April, 2020 CAS/CAC/UC

Avocado Grower Meetings

Seminar Presentation Document

The original field meeting for avocado growers was canceled this year. This was to be an opportunity for growers to get out and see, touch and interact with other growers and people experienced in the various topics that were to be covered. The meetings were to be in real groves at Cal Poly San Luis Obispo, Pine Tree Ranch in Santa Paula and Rancho Aqua Tibia in Pauma Valley. The discussion and viewing topics ranged from 1) Planting Density, 2) Pruning, 3) Root Rot Management, 4) Pest Monitoring, 5) New Scions/Rootstock and 6) the Healthy Soils Program. We had a range of speakers arranged for the different sites to show, demonstrate and spark discussion. The actual sessions were designed so that every topic got covered and everyone got to hear the highlights of the topic in a 15 minute period, before moving on to the next topic. For the Scion/Rootstock session, this would have been just a brief introduction before the more elaborate workshop of that topic which will be held in June. The enclosed outlines and references cover the topics that would have been covered at these field meetings. They are not a substitute for actually being out in the field, but hopefully they will keep you thinking about how to improve your avocado production. 

Planting Density: Tim Spann of CAC

Pruning: Gary Bender, Sonia Rios of UCCE and Gabe Felipe of Mission Produce

Root Rot Management: Ben Faber of UCCE

Pest Monitoring: Enrico Ferro, Tom Roberts, Jane Delahoyde, Jim Davis

New Scions/Rootstocks: Mary Lu Arpaia, Patricia Manosalva of UC Riverside

Healthy Soils Program, Alli Fish, Esther Mosase of UCCE; Lance Andersen of Mission RCD; Jamie Whiteford of Ventura RCD; Claire Balint of Cal Poly; Mary Matava of El Corazon Compost

READ THEIR PRESENTATIONS

Lemon growers are seeing good production this year, but the market for choice lemons has been ruined by COVID-19 food-service industry losses.

“We've seen 20 to 25 percent decrease in fruit movement, and that's mainly attributable to the decline in the food-service industry,” according to Ventura County lemon grower Will Pidduck.

Ironically, it is otherwise a good year for lemons in Ventura County. “We're growing great quality fruit. The quantity is great,” said Pidduck.

However, with the coronavirus pandemic forcing the closure of restaurants, schools, cruise lines and other commercial food-service operations, the market for an entire class and size of lemon has all but vanished.

“That's a big hit,” Pidduck said. As a fourth-generation citrus grower in the Ventura area, Pidduck also grows mandarins, oranges and avocados, but is primarily a lemon grower.

“A large portion of the choice fruit goes to the food-service industry. And without that industry, it's bad. The movement has slowed dramatically on the choice fruit,” Pidduck said. “We're still moving the fancy, the higher quality fruit. But for the choice and some smaller-sized fruit, it definitely slowed way down.”

And what is happening  in the global lemon market?  from Fresh Plaza

An Automated Delivery System for Therapeutic Materials to Treat HLB Infected Citrus

Ozgur Batuman¹ and Louise Ferguson²

¹Southwest Florida Research and Education Center, University of Florida, Immokalee, FL; ²UC Davis, Department of Plant Sciences, University of California Davis, Davis CA

Why is this research needed?

In 2005, a disease called Huanglongbing (HLB, citrus greening, was identified in Florida's commercial citrus groves. The disease is caused by a bacterium that affects all citrus cultivars by disrupting the flow of nutrients from the source of production, to the site of use, causing tree decline. HLB weakens the root system, increases early fruit and leaf drop, lowers tree productivity and fruit quality and ultimately kills the tree. The disease has spread to all the major production regions in Florida. Economic losses have exceeded more than $4 billion dollars. Currently, more than 95% of Florida's trees are infected. There is currently no cure for the disease.

Efforts to control HLB have been unsuccessful as the bacterium cannot be cultured, literally grown, in a petri dish, and once in the plant it proliferates within the citrus phloem. Phloem is the system that transports sugars from their site of production, the leaves, to plant parts that use sugars, the roots or flowers.Phloem transport is generally downward but can be upward as well.

Once the HLB bacterium is in a tree's phloem it has the potential to infect the entire tree. It is exceedingly difficult to introduce any control agent into the phloem with the conventional control methods of foliar spraying or soil drenching. 

Thus far, no treatment preventing HLB infection, or controlling the bacterium once within the tree, has been developed. Potential chemicals are being investigated, but in order to test them, direct or indirect phloem delivery, where the bacterium proliferates, is needed. Therefore, an effective method of delivering an effective volume of theraputics into the phloem is needed to evaluate potential treatments. 

What is the focus of this project?

