Pathogenicity of Washington and Oregon isolates of tobacco rattle virus on potato

Soil samples from corky ringspot (CRS) problem fields of potato in the states of Washington and Oregon were collected and planted withNicotiana tabacum ‘Samsun NN’ tobacco to bait tobacco rattle virus (TRV) and to increaseParatrichodorus allius populations, the vector of TRV. Pathogenicity of three isolates of TRV was assessed on Russet Burbank and Russet Norkotah plants usingP. allius as the vector. The most severe CRS tuber symptoms were with TRV from Pasco, WA, followed by Umatilla, OR, and Mattawa, WA, indicating a distinct variation in virulence among virus isolates. The lowest number ofP. allius that transmitted TRV resulting in CRS symptoms on Russet Norkotah was three nematodes per 250 cm³ soil. When potato plants were exposed toP. allius at different times in their growth, tubers on older plants were more resistant than younger tubers to CRS. Severity of CRS tuber symptoms was correlated with age of potato plants in pot culture. Reproduction ofP. allius on potato roots did not influence the incidence and severity of tubers symptoms. Under similar conditions, Bintje, a variety known to be resistant to CRS, served as a host forP. allius, yet remained asymptomatic. The protocol employed in these studies is appropriate for testing potato germplasm for resistance to CRS.     

Evidence for the influence of weeds on corky ringspot persistence in alfalfa and Scotch spearmint rotations

Corky ringspot disease (CRS) of potato is caused by tobacco rattle virus (TRV). The virus is transmitted by the stubby root nematode (Paratrichodorus allius) in the Pacific Northwest potato-producing regions. Alfalfa (Medicago sativa L.) and Scotch spearmint (Mentha cardiaca Baker) rarely serve as hosts for TRV. Therefore,P. allius reared on these plants for 1 to 3 months are cleansed of TRV in greenhouse trials. However, weeds in alfalfa and Scotch spearmint rotation crops may serve as hosts for the virus. In greenhouse trials, hairy nightshade (Solanum sarrachoides), prickly lettuce (Lactuca serriola), henbit (Lamium amplexicaule) and, green foxtail (Setaria viridis) grown alone were found to be suitable hosts ofP. allius, whereas Powell amaranth (Amaranthus powellii) was not. ViruliferousP. allius added to hairy nightshade, prickly lettuce, henbit, green foxtail, or Powell amaranth in mixtures with alfalfa and/or Scotch spearmint occasionally remained viruliferous over a 3-to 4-month period, whereasP. allius maintained on weed-free alfalfa or Scotch spearmint became virus-free after 1 to 2 months. Potato grown in soil containingP. allius that had been maintained on weed-alfalfa or weed-Scotch spearmint mixtures for 3 to 4 months exhibited slight to severe CRS symptoms on new tubers, whereas potato following weed-free Scotch spearmint or alfalfa were free from CRS symptoms. Severe CRS symptoms on potato tubers were only observed when potatoes were grown in soil containingP. allius that were maintained on hairy nightshade or hairy nightshade mixtures with alfalfa or Scotch spearmint. These preliminary data suggest that the presence of weeds that serve as hosts of both TRV and P.allius may nullify the positive effects of growing alfalfa or Scotch spearmint for CRS control. Targeted control efforts of known weed hosts may be required to successfully eliminate CRS from fields using alfalfa and Scotch spearmint rotational crops.     

Weed hosts ofParatrichodorus allius and tobacco rattle virus in the Pacific Northwest

The ability of several weed species to serve as hosts for tobacco rattle virus (TKV), the causal agent of corky ringspot disease of potato (CRS), and its nematode vector,Paratrichodorus allius, was investigated in greenhouse studies. ViruliferousP. allius multiplied on 24 out of 37 weed species tested, indicating they were suitable hosts of the vector. However, only 11 of these weeds were infected with TRV, as determined by ELISA. The nonhost status of a given weed species was not changed whether the viruliferous vector population originated from CRS problem fields in WA, OR, or ID. Several weeds served as hosts for the vector and virus including kochia, prickly lettuce, henbit, nightshade species (black, hairy, and cutleaf), common chickweed, and annual sowthistle. Virus-freeP. allius acquired TRV from the three nightshade species, volunteer potato grown from TRV-infected tubers, and prickly lettuce, and subsequently transmitted the virus to ‘Samsun NN’ tobacco indicator plants. Thus, some weeds may play a role in the epidemiology of CRS by perpetuating TRV and its vector in a problem field.     

