There was just a group of Florida researchers here in California sharing their experiences with ambrosia beetles and a fungal disease in avocado and other members of the laurel family. This is a pest/disease complex similar to that found here caused by a shot hole borer and fusarium.  Avocados grown in Florida are of the West Indian or West Indian cross with Mexican or Guatemalan varieties.  They are usually big, green fruit that tend to be of a lower oil content.  Some marketers promote them as “low cal” or “slimcados” as a result.  Whatever. 

One of the things that struck home during these wonderful talks was the pronunciation of the word Hass. It was “hozzz”.  The “a” was pronounced like the a in hot, not in hat.  It made me think that this is probably how our familiar fruit is probably pronounced in much of the US.  I also hear Californians (and CA growers, too) pronounce this iconic fruit “hozzz”.  The generally accepted pronunciation of this name is “HaaaaSSSSS”. Like in the verb “has” - “He has an avocado”.

The fruit variety was found by a California grower named Rudolph Hass in the 1920's. The name Hass is of German origin. How it has come to be pronounced differently from his name is not clear to me. According to Google Translate, even in German it is pronounced as “has”, though with a somewhat clipped “s” on the end.

And not only has the pronunciation of the name been changed, sometimes the spelling in many produce departments is “Haas”. I once saw it on packaging spelled this way and when I asked the produce manager how that had happened, he told me that they had asked the packer explicitly to spell it that way because that's the way the consumers wanted to see it spelled.

So, the consumer drives the market. Maybe how people say it isn't important, as long as they know what they are buying and enjoy the fruit. At least most Californians seem to know how to say the word Hass.

Can you say Hass?

 

Photo: On the left: Florida (Slimcado) avocado. On the right: Haas avocado or Lamb-Haas. From: The Gardening Cook, http://thegardeningcook.com/slimcado-information/

During the first week of August, the California Avocado Society, Inc., California Avocado Commission, University of California Cooperative Extension hosted six Laurel wilt researchers from the University of Florida.  The speakers were Randy Ploetz, Jonathan Crane, Bruce Schaffer, Daniel Carrillo, Jeff Wasielewski and Edward Evans. In addition to talking at all three seminar locations, San Luis Obispo, Ventura, and Fallbrook, the researchers were also able to tour the major avocado growing regions.

Laurel wilt is a deadly disease of redbay (Persea borbonia) and other tree species in the Laurel family (Lauraceae), which includes the avocado tree. The disease is caused by a fungus (Raffaelea lauricola) that is introduced into host trees by a nonnative insect, the redbay ambrosia beetle (Xyleborus glabratus). Native to Southeast Asia, the ambrosia beetle has similarities to our current ambrosia pest, Polyphagous shot hole borer and the Kuroshio Borer. That is why this visit from our fellow extension researchers from the University of Florida. However the fungus associated with Laurel Wilt is unlike the disease here in California. Laurel Wilt is a disease that that causes a reaction in the tree to restrict water flow in the tree and the tree collapses rapidly.  The tree dies so fast that it doesn't even have a chance for the leaves to fall of the dead branches.

The beetle has the ability to spread the specific pathogen to other ambrosia beetle vectors which happens when they feed on infected trees. Sanitation is the most effect way to manage this problem. Scouting for wilted branches and their rapid removal has been the key to early intervention and eradication. Dr. Ploetz suggests removing the tree immediately. By the time you see frass and streaks in the wood, the tree is already infected and has been for sometime. As soon as a growers see the wilt in the branches, its time to move quickly. This disease may be mistaken for Verticillium wilt or Phytophthora. It can spread throughout the grove by root grafting.

The generation time for the beetle inside avocado trees takes about 40-50 days depending upon temperatures. Their flight activity is highest in the late afternoon and early evening. Dr. Carrillo mentioned that ambrosia beetles are notoriously difficult to control because they are inside the tree most of their life cycle versus being outside the tree. Contact herbicides will not work, because the insects are primarily inside the tree. One of the first goals to avoid infection, is to keep your trees healthy. A sick tree is more attractive

to beetles. They like dense canopies, non-pruned trees with overlapping leaves and branches. Chipping infected trees is effective in reducing the spread of the disease.  Chips should be as small as possible and spread so that they dry out.  However, beware, the smell of the chipped wood can attract other wood boring insects. The researchers are currently working with different commercial formulations of insecticides that still need more testing. Carrillo strongly suggests that our industry start monitoring for this pest/disease complex.

