply commercial avocado growers. But you do have options. In general, avocado trees are big and need room. ‘Hass’ has an umbrella-shaped canopy, but ‘Reed’ is more columnar and needs less room. ‘Holiday’ is a much smaller tree than ‘Hass’ and “Littlecado’ is a somewhat smaller tree. You can also choose on the basis of when you want to pick your fruit. For example, ‘Fuerte’ is a winter fruit and ‘Reed’ is a summer fruit. That means they taste best in those periods, but they will hold on the tree for a much longer period of time. You can also choose on the basis of taste. Of course, this is very subjective, but ‘Zutano’, ‘Bacon’ and ‘Stewart’ have a lower oil content than ‘Hass’, ‘Pinkerton’ and ‘Reed’. Many consider ‘Nabal’ the best tasting fruit. It’s a summer variety and hard to find in retail nurseries, but sometimes you can order them for delivery later.
You can read more about varieties at this website: http://ucavo.ucr.edu/AvocadoVarieties/VarietyFrame.html#Anchor-47857. But remember, most of these are not available at retail nurseries.
- Author: Chris M. Webb
Ventura County UCCE Farm Advisor, Ben Faber gives advice on how to identify and treat root rot in avocados.
How to identify root rot and treat it?
The canopy is thinning. The leaves are small and yellow. There is dieback in the canopy, with leafless tips on the branches. You dig around under the canopy in the wetted area of the sprinkler and you can’t find roots within 6 inches of the soil surface or if you do find them, they are black. There is little mulch under the tree. There are weeds growing under the tree. All these are signs of root rot disease. But it is also a sign of lack of water, because that is what is happening – there are no roots to take up water. At this point, gardeners may unfortunately water the tree more, which only makes conditions worse for a diseased tree; adding more water to a tree that can not easily take it up, creates asphyxiation conditions.
So what do you do if you have disease? First of all, make sure you are irrigating to the needs of the tree by checking soil moisture before irrigating and ensuring the tree is not receiving supplemental water from another area such as a lawn sprinkler. Add and maintain a good thick layer of woody mulch under the canopy. Adding gypsum (15-20 pounds per tree), evenly spread under the canopy, can also help. There are fungicides available from retail nurseries, but reviewing and modifying irrigation practices and maintaining a mulch layer are the two most important things you can do.
Read on for details on mulch and irrigation.
Mulch and avocados?
“I just raked up all the leaves under the avocado and it looks so nice,” you say. PUT THEM RIGHT BACK. The avocado is shallow-rooted and really depends on the natural leaf mulch to protect its roots. In fact, the roots will actually colonize the rotted leaves as if it were soil. This mulch is also a first line of defense against root rot. The decomposing leaves create a hostile environment to the microorganism that causes the disease. The mulch also helps to reduce evaporative loss of water and therefore reduces water needs. Commercial growers will actually spread mulch in cases where trees are too young to produce adequate leaf drop for mulch or in windy areas where mulch has blown away. The key to remember is that the mulch should be kept at least 6 inches away from the trunk to avoid collar rot, which can be caused by keeping a moist mulch against the trunk.
How to irrigate avocados
Mature avocado trees may be large above ground, but they have very shallow roots mostly in the top 8 inches of soil. The tree therefore does not have access to a large volume of stored water. As opposed to a deep-rooted walnut, avocados require frequent, small amounts of water. A young tree in the summer might need multiple applications per week, but because the root system is small, each application may only be 5-20 gallons. An older tree with its wider rooting pattern may go a week to a month between irrigations depending on the weather and rainfall. Proper irrigation is the best way to keep the avocado from getting root rot. Both over and under irrigation can induce the conditions for root rot, although over irrigation is more common. And remember, it is not just the amount applied at an irrigation, but the timing that is important, as well. Because you are managing such a shallow root system, just poking your finger into the root system will tell you if there is adequate moisture there before you irrigate again.
