Differential attraction ofDrosophila melanogaster Meig. to potato tissue infected with two varieties ofErwinia carotovora

Experiments were conducted to determine ifDrosophila melanogaster Meig. was differentially attracted to decaying potato tissue infected withErwinia carotovora var.carotovora as compared toE. C. var.atroseptica because these insects transmit each bacterium at a different rate in nature. The results of olfactometer tests demonstrated that potato tissue infected with the two varieties of the organism do not differ in their attractiveness toD. melanogaster; also, that attraction of these flies to healthy potato tissue was not significantly different than the attraction to infected tissue.     

The presence and location ofErwinia carotovora subsp.carotovora (Jones) Bergey et al. in the gut of adultDrosophila melanogaster (Meigen)

The potato blackleg bacteriumErwinia carotovora subsp.carotovora (Jones) Bergey et al. was found in feces and regurgitated material obtained from surface sterilized adult fruit flies,Drosophila melanogaster (Meigen), that had fed for five hours on nutrient agar cultures of the bacterium. The highest levels of bacteria were obtained from fecal material and it is likely that this is the source of most of the inoculum carried by these vectors. The bacteria were found to be present in the foregut, midgut, and hindgut ofD. melanogaster. The largest number of bacteria were in the foregut with the population level decreasing as distance down the digestive tract increased. It is probable that the association between the vector and the bacterium is accidental. The probability that the insect aids survival of the bacterium by protecting it from adverse environmental conditions is suggested.     

Effect of temperature on differential persistence and insect transmission ofErwinia carotovora var.atroseptica andErwinia carotovora var.carotovora

Adult fruit flies (Drosophila melanogaster Meig.) transmittedErwinia carotovora var.carotovora (Jones) Dye (ECC) andErwinia carotovora var.atroseptica (van Hall) Dye (Eca) from vegetative material inoculated with equal proportions ofEcc andEca to uninoculated potato plants and from inoculated potato plants to sterile vegetative material at 15° and 27°C. The insects transmittedEcc andEca with approximately equal frequencies over an 18 hr period at 15°C. At 27°C the proportion ofEcc transmitted was significantly greater thanEca. At 27°C moreEcc thanEca was present in the inoculated vegetative material and potato stems, on fruit flies visiting these sites, and in injured potato tissues to which flies transmitted the bacteria. Results indicate that temperature is important in determining success ofEcc andEca transmission by insects and the relative persistence of the two organisms in plant tissues after transmission.     

Insect transmission ofErwinia carotovora var.carotovora andErwinia carotovora var.atroseptica to potato plants in the field

Adult fruit flies (Drosophila melanogaster Meig.) artificially contaminated withErwinia carotovora var.carotovora (Jones) Dye (Ecc) and/orE. carotovora var.atroseptica (van Hall) Dye (Eca) readily transmitted the bacteria to plants in the field that had been injured by crushing the stem. Injured, inoculated plants developed disease symptoms when maintained at high relative humidities.Erwinia was transmitted to ten-hour-old injuries and they became infected as frequently as freshly made wounds. Insect transmission ofEca, Ecc, and mixtures was greatest during the afternoon, which was the warmest part of the day.Ecc was transmitted significantly less frequently during the cold morning than during afternoon or evening hours. A potato cull pile placed in a commercial potato field attracted a natural insect population which increased during the season. BothEcc andEca were isolated from uninoculated rotting tubers in the cull pile and from insects associated with the pile from May through September. These naturally infested insects transmittedEcc and/orEca from the cull pile to artificially injured field plants during July and August at distances as great as 183 m from the cull pile. NoErwinia was isolated from injured plants in a neighboring control field, which lacked a cull pile, further than 6 m upwind from the cull pile. We suggest that insects are important agents in the epidemiology of potato blackleg and soft rot even in areas with low relative humidities.     