 

Scouting for laurel wilt in commercial avocado groves

1. Surveying for the symptoms of laurel wilt is a key component of limiting the spread of laurel wilt. Growers and their workers should survey their groves immediately and then weekly or more often if an infestation is detected in an adjacent grove. Pathogen sniffing dogs are currently being used, however there are less than half-dozen trained dogs for this purpose. Symptoms to look for might include:

i. Leaf and young stem wilting.

ii. Leaf color changing from light green to dark green, bluish-green or greenish-brown. Some leaves showing leaf mottling (dark and light green areas) and yellowing.

iii. Dead leaves curled hanging on the tree.

iv. A few stems and limbs with 2 to 4 ft of dieback or whole sections or entire limbs with dieback.

 v. Inspection of the trunk and major limbs may show dried sap (white, crystalline powdery material) that indicates insect boring. In any case, on symptomatic limbs remove the bark down to the sapwood and look for dark streaking. Dark streaks in the sapwood may indicate fungal infection. Normally this sapwood should be white to yellowish with no dark staining or streaking. In addition, small, dark holes in the sapwood further indicate wood boring beetles are present.

2. If the tree shows only a few stems and limbs with 2 to 4 ft of dieback, wait for confirmation of laurel wilt before removing the tree. You can remove the dead part of the limb by cutting several feet below the dead area of the limb; burn or bury the infested limb.

 

Many growers don't hear about the amount of research and extension activity that is going on surrounding the Shot Hole Borers and Fusarium Dieback.  This pest/disease complex affects not only avocados, but many other tree species, including many California natives. These are the notes from a Collaborative organized by UC ANR.  There is a lot of energy by a lot of groups addressing this massive problem.

Invasive Shot Hole Borer (Polyphagous and Kuroshio)/Fusarium Dieback

Quarterly Situation Report

January 1 through March 31, 2017

Executive Summary

Overview

No spread of invasive shot hole borer/Fusarium dieback (ISHB/FD) was reported this quarter, which may have been due to cool seasonal temperatures and resulting insect inactivity. The period served to establish new trapping sites, evaluate programs, and establish new ones.

Trapping locations expanded in Orange, Ventura, and Santa Barbara Counties. A new trapping program was started that serves the Santa Monica Mountains, Conejo and San Fernando Valleys.

An urban observational project modeled after successes in Orange County was initiated in Ventura County. The project can potentially expand monitoring by citizen scientists throughout the region to benefit tracking and treatment.

Representatives of various public, academic, and private entities from seven of the nine southern California counties participated in activities, which are summarized below

Statewide

o   ISHB/FD dialogue continued between counties, Agricultural Commissioners, and other stakeholders through conferencing hosted by the California Agricultural Commissioners and Sealers Association.   Education and outreach to counties without infestations is tantamount for detection and readiness

o   Outreach was initiated to Inyo and Mono Counties in eastern California

o   Legislative outreach and education continued to State Assembly and California Department of Food and Agriculture

o   California Department of Fish and Wildlife hired staff for ISHB monitoring, trapping, developing a wildlands field guide, and creating a management matrix

o   Media interviews about ISHB/FD were conducted for publications, many of which are circulated online.

Regional

o   Two ISHB/FD working groups met:

1. Emerging Tree Pests Education& Outreach Subcommittee (5x)
2. Natural Resources/Urban Forest SHB Coalition (3x)

o   University of California (UC) researchers continued investigations with bio-control agents and pesticides to combat ISHB/FD

o   A UC publication focused on ISHB/FD invasion to palm trees was published

San Diego County

o   The Resource Conservation District of San Diego researched funding to continue ISHB/FD monitoring, trapping, education, and outreach needs

o   Southwest Wetlands Interpretive Association continued studying the SHB infestation and found that an estimated 88% of plant material in the Tijuana River Valley was affected despite recovery following massive SHB-caused dieback in 2016

o   UC-ANR & the Emerging Tree Pests Education and Outreach Subcommittee organized and presented the Southern California Green Waste and Wood Biomass Symposium held in San Diego

