Project : USDA ARS

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Research Project: Production and Disease and Pest Management of Horticultural Crops

Location: Southern Horticultural Research Unit

2019 Annual Report

Objectives
Objective 1. Develop integrated strategies to control invasive diseases and pests within the context of small fruit production systems of the Gulf Coast. Sub-objective 1.1. Determine the importance of wild fruit hosts to the ecology and life history traits of Spotted-wing Drosophila (SWD) and other vinegar fly pests of fruit crops of the U.S. Gulf Coast, with an emphasis on fly population dynamics in surrounding landscapes. Sub-objective 1.2. Develop and evaluate Integrated Pest Management (IPM) strategies involving selective chemical application. Sub-objective 1.3. Determine pathogen lifecycle events and spread patterns of bacterial leaf scorch, a new and emerging disease of blueberries in the Gulf South. Sub-objective 1.4. Develop disease screening assays using traditional and molecular screening techniques to identify disease resistant small fruit germplasm and characterize relevant host/pathogen relationships, the influence of cultural practices, and virulence levels of pathogens. Sub-objective 1.5. Develop disease control protocols based on cultural practices of removing rosette infected primocanes, which are a source of fungal inoculum, to reduce rosette disease severity in erect blackberry cultivars. Objective 2. Develop disease and pest control strategies that can be readily integrated with existing production practices used in container-grown ornamental plant nursery systems. Sub-objective 2.1. Develop a three-step ‘push-pull’ management strategy for protecting vulnerable nursery tree stock from ambrosia beetles. Sub-objective 2.2. Examine the effect of binding and leaching potential of soil-incorporated insecticides in alternative and standard soilless substrates in container-grown ornamental plants. Sub-objective 2.3. Determine optimal timing of disinfestant to restrict pathogen dispersal through irrigation water and limit plant disease. Sub-objective 2.4. Develop a comprehensive preventive and reactive disease management strategy to control Pseudomonas, Colletotrichum, and Rhizoctonia in plant propagation facilities. Sub-objective 2.5. Develop an integrated disease management strategy to control Leyland cypress blight in ornamental plant nursery production. Sub-objective 2.6. Identify changes in spray patterns across 100 foot blocks of container-grown plants using commercial sprayer equipment that correlate with reduction in disease intensity. Objective 3. Develop and improve pollination practices on berry and vegetable farms along the Gulf Coast and increase capability to use native bees.

Approach
Develop an updated pest management program to control the spotted-wing Drosophila fly from damaging fruits and vegetables. Develop cultural and chemical controls and tolerant cultivars of several small fruit diseases, with emphasis on Phytophthora root rot, a serious existing disease, and Xylella bacterial leaf scorch, a new disease of blueberry. Identify habitat sources of ambrosia beetle, and characterize repellant and attractant strategies that prevent ambrosia beetle movement into ornamental plant nurseries. Develop updated plant disease management practices to control existing and new pathogens in propagation, to time disinfestant treatments that prevent spread of Phytophthora in irrigation water, and to produce a risk-based fungicide timing model to control Passalora blight of Leyland cypress in the nursery. Nesting habitat for native pollinators will be promoted to expand bee management practices that are critical for achieving profitable fruit and vegetable yields.

