California Ag Network

Category: Pest/Disease Management

  • New CA Blackeye Varieties Show Resistance to Cowpea Aphid

    Field trials in the Central Valley with two new varieties of blackeye beans, CB74 and CB77, show impressive resistance to cowpea aphids compared to standard CB46, CB5, and CB50 lines. Four varieties of blackeyes including CB46, CB77, CB74, and CB5 were seeded into a blackeye CB50 field, in single lines on 30-inch beds in the Sacramento Valley in May 2020 (Photo 1). By mid-summer, CB50, CB46, and CB5 were heavily infested with aphids (photo 2), whereas CB74 and CB77 were clean (photo 3).

    Photo 1. Blackeye variety trial, Sacramento Valley, 2020; left to right, CB46, CB77, CB74 (early maturing), and CB2 compared to the standard CB50 line planted in the field on the far left.

    Cowpea aphids are serious insect pests of blackeyes. These aphids can quickly colonize plants and cause injury by direct feeding and injecting toxic saliva into plants, leading to stunted growth or death of plants. Sticky honeydew released by the aphids can stimulate black mold growth on plants, reducing photosynthesis and plant health. Cowpea aphids also vector a number of viral mosaic diseases that can cause serious losses in many crops. Biological control cannot be relied on because natural enemies often appear when cowpea aphid infestations are already high and causing serious damage. Applying pesticides early in the season prevents cowpea aphid infestations but beneficial insects can be destroyed, leading to outbreaks of other insect pests. Thus, the development of cowpea aphid resistant blackeye lines is an important breakthrough in managing this pest.

    Photo 2. Heavy cowpea aphid pressure on blackeye bean CB46 leading to significant yield and quality losses.

    Blackeye beans, also known as cowpeas, or blackeye peas in southern states, are an important food crop worldwide. In California, about 8,000 acres are grown annually for dry or canned blackeye bean markets. These new blackeye bean lines are being developed by the UC Riverside blackeye breeding program, led by Drs. Phil Roberts and Bao Lam Huynh, with support from the California Dry Bean Advisory Board and the US AID Feed the Future Innovation Lab for Legume Systems Research (formerly Innovation Lab for Collaborative Research on Grain Legumes). The aphid resistance and other traits have been introgressed into California Blackeye elite backgrounds using natural selection and new molecular markers to expedite the breeding process. Compared to standard varieties, CB74 and CB77 also have more stable yields resulting from heat tolerance, better tolerance to lygus bugs, and equivalent resistance to Fusarium wilt and root-knot nematodes.

    Photo 3. Adjacent CB77 blackeye plants show high levels of resistance to cowpea aphid infestations.

    Blackeye variety observation trials are being conducted in fields by UCCE Farm Advisors Rachael Long, Sarah Light, and Nick Clark in the Sacramento and San Joaquin Valleys, in collaboration with local farmers. More information on blackeye beans can be found in the Blackeye bean production manual for California, UC ANR 21518, http://beans.ucanr.edu/files/226601.pdf. The lead UC bean breeders hope to have these lines available to farmers within the next few years. – By Rachael Freeman Long, UCCE Farm Advisor

  • Broomrape: a Parasitic Weed in CA Processing Tomato

    Figure 1: Branched broomrape infestation in a processing tomato in California.

    Branched broomrape (Phelipanche ramosa), a weedy parasitic plant that can cause devastating damage to many economically important wide range of broadleaf crops including tomato, cabbage, potato, eggplant, carrot, pepper, beans, celery, peanut and sunflower has recently re-emerged in fields in Central Valley counties in California. This weed utilizes a modified root, called haustorium, to fuse into a host plant root and extract nutrients and water which can greatly reduce productivity or even kill the host depending on the level of infestation, susceptibility of the host, and environmental conditions. Tomato is highly susceptible to branched broomrape. In the United States, California accounted for over 90% of the 12 million tons of tomatoes grown in 2018. Studies in Israel showed that at extreme infestation levels broomrape can cause processing tomato yield losses as high as 70%. The annual losses in tomato due to broomrapes in Israel and Turkey are estimated at $5 and $200 million, respectively. Up to 80% crop loss due to branched broomrape has been reported in tomato in Chile which is highly concerning given the similarity in production systems and broomrape species with California.

