The Latest on Florida's LW Disease

Recommendations for Laurel Wilt in Commercial Groves

Jonathan H. Crane, Tropical Fruit Crop Specialist1, Daniel Carrillo, Entomologist – Tropical Fruits1, Edward A. Evans, Agricultural Economist1, Randy C. Ploetz, Plant Pathologist – Tropical Fruits1, and Jeff Wasielewski, Commercial Tropical Fruit Crops Extension Agent2; 1UF/IFAS, TREC, Homestead, FL and 2Miami-Dade County Extension-UF/IFAS, Homestead, FL  Website: http://trec.ifas.ufl.edu 

Laurel wilt is a lethal disease that spreads rapidly among root-grafted avocado trees and for this reason necessitates frequent scouting to detect symptomatic trees and immediate tree removal to stop spread to adjacent trees. The research on the management of the laurel wilt pathogen and several ambrosia beetle vectors is continuing, therefore, recommendations for mitigation and control may periodically change.

This document describes the current recommendations to limit or prevent of the spread of the laurel wilt pathogen among avocado trees and to control the ambrosia beetle vectors of the disease. In addition, alternative management strategies concerning re-establishment of productive groves are discussed.

If you have trees symptomatic for laurel wilt please contact Don Pybas, LW Coordinator, (305- 247-0848; pybas@ufl.edu) or Jeff Wasielewski, Commercial Tropical Fruit Crops Extension Agent, (305-248-3311, ext. 227; jwasielewski@ufl.edu) for more information.

Disease symptoms of laurel wilt

The earliest external symptom of LW is green wilted leaves in sections of the canopy.

However, green-leaf wilting is quickly followed by desiccation and browning of these leaves, which remain attached to the plant stems for up to 12 months. Stem and limb dieback follow and eventually additional sections of the tree decline and the tree dies. Underneath the bark, the normally cream- white sapwood may have dark blueish-black streaks. Frequently, trunks and/or limbs may have numerous small diameter holes with sawdust tubes (toothpick-like protrusions from the bark), and/or sawdust clinging to the bark which is evidence of ambrosia beetle activity (i.e., boring into the tree).

Two main strategies for controlling the spread of the laurel wilt (LW) pathogen

1. Frequent scouting to detect trees with green leaves beginning to wilt (i.e., early symptoms of LW) and their immediate uprooting and destruction.
2. Infusion or injection of Tilt® fungicide into all the trees in a grove before a LW outbreak occurs. Repeated applications at an 8 to 18 month interval are necessary.

Of the two strategies mentioned above, the early detection-immediate destruction method is the most economical. Repeated infusion of fungicides may cause damage to the trees at the infusion sites and therefore may not be sustainable. In contrast, there is antidotal evidence that repeated injections are less damaging to the trees but there is a lack of information on the long-term sustainability of this method.

Primary strategy: early detection and tree sanitation (rouging)

A helicopter survey implemented by the Avocado Administrative Committee was important in alerting the industry and growers to new areas of potential LW infestation and is now important for monitoring on-going infestations. However, helicopter surveying is expensive and not frequent nor detailed enough to detect early green-leaf wilting caused by laurel wilt.

Scouting frequently is key in detecting LW affected trees at the green-wilt stage. Ground-based scouting by driving through groves via equipment (e.g., tractors, mowers, and spray equipment), trucks, and ATVs is possible. However, detecting early symptoms of LW in the tops of trees may be difficult by just driving. An alternative is the use of drones, which can cover large areas quickly, and in detail. However, there are Federal Aviation Administration requirements and regulations that must be met in order to operate commercial drones. Canine detection of LW is an additional detection method. The canines detect the LW pathogen in pre- and fully symptomatic (e.g., desiccated leaves, tree dieback) trees. In many cases, fungicide infusion of infected but pre-symptomatic (no outward signs like wilting) trees allows these trees to remain productive, however fungicide re-treatment is necessary. In addition, early detection by canines provides time to remove pre-symptomatic trees to prevent further root graft spread of the pathogen to healthy trees. The number of available trained canines and for some producers, the cost, may limit the use of canines.

