Russell L. Groves

The Role of Weed Hosts and Tobacco Thrips, Frankliniella fusca, in the Epidemiology of Tomato spotted wilt virus

Wild plant species were systematically sampled to characterize reproduction of thrips, the vector of Tomato spotted wilt virus (TSWV), and natural sources TSWV infection. Thrips populations were monitored on 28 common perennial, biennial, and annual plant species over two noncrop seasons at six field locations across North Carolina. Sonchus asper, Stellaria media, and Taraxacum officianale consistently supported the largest populations of immature TSWV vector species. The tobacco thrips, Frankliniella fusca, was the most abundant TSWV vector species collected, comprising over 95% of vector species in each survey season. Perennial plant species (i.e., Plantago rugelii and Taraxacum officianale) were often only locally abundant, and many annual species (Cerastium vulgatum, Sonchus asper, and Stellaria media) were more widely distributed. Perennial species, including P. rugelii and Rumex crispus, remained TSWV infected for 2 years in a small-plot field test. Where these perennial species are locally abundant, they may serve as important and long-lasting TSWV inoculum sources. In random surveys across 12 locations in North Carolina, TSWV infection was documented by double antibody sandwich enzyme-linked immunosorbent assay in 35 of 72 (49%) common perennial (N = 10), biennial (N = 4), and annual (N = 21) plant species across 18 plant families. Estimated rates of TSWV infection were highest in Cerastium vulgatum (4.2%), Lactuca scariola (1.3%), Molluga verticillata (4.3%), Plantago rugelii (3.4%), Ranunculus sardous (3.6%), Sonchus asper (5.1%), Stellaria media (1.4%), and Taraxacum officianale (5.8%). Nine plant species were determined to be new host recordings for TSWV infection, including Cardamine hirsuta, Eupatorium capillifolium, Geranium carolinianum, Gnaphalium purpureum, Linaria canadense, Molluga verticillata, Pyrrhopappus carolinianus, Raphanus raphanistrum, and Triodanis perfoliata. Our findings document the relative potential of a number of common annual, biennial, and perennial plant species to act as important reproductive sites for F. fusca and as acquisition sources of TSWV for spread to susceptible crops.

Two Xylella fastidiosa genotypes associated with almond leaf scorch disease on the same location in California

ABSTRACT Almond leaf scorch disease (ALSD) has recently reemerged in the San Joaquin Valley of California threatening almond production. ALSD is caused by Xylella fastidiosa, a nutritionally fastidious bacterium. Single nucleotide polymorphisms (SNPs) in the 16S rRNA gene (16S rDNA) of X. fastidiosa strains were identified to characterize the bacterial population in infected trees. Genotype-specific SNPs were used to design primers for multiplex polymerase chain reaction assays of early passage cultures. Two genotypically distinct types of X. fastidiosa strains, G-type and A-type, coexist simultaneously in the same infected almond orchard. This was substantiated by restriction fragment length polymorphism analysis of a different genetic locus, RST31-RST33, which has previously been used to identify and differentiate X. fastidiosa strains. Furthermore, unique bacterial colony morphology was consistently associated with the A-type X. fastidiosa strains. To our knowledge, this is the first report of a mixed genotype infection of X. fastidiosa disease on the same location under natural environmental conditions. The concept of mixed genotype infection could affect the current epidemiological study based on the assumption that one genotype causes ALSD on one location and, therefore, the disease management strategy.

Overwintering of Frankliniella fusca (Thysanoptera: Thripidae) on Winter Annual Weeds Infected with Tomato spotted wilt virus and Patterns of Virus Movement Between Susceptible Weed Hosts.

