Subtropical Fruit Crops Research & Education
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Subtropical Fruit Crops Research & Education

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Replant Vicissitudes

Replanting Trees in Mature Citrus Groves

By Craig Kallsen, UC Cooperative Extension Advisor, Kern County


While citrus groves are long-lived, the individual trees that compose the grove are not necessarily so. Inevitably, for many reasons, some mature trees will eventually die and be replaced with baby trees from the nursery.  The process of growing these replants into productive trees can be a slow process and frequently hazardous to the health of the replant.

When considering replanting dead or dying trees, the first relevant decision, and generally outside the scope of this article, is the economic viability of the grove. If many replants are required, perhaps the soil or location is unsuitable for citrus. It may well make more economic sense to push the grove out and switch to a different crop.

Before replanting a given tree, the grower should try to determine why the original tree or trees died.  If it was a stubborn-infected tree, neighboring host weeds, if present, such as the mustards or Russian thistle, may require control. If the original tree died from a root rot, the irrigation system may need replacement and water application efficiency or drainage may need to be addressed. Vertebrate pests, insect pests, fungal diseases or nematodes may need to be treated.  A wide variety of rootstocks are available to the grower, and changing the rootstock of the replants from what currently exists in the field might be a viable option for improving the health and productivity of the new trees in response to disease, incompatibility issues, frost hazard or  pH related nutrient-absorption problems in the grove.

After deciding to replant missing or sick trees, select the best possible replants from the nursery.  Equal, if not more, care should occur in selecting replants as occurred in buying the original trees for the orchard. There is a tendency for “left-over” trees to end up as replants. Trees should be large, but not root bound or J-rooted, vigorous, with healthy, green foliation and free from insect, mite and snail pests. 

An unexpected hurdle in some older orchards is choosing the variety to replant. Citrus is long-lived tree and the navel orange is a good case in point. For example, some navel varieties planted decades ago are no longer available. Some groves have changed hands so many times growers are not sure what selection of navel they have in their grove. In fact, navels were being planted so fast in the 1960's that it's doubtful that even the original owners were sure which rootstock or variety they were getting.  Replacing a Frost Nucellar with a Parent Washington is of little consequence; however, real differences in maturity become apparent between a Newhall versus Washington navel.  When many blocks of citrus are replanted at the same time, special care should be taken to insure that the Valencia replants end up in the Valencia groves and not in the navel groves and vice versa.  Putting different varieties on the same trailer for planting is asking for a mix-up.    

The environment for the replant in a mature grove is very different from that which young trees in a newly planted grove experience. The growth rate of the replant will be slower, simply because of shading from large, full-grown neighbors, and this is tough to mitigate. However, the grower is able to adjust the flow of water, nutrients and pesticides to the size of the replants compared to the mature trees. The water requirement of the newly replanted tree is probably 1/50th of that of the mature tree (i.e. the newly planted tree may only transpire about one gallon of water per day during the summer).  Water to the replant may be decreased by the use of emitters having a much reduced flow-rate (which have to be monitored closely, as the smaller orifices are more likely to plug) or through the use of devices, such as pulsators which interrupt the flow of water at intervals reducing the total flow rate per unit time.  If fertilizer or amendments are injected through low-volume irrigation systems, decreasing the flow of water to the trees through smaller emitter orifices will concomitantly decrease the flow of nutrients. Reducing the level of fertilization is critical for good replant growth, since for example, the nitrogen requirement of the young tree is only a fraction of that for the mature tree.

Controlling weeds adjacent to replanted citrus avoids excessive competition for light, nutrients, and water. Weeds can become especially thick around replants because of the more open, less-shaded ground around them as compared to the limited area now adjacent to mature trees. When the herbicide applicator encounters these weedy areas, the tendency is to give the replant space an especially heavy application. On young trees this can be especially damaging as the chance for both foliar and root uptake of the pre-emergent herbicides, and foliar uptake and burn from post-emergent herbicides, increases.  In heavily replanted groves, the use of only carefully applied post-emergent herbicides may be beneficial until the replants achieve sufficient size to tolerate the pre-emergent materials. Many groves have sufficient residual pre-emergent herbicides to carry them through a year or so of replant establishment without a substantial increase in weed pressure.  The hoe is a surprisingly effective tool for keeping weeds under control around replants and provides an opportunity for scheduled inspection of the replants for other possible problems.  Gophers quickly find weedy areas and, experience suggests, consider citrus roots just as appetizing as the roots of weeds.