     
    Figure 2: A close view of a flowering branched broomrape

    Branched broomrape is currently classified in California as an “A” pest, that is, “an organism of known economic importance subject to California State enforced action involving: eradication, quarantine regulation, containment, rejection, or other holding action”. As a potentially severe economic pest and as a California “A-list” pest, establishment and spread of a branched broomrape in California tomato production regions could cause severe consequences for individual growers and for the entire tomato industry. Currently, discovery of broomrape in a commercial tomato field leads to a hold order and crop destruction without harvest. In addition to branched broomrape, several fields have been reported with infestations of Egyptian broomrape (Phelipanche aegyptiaca), a Q-listed species (that is, having a temporary “A” classification pending determination of permanent rating by California State), causing similar industry and grower concerns.

     
    Figure 3: Hundreds of tiny branched broomrape seeds (0.2 – 0.4 mm) and the single capsule from which the seeds were sourced.

    In the United States, branched broomrape was first reported in 1890, and since then, over 150 occurrences of branched broomrape have been documented. In recent times, reports of branched broomrape in the United States have been increasing; from 7 occurrences in 2015 to 65 in 2019. Branched broomrape has been documented in Texas, Virginia, South Carolina, Illinois, New Jersey, Tennessee, Kentucky, Alabama and California. In California, the first reported case of branched broomrape was in Butte County (1903) and later in Alameda County (1929). Other counties in California with reported branched broomrape detections include Colusa, Sacramento, San Benito, Santa Clara, San Joaquin, Ventura and Yolo.

    Figure 4: Distribution of branched broomrape in California as of November 10, 2019. Data source: Calflora and GBIF 2019

    A severe infestation of branched broomrape in the Sacramento Valley in 1959 prompted an intervention that involved soil fumigation with methyl bromide to target the soil seedbank; this was as an industry-led effort funded through a legislative marketing order program. Branched broomrape became a less significant problem after that effort which involved research, intensive field surveys, and fumigation of infested fields and equipment from 1973 to 1982 that cost over $1.5 million. However, this parasitic weed has recently been detected in several tomato fields in Yolo, Solano (Egyptian broomrape) and San Joaquin Counties. The cause of the re-emergence of the problem remains unclear, although re-introduction or recurrence from long-dormant seed in the soil and subsequent spread have been speculated.

    Figure 5: A branched broomrape plant attached to a volunteer tomato root in a processing tomato field in mid-June.

    The re-emergence of branched broomrape in California is of concern to the processing tomato industry as: 1) the experience in other regions of the world has demonstrated the extreme vulnerability of tomato to branched broomrape parasitism, 2) broomrapes seem likely to rapidly establish and spread in California because of the similarity to the species’ native climate, (3) repeated cultivation of processing tomato in the same fields, (4) the cultivation of a wide range of hosts (e.g. carrot, sunflower, safflower) in California, (5) intensive agricultural practices that could rapidly spread broomrape seeds to uninfested fields, (6) the production of copious number of minute seeds could easily disperse via machinery and irrigation water in the highly mechanized and irrigated cropping systems of California, (7) seed longevity (> 20 years) allows the parasite to persist even in the absence of any hosts, and (8) the major part of the parasite’s lifespan occur underground, making it inaccessible to conventional means of weed control such as cultivation and contact herbicides, (9) some of the important management tools (e.g. herbicides known to be effective in controlling broomrapes) are not yet registered or tested in California, (10) regulatory and environmental challenges with soil fumigation practices.

    Research efforts are currently being made to further understand, and develop detection and control approaches for branched broomrape in tomatoes and other specialty crops in California. For more information about branched broomrape in California, please see:

    http://tomatonet.org/branchedbroomrape

    http://tomatonet.org/img/uploadedFiles/Broomrape/CTRI_2019_NEWSLETTER.pdf

    https://www.plantsciences.ucdavis.edu/news/broomrape-eradication-high-priority-uc-researchers

    – Article By O. Adewale Osipitan, Brad Hanson, Matthew Fatino & Mohsen Mesgaran (UC Cooperative Extension)

  • New Targets for Huanglongbing Treatments

    Scientists are closer to gaining the upper hand on a disease that has wiped out citrus orchards across the globe. New models of the bacterium linked to the disease reveal control methods that were previously unavailable.

    Metabolic models of organisms are like road maps of cities. “They show you all the biological processes, and how they work together,” said UC Riverside microbiology professor James Borneman. “They also show you which molecular pathways, if blocked, will kill the organism.” 