Riverside County

o   California Association of Certified Pest Control Advisors and University of California Agriculture and Natural Resources Division (UC-ANR) presented a half-day ISHB/FD workshop in Temecula

o   ISHB/FD training for State Parks rangers from the Inland Empire District was held in La Quinta

Orange County

o   California Department of Fish & Wildlife added new trapping and monitoring sites that included nature preserves, large institutional campuses, and parkways

o   California State Parks trapped at Crystal Cove, with no positive finds

o   ISHB/FD training was conducted for US Fish & Wildlife Service staff

o   Monitoring protocols for oak and sycamore were developed for the 4000-acre Audubon Starr Ranch in Trabuco Canyon

o   Orange County Master Gardeners received ISHB/FD training

o   Orange County Parks expanded trapping and monitoring activities to more parks, with Irvine Regional Park a focal point

o   Outreach continued throughout the county, with added focus to the business community

Los Angeles County

o   The Huntington continued monitoring its 207-acre parcel

o   The Resource Conservation District of the Santa Monica Mountains (RCDSMM) initiated trapping in the Santa Monica Mountains with partner National Park Service (NPS) – Santa Monica Mountains National Recreation Area

o   RCDSMM conducted trap making and trapping workshops for trapping in the San Fernando Valley and Santa Monica Mountains

o   UC-ANR met with City of Downey staff to discuss current infestations and monitorin

Ventura County

o   California Association of Certified Pest Control Advisors and University of California Agriculture and Natural Resources Division (UC-ANR) presented a half-day ISHB/FD workshop in Ventura

o   Master Gardener urban observation project began

o   RCDSMM and NPS established trapping sites in the Santa Monica Mountains/Conejo Valley area

o   UC Santa Barbara and UC-ANR coordinated an all-day meeting of the Santa Clara River Academic Coalition to discuss research being conducted in the watershed

o   Ventura County Agricultural Commissioner continued monitoring traps at avocado orchards in the Santa Paula area

o   UCSB continued trapping in riparian areas in the Santa Clara River watershed and started establishing sites in the Ventura River corridor

Santa Barbara County

o   Santa Barbara County SHB Working Group met (1x)

o   UC-ANR education and outreach was delivered by print and online media, tabling, and public events

o   UC-ANR met with Montecito Fire Department and the Santa Barbara County Fire Safe Council to discuss ISHB/FD threat

o   UCSB/UCR continued monitoring traps in the Montecito area and established new sites in the City of Santa Barbara foothills

San Luis Obispo County

o   CAL FIRE explored funding for trapping, monitoring, education and outreach

  

PSHB Working Group Coordination

Active groups – regionwide

o   There are currently nine working groups focused on shot hole borers.   PSHB/KSHB updates and discussions on research, education and outreach strategies took place during group coordination meetings held, in person and via conference calls:

1. CACASA – California Agricultural Commissioners and Sealers Association (J. Deviney, Agricultural Commissioner-Santa Clara County group conference call facilitator - 3/22)
2. Emerging Tree Pests Education and Outreach Committee (A. Cornejo, US Fish & Wildlife Service, facilitator – 1/25, 2/8, 2/22, 3/8, 3/22)
3. Natural Resource / Urban Forestry Shot Hole Borer Coalition (Chaired by California Department of Fish and Wildlife – G. Sevrens, facilitator – 1/3, 2/7, 3/7)
4. Orange County Emerging Tree Pests Group
5. San Diego Association of Governments – Emerging Tree Pests Subcommittee (SANDAG)
6. Santa Barbara County Shot Hole Borer Working Group (M. Bianchi, University of California Cooperative Extension – Santa Barbara & San Luis Obispo Counties – group facilitator, 3/23)
7. University of California Emerging Tree Pests Working Group – statewide for ISHB – (S. Drill, University of California Cooperative Extension – Los Angeles & Ventura Counties – group facilitator, 3/9)
8. Ventura County Green Waste and Pest Prevention Advisory Committee
9. Ventura County Polyphagous Shot Hole Borer Working Group (S. Drill, University of California Cooperative Extension – Ventura – group facilitator)