The major component of LW control is to remove and destroy LW affected trees at the green- wilt stage. Do not hesitate to remove and destroy a green-leaf wilted tree – waiting to see if more of the tree canopy develops symptoms or turns brown allows time for the pathogen to spread to adjacent trees through their connected root systems. Waiting to remove and destroy the LW affected tree reduces the chance tree removal alone will stop the spread of the LW pathogen to neighboring trees.

The LW pathogen can survive in declining and dying trees and the only way to stop movement of the pathogen to adjacent trees is to remove (uproot) affected trees immediately. Just stumping or hatracking LW affected trees will not stop the pathogen from moving to adjacent trees through root grafts among adjacent trees. In addition, trees that are affected by LW are very attractive to ambrosia beetles. If these ambrosia beetles come into contact with the LW pathogen inside the tree, these beetles and their offspring may be contaminated with and spread the pathogen to more trees and groves. Therefore, proper destruction of LW affected trees is an important component for reducing the spread of this disease by root grafts and by ambrosia beetles.

Steps for properly removing and destroying LW affected trees

1. Remove the entire tree by pulling or pushing. Pushing trees over before tree destruction is easier than trying to dig or uproot stumps from the ground.
2. Once the tree has been removed, it should be chipped or burned. High powered front-end grinders are now in use which quickly chip the entire tree including the trunk area. Alternatively, wood may be chipped and wood too large split then chipped or burned. Call FDACS-Div. of Forestry at 305-257-0875 or 954-475-4120 for permission to burn (burn permit). Wood cut into smaller pieces will burn faster and more completely. The large wood may need to be burned more than one time (restack to expose non-burned wood) in order to be destroyed.
3. Treating the tree stump (or stumps) with insecticides does not prevent the emergence of ambrosia beetles, which are already inside the tree and after a few weeks does not prevent further ambrosia beetle infestation. Thus, the best method to prevent beetle emergence from this wood and new boring into the wood is to dispose of the wood by chipping and/o rburning.
4. Ambrosia beetles are attracted to avocado wood chips and leaving them untreated or not burned results in increased activity of ambrosia beetles in the grove. Wood chips should be spread out, not mounded, and thoroughly sprayed twice, 7-days apart, with an approved insecticide (e.g., Hero®, Talstar®, Malathion, Danitol® or Agrimek®), plus adjuvant (i.e., NuFilm®, Vapor Gard® or Pentrabark®). Some biopesticides (e.g., BotaniGard) have also been shown to kill ambrosia beetles as well. Avocado groves under organic production should chip and burn all wood. 

Secondary strategies for preventing the spread of LW by root grafts among adjacent trees

Prophylactically treat entire planting with systemic fungicide

At present, there are no fungicides that cure LW-affected trees. The fungicide Tilt® (propiconazole) can be used, but it must be inside the tree before they are infected with the laurel wilt pathogen. Currently, Tilt® is the only labeled fungicide approved for laurel wilt control on avocado trees. Tilt® can only be used in commercial operations and is not approved for use by homeowners in their landscapes. When using Tilt®, growers must have in their possession the Section 18 special local needs label (SLN); go to http://www.syngenta-us.com/ or contact Jeff Wasielewski, Commercial Tropical Fruit Crops Extension Agent, (305-248-3311, ext. 227; jwasielewski@ufl.edu).

Infusion. Infusion consists of diluting the Tilt® with water and either passively or under pressure infusing the mixture into the flare roots at the base of the tree. Infusion of Tilt® must be repeated every 8 to 18 months. While effective, the infusion method causes damage to the flare roots making re-treatment difficult. Therefore, this application method may not be sustainable.

Injection. Injection consists of injecting the tree along the trunk and the major scaffold limbs using undiluted (or slightly diluted) Tilt®. Injection of Tilt® must be repeated every 12 to 16 months. Although a substantial number of groves have been treated in this manner, whether this method is sustainable in the long-term is not known.

Prophylactic treatment of groves, whether by infusion or injection, can be costly. As such this strategy if chosen is best suited for groves with varieties that generate relatively high returns.