Our project focuses on developing a method of delivering therapeutic liquid materials, bactericides, microbial metabolites, RNAi, or biologicals, into the citrus vascular tissues, both the xylem which conducts water and nutrients upward from the roots and the phloem, which conducts sugars and other metabolic products downward from the leaves. We are investigating diffusion, trunk punctures with a surrounding liquid reservoir for passive uptake and infusion, low pressure active injections. We are focusing on these methods as foliar sprays and root drenches have not been successful phloem delivery methods.

Who will be doing the research?

The project is led by plant pathologist Dr. Ozgur Batuman with colleagues at the Southwest Florida Research and Education Center (SWFREC) at University of Florida in Immokalee. This four-year project will also study the citrus vascular system with a multidisciplinary research team including UF Plant Pathologists Drs. Nabil Killiny and Amit Levy at Lake Alfred, SWFREC UF Plant Physiologist Ute Albrecht, Citrus Horticulturist Fernando Alferez, Precision Ag. Engineer Yiannis Ampatzidis, Agricultural and Natural Resources Economist Tara Wade, University of California-Davis Extension Specialist Louise Ferguson and Texas A&M-Kingsville Citrus Center Plant Pathologist Veronica Ancona as well as number of graduate students, postdocs, and Florida, Texas and California citrus industry members.

How will this research be done?

Our earlier research involving comparisons of delivery methods including foliar sprays, soil drenching and trunk injection determined Needle-Assisted Trunk Infusion (NATI) was the best potential delivery method (Figure.1). In initial experiments, using NATI, 1 ml of rhodamine (1%) dye was injected into the trunks of one-year-old citrus seedlings. A visible red color, indicative of rhodamine uptake and movement, was detected in the upper-most leaves within 30-60 min and an increase in color intensity was observed within 24 hours. Similar results were observed in two-year-old grafted Valencia plants within 48 hours. If the NATI delivery method can be automated, large numbers of trees could be treated quickly. Once the delivery method has been developed, implementation will be tested with potential treatments developed within other research projects.

Our proposed automated delivery would consist of a robotic arm with several modules at the end of the arm, installed on an ATV or tractor. One module with needles would grip and puncture the trunk, a second module would wrap a reservoir around the trunk below the punctures and third module would fill the reservoir. (Figure 2). Hopefully, a robotic arm plus automated system will be inexpensive enough for growers to purchase and simple enough to use.

Another approach is disease prevention; application pf prophylactic chemicals that prevent infection. In this scenario our system would be used treat healthy young trees with bactericides or boost their immune system. When infected by the ACP the bacterium would either be killed or suppressed, perhaps below the level that harms tree growth and productivity. This option is analogous to the vaccinations that prevent diseases in humans and animals.

What are the greatest challenges and opportunities.

The greatest challenge is successful phloem delivery. The greatest opportunity is that, if successful, we will have developed a method that will allow much more precise deliver of theraputics to citrus trees. For example, if an effective phloem delivery method is developed, it could be used to control insects that feed on citrus plant parts. Or, it could be used to deliver growth regulators, perhaps nutrients and carbohydrates, to roots and fruits to increase growth, development and fruit quality; much like an intravenous injection functions in an animal.

Among the questions we hope to investigate are:

• When, what kind of, and what amount of therapeutics can be applied by NATI?
• At what frequency?
• What type of citrus tree: cultivar, age, infected, healthy is the best for treatment by NATI?
• Can we kill the bacterium? How and when to assess a change in bacteria titer after treatment?
• When will become available and be economically feasible for growers?

Figure 1. Distribution of rhodamine (red dye; 1%) applied by NATI in various tissues (left) of grafted and non-grafted young citrus plants grown in the greenhouse (right). Photos taken 2 weeks after the treatments. Treatments and tissues observed are indicated. Yo = year-old.

Figure 2. Projected automated delivery system (ADS); an ATV with extendable arm with NATI and the cover placement systems on the arm guided onto the tree trunk (upper panel), and closeup of NATI and cover placement system (panel below).

Acknowledgement

The United States Department of Agriculture National Institute of Food and Agriculture (USDA-NIFA) Grant # 2019-70016-29096.

For more information, please visit this project's dedicated website:

https://swfrec.ifas.ufl.edu/programs/citrus-path/automated-delivery/

The calls have come in.  We've gone from cool to hot and Dry Root Rot of Lemon has struck,  It's shocking how fast the trees go down.

Dry Root Rot has menaced growers in Ventura County for many years. In the ‘50's and ‘60's it seemed most prevalent on older orange trees. A few years after the wet winter of 1968-69, dry root rot became an increasing problem among citrus trees of all ages. At that time, most of the damaged trees were on sweet rootstock (susceptible to Phytophthora), and growing in fine-textured soils or soils with poor drainage. A few years after another wet winter/spring (of 1983), dry root rot again reared its ugly head, but this time predominately on young lemons.