Some pre-emergent herbicides are registered for new citrus plantings and some may be injected through the irrigation system.  Because of their increased cost, growers may be reluctant to use these potentially less-phytotoxic chemicals. The grower should be aware that most label directions for many of the less-expensive pre-emergent, and thus commonly used, herbicides are much different for young trees as opposed to mature trees. For example, pre-emergent herbicides containing simazine and diuron, should not be used on citrus that has been in the ground for less than a year, and some herbicides containing both diuron and bromacil, are not labeled for use if the citrus is less than three years old.  The use of these herbicides in mature groves can greatly affect the growth of new replants, especially, if used in groves with coarse soils low in organic matter. Some applicators are cautioned to turn off their machines before spraying some pre-emergent materials adjacent to a replant, but the grower should be aware that some herbicides travel down-slope with surface-drainage water.

Inspection of replants should be an active part of the pest control procedure of any grove.  The pests of mature trees are rarely the same as for the juvenile replants. Several species of ants are capable of establishing hives in the wraps of young replants, which can result in trunk girdling.  Heavy feeding of the false chinch bug, which does not damage mature citrus, can result in the rapid death of a replant, while pests like the brown garden snail, California orangedog ( or Fuller rose beetle can set the tree back seriously.  Ground squirrels, meadow mice and rabbits can strip the bark and leaves or girdle the trunk killing the replant.

Young trees planted in mature groves appear to be more at risk from freezing than do trees of equal age in new grove establishments.  This may be partly due to the increased shading of the ground by large trees and tree litter inside mature groves, which allows for less absorption of heat for radiation back to the trees at night, or the generally, poorer health of replant trees.  Replants in cold areas definitely need to have the trunk tree-covered or trunk wrapped with an insulating material.  If possible, avoid tying the wrap to the tree as lower wind speeds within a mature grove makes tying less necessary.  If not checked, as replants commonly are not, these ties may eventually girdle the tree. 

Producing mature, productive trees from replants requires extra effort and expense. To make matters worse, the pay-off can be a number of years down the road.  However, actively growing vigorous replants, in older blocks with many missing trees, may eventually determine the difference between profit and loss.  Big, healthy replants can help sell an older orchard; an important consideration  when owners' thoughts of getting up in the middle of Christmas or New Year's Eve, to start wind machines begins to lose its appeal.

Figure 1.  Replants are subject to many vicissitudes.  This replant appears to be the victim of crows looking for something to eat in the tree wrap (photo by Craig Kallsen).

Posted on Monday, June 17, 2019 at 6:59 AM

Which Way Weather?

In 2018 the Ojai Valley Land Conservancy (OVLC) accepted a grant from the Resources Legacy Fund on behalf of Watershed Coalition of Ventura County (WCVC) for a study of projected climate changes in Ventura County.  OVLC contracted with Drs. Nina Oakley and Ben Hatchett, climatologists with the Desert Research Institute (DRI), to evaluate historic climate variability and projected changes in Ventura County.  This information is needed to “paint a picture” of future climate in the watersheds of Ventura County (Ventura River, Santa Clara River, and Calleguas Creek) to support and inform climate change-related decision-making. This study provides important information for the amendment to WCVC's Integrated Regional Water Management (IRWM) Plan

You can find a copy of the report on the DRI website at:   

To view presentations and other information from the two WCVC Climate workshops conducted with Drs. Oakley and Hatchett in October of 2018, and April of this year please visit:

Some of those most interesting findings for me, are the historical data. For example, data for the years 1896 – 2018, show a tendency toward increasing maximum temperatures over the period, especially the last 10 years (Fig 1.2). But most interesting, is the increasing minimum temperatures (Fig 1.3) as compared to the maximum temperatures. Winter where is thy sting? The 2018-19 winter was the coldest in my memory, with the heater on full time at night, but there was no general frost damage this year. I can remember 1990 and 2007.