ABSTRACT Overwintering of tobacco thrips, Frankliniella fusca, was investigated on common winter annual host plants infected with Tomato spotted wilt virus (TSWV). Populations of tobacco thrips produced on TSWV-infected plants did not differ from those produced on healthy plants, whereas populations varied greatly among host plant species. The mean per plant populations of F. fusca averaged 401, 162, and 10 thrips per plant on Stellaria media, Scleranthus annuus, and Sonchus asper, respectively, during peak abundance in May. Adult F. fusca collected from plant hosts were predominately brachypterous throughout the winter and early spring, but macropterous forms predominated in late spring. Weed hosts varied in their ability to serve as overwintering sources of TSWV inoculum. Following the initial infection by TSWV in October 1997, 75% of Scleranthus annuus and Stellaria media retained infection over the winter and spring season, whereas only 17% of Sonchus asper plants remained infected throughout the same interval. Mortality of TSWV-infected Sonchus asper plants exceeded 25%, but mortality of infected Stellaria media and Scleranthus annuus did not exceed 8%. TSWV transmission by thrips produced on infected plants was greatest on Stellaria media (18%), intermediate on Scleranthus annuus (6%), and lowest on Sonchus asper (2%). Very few viruliferous F. fusca were recovered from soil samples collected below infected wild host plants. Vegetative growth stages of Stellaria media, Sonchus asper, and Ranunculus sardous were more susceptible to F. fusca transmission of TSWV than flowering growth stages, whereas both growth stages of Scleranthus annuus were equally susceptible. In a field study to monitor the spatial and temporal patterns of virus movement from a central source of TSWV-infected Stellaria media to adjacent plots of R. sardous, the incidence of infection in R. sardous plots increased from <1% in March to >42% in June 1999. Infection levels in the Stellaria media inoculum source remained high throughout the experiment, averaging nearly 80% until June 1999 when all Stellaria media plants had senesced. Dispersal of TSWV from the inoculum source extended to the limits of the experimental plot (>37 m). Significant directional patterns of TSWV spread to the R. sardous plots were detected in April and May but not in June. R. sardous infections were detected as early as March and April, suggesting that overwintering inoculum levels in an area can increase rapidly during the spring in susceptible weed hosts prior to planting of susceptible crops. This increase in the abundance of TSWV inoculum sources occurs at a time when vector populations are increasing rapidly. The spread of TSWV among weeds in the spring serves to bridge the period when overwintered inoculum sources decline and susceptible crops are planted.

Identifying the predator complex of Homalodisca vitripennis (Hemiptera: Cicadellidae): a comparative study of the efficacy of an ELISA and PCR gut content assay

A growing number of ecologists are using molecular gut content assays to qualitatively measure predation. The two most popular gut content assays are immunoassays employing pest-specific monoclonal antibodies (mAb) and polymerase chain reaction (PCR) assays employing pest-specific DNA. Here, we present results from the first study to simultaneously use both methods to identify predators of the glassy winged sharpshooter (GWSS), Homalodisca vitripennis (Germar) (Hemiptera: Cicadellidae). A total of 1,229 arthropod predators, representing 30 taxa, were collected from urban landscapes in central California and assayed first by means of enzyme-linked immunosorbent assay (ELISA) using a GWSS egg-specific mAb and then by PCR using a GWSS-specific DNA marker that amplifies a 197-base pair fragment of its cytochrome oxidase gene (subunit I). The gut content analyses revealed that GWSS remains were present in 15.5% of the predators examined, with 18% of the spiders and 11% of the insect predators testing positive. Common spider predators included members of the Salticidae, Clubionidae, Anyphaenidae, Miturgidae, and Corinnidae families. Common insect predators included lacewings (Neuroptera: Chrysopidae), praying mantis (Mantodea: Mantidae), ants (Hymenoptera: Formicidae), assassin bugs (Hemiptera: Reduviidae), and damsel bugs (Hemiptera: Nabidae). Comparison of the two assays indicated that they were not equally effective at detecting GWSS remains in predator guts. The advantages of combining the attributes of both types of assays to more precisely assess field predation and the pros and cons of each assay for mass-screening predators are discussed.

Temperature and Precipitation Affect Seasonal Patterns of Dispersing Tobacco Thrips, Frankliniella fusca, and Onion Thrips, Thrips tabaci (Thysanoptera: Thripidae) Caught on Sticky Traps