Spot treatment. This is a combination strategy of early detection of LW symptomatic trees and treating by infusion one to two healthy trees adjacent a LW affected tree in an effort to prevent root movement of the LW pathogen to healthy trees. However, the key to success with the spot fungicide treatment is early detection of trees with symptoms of LW, their immediate uprooting and destruction and immediate infusion with Tilt® fungicide to the healthy adjacent trees. Although the spot treatment method is generally less costly than treating an entire grove approach, it may not be sustainable because of the damage to the flare roots caused by the necessity of repeated fungicide applications.

Ambrosia beetle suppression - insecticide applications in the immediate area (1 acre) surrounding LW positive avocado trees

The largest portion of the ambrosia beetle population is inside infested trees. Contact insecticides will not prevent ambrosia beetle emergence from already infested trees or wood.

Chipping and burning wood are the only ways to eliminate ambrosia beetles inside tree stumps and wood and stop their reproduction. To suppress ambrosia beetles active in the environment after chipping and/or burning infested trees, applications of contact insecticides and/or biopesticides to about one acre of trees surrounding a LW affected tree is recommended. After removing LW affected trees, make two applications of insecticide directed to the trunk, scaffold limbs and medium to large wood (not the foliage) of adjacent trees at a 10-15-day interval. The spray should be bark directed spray to the lower part of the tree and not the leaves and upper canopy. Malathion, Danitol® and Agri- Mek®SC are registered for bearing avocado trees. In order to reduce the chances of beetle resistance these products should be rotated (Table 1). For Malathion and Danitol® use an adjuvant such as NuFilm® to prolong the efficacy of the insecticide. Do not use sticker-type adjuvants like NuFilm with Agri-Mek®SC use a non-phytotoxic, non-ionic activator type wetting, spreading and/or penetrating spray adjuvant or horticultural oil (not dormant oil) – see label for details. Do not use Vapor Gard® and Pentrabark® adjuvants as they may cause phytotoxicity on leaves. For non-bearing avocado trees, Talstar®-S and Hero® may be applied (Table 2); use an adjuvant to prolong their efficacy.

The biopesticides BotaniGard® ES and Mycotrol® (the active ingredient is Beauveria bassiana

or Bb) are registered for avocado and have been shown to provide some ambrosia beetle control. The strategy is to attempt to suppress the ambrosia beetle population increase, which occurs during the spring. Research has shown trunk directed applications of Bb are capable of killing ambrosia beetles and the time-line for application of these materials is February through May when average temperatures are lower and beetle populations are increasing. However, proper trunk and major limb directed application is paramount for Bb to be effective. The concept is to place live Bb spores on the main trunk and scaffold limbs where ambrosia beetles may contact the live Bb spores.

• First, compatibility of Bb with other spray materials varies. A general recommendation is to clean spray tanks prior to tank mixing. Bb is not compatible with some fungicides (e.g., Abound® and Switch®) and insecticides (e.g., Danitol® and Malathion) (Table2).
• Spraying Bb too close to an application of a non-compatible spray material application will reduce the effectiveness of ambrosia beetle control. Therefore, time Bb applications toh ave the most time before and after any potential non-compatible spray material is used.
• Only purchase what will be used immediately. Do not leave these bio-insecticides in the tank for prolonged periods since it will kill the active Bb spores. Also be sure to store BotaniGard® ES and Mycotrol® in a cool, dry place that stays between 40°F and 85°F – otherwise the Bb spores will be killed.
• Mix the BotaniGard® ES and Mycotrol® in sufficient water to ju st wet the trunk and major limbs. Do not spray to run-off as this causes the Bb spores to wash off the bark. A light but thorough wetting is ideal.

In general, ambrosia beetles are attracted to areas of low light levels (i.e., shade). To reduce the potential for ambrosia beetle attack, selective pruning and/or annual or biannual topping and hedging should be practiced to increase light levels within the grove and tree canopies.