Precipitation in the South Coast region exhibits high interannual variability over the period examined. No notable long-term trends are observed (Fig. 1.4). Since approximately 2000, the 11-year running mean decreases, associated in part with the 2012–2019 drought. It is unclear whether this trend will continue in subsequent years.

There's a lot more information in the report.  READ On.

But something to keep in mind, is that we had a terrible heat wave last July, and it could easily happen again.  Growers who had their trees well hydrated before the heat arrived, sustain less or no damage to the trees and much less fruit drop.  Trees that were irrigated on the day it started to get hot, never had a chance to catch up with the heat.  Once the atmosphere starts sucking the tree dry, water movement through the soil, roots and trunk cant keep up with the demand.  Weather forecasting is pretty accurate 3 days out, and if heat is forecast, get those trees in shape.  You can run water to reduce the temperature and raise the humidity in the orchard to reduce transpirational demand which helps some.

Something we  learned last year.  What we saw and what to expect:


Map of elevational changes in Ventura County and how

elevation ventura
elevation ventura

Posted on Friday, June 14, 2019 at 6:27 AM
Tags: climate change (5), cold (5), frost (19), heat (6), rainfall (4), temperature (1)

Those Other Citrus Diseases

The So-Called “Leaf Fleck” Virus Diseases of Citrus

Robert R Krueger, USDA-ARS National Clonal Germplasm Repository for Citrus & Dates

Riverside, California

Huanglongbing has recently emerged as an existential threat to California citrus production. Although thus far it has been apparently confined to Southern California residential citrus plantings and has not yet been detected in Central or Northern California, its potential for destruction has resulted in most of the attention paid to citrus diseases (as well as most of the research funding) being focused on Huanglongbing. However, other citrus diseases have historically been deleterious to citrus production and their elimination is required in registration and certification programs. It is therefore important to remain knowledgeable regarding these diseases.

One such group of diseases is sometimes referred to as the “leaf fleck” diseases. This is a reference to the symptoms produced in indicator plants in bio-indexing. Bio-indexing was, until relatively recently, the only manner of detecting these diseases, which have quite different effects from each other in commercial orchards. Recent advances in understanding these diseases were presented at the XXI Conference of the International Organization of Citrus Virologists (IOCV) held in Riverside March 09 – 12, 2019. A brief over-view of these findings and their historical context will be presented in this communication.

The diseases to be discussed include Concave gum (CG), Cristacortis, Impietratura, and the newly described Citrus virus-A. These diseases for the most part have historically been associated with the Mediterranean area. CG has historically been present in California, apparently introduced with a varietal introduction before stringent guidelines were in place. CG, Cristacortis, and Impietratura all cause the so-called “oak leaf pattern” in young, tender spring flushes of sweet oranges and mandarins when temperatures are mild. However, other symptoms and the economic effects of these three diseases are different.

Concave gum causes the formation of “concavities” in the trunk and larger limbs of infected trees (Fig 1). These concavities are depressions or pits that may be up to several square inches in size. In the initial stages of concavity formation, the bark cracks and exudes gum. Gum may also be present on the exterior of long-established concavities and within the trunk under the concavities. A portion of the xylem is plugged with these gummy exudates. The overall effect on the tree is generally not death but rather a general debilitation. Higher levels of concavities are associated with a larger degree of tree debilitation and decreased yield and fruit quality (Wallace, 1978).

Cristacortis (Fig 2) also results in pits on the trunks and main branches of infected trees. However, the pits are smaller, deeper, and sharper and occur in both the scion and rootstock. As with CG, the effect is a general debilitation of the tree and decreased economic performance. Impietratura (Fig 3) differs from CG and Cristacortis in that there are no vegetative symptoms. Infected trees have large numbers of small, hard fruits. Gum deposits are present on the albedo at the stem-end of the fruit and in the stem near the fruit. In some fruits, there is surface browning with gum present beneath the surface (Wallace, 1978).