Expression of the chicken lysozyme gene in potato enhances resistance to infection byErwinia carotovora subsp.atroseptica

Infection of potato plants and tubers with the bacteriumErwinia carotovora subsp.atroseptica produces blackleg and soft rot diseases, which cause significant losses to crops and stored potatoes. In order to obtain resistance against this bacterium, the genechly encoding the enzyme lysozyme from chicken was introduced into potato plants (cv. Desirée) viaAgrobacterium- mediated transformation. Sixty-three and 69 transgenic potato clones were evaluated in the greenhouse for resistance to blackleg and soft rot diseases, respectively. Results reported in this paper indicate that 21%-29% of the potato clones showed increased resistance to infection by the bacteriumE. c. subsp.atroseptica T7, as revealed by a reduced severity of blackleg or soft rot symptoms. Nine clones showing different levels of resistance were selected for further molecular analysis. The number of copies of the transgene integrated in the plant genome of these clones was estimated by Southern blot analysis. The level of transgene expression, detected by Northern blot analysis, correlated with the level of resistance detected in these clones.     

Associative effects of leaf roll and ring rot on disease expression and yield of potatoes

Netted Gem potato plants infected with either the ring rot bacterium (Corynebacterium sepedonicum) or the potato leaf roll virus alone exhibited typical symptoms of the respective diseases. Plants infected with both the bacterium and the virus exhibited severe leaf roll. Typical ring rot symptoms were masked in dual infections, thereby increasing the difficulty in detection of the bacterial disease. The combined yield-reducing effect of the two pathogens was greater than that of the potato leaf roll virus and was similar to that caused by ring rot alone.     

Effect of temperature and latent viruses on atypical ring rot symptoms of Russet Burbank potatoes

The ring rot bacterium,Corynebacterium sepedonicum (Spieck. and Kotth.) Skapt. and Burkh. [Clavibacter michiganense subsp.sepedonicum (Spieck et Kotth.) Davis et al.], and latent potato viruses (potato virus S and potato virus X) were investigated for their effect on atypical (ATYP) ring rot symptom development on Russet Burbank potato plants at different temperatures. Plants grown at 21 C from stem cuttings root-inoculated withC. sepedonicum developed typical wilting and chlorotic symptoms of ring rot that were equally severe on virus-free (VF) and virus-infected (VI) plants. All VF and VI plants grown at 15 C from inoculated stem cuttings exhibited ATYP symptoms that included extreme stunting, resetting, and chlorotic symptoms of ring rot. More severe ATYP symptoms developed on VI than on VF plants. Up to 5 wk after inoculation withC. sepedonicum, ATYP symptom severity ratings of both VF and VI plants increased and declined thereafter. The ATYP severity ratings were highly correlated with fresh weight of plants with high severity ratings being associated with low fresh weights. These results emphasize the need to determine the role of temperature and viral pathogens on ring rot symptomology in existing and newly developed potato cultivars and thereby enable better field detection of bacterial ring rot.     

Effect of the ring rot pathogen and latent potato viruses on ring rot symptoms and yield of potatoes

The ring rot bacterium,Corynebacterium sepedonicum (Spieck. and Kotth.) Skapt. and Burkh., and latent potato viruses (potato virus S and potato virus X) were investigated for their effect on ring rot symptom development on potato plants in the greenhouse and on symptom development and yield of potatoes in the field. Both virus-free (VF) and virus-infected (VI) Red Pontiac stem cuttings root-inoculated with ring rot bacteria in the greenhouse developed typical (T) ring rot symptoms, and symptom severity did not differ between VF and VI plants. In a field study, both VF and VI Russet Burbank seed pieces knife-inoculated with ring rot bacteria produced plants with atypical (A) and T ring rot symptoms as well as a combination of both types. The data suggest that more A than T symptoms develop on VI plants and more T than A symptoms develop on VF plants. Combined infection with the ring rot pathogen and the latent potato viruses resulted in greater yield losses of total and marketable Russet Burbank tubers than infection with the bacterial or viral pathogens alone.     