    Simplified metabolic model and its striking similarity to a road map. (Metallo&Vander Heiden)

    In this case, researchers created the first models of the bacterium associated with Huanglongbing or HLB, also known as citrus greening disease. The team’s work is described in a new paper published in Nature’s npj Systems Biology and Applications.

    The research team made models for six different strains of the bacterium known as CLas and doing so enabled them to identify as many as 94 enzymes essential for the bacterium’s survival. These enzymes can now be considered targets for the creation of new antibacterial treatments.

    In addition, the team identified metabolites required for the bacteria to grow.

    “Just like when humans break down the food they eat into small components called metabolites, which feed our cells, bacterial cells also require metabolites for their growth,” Borneman said.

    Knowing the metabolites needed for CLas’ growth could enable scientists to cultivate it in a laboratory setting. It is not currently possible to grow CLas on its own, hindering scientists’ ability to study it and ultimately to manage it.

    This research project involved a collaboration between UC Riverside, UC San Diego, Texas A&M University, and the U.S. Department of Agriculture. In addition to Borneman, members of the modeling team included UCR plant pathologist Georgios Vidalakis and UCSD systems biologist Karsten Zengler.

    UC Riverside is at the forefront of efforts to combat Huanglongbing. Other important areas of research include antibacterial development and delivery, immune system fortification in citrus, engineering resistant citrus via a detailed understanding of host-microbe interactions, breeding resistant citrus, and insect management, among others.

    Because microbes tend to mutate and acquire resistance mechanisms in response to drugs and other efforts to thwart them, Borneman cautions that any one solution to the problem may be short-lived.

    Transmission electron microscope image of CLas bacterium. (J.M. Bové/INRA)

    “Microbes almost always adapt to control measures, perpetuating the ‘arms race’ between pathogens and hosts,” Borneman said. “There won’t be one thing that will fix this disease. We likely will need to address all three components associated with the disease — the bacterium, the insect that transmits it, and the citrus plants — to find a long-lasting solution.”

    To that end, the research team is constructing metabolic models of citrus and the insect, the Asian citrus psyllid.

    “We expect that this multiorganism modeling endeavor will provide new insights into the mechanisms underlying this disease, which will lead to effective and sustainable Huanglongbing management strategies,” Borneman said. — By Jules Bernstein, UC Riverside

  • Barking Up the Right Tree: Canines Detect HLB

    In September 2019, huanglongbing—also known as HLB or citrus greening—was detected in residential citrus trees located in Ventura County, in southern California (and just recently, the first HLB carrying Asian Citrus Psyllid was detected in a commercial CA citrus grove).

    During an orchard review in California, expert detector dog, Szaboles alerts his trainer by sitting next to a citrus tree infected with Huanglongbing (HLB).

    The disease, which has no known cure, is caused by a bacterium that devastates citrus plants and is transmitted by psyllids (small plant-eating insects resembling lice) that carry the bacteria from tree to tree or by the grafting of infected plant material. Common symptoms of infection include blotchy mottling of entire leaves, premature defoliation of the tree, and fruits that are usually small, with green peel at the bottom and a bitter taste. Unfortunately, these visible signs of HLB do not manifest until months or even years after the initial infection. By then, it is too late for citrus growers to save their crop from a mass infestation.

    However, the Farm Bureau of Ventura County found a rather unconventional way around this problem.

    To contain the infected trees and prevent a citrus epidemic, the Farm Bureau decided to hire a team of detector dogs specially trained to find HLB bacteria. The dogs are a result of a unique program funded by USDA and run by Agricultural Research Service (ARS) Research Leader and Plant Pathologist Timothy Gottwald (now retired) at the U.S. Horticultural Research Laboratoryin Fort Pierce, FL.

    First, the clever canines patrolled the perimeter because the edges of a grove are where the disease would first accumulate. Then, they trotted through the grove of trees to ultimately identify over 200 infected trees from a grove of 3,500.

    According to Gottwald’s research, the canine-detection method has an accuracy rate of 99 percent. While humans need several minutes to visually examine each tree and collect samples for laboratory testing (which requires considerable lab time and supplies), the dogs can travel through groves of trees in mere minutes, sensing HLB-infected trees by smell faster and far more easily. The time saved in finding the disease earlier gives citrus growers the opportunity to remove and destroy or quarantine HLB-infected trees, controlling what could’ve been a severe outbreak and loss of crop.
     