Evidence for persistent, symptomless infection of some potato cultivars with tobacco rattle virus

Summary Infection with an M-type (particle producing) isolate of tobacco rattle virus (TRV) was detected in leaves and/or roots of some plants from 11 of 13 potato cultivars grown in soil containing viruliferous trichodorid nematodes. Virus was detected in tubers of 8 of 13 cultivars, although only two (Pentland Dell and Maris Bard) developed spraing symptoms. Six cultivars (Arran Consul, Home Guard, King Edward, Romano, Santé and Wilja) were infected with TRV without developing spraing symptoms. Plants grown from virus-containing, symptomless tubers became systemically infected with M-type TRV and produced symptomless infected daughter tubers. Virus was maintained through three generations of vegetative propagation, and the plants were sources for acquisition of the virus by trichodorid nematodes. Distribution of virus in both spraing-affected and symptomless tubers was erratic. Movement of symptomlessly infected seed tubers may be a means of dissemination of the virus and of its introduction to previously unaffected sites.     

Effects of soil fumigation with Telone-C on the field inoculum potential of tobacco rattle virus and potato yields

Two Julesburg sandy-loam fields in Weld County, Colorado, with a history of Corky Ringspot (CRS) caused by tobacco rattle virus (TRV) were injected with Telone-C as fall or spring applications and planted to potatoes. Deep soil sample cores taken from the experimental fields were planted toNicotiana tabacum L. var. Samsun serving as TRV bait and indicator plants. Systemic infection of bait plants and assay of the bait plant roots for TRV indicate that potential TRV inoculum was present which escaped the effects of fumigation treatments. TRV was found to be widely but erratically distributed throughout the experimental fields. The low incidence of CRS in tubers harvested from the treated fields suggests that fumigation may have short term benefits but potential inoculum exists which could lead to infection after the effects of fumigation have dissipated. The total yield of tubers was increased in the two cultivars planted (Norgold and Norland) in the fumigated fields receiving spring or fall applications. The yield of U.S. No 1 Grade potatoes in the Norland cultivar was increased significantly by both fall and spring fumigation. These yield increases were directly related to the reduced incidence ofVerticillium wilt observed in the fumigated soils and reported as a personal communication from unpublished data.     

Potato germplasm resistant to corky ringspot disease

Potato germplasm was assessed for resistance to corky ringspot disease (CRS) incited by transmission of tobacco rattle virus (TRV). TRV is transmitted by the stubby root nematode,Paratrichodorus allius, in the Pacific Northwest, and characterized by necrotic areas in the tuber. Four tests were conducted at two different problem fields in Umatilla, OR and Pasco, WA. The fields differed in the virulence of the virus isolates. Some clones and named varieties showed resistance only in the field with the relatively mild Umatilla isolate, while others remained symptomless at both sites. The host suitability of test potatoes to threeP. allius populations from the region was determined in greenhouse pot tests, and expressed as reproductive values [R_f = (final population of nematode at 55 days) / (initial population)]. TheR _f values for the nematodes were not correlated with resistance ratings based on visual scoring of CRS symptoms in field grown potatoes. Thus, it appears that CRS resistance is based on reaction of potato genotypes to virus and not the vector. The availability of resistant clones inin vitro form is indicated.     

Tracking Foliar Symptoms Caused by Tuber-Borne Potato Yellow Vein Virus (PYVV) in Solanum Phureja (Juz et Buk) Cultivar “Criolla Colombia”