The effect of an alternate weed host,hairy nightshade,Solanum sarrachoides (Sendtner) on green peach aphid distribution and Potato leafroll virus incidence in potato fields of the Pacific Northwest

Hairy nightshade, Solanum sarrachoides (Sendtner) is an annual solanaceous weed prevalent in potato farmlands of the Pacific Northwest. S. sarrachoides is also a reservoir for Potato leafroll virus (PLRV) and its most important vector, the green peach aphid, Myzus persicae (Sulzer). Green peach aphids prefer S. sarrachoides than potato, Solanum tuberosum (L.), and produce more nymphs on S. sarrachoides than on potato. Increased green peach aphid preference for S. sarrachoides and greater fecundity on this plant could lead to overcrowding and movement toward neighboring potato plants. To test our hypothesis, field trials were conducted at Kimberly, Idaho during the summers of 2003 and 2004. A row of S. sarrachoides seedlings was planted in the middle of potato plots and naturally occurring green peach aphid populations were monitored weekly on potato plots with and without S. sarrachoides. More aphids were consistently found on S sarrachoides plants than on potato plants. More aphids were found on potato plants in plots with S. sarrachoides plants than without S. sarrachoides. Targeted insecticide applications on S. sarrachoides rows alone reduced the number of aphids on potato plants in those plots, suggesting that insecticide sprays prevented aphid movement from S. sarrachoides to potato. Testing of potato tuber sprouts 90 days after harvest by double antibody sandwich ELISA indicated a greater PLRV infection rate on plots with S. sarrachoides than plots without S. sarrachoides. Therefore, the presence of S. sarrachoides in potato plots will likely increase aphid populations and PLRV incidence on neighboring potato plants.     

Evaluation of an avirulent strain ofPseudomonas solanacearum for biological control of bacterial wilt of potato

An avirulent strain ofPseudomonas solanacearum, B82, was tested for its ability to protect the potato cultivar, Ontario, from bacterial wilt caused by virulent strains of this bacterium. Strain B82 was not antagonistic to 124 virulent strains ofP. solanacearum and was not bacteriocinogenic. When potato seedpieces were soaked for 5 hr in suspensions of B82 (10⁸ to 10⁹ cfu/ml), reduction in disease severity (up to 50%) was noted in some experiments. The treated plants, grown in the greenhouse or growth room for 4–5 wk, were challenged by stem inoculation, soil drenching, or root-to-root infection with virulent strains (276 or Br5) of the bacterium. Protection was not obtained consistently, however, and much of the variability could be attributed to differences in ambient temperature and light conditions in the greenhouse between experiments. In general, high ambient temperatures and/or reduced sunlight resulted in no protection. Protection appeared to depend on the ability of strain B82 to multiply in the vascular system in the crown region and to colonize the rhizosphere of treated plants. Soil temperatures above 24 C affected the ability of strain B82 to survive in the rhizosphere. The close dependence of protection on environmental conditions suggests that this method of control may not be practical under field conditions.     

Multiplication ofPseudomonas solanacearum in resistant potato plants and the establishment of latent infections

When 1-mo-old plants of a wilt-resistant clone ofSolanum phureja (1386.15) were stem-inoculated with three strains ofPseudomonas solanacearum (K60, S123, and S206), the bacteria multiplied rapidly at the point of inoculation and then moved in the vascular system to other parts of the stem. Resistant plants showed a remarkable ability to support relatively high populations of the bacterium in the absence of disease symptoms. Although multiplication in this resistant clone was substantially less than in susceptible Russet Burbank potato plants, large numbers of bacteria (up to 624 × 10⁴ cells of K60 per 5-cm stem segment) reached the base of the stem of plants maintained at high temperature (28°C) for 20 days after stem inoculation. From the base of the stem, the bacteria moved rapidly into the roots and tubers. Strains ofP. solanacearum differed in their ability to cause latent tuber infection in different resistant potato clones. When 11S.phureja ×S. tuberosum hybrids were stem-inoculated, maintained at 28°C for 3 wk and then grown to maturity at 20°C., most of the clones yielded tubers infected by one or more strains. The race 1 strain (K60) was the most infectious; 53.8% of all tubers harvested from all plants inoculated with this isolate carried latent infections. Because one clone (BR 53.1) never yielded infected tubers, there appear to be genetic factors which may be useful in breeding programs aimed at eliminating latent tuber infection.     