Abstract Effects of temperature and precipitation on the temporal patterns of dispersing tobacco thrips, Frankliniella fusca, and onion thrips, Thrips tabaci, caught on yellow sticky traps were estimated in central and eastern North Carolina and eastern Virginia from 1997 through 2001. The impact that these environmental factors had on numbers of F. fusca and T. tabaci caught on sticky traps during April and May was determined using stepwise regression analysis of 43 and 38 site-years of aerial trapping data from 21 and 18 different field locations, respectively. The independent variables used in the regression models included degree-days, total precipitation, and the number of days in which precipitation occurred during January through May. Each variable was significant in explaining variation for both thrips species and, in all models, degree-days was the single best explanatory variable. Precipitation had a comparatively greater effect on T. tabaci than F. fusca. The numbers of F. fusca and T. tabaci captured in flight were positively related to degree-days and the number of days with precipitation but negatively related to total precipitation. Combined in a single model, degree-days, total precipitation, and the number of days with precipitation explained 70 and 55% of the total variation in the number of F. fusca captured from 1 April through 10 May and from 1 April through 31 May, respectively. Regarding T. tabaci flights, degree-days, total precipitation, and the number of days with precipitation collectively explained 57 and 63% of the total variation in the number captured from 1 April through 10 May and from 1 April through 31 May, respectively.

Environmental fate of soil applied neonicotinoid insecticides in an irrigated potato agroecosystem.

Since 1995, neonicotinoid insecticides have been a critical component of arthropod management in potato, Solanum tuberosum L. Recent detections of neonicotinoids in groundwater have generated questions about the sources of these contaminants and the relative contribution from commodities in U.S. agriculture. Delivery of neonicotinoids to crops typically occurs as a seed or in-furrow treatment to manage early season insect herbivores. Applied in this way, these insecticides become systemically mobile in the plant and provide control of key pest species. An outcome of this project links these soil insecticide application strategies in crop plants with neonicotinoid contamination of water leaching from the application zone. In 2011 and 2012, our objectives were to document the temporal patterns of neonicotinoid leachate below the planting furrow following common insecticide delivery methods in potato. Leaching loss of thiamethoxam from potato was measured using pan lysimeters from three at-plant treatments and one foliar application treatment. Insecticide concentration in leachate was assessed for six consecutive months using liquid chromatography-tandem mass spectrometry. Findings from this study suggest leaching of neonicotinoids from potato may be greater following crop harvest in comparison to other times during the growing season. Furthermore, this study documented recycling of neonicotinoid insecticides from contaminated groundwater back onto the crop via high capacity irrigation wells. These results document interactions between cultivated potato, different neonicotinoid delivery methods, and the potential for subsurface water contamination via leaching.

Integrated Control of Potato Pathogens Through Seed Potato Certification and Provision of Clean Seed Potatoes

Long-term data sets are rare in agriculture, and the impact of plant diseases on food production is challenging to measure, which makes it difficult to assess the impact of policy changes or research-based disease control efforts. Despite this challenge, it is clear that one of the largest impacts of biological research on food security over the past century has been in production of vegetatively propagated fruit and vegetable crops such as potato. The yield and quality of these crops is higher in countries that have effective plant propagation and certification systems. Of these systems, seed potato production and certification is among the most developed. We analyzed a dataset from a century-old seed potato certification program in Wisconsin to assess the efficacy for potato disease control and the cost of this program compared to other disease control and potato production costs. We found that over the past century, certification has gradually reduced the incidence of mechanically transmitted vascular potato pathogens that lack insect vectors to undetectable levels, and much of this reduction occurred prior to the use of tissue culture and the development of immunoassays. Rejection of seed lots from certification is now rare, with Potato virus Y (PVY), a virus spread nonpersistently by numerous, noncolonizing aphid species, and farmer errors being the main causes of rejection. PVY level increases occurred in 2000, coincident with the first detection of a new invasive vector, soybean aphid, in the Midwest. The increased PVY incidence was more pronounced in varieties that exhibit mild foliar symptoms. Starting in 2004, a decrease in PVY incidence occurred following comprehensive science-based changes to early generation seed potato production. The cost of the certification program has not increased in two decades, and the fees charged are comparable to those in 1913. The cooperative nature of the seed potato certification program has contributed to its sustainability across generations. However, looming soilborne disease problems are not easily addressed by certification and will likely cause significant challenges in the future.