Summary

Ambrosia beetle control is a part of the strategy to control laurel wilt. However, it is imperative to implement early scouting for laurel wilt symptomatic trees and for tree removal and destruction to occur quickly. Strategies to suppress the ambrosia beetle population include selective pruning and/or regular topping and hedging to reduce the shading within and among avocado trees. This may reduce the attractiveness of trees to ambrosia beetles. Actively controlling ambrosia beetles in the immediate vicinity of an active laurel wilt outbreak may reduce new laurel wilt outbreaks.

Economic considerations

Rogue (remove and destroy) and replace

Economic modeling indicates that early detection and removal of the infected avocado tree (primary strategy) plus replacement (i.e., replanting) provides the best option for the operation to remain profitable. Immature trees are less likely to be attacked by ambrosia beetles and are not root grafted to adjacent trees. Harvesting can commence from third or fourth year. 

Stumping and top-working trees

Stump healthy trees to 3 to 5 ft in height (~$25/tree) and allow trees to resume production.

Alternatively stump trees and topwork (~$35/tree) trees to a more profitable cultivar. Production may resume in 2-4 years depending upon cultivar and production practices. Their production level will be at a higher level than young (1-3 years-old) trees. The drawback for maintaining these older trees is they remain root grafted to adjacent trees and LW could move from tree to tree via the root system. It is not recommended to stump LW affected trees to topwork.

Replanting

Remove a block or grove of old avocado trees (~$50/tree) and replant avocado trees (~$30/tree) with a more profitable cultivar. To establish a new grove estimated to be ~$8,000/acre. The advantages to removing mature root-grafted avocado trees and replanting to young avocado tree includes: (1) young trees are less attractive to AB (small diameter trunks and lots of light in grove) and (2) young trees are not and will not be root grafted to adjacent trees for years – so LW pathogen cannot spread tree to tree by the roots. This strategy buys time to find better control solutions to LW-AB. The disadvantages to remove and replanting with young trees is the cost of removing old trees and replanting with young trees, fruit production will not begin for 3-4 years, and will not be substantial until 7-8 years.

Alternative options for LW affected groves

Remove trees (~$50/tree) and enter an alternative agricultural business or fruit crop.

Table 1. Contact insecticides for ambrosia beetle control. Direct applications to the trunk and medium to large wood to about one acre of trees surrounding the LW affected tree. Rotate pesticides products reduce the potential for insect resistance. MixDanitol®,Malathion and Hero® with anadjuvant (e.g.,NuFilm®) to prolong their efficacy. ForAgri-Mek®SC do not useNuFilm® or similar, use a non-phytotoxic, non-ionic activator type wetting, spreading and/or penetrating sprayadjuvant or horticultural oil (no a dormant oil) – see label for details. The biological control insecticidesBotaniGard® ES andMycotrol® do not need anadjuvant and should be stored in a cool (<85°F), dry place.

 (c://tree destruct and LW control recommendations 12-22-17.doc)

1. You need to get a Remote Pilot Airman Certificate (FAA Part 107) if you are going to do anything commercial with it. That includes even if you are using it on a job where you will not be paid.
2. You can use it for scouting but you can't fly it over people (unless you get a waiver). You also always have to have it in your or an assistant's (the spotter with radio communication) line of sight.
3. Even if you have a drone that is capable of doing a spray application, the Dept. of Pesticide Regulation currently requires that the drone pilot have an Airman's Certificate (Pilot's license). This is addition to the Pest Control Aircraft Pilot Certificate from DPR. However, things may change in 2018. See pages 3-47 to 3-52 in http://www.cdpr.ca.gov/docs/enforce/compend/vol_1/entirerep.pdf

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Have you ever thought about it? I'm sure a lot of you have…what's the biggest living thing on earth? If you're like me, you immediately think of animals like the blue whale. In this case, we're just talking about organisms that spend their life predominantly or entirely on land. Anybody thinking of the rhinoceros, or maybe the African elephant? Nope!

The answer to this question might surprise you. The largest terrestrial organism on the planet is actually a fungus! Not your typical, garden-variety mushroom, but a fungus, nonetheless. And while it is edible, it comes with a few problems. Its common name is honey fungus.