Progress Report
This is a bridging project which continues research from Project number 6062-21430-002-00D, which is undergoing OSQR review. In 2019, ARS entomologists at the Thad Cochran Southern Horticultural Laboratory in Poplarville, Mississippi and the Horticultural Crops Research Laboratory in Corvallis, Oregon discovered that some wild fruit blueberry hosts, especially species found within the pest’s native range as well as select blueberry cultivars, showed resistance to spotted-wing Drosophila (SWD) attack. Smaller-sized wild fruit and very large-sized cultivated fruits displayed the most resistance. The research revealed SWD can use both optimal and sub-optimal habitats (host berries) for laying eggs at some cost to their reproductive success. Potentially, new varieties of blueberry will be released with some resistance to SWD attack. An ARS entomologist at the Thad Cochran Southern Horticultural Laboratory in Poplarville, Mississippi and an ARS chemist at the Southern Regional Research Center in New Orleans, Louisiana have shown that erythritol, an affordable-sugar alcohol, acts as a synergist by doubling the effectiveness of a commercial microbial-based biopesticide in controlling SWD under laboratory and field studies. An ARS entomologist at the Thad Cochran Southern Horticultural Laboratory in Poplarville, Mississippi and an ARS plant pathologist and chemist at the Natural Products Utilization Research Laboratory in Oxford, Mississippi completed essential oil screenings and found that extracts from two species of abundant New World Jatropha (belly plants) displayed better control of ornamental insect pests than commercially available neem oil or malathion. These findings suggest that these plant species may be potential sources of new biopesticides. In 2019, ARS plant pathologists at the Thad Cochran Southern Horticultural Laboratory in Poplarville, Mississippi and the Genetic Improvement for Fruits and Vegetables Laboratory in Chatsworth, New Jersey genetically characterized blueberry clones for resistance to stem blight and blueberry leaf rust and clarified pathogen identification. An ARS plant pathologist and plant geneticist at the Thad Cochran Southern Horticultural Laboratory in Poplarville, Mississippi developed protocols for screening strawberry seedlings for resistance to anthracnose disease using traditional and molecular approaches. The 66 strawberry breeding clones expressed similar disease resistance responses to each pathogen species, even though one Colletotrichum species was more aggressive. It was determined that two molecular markers previously reported to indicate resistance to one species of the pathogen were only moderately effective at identifying resistance to the other two Colletotrichum species. Additionally, visual ratings of disease severity were found to be more quickly assessed and as effective and reliable as ratings based on computer image analysis. ARS plant pathologists at the Thad Cochran Southern Horticultural Laboratory in Poplarville, Mississippi evaluated protocols for control of blackberry rosette disease in erect blackberry cultivars based on removal of rosette infected primocanes (the initial source of fungal inoculum). This year, fruit was plentiful on both cultivars and rosette incidence was heavy. Data severity is being analyzed. In 2019, ARS entomologists at the Thad Cochran Southern Horticultural Laboratory in Poplarville, Mississippi have shown that intercept traps placed around the perimeter of a tree nursery and from 0 to 20 m into the field can capture and kill 80 to 90% of female ambrosia beetles before they have an opportunity to attack host trees and infect them with deadly symbiotic fungi. In 2019, an ARS plant pathologist at the Thad Cochran Southern Horticultural Laboratory in Poplarville, Mississippi evaluated efficacy of disinfestants and copper to eliminate epiphytic populations of Pseudomonas amygdali, the cause of Loropetalum bacterial gall, from production surfaces. Sodium hypochlorite (bleach) and potassium peroxymonosulfate eliminated the bacterium from all production surfaces, while two quaternary ammonium products only eliminated it from metal surfaces, such as pruning tools. The efficacy and phytotoxicity of these products on Loropetalum stem cuttings are being evaluated. An ARS plant pathologist and plant geneticist at the Thad Cochran Southern Horticultural Laboratory in Poplarville, Mississippi and molecular biologist at the National Peanut Research Laboratory in Dawson, Georgia had the genome of Passalora sequoiae, the cause of Leyland Cypress blight, sequenced using the PacBio and Illumina platforms. An annotated whole genomic sequence has been assembled in collaboration with plant pathologists at Colorado State University. Annotation is being used to identify genes associated with virulence, aggressiveness and fungicide resistance. An ARS plant pathologist at the Thad Cochran Southern Horticultural Laboratory in Poplarville, Mississippi delayed work to compare chlorine dioxide activity under different water quality conditions to control Phytophthora nicotianae in irrigation water due to a critical vacancy, however, the work is scheduled to be completed by early 2020.

Accomplishments
1. Honey bees put up a good fight against Selenium. Honey bees are known to be susceptible to elevated levels of naturally occurring Selenium (Se) in the environment. An ARS researcher in Poplarville, Mississippi, along with University of Southern Mississippi partners, investigated how two inorganic forms of Se effected their biological traits, oxidative stress, and gene regulation. They found that honey bees were negatively affected by small increases of Se in their diet (0.6–6 µg Se /mL) but bee mortality increased at 6 µg Se /mL. The highest accumulation of Se residue that the bees could tolerate was 0.12 µg Se /bee. Selenium induces oxidative stress in honey bees that leads to protein damage at high concentrations and in turn can be fatal. In an effort to alleviate oxidative stress, honey bees upregulated considerable sets of antioxidant genes, including uncharacterized “selenoprotein genes” and more particularly a Selenium binding protein gene required for the translation of selenoproteins mRNA. This information will help scientists and beekeepers make informed decisions on what is best for honey bees and possibly hive placement and location as it relates to Selenium in the environment.

2. The developmental stage of honey bees affects their susceptibility to pesticides and pests. ARS researchers in Poplarville, Mississippi, along with Mississippi State University and University of Ljubljana partners, investigated the combined effects of a neonicotinoid (imidacloprid) pesticide on developing honey bees that were later infested with the parasitic mite, Varroa destructor. Our results show that when a realistic field dose of imidacloprid was consumed in the larval stage, the immune response decreased in the early developmental stages of the honey bee then increased in the later developmental stages. In addition, the gene expression patterns of Varroa infested honey bees changed during their development, with an increasing number of genes being differentially expressed from the pupae to very young adult stage. Both stressors affected the antimicrobial peptides and genes involved in their defensive behavior and that also varied in different developmental stages. Thus, changes in the immune-related gene expressions in honey bees exposed to imidacloprid and Varroa mites alone or combined differed in each of the three developmental stages. These finding indicate that an extensive diversity of stress factors can interfere with the immune system of developing honey bees.