What these three diseases have in common is the “oak leaf pattern” of leaf clearing seen in the leaves of sweet orange and mandarin under appropriate conditions (Fig 4). These symptoms can often be seen in the field and this led to the development of a biological index for this pattern (Roistacher, 1995). This consists of the use of ‘Dweet' tangor as an indicator, held under cool (65 – 75 ºF) temperatures in the greenhouse. ‘Dweet' proved to be a more sensitive indicator than other mandarins or sweet oranges. A problem is that the patterns in the indicator leaves are so similar that differentiation is difficult or impossible. Isolates are maintained based on the identification of the source trees in the field. Other diseases, notably psorosis, produce similar symptoms in indicators but the symptoms differ from the oak leaf pattern (Fig 4). This led to the association of these diseases and some others as part of a “psorosis complex” for many years. Since for most of these diseases, a causal agent had not been definitively established, disconnecting of the oak leaf pattern-forming presumed viruses was done based upon transmissibility, ability to cross protect, epidemiology, etc (Timmer and Beñatena, 1977; Wallace, 1978).

Recently, the di Serio group in Italy (Navarro et al, 2018a, b) and Vives in Spain (presentation at IOCV, 2019) have identified viruses associated with some of the leaf-flecking diseases and have developed laboratory assays for them. Navarro et al (2018a) identified a CG-infected tree by bio-indexing and excluded psorosis by molecular methods. Next-generation seque3ncing (NGS) identified an apparently new negatively stranded RNA virus, Citrus concave gum associated virus (CCGaV). CCGaV was originally said to be a member of the genus Phlebovirus, previously only reported in insects (Navarro et al, 2018a). However, further phylogenetic studies led to a proposal to create a new genus Coguvirus to accommodate CCGaV. A second virus from the proposed new genus Coguvirus was isolated and identified as Citrus Virus A (CiVA). A field survey in Southern Italy encompassing 71 trees showed 15 trees with CiVA present and 5 trees infected with both CCGA and CiVA. Ten of the trees were infected by CiVA and not CCGaV and were asymptomatic. CiVA did not produce symptoms in inoculated plants of ‘Dweet' tangor, ‘Madame Vinous' sweet orange, or other potential indicator plants (Navarro et al, 2018b).

At the IOCV conference, Vives reported Phlebo-like viruses associated with CG, Cristacortis, and Impietratura. A CG isolate (CG-24, originally from California) and an Impietratura isolate I-501 showed homology with CiVA, whereas Cristacrotis isolate C-601 (from Corsica) showed homology with CCGaV, based upon the sequences published by the de Serio group. At the same meeting, several other possibly-related viruses were discussed. Park from Texas presented an oak-leaf inducing virus that acted similar to a CG isolate CG-301 but grouped with CiVA was dubbed Citrus oak leaf associated virus (COLaV). Bester from South Africa described field trees that had psorosis-like trunk and limb symptoms but were negative for psorosis. Some apparent viruses were sequenced, some more like CCGaV and some more like CiVA. Cao from China described five new viruses that would also be related to CCGaV and CiVA that, converse to the South African report, produced leaf symptoms but no trunk symptoms.

These new developments are starting to shed some light on these previously mysterious diseases, but are also opening up new questions. Of particular interest in California is what we are calling CG isolates may in fact be CiVA isolates. This is confusing because the trees producing these isolates were those identified as CG trees based upon the field observations. Some of these trees are still maintained as field trees in Riverside. It is possible that the CG tree used by Navarro et al (2018a) to identify CCGaV had symptoms similar to our California CG trees but actually were caused by a different causal agent. However, our California CG isolates consistently produce symptoms in the ‘Dweet' indicator whereas CiVA did not (Navarro et al, 2018b). In any case, the new NGS methods have revealed interesting new insights into these interesting old diseases. 