    However, there are still some considerations to keep in mind when deploying the canines as an early detection technology. Each dog team can only work 30 minutes at a time before they must rest, and a new team takes over. Teams can work for about 6 hours a day. In addition, much like any lab equipment, dogs are periodically calibrated to the HLB “scent signature” to remain finely honed optimized detectors.

    Still, the dogs are more sensitive and accurate than any other available technology in the field. The employment of detector dogs is still a voluntary program, which means fruit growers and farmers have the choice to either use dogs to find HLB early or continue using a USDA-approved DNA test called polymerase chain reaction (PCR).

    “While PCR is an effective way to detect infections, it isn’t efficient,” Gottwald explained. “It’s difficult to pinpoint incomplete infections—or infections that are only on one part of a plant (for example, a single leaf out of an entire tree with possibly thousands of leaves)—using PCR because it requires numerous samples from all over the suspect specimen.”

    “On the other hand,” Gottwald continued, “detector dogs identify pathogens ‘holistically,’ easily locating minute infections regardless of what small part of the tree they are infecting.” With this increased scope and accuracy rate, Gottwald and his colleagues believe that the canines’ abilities will revolutionize how growers protect their crops.

    “These specially trained canines may also be used to detect diseases and infections in other fruits and vegetables, such as grapes, peaches, plums, and tomatoes,” he said. “Our research has shown that they can sense pathogens like plum pox virus or squash vein yellowing virus, which both can cause severe economic losses to agriculture industries.”

    The canine-detection method was validated in blind tests by USDA ARS in collaboration with the California Department of Food and Agriculture, with results published in 2020.— By Georgia Jiang, USDA-ARS

  • CLas-positive Asian Citrus Psyllid Found in Riverside Commercial Grove

    An Asian citrus psyllid (ACP) sample – confirmed positive for Candidatus Liberibacter asiaticus (CLas) the bacteria that causes Huanglongbing (HLB) – was collected from a commercial citrus grove in the Woodcrest area of Riverside County. Confirmed by Citrus Research Board’s Jerry Dimitman Laboratory, this single adult psyllid is the first CLas-positive ACP found in a commercial citrus grove in California.

    While a positive ACP detection in a commercial grove is cause for serious concern, as of today, HLB has not been detected in any California commercial groves. That said, it is more crucial than ever that we stop the disease from spreading by eradicating the Asian citrus psyllid in commercial groves. The cost to manage the Asian citrus psyllid is far less than any potential costs or loss to the industry should HLB take hold throughout our state.

    An expansion of the HLB quarantine zone will not be established as a result of the CLas-positive ACP detection and CDFA staff is swiftly conducting surveys and collecting samples per the ACP/HLB Action Plan  from the perimeter of all commercial groves and all residential HLB host plants that are located within a 250-meter radius around the find.

    While treatment is not mandatory as a result of the detection, all growers within 250-meters of the find site will be notified to apply insecticides to all HLB host material within the designated area with materials recommended by the University of California (UC).

    Currently, the best way to stop the disease from spreading is to stop the ACP. To stop the ACP, we must restrict its movement and suppress existing psyllid populations. It is critical to follow best practices and review recommendations from the UC on how to protect commercial citrus groves from HLB. Regulations are in place to help prevent the spread of the pest and disease. All growers, packers and haulers must comply with all California Department of Food and Agriculture, county and federal regulations, including quarantines.

    Growers in Riverside County may contact the County Agricultural Commissioner’s office or the CDFA Pest Hotline at 800-491-1899 for additional information. If you see or suspect ACP or HLB symptoms in your grove, please notify the CDFA hotline. – By the Citrus Pest & Disease Prevention Program

  • Progress on Navel Orangeworm Sterile Insect Technique Research

    Most tree nut growers have heard about the sterile insect technique research underway to combat Navel Orangeworm, the number one pest threat to almond and pistachio growers.  But what is the progress on this program and how soon can we expect to see these sterile insects released on a large scale to support California growers?  Watch this brief interview with Houston Wilson from the Kearney Ag Research & Extension Center, UC Cooperative Extension, to learn more.
    Please thank this video’s sponsor Suterra by taking this brief Survey.