John Boland, Southwest Wetlands Interpretive Association (SWIA) – Imperial Beach

o   Continued studying KSHB in the Tijuana River Valley riparian forest, with funding from the US Navy and US Fish & Wildlife Service

Sabrina Drill, UC Cooperative Extension – Ventura & Los Angeles Counties

o   Met with Santa Clara River Academic Coalition to discuss SHB research, monitoring, trapping efforts in the watershed – (3/1)

o   Coordinated Cooperative Extension/UC Riverside SHB coordination working group meeting in Riverside – (3/9)

o   Participated in California Agricultural Commissioners & Sealers Association (CACASA) statewide call – (3/22)

Tom Dudley, UC Santa Barbara – Marine Science Institute

o   Coordinated Santa Clara River Academic Coalition meeting for the watershed – (3/1)

Jan Gonzales, UC Cooperative Extension – San Diego

o   Participated in Coordinating Work Group Meetings/Calls:

• Natural Resources/Urban Forestry SHB Coalition (1/3, 2/7, 3/7)
• Emerging Tree Pests Education and Outreach Committee (1/25, 1/11, 3/8, 3/22)
• Statewide PSHB Group (3/22)

o   Coordinated visit to the San Diego Zoo and gave briefing on ISHB during field tour for UC ANR's Associate Vice President, Wendy Powers; Chris Greer, Vice Provost of Cooperative Extension and guests from the National eXtension program (1/24)

 

Funding, Legislation, Planning, Policy and Regulation

John Kabashima, UC Cooperative Extension – Orange County

o   Gave a team talk with the California Department of Food and Agriculture (CDFA) Director of Plant Health to the Invasive Species Council of California (ISCC) on PSHB/KSHB/FD as an example of emerging tree pests that the present system isn't addressing effectively. Ideas to address this by CDFA and the ISCC Advisory committee are incorporated in their interagency "Charting the Pest Prevention System in California" plan.  The California Assembly Speaker's office suggested we identify an Assembly Democrat who would ask the State agencies to address this issue and recommend a plan and budget.  This would be determined through either a special summit meeting or Assembly hearing. Several approachable Assemblymembers were identified to collaborate on this matter (1/5)

o   Discussed SHB funding needs with collaborators, including the Resource Conservation District of San Diego County, University of California Cooperative Extension San Luis Obispo

Surveying/Monitoring/Detection/Mapping

Dan Berry & Tim Thibault, The Huntington Gardens – Pasadena

o   Continued plant collection monitoring for 207 acres

Rosi Dagit, Resource Conservation District (RCD) of the Santa Monica Mountains – Topanga

• Conducted Santa Monica Mountains SHB trapping coordination meeting with the RCD and Santa Monica Mountains National Recreation Area staff (1/24)
• Taught a SHB trap making and monitoring workshop at Malibu Creek State Park for– (3/1
• Sandy De Simone, California Audubon – Trabuco Canyon

o   Met with Audubon Starr Ranch seasonal riparian assistants in January to devise a protocol for monitoring oaks and sycamores for pests and disease on the 4,000 acre property located in southeast Orange County 

Sabrina Drill, UC Cooperative Extension, Ventura and Los Angeles Counties

o   Met with UCCE Ventura Master Gardener Program Coordinator Leah Hanson to discuss upcoming SHB observational training and urban monitoring sites throughout the county as part of the new Tree Detective program in sycamores – 3/27/17

John Kabashima, UC Cooperative Extension - Orange County

o   Discussed SHB infestations and monitoring with City of Downey officials (1/18)

Lana Nguyen, State Parks – Orange Coast District

o   Received confirmation from the Eskalen Lab that tree specimen from Crystal Cove was negative for Fusarium dieback

Cathy Nowak, Orange County Parks – Irvine

o   UCCE completed surveys & mapping for Aliso & Wood Canyons Wilderness Park, Key Ranch, Heritage Hill, and O'Neill Regional Park to Oso Parkway

o   Monitoring continues at Irvine Regional Park

o   Twelve monitoring stations were established at Whiting Ranch Wilderness Park – Trabuco Canyon