Literature Cited

Navarro B, Minutolo M, de Stradis A, Palmisano F, Alioto D, di Serio F. 2018a. The first phlebo-like virus infecting plants: a case study on the adaptation of negative-stranded RNA viruses to new hosts. Mol Plant Pathol 19:1075-1089.

Navarro B, Zicca S, Minutolo M, Saponari M, Alioto D, Di Serio F. 2018b. A negative-stranded RNA virus infecting citrus trees: the second member of a new genus within the order Bunyavirales. Front Microbiol 9:2340. doi: 10.3389/fmicb.2018.02340.

Roistacher, CN. 1995. A historical review of the major graft-transmissible diseases of citrus. FAO Regional Office for the Near East, Cairo.

Timmer, LW, and Beñatena, HN. 1977. Comparison of psorosis and other viruses causing leaf flecking in citrus. Proc Int Soc Citriculture 3:930-935.

Wallace, JM. 1978. Virus and viruslike diseases. Pp 67 – 184 in Reuther, W, et al (eds). The Citrus Industry. Rev ed. IV. Crop protection. University of California Div of Agr Sci, Berkeley, CA.

Fig 1.  Concave Gum

Fig 2. Cristacortis

Fig 3. Impietratura

Fig 4. Oak Leaf Pattern


Posted on Wednesday, June 12, 2019 at 6:06 AM
Tags: citrus (318), concave gum (1), cristacortis (1), impietratura (1), psorosis (1), virus disease (1)

A Great Bee Guy Dies

Dr. Robbin Thorp was the world authority on Franklin's bumble bee, pictured on his computer screen. (Photo by Kathy Keatley Garvey)
Dr. Robbin Thorp, a global and legendary authority on bees and a distinguished emeritus professor of entomology at the University of California, Davis, passed away Friday, June 7 at his home in Davis, surrounded by family. He was 85.

Dr. Thorp, a member of the UC Davis entomology faculty for 30 years, from 1964-1994, achieved emeritus status in 1994 but continued to engage in research, teaching and public service until a few weeks before his death.

A tireless advocate of pollinator species protection and conservation, Dr. Thorp was known for his expertise, dedication and passion in protecting native pollinators, especially bumble bees, and for his teaching, research and public service. He was an authority on pollination ecology, ecology and systematics of honey bees, bumble bees, vernal pool bees, conservation of bees, native bees and crop pollination, and bees of urban gardens and agricultural landscapes.

“Robbin's scientific achievements during his retirement rival the typical career productivity of many other academic scientists,” said Steve Nadler, professor and chair of the UC Davis Department of Entomology and Nematology. “His contributions in support of understanding bee biodiversity and systematics are a true scientific legacy.”

Lynn Kimsey, director of the Bohart Museum of Entomology and UC Davis professor of entomology, said: "I've known Robbin since I was a graduate student at UC Davis. Even though he wasn't my major professor, my project was on bees and he was incredibly helpful and supportive. His enthusiasm about pollinators and bees in particular actually grew after he retired, and he continued helping students and researchers and was the backbone of so much research. His support and kindness was matched by his undemanding assistance and expertise. What a terrible loss to his family and to the research and conservation communities."

Colleague Norman Gary, UC Davis emeritus professor of entomology, commented earlier this year in a letter of support for a College of Agricultural and Environmental Sciences award: “Robbin is recognized internationally for his expertise and research on bees, especially non-Apis species, known as wild bees. I doubt that there is anyone else in the world who can compete with his expertise in the systematics of the 20,000 species of bees on this earth. He has the perfect balance of research of field research on the biology and behavior as well as laboratory research on the taxonomy of bees.” He was the go-to person to identify a bee by species.

Professor Neal Williams, who organized a symposium in Dr. Thorp's honor at the 2019 Pacific Branch, Entomological Society of America meeting in San Diego, said: ‘Through his tireless efforts in research, advocacy and education, he has inspired a new generation of bee researchers…I like many others, feel truly honored, to have received the mentoring of Robbin and to have him as a colleague.”