- Author: Chris M. Webb
The California Avocado Commission (CAC) funded a multiyear study lead by Greg W. Douhan, a UC Plant Pathology and Microbiology researcher, to control avocado root rot. Control of this destructive pathogen will ultimately be accomplished with resistant rootstocks.
The research team is focusing on crossing already-resistant rootstocks, and has identified several that have greatly improved yields on land infested with Phytophthora cinnamomi. Six plots are being tended this year including one in Santa Paula. They have plans for additional plots for the study. Work is being done at the UC South Coast Research and Extension Center in Irvine and at University of California, Riverside (UCR).
"Seed screening seeds from our breeding blocks will continue…We have also collected new isolates of P. cinnamomi to determine how diverse the population is so that appropriate isolates can be used in the initial screening process. My graduate student is finishing up this summer and will hopefully have most of our advanced lines characterized genetically. I am also submitting a USDA grant to work on the parentage analysis of our advanced lines as well as continue to work on genetically characterizing avocado rootstocks.”
To read the entire study report, please visit http://www.avocado.org/growers/symposiumcontent.php?research=b3.
- Author: Chris M. Webb
Ventura County UCCE Staff Research Associate Maren Mochizuki shares with us how monitoring spore traps in avocado orchards can lead to better understanding and management of disease.
An important component of integrated pest management is frequent monitoring to understand which, if any, pests are present and at what time of year. In collaboration with Akif Eskalen a researcher at UC Riverside, Ben Faber, Ventura County UCCE Farm Advisor and Maren Mochizuki, Ventura County UCCE Staff Research Associate are sampling in three avocado orchards in Ventura and Santa Barbara counties using traps that catch spores, reproductive structures for many disease-causing organisms in avocado such as Dothiorella and Phytophthora
The spore traps consist of glass microscope slides suspended at two heights in the avocado tree canopy. Every two weeks, we remove the slide and replace it with a fresh one; researchers at UC Riverside examine and identify the spores under a microscope. We hope to improve our understanding of the life cycles of these spore-producing organisms for more effective disease management.
Young avocado tree killed by Dothiorella canker,
White, powdery exudates from a Dothiorella canker
Healthy green avocado foliage (right) next to the pale, yellow foliage of a tree with Phytophthora root rot
- Author: Chris M. Webb
Ventura County University of California Cooperative Extension (UCCE) is here to extend science-based research to people in our community. We do this in a variety of ways, one of which is newsletters.
Landscape Notes is written for people working in the commercial landscape industry. The last issue is all about establishing landscape trees. It is full of fabulous, practical information that will help establish healthy trees.
Clover Lines is a newsletter published for 4-H members and leaders in Ventura County. It contains events, activities, and opportunities for youth aged 5-19.
Topics in Subtropics is a combined effort by University of California Cooperative Extension Farm Advisors from many counties in the state. It emphasizes citrus and avocado, but also discusses the minor subtropicals. The last issue covered:
- Avocado Research in Ventura County
- Laurel Wilt Disease Conference and Tour in Florida and Georgia
- Managing Insecticide Resistance will be Key to the Future of Effective Citrus Pest Management
- Smart Sprayers Make Sense
Farm Water Quality News delivers the latest news on integrating environmental quality with crop production practices. The last issue covered:
- Regulatory Update
- Industry Update
- Technical Tips
- Research Update
UC Cooperative Extension Report is our department newsletter. This newsletter includes upcoming events, highlight summaries of research and outreach activities, interesting facts and more.
Santa Clara River Watershed Times covers topics vital to anyone who lives, works, and recreates in the Santa Clara River watershed, the largest river system in Southern California. An amazing amount of information is extended in this newsletter covering a wide range of issues, opportunities, regulations, and accomplishments in an easy to read format with great photos. Links for more information are scattered throughout.
Our newsletters can be found by clicking this link. Once there, you can read current and back issues. You can also sign up for email notification to let you know when a new issue has been posted.