Gail Sevrens, California Department of Fish & Wildlife, South Coast Region – San Diego

o   Hired two new Scientific Aides, Ryan Lawler and Aidan Renner. They are assembling an expanded field guide to identifying SHB in wildland settings as well as a management matrix to determine the best course of action in the event that it is found.   Theywill begin monitoring and trapping for SHB in the region

o   Worked with the SHB Survey Committee, to investigate what kind of and how frequently surveys are being done

Research Activities and Findings

Dan Berry and Tim Thibault, The Huntington – Pasadena

o   Traveled with Akif Eskalen and Richard Stouthamer to Taiwan for the purpose of PSHB bio-control and insect agents (January)

o   Finalized stress test with Colin Umeda (UC Riverside) 

John Boland, Southwest Wetlands Interpretive Association (SWIA) – Imperial Beach

o   The Tijuana River Valley is the first natural habitat to be substantially invaded by the KSHB. It first appeared in the valley in 2015 and almost immediately caused extensive damage to the riparian forests.   It is estimated a total of 355,510 willows, or 88% of the population in the valley, have already been infested. More than two billion (2 x 109) beetles were born in the valley during 2015-16. This was likely the single largest production of offspring by this beetle in the US so far.

o   KSHB has not killed all of the willow trees it infested. Many heavily damaged by the KSHB during 2015 re-sprouted in 2016. In some sites, where the willow canopy was destroyed by the KSHB, there is now an extensive short canopy of re-sprouting willow “shrubs” up to four meters tall.

o   A final report on this research will be released in April 2017

Tom Dudley, University of California Santa Barbara – Marine Institute

o   Interfaced with Institute staff and graduate researchers to monitor, trap, and research SHB in Santa Barbara and Ventura Counties

o   Attended an interagency invasive species meeting in January on the east coast, noting several talks and posters on SHB. There is a continued research effort on semiochemicals, which UCSB may be involved at the research testing level

Akif Eskalen, UC Cooperative Extension – Riverside County

o   Traveled with Richard Stouthamer (UC Riverside), Dan Berry and Terry Thibault (both from The Huntington) to Taiwan for the purpose of PSHB bio-control and insect agents (January)

Nathan Gregory, Irvine Ranch Conservancy – Irvine

o   The Irvine Ranch Conservancy surveyed lower Agua Chinon at Limestone Canyon Nature Preserve in Silverado (northern Orange County foothills) and found no signs of PSHB. However, 12 potentially infested trees were identified in Bommer Canyon Community Park, Irvine (near Turtle Ridge) with samples taken and submitted to the Eskalen Laboratory at UC Riverside. Of those 12, five were surveyed last year and found to be newly infested this year. The other seven are newly surveyed this year.

Cathy Nowak, Orange County Parks – Irvine

o   Tim Paine continued pesticide treatments at Mason Regional Park - Irvine that examine five treatments on ten trees. Focus is on compounds that have shown promise in previous trials at UC Irvine and in the laboratory

o   Akif Eskalen's final report on the pesticide trials conducted at three regional parks is forthcoming

Richard Stouthamer, UC Riverside

o   Traveled with Akif Eskalen (UCCE-Riverside), Dan Berry and Terry Thibault (both from The Huntington) to Taiwan for the purpose of PSHB bio-control and insect agents (January)

Management and Restoration

Dan Berry and Tim Thibault, The Huntington – Pasadena

o   Removed and replaced two PSHB-infested Quercus robur (English oak)

Gail Sevrens, California Department of Fish and Wildlife (CDFW), South Coast Region – San Diego

o   Inquired about trunk injection of pesticides on creek banks and in dry creek beds. CDFW pesticide unit was consulted. Learned that applications consistent with label instructions are alright to use, as long as trees are not growing in the water. Can be okay if next to water. Said ecological risk outweighs risk of insects.

o   Updating and gathering input on Southern California Shot Hole Borers/Fusarium Dieback Management Strategy for Natural and Urban Landscapes