Seasonal patterns of adult thrips dispersal and implications for management in eastern Virginia tomato fields

Seasonal flight activity of thrips was examined in commercial tomato fields, Lycopersicon esculentum L., on Virginia’s (USA) Eastern Shore in 2000 and 2001. In each of three regions along the Shore, populations of adult thrips infesting tomato flowers and dispersing within tomato fields were monitored weekly. Frankliniella fusca (Hinds) was the only thrips species captured that is currently considered as a competent vector of Tomato spotted wilt virus (TSWV) in most of the mid-Atlantic US region. Seasonal patterns of F. fusca capture were dissimilar among tomato fields across all regions; yet, more F. fusca were captured between midMay and mid-June in all regions compared with those captured between transplanting and mid-May each year. Despite the relatively low observed dispersal activity of F. fusca before mid-May, the threat of TSWV transmission warrants protection of the crop from immigrating F. fusca from transplanting until the end of marketable fruit set. Frankliniella tritici (Lindeman) was the most frequently encountered thrips species dispersing in tomato fields and infesting tomato flowers. Seasonal patterns of F. tritici capture were similar among tomato fields in all regions in 2000, but were dissimilar among regions in 2001. Dispersal of F. tritici was most pronounced between mid-May and mid-June across all fields in each year. Because F. tritici was the dominant species recovered from tomato flowers, it is likely responsible for cosmetic injury to tomato fruit and should be managed to reduce fruit injury when infestations are highest, between mid May and mid June. r 2003 Elsevier Science Ltd. All rights reserved.

Soft Rot Disease Severity Is Affected by Potato Physiology and Pectobacterium taxa

Pectobacterium species cause disease worldwide in many crop and ornamental plants, including potato. A new Pectobacterium subspecies, P. carotovorum subsp. brasiliensis was recently described in Brazil and later found in the United States, Israel, and South Africa. Its virulence traits and host range remain unknown. A comparison of three taxa commonly found on potato showed that both P. carotovorum subsp. carotovorum and subsp. brasiliensis are more aggressive in causing tuber and stem soft rot than P. atrosepticum. Also, despite bacterial growth inhibition in vitro of P. carotovorum subsp. carotovorum and P. atrosepticum strains by P. carotovorum subsp. brasiliensis, this new subspecies and P. carotovorum subsp. carotovorum are able to co-colonize in the same infected tissue. Both subspecies were motile in lesions. Pathogenesis assays showed that host ranges of all three overlap, but are not identical. The host ranges of individual strains of P. carotovorum subsp. carotovorum and subsp. brasiliensis are limited, whereas P. atrosepticum can macerate many plant species in addition to potato. There was high variability in virulence assays with potato tuber; thus physiological factors were investigated. Tuber size, maturity, and field location had significant effects on susceptibility to soft rot, with larger, more mature tubers being more susceptible.

Distribution and Management of Citrus in California: Implications for Management of Glassy-Winged Sharpshooter

Abstract The epidemiology of Pierce’s disease of grape (Vitis spp.) in California has changed over the past 10 yr due to the introduction of an exotic vector, Homalodisca vitripennis (Germar), the glassy-winged sharpshooter. Although this insect is highly polyphagous, citrus (Citrus spp.) is considered a preferred host and proximity to citrus has been implicated as a significant risk factor in recent epidemics of Pierce’s disease in southern California. Consequently, a detailed knowledge of the distribution and management of citrus in relation to grape is needed to improve insect and disease management. Analysis of data on the area planted to these two commodities indicates that only five counties in California concomitantly grow >1,000 ha of grape and >1,000 ha of citrus: Riverside, Kern, Tulare, Fresno, and Madera counties. Comparison of the distribution of grape and citrus within each of these counties indicates that the percentage of grape that is in proximity to citrus is greatest for Riverside County, but the total area of grape that is in proximity to citrus is greater for Fresno, Kern, and Tulare counties. The use of carbamates, neonicotinoids, organophosphates, and pyrethroids as part of the citrus pest management program for control of key insect pests was compared among the same five counties plus Ventura County from 1995 to 2006. Ventura County was included in this analysis as this county grows >10,000 ha of citrus and has established glassy-winged sharpshooter populations. The use of these broad-spectrum insecticides was lowest in Riverside and Ventura counties compared with the other four counties. Analysis of historical trapping data at the county scale indicates a negative association of broad-spectrum insecticide use with glassy-winged sharpshooter abundance. These results are used to retrospectively analyze the Pierce’s disease outbreaks in Kern and Riverside counties.