  • Fusarium Root Rot in Seedling Lima Beans

    In May, I looked at a lima bean field in the Sacramento Valley that showed poor seedling emergence scattered throughout the field (photo 1). I sent samples to the UC Davis Plant Pathology lab and the main pathogen consistently recovered from the roots was Fusarium root rot, a fungal disease caused by Fusarium solani f. sp. phaseoli. This pathogen is specific to beans and field peas and will not infect other field crops. A few bean seedlings also had Rhizoctonia and Pythium (also fungal pathogens).

    Finding Fusarium root rot in a lima bean seedling field was a surprise because this disease is most commonly encountered in established fields during mid- to late season, where it is one of the causes of early maturity (“cut out”). Rhizoctonia and Pythium can cause seedling damping-off in dry beans. However, plants usually outgrow these pathogens, particularly if the seed is treated with a fungicide and conditions favor rapid emergence.

    Fusarium solani attacks underground stems and roots of plants. In established plants, early infection is characterized by elongated reddish streaks on the roots. As the disease progresses, these eventually form reddish-brown lesions that will surround the entire root, causing decay. The above ground plant symptoms of affected plants included yellowing, wilting, stunting, and dieback. On seedling plants in the affected field, I observed dieback of the growing point, stems that were a bit swollen, and roots that were brownish and not well developed (Photo 2, diseased roots on left, healthy on right).

    Fusarium root rot causes little damage to healthy plants, but under conditions of plant stress due to drought, poor nutrition, or oxygen-stressed, waterlogged soils, Fusarium root rot can cause plant dieback and yield losses, particularly in fields with a long history of bean production. In this particular lima bean field, soil moisture was lost, causing plants to be extremely water stressed. Crop rotation, use of seed treatments, and closely watching field conditions to ensure plants are not stressed will help manage Fusarium root rot. This disease tends to be a problem in fields with a long history of bean production. More information on diseases in dry beans can be found on the newly revised UC IPM guidelines for dry beans. — By Rachael Freeman Long, UC Cooperative Extension

    Photo 2. Lima bean seedlings infected with Fusarium root rot (4 left plants) compared to healthy roots (3 plants on right).

  • More USDA Walnut Purchases Requested Amidst Pandemic

    Representative Josh Harder (CA-10) today led a bipartisan letter to United States Department of Agriculture (USDA) Secretary Sonny Perdue, asking him to use his authority to utilize all purchasing authorities to buoy the struggling walnut industry. The walnut industry – based exclusively in California – has seen prices plummet in the last two years to near or below the costs of production. Even prior to the outbreak of the Coronavirus, walnut farmers were harmed by retaliatory tariffs levied by top export partners including China, India, and Turkey. Rep. Harder is joined on the letter by fellow California Reps. Devin Nunes, Jim Costa, TJ Cox, Salud Carbajal, John Garamendi, Jerry McNerney, Jimmy Panetta, and Doug LaMalfa.

    “Walnut growers are being pinched on all sides – they’re up against tariffs from other countries, falling demand, and general chaos in food markets,” said Rep. Harder. “We have USDA programs designed for use in emergencies just like this – the Secretary should use them right away to help our walnut farmers.”

    “We are incredibly grateful to Congressman Harder and colleagues for recognizing the need to support California’s walnut growers,” said Michelle McNeil Connelly, Executive Director of The California Walnut Board. “We continue to face challenging times and greatly need Section 32 to provide relief.  As an essential industry we have continued to do our part and believe the relief funds provided to USDA will provide an excellent source on nutrition for millions of hungry Americans while, in tandem, ensuring the viability of our producers.”

    Michelle McNeil Connelly, Executive Director of The California Walnut Board

    The letter asks Secretary Perdue to use two legal authorities to increase walnut purchases. The first, Section 32 of the Agricultural Adjustments Act allows USDA to support the ag industry by purchasing their products at market rates and then distributing the products to people in need. The letter also asks the Secretary to use his authority under the Food Purchase and Distribution Program, which was created last year to help farmers harmed by trade wars with other countries.

    Rep. Harder is a leader in efforts to protect walnut growers. Last year, he led a bipartisan letter with nearly two dozen of his colleagues asking USDA to include tree nuts and other specialty crops in the Market Facilitation Program, another program designed to help farmers harmed by retaliatory tariffs. After the members of Congress made their request, USDA added walnuts and other tree nuts to the program.

    The text of the letter is below and an original copy is available here.