The soil is filled with living organisms, large and small. Bacteria are the most numerous soil organism – there can be more than a million in a teaspoon of soil! There are over 10 trillion types of soil bacteria!

There are “only” 100 billion types of soil fungi, but they play in important role in the life cycle within the soil. Fungi break down decaying plant material into nutrients that other plants can use to grow. They soak up water in water-logged soils. You might be familiar with seeing mushrooms growing on moist logs, or in soil with wet leaves.

Fir trees were particularly hard hit by the effects of honey fungus. Credit: Böhringer Friedrich https://commons.wikimedia.org/w/index.php?curid=10391442

Honey fungus is a bit different from many other fungi. It's not one that landowners will welcome because it can kill trees. In fact, it gained fame in the late 1990s as the “culprit” that was killing fir trees in the Pacific Northwest. (Fir trees are widely sought after as beautiful landscape plants,  Christmas trees, and for lumber used in building homes.)

Honey fungus is a parasite – Armillaria solidipes. Parasites are organisms that live off other organisms, sometimes hurting them. Honey fungus is widely distributed across the cooler regions of the United States and Canada. It is very common in the forests of the Pacific Northwest.

These fungi grow in individual networks of above and below-ground fibers called mycelia. Mycelia work in a similar fashion to plant roots. They draw water and nutrients from the soil to feed the fungus. At the same time, they make chemicals that are shared with other organisms in the soil. Sadly for the fir trees, the honey fungus can kill already weakened or stressed trees. The natural action of this fungus can be destructive in forests, leading to widespread die-off in timber stands.

One cool feature of the underground network of mycelia of all fungi is that they help hold soil particles together. Just like plant roots, mycelia work to prevent soil particles from blowing away in the wind or being taken away by running water. They are a very important part of the soil ecosystem.

Scientists have shown that Armillaria solipipes identify and connect to each other. That's right: when mycelia from different individual honey fungus bodies meet, either in or on the soil surface, they can attempt to fuse to each other. The fungi must be genetically identical honey fungi. When the mycelia successfully fuse to each other, they link very large fungal bodies together. This, in turn, changes extensive networks of fungal “clones” into a single individual.

The largest honey fungus that has been identified in North America is located in Oregon. It measures 3.4 miles across! Scientists also believe that this particular honey fungus may be over 2,000 years old. The next largest honey fungus is in the neighboring state of Washington.

While honey fungus is impressive in both age and size, it isn't always a favorite of scientists and landowners because of its parasitic nature. But it is the largest organism on earth, and scientists have had only a few decades of research to understand how mycelia fuse and communicate. Perhaps one day, research discoveries about honey fungus could lead to a new medicine (think Penicillin), or new ways to grow food – the possibilities are endless!

And this is the largest terrestrial organism. Who knows what lurks in the depths of the ocean?

P.S.

Armillaria solipipes is one of the many species that was once lumped as Armillaria mellea, oak root fungus.  So many woody perennials are susceptible to this fungus.  It turns out that avocado for whatever reason tends to be somewhat more tolerant of oak root fungus than many other trees.  Under stress, it too will succumb, though.

As firefighters in Southern California worked to achieve full containment on the Thomas Fire, agricultural officials in Ventura County issued their first estimate of damage to crops and farm structures, reporting that losses will exceed $171 million, with more than 70,000 acres of cropland and rangeland affected.

The Ventura County agricultural commissioner's office based its initial assessment on information about agricultural locations within the perimeter of the fire, which started on Dec. 4 and grew into the state's largest wildfire, burning nearly 282,000 acres as of late last week.

In a preliminary disaster report to the U.S. Department of Agriculture, Ventura County Agricultural Commissioner Henry Gonzales estimated more than 10,289 acres of irrigated cropland and another 60,000 acres of rangeland in the county had been affected by the fire. He estimated the cost of the damage to current and future crops, machinery and equipment, dwellings, service buildings and other structures at nearly $171.3 million, with avocados and citrus crops suffering the highest losses.

The estimates do not include wage losses of farm employees, the report said, and “nothing can measure the pain and suffering of the farmers that have lost much of their livelihood.”