Reevaluation of the potato aphid,Macrosiphum euphorbiae (Thomas), as vector of potato virus Y

The effectiveness of the potato aphid,Macrosiphum euphorbiae (Thomas), to transmit potato virus Y (PVY) to potato has generally been overestimated because tobacco has been used as the indicator host. Our results demonstrate that, although apterousM. euphorbiae can acquire PVY from potato and tobacco plants and transmit it to tobacco plants, it does not readily transmit it to potato plants. Alatae only transmitted the virus to 4.5% of potato plants. This relative inability to transmit the virus to potato seems independent of potato cultivar. Results suggest that the role of the potato aphid in the spread of PVY in potatoes may be negligible.     

The bionomics ofErwinia carotovora in North Dakota

A study into the ecology ofErwinia carotovora in North Dakota was performed to determine possible sources ofErwinia spp. which recontaminate stem cut derived seed stocks (SCDS).Erwinia carotovora pv.carotovora (Ecc) was found to overwinter in naturally infested agricultural soils that had been planted to potato the previous cropping season. However, this bacterium was only present in the 30–71 cm sampling depth.E. carotovora pv.atroseptica (Eca) and Ecc were also found in the rhizospheres of a few weed and crop species but the frequency of recovery was low. Ecc and other strains not identifiable to pathovar, but notE. chrysanthemi, were readily recovered from a variety of water sources and from sunflowers with stem and head soft-rot symptoms. AllErwinia strains were characterized, but not all could be identified to pathovar. WateringErwinia-free potato plants with water naturally infested withErwinia spp. resulted in progeny tubers contaminated withErwinia. The importance of this information with regard to the recontamination of SCDS is discussed.     

Evaluation of six insect pests for transmission of potato spindle tuber viroid

Six insect pests of potato, green peach aphid,Myzus persicae (Sulzer); potato leafhopper,Empoasca fabae (Harris); Colorado potato beetle,Leptinotarsa decemlineata (Say); tarnished plant bug,Lygus lineolaris (Palisot de Beauvois); red-legged grasshopper,Melanoplus femurrubrum (DeGeer); and southern army worm,Prodenia eridania (Cramer), were tested for their ability to transmit potato spindle tuber viroid (PSTV). PSTV infection was determined by a gel electrophoretic assay made 6 weeks after transfer of insects from infected to non-infected plants of the cultivar, ‘Katahdin.’ Plants grown from tubers of the original non-infected hosts were also assayed. PSTV was detected in only 2 of 183 test plants; these 2 plants had been infested with the tarnished plant bug. These results suggest that the insect species studied are relatively insignificant as vectors of PSTV.     

Enhanced <Emphasis Type="Italic">Fusarium oxysporum</Emphasis> f. sp. <Emphasis Type="Italic">tuberosi</Emphasis> Resistance in Transgenic Potato Expressing a Rice GLP Superoxide Dismutase Gene