The disease is caused by the fungus, Fusarium solani. This fungus is most likely present in all citrus soils in California. It is a weak pathogen in that by itself it will not attack a healthy tree. However, experiments conducted in the early 1980's by Dr. Gary Bender, showed that when seedlings were girdled, root invasion occurred. In the field, the fungus can infect trees once gophers have girdled the roots or crown. A Phytophthora infection will also predispose trees to Fusarium, as will asphyxiation. Therefore, the mere presence of the fungus in the orchard soil will not lead to the disease.

Description

Fusarium is a soil borne fungus that invades the root system. Once infected, the entire root will turn reddish-purple to grayish-black. This is in contrast to a Phytophthora infection which, in many cases, will attack only the feeder roots, but when larger roots are infected, only the inner bark is decayed and it does not discolor the wood. In addition, when observing the cross section of a dry root rot infected trunk, a grayishbrown discoloration in the wood tissue can be observed.

Dry root rot is a root disease, but symptoms of the root decline are seen above ground. They are similar to any of the root and crown disorders such as Phytophthora root rot, oak root rot fungus (Armillaria) and gophers. The trees lack vigor, leaves begin to turn yellow and eventually drop (especially in hot weather) causing twig dieback. Finally, the foliage will become so sparse that one will be able to see through the canopy of the tree. A period of two to three years may pass from the time of invasion until noticeable wilt. Many times, the tree will collapse in the summer, after a period of prolonged heat. In the case of dry root rot, the collapse is so rapid that the tree dies with all the leaves still on the tree. When looking for symptoms of dry root rot, keep an eye out for symptoms of other maladies as well — Phytophthora, oak root rot fungus and gophers being the most prevalent.

As mentioned previously, in order for Fusarium to infect a tree, there must be a predisposing factor such as girdling from gopher feeding. However, since many trees collapse from dry root rot without any apparent predisposing factor, there are obviously other factors which we have yet to identify. Therefore, in 1998, a grower survey was developed, along with intensive soil and leaf sampling, to attempt to identify as many new predisposing factors as possible. They might be elements in the soil, either deficiencies or excesses, or specific cultural practices such as irrigation patterns or fertilizer practices. Twenty orchards were identified from which 20 soil and 20 leaf samples were taken in diseased areas and another 20 soil and 20 leaf samples were taken from adjacent healthy areas. The owners or managers of the properties were given a questionnaire to complete regarding a variety of cultural operations. The objective was to identify those factors that would correlate well to trees becoming infected with dry root rot.

Survey Results

Soil analysis - The following laboratory procedures were conducted to see if there was any correlation between the disease and either deficiencies or toxicities of these elements or
conditions: sodium, boron, salt level, pH and soil type (sand, loam, clay). For these elements or conditions, no correlation was found. It would appear that for our sampling sites, these conditions, whether favorable or not (toxic or deficient), did not play a major role in predisposing the tree to dry root rot.

Leaf analysis - The following elements were analyzed for their concentration within the leaf: nitrogen, potassium, phosphate, manganese, magnesium and zinc. Of these, three correlations were found. Zinc and manganese levels were substantially higher in diseased trees. The third correlation showed a potassium deficiency in diseased trees. However, we do not believe that dry root rot is caused by elevated levels of zinc or manganese, or by potassium deficiency, but rather are a result of the disease. Unfortunately, it seems that we have still not identified any elements in leaf analysis that truly correlates and points to a predisposing factor for disease development.

Control Measures – What Works and What Does Not
Early experiments conducted by Menge, Ohr and Sakovich showed that the following circumstances or operations do not influence the incidence of this disease: fungicidal treatments, wounding the tap root at time of planting, sandy versus clay textured soils, spring versus fall planting and soil mounding.

1. In choosing your nursery tree, the choice of rootstock is not important in that, as far as we know, all rootstocks are susceptible to this disease. However, since Phytophthora is a major component in dry root rot development, choosing a rootstock like sweet orange would certainly put those trees in a high risk category. We recommend that growers use Phytophthora resistant rootstocks like C35 or Citrumelo.

Phytophthora. Publications written in the 1970's, and again noted by our survey, showed that Phytophthora is a major culprit in the dry root rot complex. To control dry root rot, it is essential that the Phytophthora, when present, be controlled. This can be accomplished by fungicidal treatments, and by the proper application and timing of irrigation water. Overwatering creates a favorable environment for the multiplication of the Phytophthora fungus.

Gophers. It is well known that gopher damage provides entry points for Fusarium. Controlling gophers is an important factor in reducing the potential of infection by Fusarium.

Control

We presently have no direct control for dry root rot. To control the disease, we must control the predisposing factors such as gophers, Phytophthora, poor drainage and over-watering. If the predisposing factor(s) cannot be identified for a given diseased orchard, it will indeed be difficult to control the disease. Two things are certain though: 1.) There are no chemicals to date which will control this disease; and 2.) Presently, there are no rootstocks resistant to the disease.

Listen to Akif Eskalen tell the Dry Root Rot story

https://www.youtube.com/watch?v=K2fyBcC1HXk&feature=youtu.be