Wood Utilization, Sanitization, and Disposal

Dan Berry and Tim Thibault, The Huntington – Pasadena

o   Continued PSHB-infested green waste grinding and material redistribution

Education and Outreach/Media Coverage

Dan Berry and Tim Thibault, The Huntington – Pasadena

o   Gave PSHB presentation to 20 physicians at the Huntington Medical Research Institute

o   Engaged with Earthwatch during an extended meeting about a citizen science initiative

Mary Bianchi, UC Cooperative Extension – Santa Barbara & San Luis Obispo Counties

o    Began education and outreach in the greater Santa Barbara area following first population of KSHB located in Montecito. Wrote and published two online articles:

1. http://www.news.ucsb.edu/2017/017516/holy-guacamole
2. https://www.edhat.com/site/tidbit.cfm?nid=179674

o   Met with Kerry Kellogg from Montecito Fire and some of their staff, along with UC statewide Fire Specialist Max Moritz (1/17) 

o   Articles will be published in the Montecito Journal, newsletters of the Montecito Association, Ennisbrook Homeowner's Association, and Westmont College

o   Gave a presentation at the Santa Barbara Fire Safe Council Meeting (2/16)

o   Outreach and tabling were conducted at the Montecito Farmer's Market and Lotusland in Montecito

California Association of Pest Control Advisers (CAPCA) – Sacramento

o   Hosted two half-day SHB educational events for agricultural and general audiences:

1. January 25 in Ventura
2. January 26 in Temecula

• Speakers included UC entomology experts Richard Stouthamer, Akif Eskalen, Frank Byrne, Tim Paine, and John Kabashima

Julie Clark De Blasio, UC Cooperative Extension – Ventura

o   Participated
in bottle-trap making workshop at Malibu Creek State Park (3/18).   Traps will be set in wildland-urban interface areas of Brentwood in the Santa Monica Mountains  

Rosi Dagit, Resource Conservation District (RCD) of the Santa Monica Mountains – Topanga

o   Coordinated a half-day SHB bottle-trap making workshop at Malibu Creek State Park (3/18). The 15 participants will support trapping efforts in the Santa Monica Mountains and San Fernando Valley

Sabrina Drill, UC Cooperative Extension – Ventura & Los Angeles Counties

o   Gave interview for California Agricultural Pest Control Advisers (CAPCA) video series on SHB (1/25)

o   Gave interview for ‘Central Coast Farm and Ranch'(2/3):

• Bug at Bay story published Spring 2017, V14N3, pp6-8

Tom Dudley, University of California Santa Barbara – Marine Science Institute

o  Coordinated the Santa Clara River (SCR) Academic Collaboration meeting held at Hansen Agricultural Research and Extension Center, Santa Paula (3/1). UCSB is establishing numerous SHB trapping sites in wildland watersheds throughout Ventura County, with a focus on areas near infestations in the SCR   

Akif Eskalen, UC Cooperative Extension – Riverside County

o   Gave ISHB presentation at two half-day educational events for agricultural and general audiences sponsored by California Agricultural Pest Control Advisors:

• January 25 in Ventura
• January 26 in Temecula
• Jan Gonzales, UC Cooperative Extension - San Diego County

o   The Southern California Green Waste-Wood Biomass Symposium was held February 9 at the Handlery Hotel in San Diego. Members of the Emerging Tree Pests Education and Outreach Committee coordinated the day-long event. It was intended to engage multiple industry and practitioner stakeholders from public and private sectors, along with researchers and policy makers. Discussions on how to best address larger regional issues related to managing residual materials of invasive pest infested and dead trees were the focus. For more information, including presentation slides see http://ucanr.edu/sites/SoCA_GWWood__Mgmt_Symposium. 108 individuals attended, with a 54% evaluation survey response rate.