Germin like proteins (GLPs) are a large group of related and ubiquitous plant proteins which are considered to be involved in different processes important for plant development and defense. Multiple functional copies of this gene family have been reported in a number of species (wheat, barley, rice, soybean mosses and liverwort), and their role is being evaluated by gene regulation studies and transgenic approaches. To analyze the role of a rice (Oryza sativa) root expressed germin like protein1 OsRGLP1, for its antifungal activity, transgenic potato plants were developed. These transgenic potato plants were molecularly characterized and biologically assessed after inoculation with Fusarium oxysporum f. sp. tuberosi. Functional analysis showed high accumulation of H₂O₂, increased Superoxide Dismutase (SOD) activity and no oxalate oxidase activity (OxO) in transgenics in comparison to nontransformed control. This increased SOD activity, resistance to heat and sensitivity to H₂O₂ suggest it is a Fe-like SOD. OsRGLP1 expression in potato plants exhibited enhanced resistance in comparison to nontransformed wild type plants suggesting its role in providing protection against Fusarium oxysporum f. sp. tuberosi through elevated SOD level. Overall, results suggest that OsRGLP1 is a candidate for the engineering of potato plants with increased fungal tolerance however, the greater height and tuber number was observed. This phenotype associated with the resistance needs to be evaluated to determine if this is a positive or negative feature.     

Germplasm transfer from the wild tetraploid speciesSolanum acaule bitt. to the cultivated potato,S. tuberosum L. using 2N eggs

Solanum acaule Bitt. (acl ¹, 2n=4x=48 and 2n=6x=72) is a wild potato species in which resistance to adverse biotic and abiotic agents has been found. The tetraploid subspeciesacaule andaemulans cannot be easily crossed with the common potato,S. tuberosum L. Grouptuberosum (tbr, 2n=4x=48) because the development of the endosperm in the hybrid seed is abnormal. Since 4xacl behaves as a diploid in intra and interploidy crosses, it was postulated that the hybridization barrier withtbr could be overcome if the wild species produced 2n gametes. One hundred and ninety-seven plants of 4xacl (22 introductions) were screened for pollen of heterogeneous size; only two plants produced this type of pollen, with 3% and 5% of pollen in the large class, respectively. On the other hand, 427 plants (47 introductions) were screened for 2n eggs through controlled crosses withtbr. Forty-one plants (13 introductions) produced 219 berries of which 46 contained at least one plump seed. Seed from berries with low seed set gave rise to hexaploid hybrids possibly through the functioning of 2n eggs from 4xacl. A sample of these hybrids was successfully backcrossed as female parents totbr, giving rise to 5x BC₁ progenies. These BC₁ progenies could be easily back-crossed totbr, yielding tetraploid or near tetraploid BC₂ plants.     

Haplotypes of the Potato Psyllid, Bactericera cockerelli, on the Wild Host Plant, Solanum dulcamara, in the Pacific Northwestern United States

‘Candidatus Liberibacter solanacearum’ (Lso) is a bacterium that infects solanaceous crops and causes plant decline and yield losses, especially in potato and tomato. Lso is transmitted to these hosts by the potato psyllid (Bactericera cockerelli Sulc) vector. B. cockerelli host plants are not limited to crop plants, but also include many wild, solanaceous weeds. These wild hosts could potentially impact overwintering and breeding of the psyllids and serve as reservoirs for Lso. In the Pacific Northwestern United States, B. cockerelli was recently reported to overwinter on bittersweet nightshade (Solanum dulcamara L.). The present study utilized high resolution melting analysis of the B. cockerelli mitochondrial cytochrome c oxidase I gene to assess the psyllid populations occurring on S. dulcamara during the summer and winter months in Washington, Oregon, and Idaho. This technique has previously been used to analyze the cytochrome c oxidase I gene of B. cockerelli, and has identified four psyllid haplotypes. Lso infection was also determined for the psyllids collected from S. dulcamara. During both the summer and the winter months in the Pacific Northwest, the Northwestern psyllid haplotype was the predominant population found living on S. dulcamara. However, low levels of the Western psyllid population were also present in Washington and Oregon during the same period. No overwintering psyllids tested were Lso-infected, suggesting that these populations do not pose an imminent threat of Lso transmission to newly emerging potatoes and other solanaceous crops in the region, unless a source of Lso becomes available.     