In his retirement, Dr. Thorp co-authored two books Bumble Bees of North America: An Identification Guide (Princeton University, 2014) and California Bees and Blooms: A Guide for Gardeners and Naturalists (Heyday, 2014). Locally, he was active in research projects and open houses at the Bohart Museum of Entomology and the Häagen-Dazs Honey Bee Haven. In his research, he monitored bees in the Häagen-Dazs Honey Bee Haven, a half-acre bee garden on Bee Biology Road operated by the UC Davis Department of Entomology and Nematology. He established a baseline in 2008 and detected more than 80 species of bees. 

Born Aug. 26, 1933 in Benton Harbor, Mich., Dr. Thorp received his bachelor of science degree in zoology (1955) and his master's degree in zoology (1957) from the University of Michigan, Ann Arbor. He earned his doctorate in entomology in 1964 from UC Berkeley, the same year he joined the UC Davis entomology faculty. He taught courses from 1970 to 2006 on insect classification, general entomology, natural history of insects, field entomology, California insect diversity, and pollination ecology.

Every summer from 2002 to 2018, Dr. Thorp volunteered his time and expertise to teach at The Bee Course, an annual workshop sponsored by the American Museum of Natural History and held at the Southwestern Research Station, Portal, Ariz. The intensive 9-day workshop, considered the world's premiere native bee biology and taxonomic course, is geared for conservation biologists, pollination ecologists and other biologists.

Highly honored by his peers, Dr. Thorp was named a fellow of the California Academy of Sciences, San Francisco in 1986; recipient of the Edward A. Dickson Emeriti Professorship of UC Davis in 2010; and recipient of the UC Davis Distinguished Emeritus Award in 2015. Other honors included: member of the UC Davis Bee Team that won PBESA's Team Award in 2013. In addition, he was a past president (2010-2011) of the Davis Botanical Society, and former chair (1992-2011) of the Advisory Committee for the Jepson Prairie Reserve, UC Davis/Natural Reserve System.

Leslie Saul-Gershenz, who received her doctorate in entomology from UC Davis and is now an associate director of research with the Wild Energy Initiative, John Muir Institute of the Environment, said “I am heartbroken.  I really hoped with all my heart that Robbin was going to get better and we would have more time with him, more of his sweetness, his kindness, his caring.  He helped so many people over so many decades, his contributions were immense in his scientific contributions but also in his positive support of students, and colleagues alike and encouragement of public engagement. I miss him so much. “

Williams said the PBESA symposium was “perhaps the greatest honor one can receive from close colleagues--a special symposium honoring him and his contributions to the field of bee biology and pollination. We designed the symposium to honor the impact of Dr. Thorp, on the field of bee biology and conservation, but at the same time present innovative research that brings together bee and pollination biology researchers."

Richard Hatfield, a senior conservation biologist with Xerces' Endangered Species Program  presented him with a framed illustration of Bombus franklini, the work of artist April Coppini of Portland, Ore. An authority on Franklin's bumble bee,  Dr. Thorp began monitoring the bumble bee population in 1998 in its narrow distribution range of southern Oregon and northern California. He has not seen it since 2006 and was instrumental in placing Franklin's bumble bee on the Red List of Threatened Species of the International Union for Conservation of Nature and Natural Resources (IUCN). Long active in the North America IUCN Bumblebee Specialist Group, Dr. Thorp served as its regional co-chair, beginning in 2011.

In August of 2016 a documentary crew from CNN, headed by John Sutter, followed him to a meadow where Dr. Thorp last saw Franklin's bumble bee. Sutter wrote about Dr. Thorp, then 82, in a piece he titled "The Old Man and the Bee," a spinoff of Ernest Hemingway's "The Old Man and the Sea."

 “Robbin has done so much for me over the years,” Hatfield said. “I'm pleased to give back even a small fraction.” Hatfield praised him as a “a Living Legend of North American Bee Conservationin a Xerces Society blog during Earth Week. “He has made lasting contributions to the bee conservation community in ways that might never be measured, but will certainly be felt.”

