Bacterial endosymbionts identified from leafhopper (Hemiptera: Cicadellidae) vectors of phytoplasmas

Insects often harbor bacterial endosymbionts that provide them with nutritional benefit or with protection against natural enemies, plant defenses, insecticides, and abiotic stresses. Certain endosymbionts may also alter acquisition and transmission of plant pathogens by insect vectors. We identified bacterial endosymbionts from four leafhopper vectors (Hemiptera: Cicadellidae) of ‘Candidatus Phytoplasma’ species by direct sequencing 16S rDNA and confirmed endosymbiont presence and identity by species-specific conventional PCR. We examined three vectors of Ca. Phytoplasma pruni, causal agent of cherry X-disease [Colladonus geminatus (Van Duzee), Colladonus montanus reductus (Van Duzee), Euscelidius variegatus (Kirschbaum)] – and a vector of Ca. Phytoplasma trifolii, the causal agent of potato purple top disease [Circulifer tenellus (Baker)]. Direct sequencing of 16S identified the two obligate endosymbionts of leafhoppers, ‘Ca. Sulcia’ and ‘Ca. Nasuia’, which are known to produce essential amino acids lacking in the leafhoppers’ phloem sap diet. About 57% of C. geminatus also harbored endosymbiotic Rickettsia. We identified ‘Ca. Yamatotoia cicadellidicola’ in Euscelidius variegatus, providing just the second host record for this endosymbiont. Circulifer tenellus harbored the facultative endosymbiont Wolbachia, although the average infection rate was only 13% and all males were Wolbachia-uninfected. A significantly greater percentage of Wolbachia-infected Ci. tenellus adults than uninfected adults carried Ca. P. trifolii, suggesting that Wolbachia may increase this insect’s ability to tolerate or acquire this pathogen. Results of our study provide a foundation for continued work on interactions between leafhoppers, bacterial endosymbionts, and phytoplasma

Commercial sugar beet cultivars evaluated for rhizomania resistance and storability in Idaho, 2018

Rhizomania caused by Beet necrotic yellow vein virus (BNYVV) and storage losses are serious sugar beet production problems. To identify sugar beet cultivars with resistance to BNYVV and evaluate storability, 30 commercial cultivars were screened by growing them in a sugar beet field infested with BNYVV in Kimberly, ID during the 2018 growing season in a randomized complete block design with 6 replications. At harvest on 26-27 September 2018, roots were dug and evaluated for symptoms of rhizomania and also placed in an indoor commercial sugar beet storage building. After 136 days in storage, samples were evaluated for surface rot, weight loss, and sucrose loss. Surface root rot ranged from 17 to 81%, weight loss ranged from 22 to 32%, sucrose losses ranged from 44 to 87%, and estimated recoverable sucrose ranged from 803 to 7,207 lb/A. Given these response ranges, selecting cultivars for rhizomania resistance and combining this resistance with storability will lead to considerable economic benefit for the sugar beet industry

What is new with rhizomania and curly top management and effects of these viruses on storage