A more-exact assessment won't be completed until late spring to early summer, Gonzales said, as his office continues to conduct on-the-ground surveys and receive surveys back from growers. What's uncertain at this point, he said, is how well the tree crops will recover.

“The rather complicated part about damage on avocado and lemon trees is that the damage isn't readily apparent,” he said. “You can't really tell the damage until sometime in the future.”

Farmers will have a better idea in the spring, when their trees either bloom or won't bloom, Gonzales said. Even then, they won't know the extent of the trees' recovery until summer, when they can evaluate the crop itself. Aerial photography, he noted, will allow officials to see crop damage as it manifests.

In Santa Barbara County, which was also affected by the Thomas Fire, Assistant Agricultural Commissioner Rudy Martel said it will be a few more weeks before his office can provide an initial damage assessment. He noted it had been less than two weeks since mandatory evacuations in the region had been lifted, and growers had just begun to return to their properties.

“The growers have not been in contact with us,” he said. “We rely on their information, and it takes them a while to get those numbers to us because they're trying to get back into production, to start their business back up.”

He said he hopes a grower assistance workshop in Carpinteria later this week will provide the office its “first opportunity to touch base” with growers and gather initial damage estimates.

In Ventura County, the fire has so far cost avocado growers an estimated $10.2 million in losses, Gonzales reported. The county has about 18,500 acres of avocados, with 6,603 acres affected by the fire and 1,250 acres that suffered damage. Some 4,030 tons of fruit have been lost.

For citrus fruit, the county's lemon crop saw more than $5.8 million in losses, with 7,591 tons of lost fruit. Of the county's 14,800 acres of lemons, 1,767 acres were affected by the fire and 400 acres were damaged. Oranges suffered losses of about $3.4 million, while the cost to mandarins reached $491,022 and grapefruit losses totaled $35,930.

Other affected commodities include vegetables, with damage and losses totaling $4.6 million; miscellaneous fruit, $1.4 million; nursery crops, $1.4 million; strawberries, $486,416; rangeland, $480,000; apiaries, $139,500; hay and grain, $129,345; cattle, $125,000; cut flowers, $61,966; and raspberries, $55,420.

The preliminary estimate of nearly $171.3 million in agricultural losses in Ventura County from the Thomas Fire includes $25 million in lost farm machinery and equipment.
Photo/John Krist

Ventura County farmers and ranchers also face costs to replace and repair destroyed or damaged buildings, equipment and other structures. Gonzales estimated the fire destroyed 260 agricultural dwellings, service buildings and other structures. Another 215 buildings and structures suffered major or minor damage. Together, they were valued at a loss of $113 million.

The report estimated the cost to replace farm machinery and equipment at $25 million. Cost of land damages, including cost to replace trees, reached $3.4 million, while loss of irrigation systems totaled another $1.1 million.

“There were many homes, many structures that were lost,” Gonzales said. “Our very water-efficient irrigation systems, because they're made out of plastic, melted, so all of those will have to be replaced.”

Restoring irrigation systems is critical, he said, because of the region's ongoing drought conditions and lack of precipiation this season. Using water trucks is not an option, he added, because of the large number of affected acres that need watering. Because of high demand, he said there's been a local shortage of irrigation components, and some growers have had to order them or go out of the area to buy parts.

Mallory Salant, district manager of Fruit Growers Supply in Santa Paula, said the fire led to skyrocketing demand for certain items.

“Aboveground irrigation parts are flying off the shelves,” she said. “A couple of days after the fire started, we were completely out of stock and it was very chaotic in the showroom.”

Irrigation tubing is currently the most sought-after item. At other times, a typical order might be 10 or 20 rolls of tubing, she said, but now farmers are ordering 200 to 400 rolls because they need to replace all of the tubing on their farm.

Despite the high demand, Salant said the store has managed to “stay on top of it” by working with vendors to restock quickly.

“At this point, our shelves are full,” she said.

by Ching Lee, assistant editor of Ag Alert/credit California Farm Bureau Federation