• 94.8% reported their awareness about the scope, continued threat and challenges to managing surplus green waste-wood biomass increased by a moderate to great extent
• 87.9% responded they learned more about management options and strategies for green waste-wood residuals by a moderate to great extent
• 84.5% replied they are very likely to share with others what they learned at this symposium

o   Park Rangers Association of California Training and Conference

Co-presented with Gina Moran (CA State Parks) on Invasive Tree Pests including ISHB/FD, gold-spotted oak borer and South American palm weevil, February 28, 2017, La Quinta, CA.

o   Distributed ISHB and related handout materials:

• 200 SHB general audience handouts
• 50 SHB ID and Symptoms handouts
• 50 SHB Management handouts
• 25 SHB ID Field Guides
• 750 Buy It Where You Burn It firewood awareness rack cards

 

This report is a summary of known activity relating to Polyphagous and Kuroshio Shot Hole Borer/Fusarium Dieback for the stated 3-month period. If there are any additions, corrections or comments, send them to Sabrina Drill, sldrill@ucanr.edu and/or post information to the PSHB Working Group Collaborative Tools group. Report compiled by Jan Gonzales, UCCE-San Diego

PSHB Working Group Stakeholder Organizations

 

APHIS (USDA Animal and Plant Health Inspection Services)

California Agricultural Commissioners and Sealers Association (CACASA)

California Audubon, Starr Ranch

California Avocado Commission CAL FIRE (California Department of Forestry and Fire Protection) California Department of Fish and Wildlife California Department of Food and Agriculture California Firewood Taskforce California Native Plant Society California Department of Parks and Recreation California State Senate Center for Invasive Species Prevention Center for Natural Lands Management City of Pasadena City of San Diego Davey Tree Service Everest Consultants Farm Bureau-San Diego County Farm Bureau-Ventura County Los Angeles County Agriculture, Weights and Measures Los Angeles County Arboretum Los Angeles County Fire and Forestry Los Angeles County Parks National Park Service Native American Environmental Protection Coalition Nature Reserve of Orange County North East Trees NRCS (USDA Natural Resource Conservation Service) Orange County Agricultural CommissionerOrange County Fire Authority Orange County Parks Orange County Public Works Rebecca Latta Consulting ArborisResource Conservation District of San Diego Resource Conservation District of the Santa Monica Mountains River Partners San Diego County Agriculture, Weights and MeasuresSan Diego County Parks SANDAG (San Diego Association of Governments) Santa Barbara County Agriculture DepartmentSanta Barbara County Master Gardeners Santa Clara County Agricultural Commissioner/CACASA Pest Prevention Committee Southwest Wetlands Interpretive Association (SWIA) Sycuan Band of the Kumeyaay Nation The Huntington Library, Art Collections and Botanical Gardens The Nature Conservancy Tree People University of California Cooperative Extension University of California Division of Agriculture and Natural Resources (UCANR)

University of California, Irvine

University of California, Riverside

University of California, Santa Barbara

University of California, Santa Cruz

USDA Forest Service USDA Forest Service-Cleveland NF USDA Forest Service-Forest Health Protection USDA Forest Service-State and Private Forestry, Region 5

USDI Bureau of Indian Affairs

USDI Bureau of Land Management

USDI Fish and Wildlife Service

USDI National Park Service-Santa Monica Mountains National Recreation Area

Valley Crest Tree Company

Ventura County Agricultural Commissioner West Coast Arborists, Inc. Western Chapter of International Society of Aboriculture

 

Photo: Sun Blotch Branch

 

Sun-blotch, a viroid disease affecting the avocado, was observed as early as 1914 in Southern California. Sun-blotch is widespread in California, and likely to be found in all districts where avocados are being grown. The main counties in which the disease is found are San Diego, Ventura, Los Angeles, Orange, Riverside, and Santa Barbara.

Symptom's and signs

Sunblotch causes a wide variety of symptoms or may exhibit no symptoms in some hosts. The rate of spread of sunblotch virus appears to be extremely slow due to improved cultural practices in the nursery and field, and the apparent absence of an insect vector. Symptoms of sunblotch include necrotic, red, yellow, or white discolorations on fruit, often in depressions or scars in the fruit surface. Twigs can develop narrow, necrotic, red or yellow streaks on their surface or in shallow lengthwise indentations along the twig. Leaves may have white or yellowish variegated areas and be deformed, but leaf symptoms are uncommon. Rectangular cracking and checking of the bark, called "alligator bark," often occurs on the trunk and larger branches. Infected trees may be stunted and have a disproportionate amount of horizontal growth or sprawling lateral low limbs. Trees with visible sunblotch symptoms often have reduced yields. Infected trees can also be symptomless, although large reductions in yield of previously vigorous trees may indicate the presence of the viroid in otherwise symptomless carriers.