Rhizomania is a serious yield limiting viral disease in sugarbeet first identified in California, USA in 1984. The disease has since spread to all major production areas in the United States. Rhizomania is caused by Beet necrotic yellow vein virus (BNYVV) and vectored by the plasmodiophorid (a fungal-like organism), Polymyxa betae. The virus survives inside the thick-walled resting spore of the vector in the soil, which can remain viable for many years. As a result, once a field is infested, using crop rotation and non-host crops will not be effective for controlling the disease. In the spring with near saturated soil conditions, the resting spore will germinate to release zoospores when in close proximity to sugarbeet roots. The zoospores will attach to the root and transmit the virus to the sugarbeet root. Therefore, the primary control measure will be to grow a sugarbeet cultivar with high resistance to BNYVV. Cultivars with the Rz1 resistance gene are available, but resistance breaking strains of the virus have been found in CA, CO, ID, MN, and OR. In the disease screening nursery in Kimberly, ID, check cultivars with just the Rz2 gene for resistance frequently exhibit symptoms on 10 to 40% of the plants, while cultivars with just the Rz1 gene tend to have just occasional blinking plants (plants with yellow narrow upright leaves). In the nursery and commercial fields, the Rz1 gene seems to be necessary to maintain an acceptable level of resistance, even if the cultivars contain other sources of resistance. Since resistance breaking strains are known to occur and resistance genes only allow for partial resistance to this disease, it would be wise to grow cultivars with additional sources of resistance to help protect Rz1. If inoculum levels are quite high, resistance in the cultivars may breakdown. Another control measure to consider would be to reduce irrigation frequency, so the soil surface dries between irrigations. Genetic engineering approaches have been studied for rhizomania, so hopefully in the near future cultivars with resistance based on transgenic strategies will become available. Curly top is another serious yield limiting viral disease for sugarbeet grown in semiarid production areas in the United States. Curly top is vectored by the beet leafhopper and can be caused by a number of Curtovirus species: Beet severe curly top virus (BSCTV; formerly CFH strain), Beet mild curly top virus (BMCTV; formerly Worland strain), and Beet curly top virus (BCTV; formerly Cal/Logan strain). Other Curtovirus species have been documented or at least proposed in recent years and some have been shown to occur on sugarbeet in other countries. A survey of the western United States showed BSCTV, BMCTV, and BCTV were present in sugarbeet (6). However, samples in this survey along with some collected in 2012 in Idaho show a virus different from these three was also present in sugarbeet. This “unknown” virus amplifies with the coat protein primers, but does not amplify with the species specific primers. This “unknown” virus is currently being investigated further at the USDA-ARS laboratory in Kimberly, ID. The curly top virus species are carried between growing seasons by adult female beet leafhoppers that overwinter on weeds in desert areas and poorly managed residential areas. The females lay eggs in the spring, leading to the start of approximately three generations under Idaho conditions. When the winter host plants desiccate in the spring, the beet leafhoppers move into crop areas carrying the curly top viruses. Most commercial sugarbeet cultivars in the western United States carry partial resistance to the curly top viruses, but the low to intermediate resistance carried by the cultivars tends not to be as protective prior to the eight-leaf growth stage. Thus, the earlier plants become infected the higher the yield loss. In-furrow, foliar, and seed-treatment insecticides have been used to supplement this host resistance. Based on studies by the USDA-ARS Kimberly sugarbeet program (1,9), the neonicotinoid seed treatments (Poncho, Cruiser, and NipsIt) were established as being effective at reducing curly top through early season control of the beet leafhopper vector. Root yield increases of 17% or more have been observed for Idaho (based on USDA-NASS statistics) and other states with semiarid production areas through the use of these neonicotinoid seed treatments (1,9). These seed treatments provide at least 59 days of beet leafhopper protection after planting along with early season control of leafminer and aphids (1,3,4,7,9). In 2012, research indicated that some labeled foliar insecticides (Asana® and Mustang®) may be used to potentially extend this coverage period. However, foliar insecticides would not be recommended to replace the seed treatments, because of their short efficacy period. Currently genetic engineering approaches are being developed for curly top, so in the future cultivars with resistance based on transgenic strategies may become available. In storage, sugarbeet roots have been shown to lose an average of 0.2 to 0.3 pounds of sucrose per ton of roots per day when stored under ambient conditions, but losses can be as high as 0.5 pounds depending on cultivar. Thus, cultivar selection for storage could improve storability and reduce sucrose losses in storage (5). However, pathogen (2,5,7,8), pest (3), and environmental influences (over or under watering, frost, etc.) during the growing season can also negatively impact root storability. In particular, rhizomania (5,8) and curly top (7) have been documented to negatively influence root storability. Also, placing roots infested with Rhizoctonia solani and bacteria such as Leuconostoc into storage piles has been shown to reduce sucrose in neighboring healthy roots (2). Thus, keeping sugarbeet plants healthy and as stress free as possible during the growing season will also reduce losses in storage