Potato yellow vein virus (PYVV) causes vein yellowing in leaves and reduces potato yield by 50 %. In Andigena potato varieties, PYVV infections result in both symptomatic (S) and symptomless (NS) plants. The present study tracked PYVD symptoms over two field generations (G₁ and G₂) derived from mother plants (G₀) of the Solanum phureja cultivar “Criolla Colombia.” PYVV was detected in the leaves of G₀ plants by RT-PCR. The experiment began with tubers of 39 S plants from a commercial crop and 94 certified NS plants (PLRV, PVY, PVX, PVS free), yielding a total of 3561 plants analyzed over the two generations. The greenhouse whitefly vector Trialeurodes vaporarioum was controlled chemically. S plants produced large proportions of NS plants, 62 % and 84 % in G₁ and G₂, respectively. NS plants, whether RT-PCR positive or negative, generated greater than 96 % NS plants, but in some cases, produced a proportion of S plants ranging between 0.2 % and 3.4 %. The presence of PYVV was evaluated by RT-PCR and qPCR with Taqman probes in sprouts of the tubers of S and NS plants. One to 6 sprouts were analyzed per tuber. In 113 sprouts of the tubers of S RT-PCR positive/ negative plants and 36 of NS RT-PCR positive/negative plants, the viral RNA copies ranged between 3.42?×?10² to 6.01?×?10⁸ copies/mg of plant extract. These results show that PYVV is present not only in symptomatic plants but also in some symptomless plants. PYVV is passed from symptomless mother plants to some of its tubers along with unknown putative silencing or latency mechanisms that prevent the virus from expressing in most of the infected progeny. Late vector transmission, heterogeneous viral concentration in tubers and virus latency are also discussed.     

Diagnosis of tobacco rattle virus (TRV) and other viruses in weed and rotation crops in potato fields

The methodology for the detection of tobacco rattle virus (TRV) and for the incidence of TRV and other viruses was determined in weed species associated with potatoes. Symptoms onNicotiana tabacum L. cv. ‘Samsun NN’and Phaseolus vulgaris L. cv. ‘Bountiful’ were used to detect the viruses. Identity was confirmed by comparison with symptoms produced on plants by standard cultures of TRV and potato virus X (PVX), by electron microscopy, and by serology.Portulaca oleracea L.,Solatium sarachoides Sendt., andAmaranthus retroflexus L. were hosts of TRV; the latter two also carried PVX. Most TRV carriers were found adjacent to infected potatoes. The roots of naturally infectedS. sarachoides contained the highest concentration of TRV encountered in these studies. Mechanical inoculation of TRV to A.retroflexus andS. sarachoides produced variable symptoms that included red spots on the former and chlorosis and stunting on the latter. TRV moved systematically in both species. Assay methods for detection were adequate but low concentrations of TRV limited the usefulness of electron microscopy and serology.     

Weed hosts of the tobacco rattle virus in Idaho

The corky ringspot disease of potato (Solanum tuberosum) caused by the tobacco rattle virus (TRV) is a serious problem in certain Idaho localities. To investigate natural TRV sources, weeds and plants from rotation crops were randomly collected from six fields with a history of corky ringspot symptoms. Transmission trials, Ouchterlony agar double-diffusion tests, and electron-microscopy showed evidence of TRV in three Idaho counties (Fremont, Ada, and Canyon). Inoculations were made from 27 species and TRV was recovered from the roots of three:Solanum nigrum, Brassica campestris, andErodium cicutarium. Solanum nigrum was the most consistent source of TRV and it generally incited the severest symptoms onNicotiana tabacum (Samsun NN). The results show thatS. nigrum, besides being an important wild host of TRV, may be a useful naturallyoccurring bait plant for detection of TRV in field soils.     

Prevalence of potato yellow vein virus (PYVV) in Solanum tuberosum Group Phureja Fields in Three States of Colombia

Potato yellow vein virus (Crinivirus) causes leaf yellowing, is limited to the phloem and is transmitted by Trialeurodes vaporariorum and tubers. PYVV causes up to 50 % yield reduction in S. tuberosum. The prevalence of PYVV was established in Group Phureja in 2008 in the Colombian states of Cundinamarca, Antioquia and Nariño. The presence (S) or absence (NS) of symptoms was established in 24,930 plants. Symptom prevalence was as follows: Cundinamarca 0.74 %?±?0.03, Antioquia 5.4 %?±?12 and Nariño 6.2 %?±?0.11. Percentages of S plants were highly variable among different states and municipalities. PYVV was diagnosed by RT-PCR in a subsample of 1,545 plants from Cundinamarca and Nariño. RT-PCR showed that 2.4 %?±?8.3?×?10^?5 and 41 %?±?0.0009 % of the symptomless plants of Cundinamarca and Nariño respectively were positive. This suggests possible late infection of plants by the vector which did not have enough time to express symptoms, infections with two or more viruses or “virus latency”. PYVV symptomatic prevalence in Group Phureja was lower than that reported for Diacol-Capiro (Group Andigena), but increases if molecular results are taken into account. This is the first known study aimed at determining PYVV prevalence using molecular tools and extensively sampling of S.tuberosum Group Phureja plants in grower’s fields in three states and multiple municipalities of Colombia.     