    Dear Secretary Perdue:

    Thank you for your leadership overseeing the U.S. Department of Agriculture (USDA). We write today in support of USDA purchases of California walnuts and encourage utilizing all purchasing authorities, including Section 32 and the Food Purchase and Distribution Program, to provide much-needed support to California’s walnut industry.

    Representative Josh Harder (CA-10)

    The COVID-19 pandemic has come on top of an already difficult time for our 4,500 California walnut producers who have, and continue, to suffer from the effects of retaliatory tariffs in India, Turkey and China.  From tariff actions alone, what was a $1.5 billion industry just two years ago, has declined by more than 41 percent to $878.8 million. Producer prices have been near or below the costs of production for the last two years (0.65/cents per pound) and the outlook is bleak.  The continued impacts of COVID-19 have resulted in excess inventories, with a record carry-out of nearly 90,000 tons from the current crop compounded by an anticipated record crop in excess of 700,000 tons which will begin harvest in September. The California Walnut Board estimates initial farm gate losses from COVID-19 at nearly $300 million, and as with this pandemic, are evolving and eroding quickly.

    Global walnut demand has slowed resulting from port disruptions, distribution challenges, the sharp decline of the food manufacturing and food service sectors, and consumer economic uncertainty. With the trajectory of COVID-19 lasting well into 2021, walnut producers are in need of assistance from any and all programs available to protect the 85,000 full-time jobs attributable to the walnuts industry. Our California’s walnut farmers represent ninety-nine percent of U.S. production of walnuts, were the 5th leading export from the state prior to these challenges. Despite falling to the 13th leading export from the state, the California walnut industry contributes over $6 billion to the state’s economy.

    Demand for nutrition programs has grown immensely, with Feeding America reporting that 98 percent of food banks reported an increase in need for food assistance.  In California alone, food bank demand has grown by 73 percent, while farmers and ranchers have seen market declines of over 50 percent. Walnuts provide a shelf stable source of protein and essential omega-3 fatty acids, providing nutrition to feed America’s hungry through meal inclusion and snacks, while also aiding our farmers.

    We appreciate your previous support for the industry, through purchases and the Market Facilitation Program, and hope your support will continue through this pandemic.  We thank you for your continued support during this unpredictable time and urge you to give all due consideration to California’s walnut industry’s purchase request.

  • Pest Colonizes Grape Vines, Forms Tumors, Repels other Predators

    Following a decade-long effort, scientists have mapped out the genome of an aphid-like pest capable of decimating vineyards. In so doing, they have discovered how it spreads — and potentially how to stop it.

    The research team’s work on the genome was published this past week in a BMC Biology paper. In it, they identified nearly 3,000 genes enabling the insect, phylloxera, to colonize and feed on grape vines by creating what are essentially nutritionally enhanced tumors. The insects live in and feed off of the structures they create.

    Phylloxera feeding on a grape root.

    “In effect, phylloxera creates its own refrigerator on the plant that it can feed from whenever it wants,” said Paul Nabity, an assistant professor of plant-insect ecology at UC Riverside. In addition to feeding the insects, these structures also protect them from attack by other parasites.

    A heavy phylloxera infestation, as occurred in the Pacific Northwest last year, could cause grapevines to lose their leaves. If the infestation reaches the roots, the plants could die.

    The tumor-like structures, known as galls, disrupt the vine’s ability to move nutrients and feed itself. They also create wounds in roots that make grapevines more susceptible to fungi and other pathogens, ultimately killing the vines.

    Claude Rispe from the French National Institute for Agriculture, Food, and Environment led the research team, while Nabity helped identify how phylloxera secrete molecules that can change the immune system of grapevines.

    “These molecules alter the plant’s defense systems and make it so that the plant doesn’t know it’s being attacked,” Nabity said.

    Native North American grapevines co-evolved with phylloxera and are now resistant to it. However, most of the grapes we eat and drink are European varieties. As a result, growers have to graft North American roots onto their European grapevines to give them tolerance to this insect.

    Now that the genes involved in the attack on non-native grapes have been identified, it may be possible to engineer phylloxera-resistant grapevines.

    “Growers currently have to graft roots to make their plants viable,” Nabity said. “A lot of money and effort could be saved with pest-resistant rootstocks.”