Experimental sugar beet cultivars evaluated for rhizomania resistance and storability in Idaho, 2018

Rhizomania caused by Beet necrotic yellow vein virus (BNYVV) and storage losses are serious sugar beet production problems. To identify sugar beet cultivars with resistance to BNYVV and evaluate storability, 12 experimental cultivars were screened by growing them in a sugar beet field infested with BNYVV in Kimberly, ID during the 2018 growing season in a randomized complete block design with 6 replications. At harvest on 26-27 September 2018, roots were dug and evaluated for symptoms of rhizomania and also placed in an indoor commercial sugar beet storage building. After 136 days in storage, samples were evaluated for surface rot, weight loss, and sucrose loss. Surface root rot ranged from 22 to 79%, weight loss ranged from 22 to 32%, sucrose losses ranged from 50 to 82%, and estimated recoverable sucrose ranged from 803 to 6,246 lb/A. Given these response ranges, selecting cultivars for rhizomania resistance and combining this resistance with storability will lead to considerable economic benefit for the sugar beet industry

Experimental sugar beet cultivars evaluated for rhizomania resistance and storability in Idaho, 2017

Rhizomania caused by Beet necrotic yellow vein virus (BNYVV) and storage losses are serious sugar beet production problems. To identify sugar beet cultivars with resistance to BNYVV and evaluate storability, 7 experimental cultivars were screened by growing them in a sugar beet field infested with BNYVV in Kimberly, ID during the 2017 growing season in a randomized complete block design with 6 replications. At harvest on 2-3 October 2017, roots were dug and evaluated for symptoms of rhizomania and also placed in an indoor commercial sugar beet storage building. After 147 days in storage, samples were evaluated for surface rot, weight loss, and sucrose loss. Surface root rot ranged from 16 to 85%, weight loss ranged from 17 to 28%, sucrose losses ranged from 25 to 87%, and estimated recoverable sucrose ranged from 596 to 8,518 lb/A. Given these response ranges, selecting cultivars for rhizomania resistance and combining this resistance with storability will lead to considerable economic benefit for the sugar beet industry

Improving Sugarbeet Storability

Storing sugarbeets, in piles, under ambient con- ditions, in production areas with mild climates, allows for longer and more productive factory campaigns. In southern Idaho, approximately one-third of the roots are directly processed, one-third are held in short-term storage, and one-third are held in long-term storage (greater than 90 days). Some beets in long-term storage will be held up to 150 days, leaving roots sus- ceptible to a number of negative influences. Extreme temperature fluctuations, excessive moisture, restricted air flow (snow, soil, weeds, and rotted roots), microbial development, res- piration rate, and buildup of impurities can all negatively impact sucrose recovery. In addition to disease and water-related problems in the field, wounding during harvest and transport will also negatively influence beet storability; therefore, saving sucrose in storage begins with cultivar selection for disease resistance and storability along with good field and harvest management

Commercial sugar beet cultivars evaluated for rhizomania resistance and storability in Idaho, 2019

Rhizomania caused by Beet necrotic yellow vein virus (BNYVV) and storage losses are serious sugar beet production problems. To identify sugar beet cultivars with resistance to BNYVV and evaluate storability, 37 commercial cultivars were screened by growing them in a sugar beet field infested with BNYVV in Kimberly, ID during the 2019 growing season in a randomized complete block design with 6 replications. At harvest on 7-8 October 2019, roots were dug and evaluated for symptoms of rhizomania and also placed in an indoor commercial sugar beet storage building. After 145 days in storage, samples were evaluated for surface rot, weight loss, and sucrose loss. Surface root rot ranged from 10 to 68%, weight loss ranged from 14 to 25%, sucrose losses ranged from 22 to 72%, and estimated recoverable sucrose ranged from 2,442 to 9,769 lb/A. Given these response ranges, selecting cultivars for rhizomania resistance and combining this resistance with storability will lead to considerable economic benefit for the sugar beet industry