Development of a PCR Assay for the Rapid Detection and Differentiation of ‘Candidatus Liberibacter solanacearum’ Haplotypes and Their Spatiotemporal Distribution in the United States

Zebra chip, or zebra complex (ZC) has become an important invasive disease of potato in the United States and New Zealand and is caused by a phloem-limited bacterium, ‘Candidatus Liberibacter solanacearum’ (Lso). A PCR assay using a single pair of simple sequence repeat (SSR) primers was developed for simultaneous detection and genotype differentiation of Lso haplotypes associated with zebra chip disease of potato. The sensitivity of the SSR PCR was similar to a 16S PCR assay, with detection limit of 100 copies of the Lso genome in haplotype A infected potato and psyllid samples and 10 copies of Lso genome in haplotype B potato and psyllid samples. The Lso detection frequency of the SSR PCR assay was 79.1 % in potato and 26.4 % in psyllid samples, respectively; whereas the detection frequency of the 16S PCR assay 59.0 % in potato and 25.9 % in psyllid samples, respectively. Samples of Lso positive potato plants and psyllids from multiple states in the US were demonstrated to have either haplotype A or haplotype B Lso and occasionally both haplotypes were found in individual samples. This is the first report that co-infection of the two haplotypes of Lso exists in potato and potato-psyllid samples. Only haplotype A Lso was detected in North Dakota psyllid samples collected in 2010, in Idaho and Washington ZC potato samples sampled from storage in 2011, and in Idaho ZC potato samples in 2012. Haplotype A Lso was also detected in New Zealand ZC affected potato samples and psyllid samples collected in 2010 and 2011. The PCR assay developed is as sensitive as previously developed assays and has the advantage of simultaneously detecting and differentiating Lso haplotypes of the ZC bacterium, thus making it extremely useful for epidemiological studies.     

Latent infection of potato tubers byPseudomonas solanacearum

Strains ofPseudomonas solanacearum differed in their ability to infect tubers of different resistant potato clones grown in infested soil. When eight resistant clones (Solanum phureja orS.phureja ×S. tuberosum hybrids) were grown at 24–28°C in soil infested with a race 1 or a race 3 strain of the bacterium, relatively few plants had wilt symptoms at harvest, but 26.7% and 9.2% of the tubers harvested from plants infected with the race 1 and race 3 strains, respectively, carried latent infections. Some infected clones never yielded diseased tubers, however. The development of symptoms above ground was not correlated with the incidence of tuber infection in any particular clone. No tuber infection occurred in tolerant or resistant clones grown in infested soil at cool (12–22°C) temperatures. Tubers were inoculated directly in an attempt to evaluate the ability of bacteria to multiply in these tissues at different temperatures. Highly virulent strains ofP. solanacearum survived in susceptible tubers in higher numbers and for longer periods than in resistant ones. Low temperature (4°C) had a deleterious effect on survival of the bacterium in tubers, but did not completely eliminate the pathogen even after 40 days.     

Experimental host range and natural reservoir of sweet potato leaf curl virus in the United States

Sweet potato leaf curl virus (SPLCV), a sweet potato whitefly (Bemisia tabaci) transmitted begomovirus, causes serious yield losses to many sweet potato cultivars. Using experimental whitefly transmissions in a greenhouse (choice tests) and in a growth chamber (no-choice tests), we evaluated 111 plant species in 30 families to determine the host range of SPLCV. The host range was limited to plants in the genus Ipomoea within the family Convolvulaceae. In total, 38 of 45 Ipomoea species tested were susceptible to SPLCV infection. Surveys were conducted during the 2007-2009 sweet potato growing seasons in Mississippi and South Carolina to evaluate morning glory species as potential reservoir hosts for SPLCV. In the sweet potato experimental fields and surrounding areas, a large proportion of volunteer sweet potatoes, as well as a high percentage of annual and perennial morning glories tested positive for SPLCV. Understanding the host range and potential virus reservoir host plants will ultimately help in the development of an effective disease management strategy that is based on the consideration of agroecological factors.