"It was great to see Robbin interacting and enjoying the conference and the company," said Professor Gordon Frankie of UC Berkeley, one of the speakers at the Robbin Thorp Symposium. "We all have learned much from him over the years, and this was a good occasion to say thanks and acknowledge Robbin's many contributions."

Professor Diane Ullman, former chair of the UC Davis Department of Entomology, said: “Robbin was my faculty advisor when I was a student! He gave me the courage to stay in graduate school and was a wonderful supporter when I came back to Davis as a faculty member. He was an amazing and passionate scientist and an extraordinary person.”

More tributes:

Professor Claire Kremen of the University of British Columbia, formerly of UC Berkeley, praised him in her letter of support for his nomination for distinguished emeritus professor in 2014:

"I have had the privilege of working with Dr. Thorp as a close colleague since 1999. I can definitely say that without his contributions, I could never have developed as extensive and impactful a research program on pollinator conservation and pollination services. It is even more noteworthy that Dr. Thorp's contributions to this research program have all occurred since his  'retirement'– he has had a very active retirement indeed."

"Dr. Thorp has contributed in three main ways. First, he has provided expert input into the design of protocols for the research, including assays for pollinator effectiveness, developing citizen science methods, rearing experimental bumble bee colonies, monitoring bumble bee colony properties in the field, and developing pollinator survey methods. Second, he has provided expert taxonomic services, including personally identifying over 100,000 native bee specimens that we have collected during this work, and working with us to develop a bee traits database. Third, he has trained numerous field assistants and graduate students from my lab in different aspects of bee biology. He's spent long hours with many of my graduate students helping them learn to identify bees. He also helped us develop methods and information sheets for teaching field and lab teams to recognize key generic and family characters for identifying bees in the field and sorting them in the lab. He's advised many of my graduate students on different aspects of their work.

"Collectively, Dr. Thorp's contributions have impacted 35 publications that have emerged from this research program to date, with many more either submitted or nearing the submission stage. He has also been a co-author on a number of these publications. Not only has Dr. Thorp had such a significant effect on the work of my lab, but he conducts his own primary work documenting the status of rare bumble bee species like Bombus franklini and B.occidentalis and contributes at a similar level to other research labs such as with his long-time collaborators Dr. Gordon Frankie and Dr. Neal Williams. It's really quite amazing how he manages to do it all."

In another letter of support, Katharina Ullmann, director of the UC Davis Student Farm, Agricultural Sustainability Institute, and a UC Davis alumnus (doctorate in entomology) wrote:

"I met Robbin Thorp in 2007 and assumed that he was an active professor because of his continued contribution to the field of entomology, teaching activities, publishing in peer-reviewed journals and vocal support of pollinator conservation efforts. For these reasons I considered asking if I could join his lab. I remember telling someone my plan and they said 'That probably won't work. Robbin is retired.' He does so much in the field of pollination ecology that I didn't even realize he was retired.

"...The entire time that I've known Robbin I've been impressed with (1) his depth and breadth of knowledge about bees and crop pollination, (2) his willingness to share what he knows, and (3) how approachable he is. It doesn't matter if you're a MacArthur Genius or a field technician just learning about bees, Robbin always makes time to talk with you and answer your questions.

"Robbin is one of the few people in North America who can identify bees down to the species level,” Ullmann said. As a result he's in high demand and has identified thousands of specimens for numerous lab groups since his retirement. However, he doesn't just identify the specimens. Instead, he's willing to patiently work through dichotomous keys with you so that you can learn those skills."

Research entomologist James Cane of USDA's Agricultural Research Service, Logan, Utah, wrote in 2014 that “Dr. Robbin Thorp should be the first scientist to be cloned, so valuable and broadly integrated are his knowledge about bees and pollination.  No one else I know has his combination of skills; normally several people would be needed.  Thus, he is a taxonomist of several genera of bees, a competent pollination biologist studying both native bees and honey bees in both natural and agricultural realms (with research experience in several crops), and a conservation advocate for bees.  Moreover, I have watched his considerable teaching skills while helping in The Bee Course over the years.  There I also get to see what a model human being Robbin is: thoughtful, considerate, a great listener, playful, polite unpretentious, all traits that the students gravitate towards.  I have looked to Robbin as a role model for over 30 years, listen carefully to what he has to say, and always look forward to being in his presence.  UC Davis is very lucky indeed to have attracted and retained such a fabulous faculty member.”

