San Luis Obispo County
University of California
San Luis Obispo County

Topics in Subtropics Blog!

Controlling Shot Hole Borer

A recent study by Joey Mayorquin and team shows a promising chemical response in sycamore trees infected with Fusarium Dieback. This holds hope for the pest/disease complex in avocado, as well.

Chemical Management of Invasive Shot Hole Borer and Fusarium Dieback in California Sycamore (Platanus racemosa) in Southern California

Fusarium dieback (FD) is a new vascular disease of hardwood trees caused by Fusariumspp. and other associated fungal species which are vectored by two recently introduced and highly invasive species of ambrosia beetle (Euwallacea spp. nr. fornicatus). One of these ambrosia beetles is known as the polyphagous shot hole borer (PSHB) and the other as the Kuroshio shot hole borer (KSHB). Together with the fungi that they vector, this pest–disease complex is known as the shot hole borer–Fusarium dieback (SHB-FD) complex. Mitigation of this pest–disease complex currently relies on tree removal; however, this practice is expensive and impractical given the wide host range and rapid advancement of the beetles throughout hardwoods in southern California. This study reports on the assessment of various pesticides for use in the management of SHB-FD. In vitro screening of 13 fungicides revealed that pyraclostrobin, trifloxystrobin, and azoxystrobin generally have lower effective concentration that reduces 50% of mycelial growth (EC50) values across all fungal symbionts of PSHB and KSHB; metconazole was found to have lower EC50 values for Fusarium spp. and Paracremonium pembeum. Triadimefon and fluxapyroxad were not capable of inhibiting any fungal symbiont at the concentrations tested. A 1-year field study showed that two insecticides, emamectin benzoate alone and in combination with propiconazole, and bifenthrin, could significantly reduce SHB attacks. Two injected fungicides (tebuconazole and a combination of carbendazim and debacarb) and one spray fungicide (metconazole) could also significantly reduce SHB attacks. Bioassays designed to assess fungicide retention 1 year postapplication revealed that six of the seven fungicides exhibited some level of inhibition in vitro and all thiabendazole-treated trees sampled exhibiting inhibition. This study has identified several pesticides which can be implemented as part of an integrated pest management strategy to reduce SHB infestation in low to moderately infested landscape California sycamore trees and potentially other landscape trees currently affected by SHB-FD.

Posted on Monday, June 18, 2018 at 6:47 AM

Irrigating with an Atmometer?

Say what? Atmosphere meters can be used to assess irrigation requirement. Here's a great little report from Mark Battany, our advisor in Santa Barbara and San Luis Obispo Counties.



  • Author: Mark Battany

Efficient and precise irrigation management is becoming increasingly important inCaliforniaagriculture, both for maximizing crop quality and for conserving water. The most advanced irrigation scheduling strategy is based on local measurements of reference evapotranspiration (ETo), which is converted to crop evapotranspiration (ETc) with an appropriate crop coefficient (kc).

To be able to use this method, an irrigation manager needs to have locally accurate ETo values throughout the growing season. However, the highly variable microclimates that characterize many farming areas often make it difficult to use data from distant weather stations; therefore an accurate local measurement may often be preferable to relying on a regional value.

One inexpensive option for measuring ETo locally is to use a simple atmometer. Atmometers are water-filled devices, in which the actual evaporation of water is measured over time. In their simplest form, the atmometer is outfitted with a graduated sight glass on the water supply tank which allows the user to easily measure the evaporation that occurred over a given period. In practice, this type of atmometer is most suited for making readings at multiple day intervals, for example once per week, or on days when irrigation is applied.

The performance of atmometers versus more expensive weather stations was evaluated on the CentralCoastin 2003. In this study, atmometers were placed adjacent to seven weather stations throughout the area, and weekly values for both methods were compared. The results indicate that the atmometers and weather stations have very comparable ETo readings, with the atmometers indicating somewhat lower ETo values under conditions of lower evapotranspiration.