    About UC Riverside

    The University of California, Riverside (www.ucr.edu) is a doctoral research university, a living laboratory for groundbreaking exploration of issues critical to Inland Southern California, the state and communities around the world. Reflecting California’s diverse culture, UCR’s enrollment is more than 24,000 students. The campus opened a medical school in 2013 and has reached the heart of the Coachella Valley by way of the UCR Palm Desert Center. The campus has an annual statewide economic impact of almost $2 billion. To learn more, email news@ucr.edu.

  • Recurrent Selection with Glufosinate at Low Rates Reduces Italian Ryegrass Resistance

    Italian ryegrass is a major weed in orchards, vineyards, field crops, and fallow fields of California (Figure 1). Several different herbicides are used to control ryegrass and had been effective in reducing infestations until resistance evolved in many populations following repeated use of the herbicides. To date, resistance to glyphosate, paraquat, and some ACCase and ALS inhibitors has been confirmed in ryegrass infestations across the agricultural landscape of California. To make matters worse, resistance to multiple postemergence herbicides with different modes of action has been confirmed within the same orchard, vineyard, or field in some areas. Consequently, management of Italian ryegrass in California annual and perennial cropping systems has become a major challenge.

    Figure 1. High infestation of Italian ryegrass in a peach orchard (photo credit: Maor Matzrafi).

    Glufosinate is an alternative non-selective postemergence herbicide that can still be used to control herbicide-susceptible and most herbicide-resistant Italian ryegrass in California as only two populations with resistance to glufosinate have been documented to date. However, the higher cost of glufosinate relative to other herbicides may drive farmers to apply glufosinate at reduced rates as has occurred in other cropping systems, such as the Australian wheat belt, with other herbicides. The lower rates and other drivers such as herbicide applications at non-optimal weed size, inappropriate weather conditions, and insufficient spray coverage may result in sublethal rate selection of ryegrass by glufosinate.

    To evaluate the potential for low glufosinate rates to select for reduced susceptibility to the herbicide, and to determine if selected populations are cross-resistant to herbicides with other modes of action as has been observed in a few studies, we conducted a greenhouse study using a herbicide-susceptible parent population originally collected from a vineyard in Sonoma County. Plants were grown in the greenhouse to the 3-4 leaf stage and treated with low glufosinate rates for three generations. For the first round of selection, plants were treated with glufosinate at 1/8X, 1/4X, and 1/2X of the labelled field rate (984 g ai ha-1). Surviving plants were grown to reproductive maturity and allowed to cross-pollinate. Seeds were harvested from all plants, pooled, germinated, and plants grown in the greenhouse for the next round of selection at slightly higher rates (1/2X, 3/4X, and 1X). For the third round of selection, plants were treated at 3/4X, 1X, and 1.25X of the labelled field rate.

    Results showed that susceptibility to glufosinate was reduced in offspring in comparison with the susceptible parent population following only three generations of selection (Figure 2). Comparing the susceptible parent population with the offspring from the second and third selection cycle, the percentage of surviving plants increased to values of LD50 (1.31 and 1.16, respectively) and LD90 (1.36 and 1.26, respectively).

    Figure 2. Dose-response of the Italian ryegrass susceptible parent population (P0) and three successive generations (P1, P2, P3) of offspring following selection with low glufosinate rates in the greenhouse. Lines are the predicted values for percent survival. Red arrow indicates the labelled field rate (984 g ai h-1). Adapted from Matzrafi et al., 2020 (https://www.biorxiv.org/content/10.1101/2020.07.04.182733v1).

    When treated with alternative postemergence herbicides (glyphosate, paraquat, or sethoxydim), no plants of either the parental or successive offspring populations survived treatment with 0.75X or higher rates of these herbicides (see Matzrafi et al., 2020 (https://www.biorxiv.org/content/10.1101/2020.07.04.182733v1).

    The magnitude of increases in resistance levels over three generations of recurrent low-rate glufosinate selection observed is relatively low compared with higher levels of resistance observed in response to low-rate selection with other herbicides (three-fold and greater). However, under field conditions, even low levels of resistance within weed populations may reduce control. This study shows that repeated selection with glufosinate at low rates can reduce the susceptibility of Italian ryegrass populations to glufosinate, and points to the importance of incorporating a diversity of approaches, both chemical and non-chemical, in the management of ryegrass in annual and perennial cropping systems of California. – By Marie Jasieniuk & Maor Matzrafi, UC Weed Science