Ron McGinley, an instructor an organizer of The Bee Course, said “Robbin Thorp converted me from bugs to bees. To say that Robbin changed my life would be a vast understatement! In retirement, Robbin continues to be one of the most highly regarded bee workers in the world. He also continues his outstanding educational/mentoring skills.”

McGinley shared several comments from The Bee Course alumni:

  • It was really a pleasure to learn from one of the best "Bee Dudes” out there. 
  • Professors with a great deal of experience can sometimes find it difficult to teach students who are just being introduced to the material. Their command of the material is so great that it seems second nature and they can forget how to guide students through the labyrinths of taxonomic structures. Not so for Dr. Robbin Thorp. The paths laid out for students in The Bee Course were clear and the light at the end of the tunnel, although sometimes faint, was always visible.
  • Robbin even bought me a Dairy Queen treat for being the first to find an active bee nest….Centris.   I said ‘Robbin, you don't have to buy me this treat' and he answered ‘Yes, I do, it's tradition.'
  • Robbin is fun to be with in the field and welcomes questions for which he gives very clear answers.
  • I sincerely hope that I will meet and work with Robbin again in the future.  He is just a joy to be with. 

Dr. Thorp was known for his public service, his response to all requests for bee identifications, and his friendships.

Insect photographer Allan Jones of Davis: “I feel particularly privileged as an outsider to have drawn his friendship, attention and support.  A picture is said to be worth a thousand words. But a picture with accurate text is a treasure. He has been adding depth and meaning for all of the visitors and friends he has kindly touched.”

Naturalist and bumble bee enthusiast Gary Zamzow of Davis said that “Dr. Thorp frequently helped the Wisconsin Arboretum staff and volunteers. He identified the bumble bees photographed at the Arboretum and other areas. He would confirm our identifications. A great learning experience for all of us. We could have not done it without Dr. Thorp's help.” 

His wife, Joyce, 84, preceded him in death on Dec. 9, 2018. Survivors include three children, Kelly, Katie and Jeff, and stepchildren Donna Gary and Steve Gary. 

Dr. Robbin Thorp, distinguished emeritus professor of entomology (Photo by Kathy Keatley Garvey)
Dr. Robbin Thorp, distinguished emeritus professor of entomology (Photo by Kathy Keatley Garvey)
Dr. Robbin Thorp with two of the books he co-authored in 2014. (Photo by Kathy Keatley Garvey)
Dr. Robbin Thorp with two of the books he co-authored in 2014. (Photo by Kathy Keatley Garvey)

Focus Area Tags: Agriculture, Environment, Natural Resources, Yard &

Posted on Monday, June 10, 2019 at 6:29 AM

The Skinny on Avocado Pests and Diseases


"A Field Day with Avocado Pest and Disease"
was a program sponsored by:
CA Avocado Society
CA Avocado Commission
University of CA Coop Extension
And these are the handouts to highlight
the talks given by:
Jim Davis, Jane Delahoyde, Ben Faber, Enrico Ferro,
Gabe Filipe, Anna Howell, Patricia Manosalva, Tom Roberts, and Tim Spann.

Avocado Lace Bug

Anthracnose, Sunblotch, and Armillaria Root Rot

Avocado Trunk Canker and Collar Rot

Avocado Branch Canker (Botryosphaeria)

Persea Mite

Phytophthora Root Rot of Avocado and Management Strategies

Shothole Borer


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Posted on Monday, June 10, 2019 at 6:24 AM
Tags: avocado. pests (1), disease (57), field day (5), mites (5), root rot (21), thrips (3)

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