                                                  Photo: Sunblotch fruit

Sunblotch is caused by dozens of variants of submicroscopic particles of genetic material (viroids, smaller than a virus with no protein coat and only nucleic acid) that alter development and growth of infected plants. Sunblotch viroid can move systemically within avocado, and it persists in host tissues. Trees that do not show symptoms even though the viroid is present are known as "symptomless carriers." Nearly all cuttings and seed from symptomless carriers will be infected with viroid. However, seedlings from symptomless carriers do not show symptoms of sunblotch when they are used as rootstocks, but the disease often appears on scions grafted to them. Conversely, most seed from trees with symptoms are not infected, and budwood and shoot cuttings from sunblotch leaf symptomatic trees often do not contain viroid. [BF1] The viroid is transmitted in pollen, but pollen only infects the fruit and seed produced from it. Unless a tree is infected by grafting or some way other than through pollen, there will be no viroid in budwood, root grafts, and shoot cuttings from that tree. [BF2] Transmission of the viroid most often occurs during grafting by using infected budwood or rootstock seedlings from infected trees with or without symptoms. Natural root-to-root grafts are important in transmitting sunblotch in groves. Mechanical transmission through wounds caused by contaminated harvest clippers, pruning tools, and injection equipment can also be important if infected trees are in the grove. [BF3] Spread via pollen from an infected tree to the flower ovule of a noninfected avocado, resulting in infected seed, can cause fruit to be culled, but does not further spread the disease unless seed is propagated. There is no evidence of insect transmission.

Management

Nursery

Careful propagation of nursery stock to eliminate viroid has greatly reduced sunblotch to a relatively minor disease. However, ongoing monitoring and management is required in nurseries and established groves. Sunblotch can be easily overlooked, and there are many ways that trees can become infected. Look for diseases and disease-promoting conditions regularly throughout the grove. In the nursery, carefully select disease-free scions and seed sources. Use stringent sanitation and frequent disinfection to avoid spreading pathogens. Periodically confirm that propagation sources are disease-free (indexing) by grafting propagative source material to young Mexican seedlings and observing leaves and twigs for sunblotch symptoms, or by performing a genetic test. Plant only indexed nursery stock that is registered as disease-free.

Field

Promptly remove symptomatic trees from the grove and chemically kill the stumps. Do not retain infected, symptomless trees just because yield does not seem to be affected; symptomless carriers are a highly infective source that can dramatically reduce yield on other trees. If only fruit and seed are infected (from infected pollen), it may not be necessary to remove that tree if indexing indicates the rest of the tree is not infected. However, trees with only fruit and seed infection indicate that other infected (possibly symptomless) trees nearby need to be indexed or removed. The danger of spreading viroid increases in established orchards where mature trees are pruned to reduce tree size and restimulate or maintain fruit production. Severe pruning of symptomless carriers, and perhaps other severe causes of tree stress, are suspected of causing viroid to become active in the new growth, inducing previously symptomless trees to exhibit symptoms. Disinfect pruning tools, harvest clippers, and injection equipment before beginning work on a new tree. Scrubbing tools clean and then soaking them in a 1.5% sodium hypochlorite solution is effective. Growers must use a registered disinfectant and follow label directions. 

                                                                                          Photo: Sunblotch leaf

As sunblotch is systematic in the avocado tree, it can exist in the roots as well as the above-ground parts. Thus, it might be transmitted through natural root graftage between a healthy and diseased tree. It is known that tree roots will sometimes become forced together in some manner until finally a natural root graft has taken place. The natural grafting between roots of trees has been attributed to a number of causes, some of which are as follows: (1) Roots growing in a shallow soil underlaid by a hardpan when crowded for growing space will sometimes become grafted together. (2) Two roots growing between two rocks or other barriers when wedged together tightly may form a graft. (3) Roots from mature avocado trees which are planted close together in a grove may, if crowded for growing space, form a graft. It is believed, through evidence existing in certain avocado orchards, that sun-blotch disease has infected healthy trees through natural root graftage.

 

Photo: Sun Blotch branch with salt-affected leaves