Like any technique, using atmometers has advantages and disadvantages. Advantages include their very low cost and ease of operation, with no computer or power required. Disadvantages include the potential for damage by freezing weather, the need to refill the water supply (every three to six weeks), and the need to read the gauge manually. Also, if they are installed in a large open area, birds may tend to perch on the evaporating surface and foul it with their droppings; for this reason several wires are installed on top of the device to discourage birds from perching there. In general, atmometers function quite reliably with few problems.

Converting atmometer ETo readings to the amount of irrigation run time required to replenish the soil moisture lost to evapotranspiration is fairly straightforward. A relatively simple example for a sprinkler-irrigated field is presented below in Table 1.

Table 1. Example conversion of ETo to irrigation run times   for a sprinkler irrigated field




A.  Measured atmometer ETo for one week



B.  Crop coefficient (kc)



C.  Calculated ETc for the week (=AxB)



D.  Sprinkler application rate



E.  Hours of irrigation required (=C/D)




(Note: To convert Gallons to Inches: Gallons ÷ Area (square feet) ÷ 0.6234 = Inches. To convert Inches to Gallons: Inches * Area (square feet) ÷ 1.604 = Gallons)

 Regression table showing relationshiop between CIMIS weather station and atmometer

Atmometer in


 Atmometer in the field

Posted on Friday, June 15, 2018 at 6:28 AM

Summer time and the livin is eatin.

Summer—it's a time for swimming, BBQs, camping, and eating invasive species

Last week during California Invasive Species Action Week (June 2 – June 10), we highlighted several pests, but there are many more invasive species out there. Now that you know about them, share your knowledge of invasive species with others. And no matter what your summer plans, here are some things YOU can do about invasive species from the California Department of Fish and Wildlife and California Department of Food and Agriculture.






  • Get to know your local invaders.
  • Learn about California's invasive plants.
  • Find out which species are threats to California.
  • Learn alternatives to releasing unwanted fish, aquatic plants, and other pets.
  • Eat them. Yum. Check out these websites to find out who is edible and how to prepare them.

If you missed it this year, help in the fight next year by learning and participating during California Invasive Species Action Week.

Posted on Wednesday, June 13, 2018 at 12:54 PM
  • Author: Tunyalee Martin

California Citrus Directions

Press Release


For more information, contact:
Bruce Babcok:
Joel Nelsen:
Gary Schulz:


 Citrus Research Board Quantifies Industry Importance

May 16, 2018 – Visalia, Calif. – The total economic impact of California's iconic citrus industry is $7.117 billion according to a new study commissioned by the Citrus Research Board (CRB).

     “In updating our economic analysis, we selected a well-known expert, Bruce Babcock, Ph.D., a professor in the School of Public Policy at the University of California, Riverside, to conduct the research. His findings quantified the significant impact of citrus on California's economic well-being,” said CRB President Gary Schulz.

     According to Babcock, the California citrus industry added $1.695 billion to the state's Gross Domestic Product (GDP) in 2016. “California citrus is a major contributor to the economic value of the state's agricultural sector and is much larger than just the value of its sales,” he said. “Estimated full-time equivalent California citrus jobs totaled 21,674 in 2016-17, and estimated wages paid by the industry during that same timeframe totaled $452 million.”

     Babcock added, “The application of management skills and capital equipment to efficiently utilize land and water to produce high-quality citrus also generates upstream and downstream jobs and income that magnify the importance of citrus production beyond its farm value.”

     In 2016-17, the most recent marketing year of data compilation, Babcock found that the total direct value of California citrus production was $3.389 billion. This value generated an additional $1.263 billion in economic activity from related businesses that supplied materials and services to the citrus industry. Layered on top was another $2.464 billion in economic activity generated by household spending income that they received from California's industry, according to Babcock, thus rendering a total economic impact of $7.117 billion.

     The study revealed that 79 percent of California's citrus was packed for the fresh market and 21 percent was processed in 2016-17, which is economically significant because fresh market fruit has a higher value than processed fruit.”