Root rot in sugar beet piles at harvest

Sugar beet root rots are not only a concern because of reduced yields, but can also be associated with losses in storage. Our primary sugar beet root rot disease problem in the Amalgamated production area is Rhizoctonia root rot. However, this rot frequently only penetrates a short distance past the surface of the root before a bacterial complex stops the fungus and continues the rot process. This rot complex leads to direct yield loss at harvest time along with additional costs in factory processing. When rotted roots make it into storage piles, they have been shown to compromise surrounding healthy roots. A recent end-of-harvest storage pile survey of 74 to 76% of the piles at receiving stations in Treasure Valley and Magic Valley has identified rotted roots entering storage. In Treasure Valley, the number of piles in the High category was almost cut in half, while the number of piles in the Low category was more than doubled. In the Magic Valley, the number on piles in the High category was eliminated and those in the Intermediate category were reduced by 66%. Thus, it appears that the efforts at harvest to keep rotted roots out of piles were successful. Controlling root rots in the field improves yield, but keeping rotted roots out of storage should increase profits as well

A robust SNP-haplotype assay for Bct gene region conferring resistance to beet curly top virus in common bean (Phaseolus vulgaris L.)

Beet curly top virus (BCTV), which is synonymous with curly top virus (CTV), causes significant yield loss in common bean (snap and dry beans) cultivars and several other important crops. Common bean cultivars have been found to be resistant to CTV, but screening for resistance is challenging due to the cyclical nature of epidemics and spotty feeding by the leafhopper that vectors the virus. We used an SNP dataset for the Snap Bean Association Panel (SnAP) agroinoculated with CTV-Logan (CA/Logan) strain to locate the Bct gene region to a 1.7-Mb interval on chromosome Pv07 using genome-wide association study (GWAS) analysis. Recombinant lines from the SnAP were used to further narrow the Bct region to a 58.0-kb interval. A missense SNP (S07_2970381) in candidate gene Phvul.007G036300 Exonuclease V (EXO5) was identified as the most likely causal mutation, and it was the most significant SNP detected by GWAS in a dry bean population (DBP) naturally infected by the CTV-Worland (Wor) strain. Tmshift assay markers developed for SNP S07_2970381 and two linked SNPs, S07_2970276 and S07_2966197, were useful for tracking different origins of the Bct EXO5 candidate gene resistance to CTV in common bean. The three SNPs identified four haplotypes, with haplotype 3-1 (Haplo3-1) of Middle American origin associated with the highest levels of CTV resistance. This SNPhaplotype assay will enable breeders to track resistance sources and to develop cultivars with better CTV resistance

Commercial sugar beet cultivars evaluated for rhizomania resistance and storability in Idaho, 2021

Rhizomania caused by Beet necrotic yellow vein virus (BNYVV) and storage losses are serious sugar beet production problems. To identify sugar beet cultivars with resistance to BNYVV and evaluate storability, 26 commercial cultivars were screened by growing them in a sugar beet field infested with BNYVV in Kimberly, ID during the 2021 growing season in a randomized complete block design with 6 replications. At harvest on 4-5 October 2021, roots were dug and evaluated for symptoms of rhizomania and also placed in an indoor commercial sugar beet storage building. After 132 days in storage, samples were evaluated for surface rot, weight loss, and sucrose loss. Surface root rot ranged from 12 to 53%, weight loss ranged from 14 to 23%, sucrose losses ranged from 28 to 68%, and estimated recoverable sucrose ranged from 766 to 9,622 lb/A. Given these response ranges, selecting cultivars for rhizomania resistance and combining this resistance with storability will lead to considerable economic benefit for the sugar beet industry