3. Pesticide interactions can have unforeseen health effects on honey bees. Insecticides, particularly neonicotinoids, are implicated in global honey bee decline. However, honey bee health can be more complicated than simple facts show. Coumaphos, an organophosphate, is a miticide used to control Varroa mites in honey bee colonies, but it can stress bees sufficiently to increase mortality. Paradoxically, imidacloprid, a neonicotinoid, at sublethal rates can suppress a bee’s appetite enough to reduce its uptake of food and of coumaphos, which inadvertently results in worker bee's living longer. ARS researchers in Poplarville, Mississippi, along with researchers at Mississippi State University and the University of Southern Mississippi examined the effects of imidacloprid and coumaphos on honey bee lifespan, food consumption, mortality, and expression of antioxidant genes. Newly emerged worker bees were exposed to either coumaphos or neonicotinoid alone or in-combination. The reduced appetite in honey bees was linked to midgut cell damage, which may be beneficial in reducing the spread of toxins within a bee’s body. These findings reveal unforeseen interactions between two pesticides that honey bees are likely to encounter in the field and hive. This information highlights the need to understand the role of insecticides in bee decline, as it is more complicated than once thought.

4. Identification of molecular markers in strawberry associated with resistance to anthracnose. Three fungal species of Colletotrichum cause devasting anthracnose diseases of strawberry. As strawberry plant breeders seek to develop anthracnose resistant cultivars, they often rely on molecular markers to identify resistant germplasm. ARS and Mississippi State University scientists in Poplarville, Mississippi, investigated the utility of two previously reported molecular markers and found that one had marginally more significant effects on anthracnose resistance caused by one species of the anthracnose pathogen. This knowledge will be used by strawberry breeders when selecting molecular markers to identify plants with anthracnose resistance genes in their breeding programs.

5. A push-pull approach to stop ambrosia beetle entry into tree nurseries. Non-native ambrosia beetles are destructive tree pests in ornamental nurseries and fruit orchards. Females vector fungi that even in small quantities will infect, grow, and invariably kill whole trees, which is a great financial loss to producers. ARS researchers in Poplarville, Mississippi and Wooster, Ohio along with University cooperators at Tennessee State University and Virginia Polytechnic Institute and State University demonstrated that adult female ambrosia beetles are repelled by a plant volatile called verbenone and strongly attracted to ethanol. This “push–pull” strategy uses verbenone emitters to “push” beetles away from vulnerable trees and ethanol lures to “pull” them into liquid traps. The ethanol-baited traps provided the strongest protection of nursery grown trees by intercepting as many as 80-90% of the ambrosia beetles. Perimeter traps in combination with locating susceptible trees 50 m or more from the edge of surrounding ambrosia beetle habitats could reduce beetle populations migrating into the nursery and hence the likelihood of fungal infections in vulnerable tree species.

6. Characterization of the response of blueberry accessions to leaf rust. Leaf rust is one of the most important diseases of blueberry worldwide. To discover new sources of resistance to the leaf rust pathogen, ARS scientists in Poplarville, Mississippi tested the susceptibility of various southern highbush accessions, interspecific hybrids, and accessions from wild blueberry species to an isolate of the blueberry leaf rust pathogen. Two southern highbush accessions showed resistance to the pathogen whereas the accessions of two wild species (sparkleberry and Darrow's blueberry) displayed a high level of resistance to the pathogen. This knowledge will be used by blueberry breeders to select plants to include in their breeding programs.

7. Pathogenicity of stem blight fungal isolates on blueberry. Stem blight, caused by species of Botryosphaeria and Neofusicoccum, is one of the more common diseases limiting the establishment of blueberry planting in southeastern USA. Accurate identification of these fungal species is essential for development of effective disease management practices. ARS scientists, located in Poplarville, Mississippi, used a multigene sequencing strategy to distinguish between six stem blight isolates collected from two regions of the USA. A temperature growth study revealed that the optimum temperature for growth of five of the tested isolates ranged from 25 to 30ºC, and in vitro fungicide assays showed four fungicides to be effective against tested isolates. A detached stem assay indicated that none of the 39 tested blueberry accessions displayed a high level of resistance to the two tested isolates, and no significant difference in lesion length was detected among the seven Vaccinium species inoculated with the two isolates. Further research is needed to test the efficacy of these fungicides in the field. The knowledge generated in this study should lead to better management of stem blight in blueberry orchards and ultimately to more abundant and higher quality blueberry for the American consumer.

U.S. DEPARTMENT OF AGRICULTURE

Southern Horticultural Research Unit: Poplarville, MS