     Of further note, California produced about 95 percent of all U.S. mandarins in the most recent reporting season.

     California Citrus Mutual President Joel Nelsen commented, “The “wow” factor in this report is something as it relates to gross revenues and positive impact for the state, people and local communities.  This enthusiasm must be tempered by the fact that huanglongbing (HLB) can destroy all this in a matter of a year if the partnerships that exist between the industry and government cannot thwart the spread of this insidious disease.  Just this week, coincidentally, Brazil authorities reported a 20% reduction in fruit volume.  Reading how that would affect our family farmers, employees and the state is sobering.”

     The CRB study also looked at the possible impact of a potential 20 percent reduction in California citrus acreage or yield or a combination of the two that could result from increased costs associated with meeting government regulations, combatting the Asian citrus psyllid (ACP) and warding off the invasion of HLB, a devastating disease that has decimated citrus production in many other growing regions such as Florida. Babcock calculated that such a reduction could cause a loss of 7,350 jobs and $127 million in associated employment income and could reduce California's GDP by $501 million in direct, indirect and induced impacts. The CRB currently is devoting most of its resources to battling ACP and HLB to help ensure the sustainability of California citrus.

     Babcock is a Fellow of the Agricultural and Applied Economics Association and has won numerous awards for his applied policy research. The economist received his Ph.D. in Agricultural and Resource Economics from the University of California, Berkeley and his Masters and Bachelors degrees from the University of California, Davis.

     The CRB administers the California Citrus Research Program, the grower-funded and grower-directed program established in 1968 under the California Marketing Act as the mechanism enabling the State's citrus producers to sponsor and support needed research. More information about the Citrus Research Board and the full report on the “Economic Impact of California's Citrus Industry” may be found at







Posted on Tuesday, June 12, 2018 at 4:20 PM

Colletotrichum Shoot Dieback of Citrus

Figure 1: Citrus shoot dieback (top) and gummosis (bottom) caused by Colletrotrichum.

A new disease of citrus has been found in the main growing regions of the Central Valley of California. The causal agents of this disease were identified as species of Colletotrichum, which are well-known pathogens of citrus and other crops causing anthracnose diseases. Several growers and nurserymen in various orchards in the Central Valley first noticed the disease in 2013. Symptoms include leaf chlorosis, crown thinning, gumming on twigs and shoots dieback, and in severe cases, death of young trees. The most characteristic symptoms of this disease are the gum pockets, which appear on young shoots either alone or in clusters and the dieback of twigs and shoots (Fig.1). Field observations indicate that symptoms initially appear during the early summer months and continue to express until the early fall. These symptoms were primarily reported from clementine, mandarin, and navel orange varieties. In order to determine the main cause of this disease, field surveys were conducted in several orchards throughout the Central Valley. Isolations from symptomatic plant samples frequently yielded Colletotrichum species. Morphological and molecular phylogenetic studies allowed the identification of two distinct species of Colletotrichum (Colletotrichum karstii and Colletotrichum gloeosporioides) associated with twig and shoot dieback. Interestingly, these Colletotrichum species were also isolated from cankers in larger branches. Although C. gloeosporioides is known to cause anthracnose on citrus, a post-harvest disease causing fruit decay, it has not been reported to cause shoot dieback of citrus. C. karstii however has not been reported previously from citrus in California and our research team is currently conducting field and green house studies to determine the pathogenicity of this species in citrus. At present, it is unclear how widespread this disease is in California orchards or how many citrus varieties are susceptible to this disease. Pest control advisors are monitoring citrus trees for the presence of the disease in the Central Valley (particularly clementine, mandarin, and navel varieties) during the early summer months. Continuing research led by Dr. Akif Eskalen in collaboration with Dr. Florent Trouillas is focused on further understanding the biology of the fungal pathogens as well as factors influencing disease expression in order to develop management strategies against this emerging disease.

Posted on Monday, June 11, 2018 at 6:10 AM
  • Author: Joey Mayorquin, Mohamed Nouris, Akif Eskalen and Florent Trouillas

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