Potato mop-top virus in cork-diseased Peruvian potatoes

Superficial, circular necrotic lesions develop on cultivar Ramacimiento (Solanum tuberosum ssp.andigena) tubers in plantings at elevations above 2,500 m in the Peruvian Andes. After drying or storage the lesions become sunken or cankerous. A virus was transmitted mechanically toChenopodium quinoa andC. amaranticolor with leaf and fruit saps from Renacimiento plants grown from symptomed tubers. The virus is a strain of potato mop-top virus which causes milder tuber symptoms in European cultivars.C. quinoa was better thanC. amaranticolor as an indicator host; its spreading local lesion reaction was most sensitive at 15 C, less at 10 and 20, and nil at 25 C. Sap inoculum from fruit resulted in symptoms in 7 days, that from leaves in 13 days, and that from tubers produced no symptoms.     

Foliar Symptoms Caused by Potato mop-top virus on Potato Plants During Vegetative Propagation in Scotland and Their Association With Tuber Yield, Spraing and Tuber Infection

The effect of foliar symptomatic infection by Potato mop-top virus (PMTV) on yield of tubers, spraing and infection in daughter tubers, and foliar symptom development and tuber infection in the following generations of propagation was investigated in commercial seed potato crops in Scotland. Six crops covering cvs Atlantic, Hermes, Nicola and Cara were studied between 2000 and 2006 by labelling paired replicates of plants with foliar symptoms and plants with no symptoms. Tubers from plants with no symptoms rarely produced plants with foliar symptoms in the following generation. Plants with no symptoms produced more infected tubers if they had been derived from plants with foliar symptoms the previous year than from plants with no symptoms. The proportion of daughter plants with foliar symptoms produced by tubers from plants with foliar symptoms in year 1 ranged from 19–41% and seemed to be associated with the severity of foliar symptoms. The detection of PMTV by ELISA in samples of leaves from plants with foliar symptoms ranged from 13% for cv. Cara to 59% for a crop of cv. Atlantic in 2004. The amounts of spraing were generally low but tended to be greater for tubers from plants with foliar symptoms than those from plants with no symptoms. These results indicate that roguing plants with foliar symptoms in seed potato crops could achieve an improvement in crop health but might be impractical when diseased plants are too prevalent.     

Evaluating Incidence of Helminthosporium solani and Colletotrichum coccodes on Asymptomatic Organic Potatoes and Screening Potato Lines for Resistance to Silver Scurf

Silver scurf, caused by Helminthosporium solani, and black dot, caused by Colletotrichum coccodes, cause tuber blemishes on potato (Solanum tuberosum L.) which affect processing and fresh market trade. Tubers from ten cultivars were collected at harvest from three organic farms in Wisconsin and categorized as symptomatic or asymptomatic based on visual symptoms of silver scurf and black dot and/or signs of H. solani and C. coccodes. Tuber incubation and PCR assays were performed on asymptomatic tubers to detect H. solani and C. coccodes. Tuber incubation and PCR assays were in slight to fair agreement (kappa coefficient <0.4) for detecting both pathogens. Most asymptomatic tubers tested were positive by one or both assays for H. solani (75 %) or C. coccodes (94 %). Minituber inoculation assays were also performed to screen potato lines for resistance to silver scurf. Of the 14 lines tested, a diploid interspecific hybrid, C287, had consistently low sporulation, suggesting it has partial resistance to silver scurf. Since the majority of tubers harvested are already infected with one or both pathogens further research should focus on organically acceptable management practices that may inhibit disease development in field and in storage.     

The effect of temperature on the development of wilting and on progeny tuber infection of potatoes inoculated with South African strains of biovar 2 and 3 ofPseudomonas solanacearum

Summary Potato plants, after inoculation with a South African strain of biovar 2 or a strain of biovar 3 ofPseudomonas solanacearum were grown in growth chambers at temperatures ranging from 14/16°C (dark/light) up to 25/30°C. When grown at 14/16°C or higher, plants inoculated with the biovar 2 strain developed wilting symptoms and internal symptoms on progeny tubers and the disease was transmitted by the progeny tubers. When the plants were inoculated with the biovar 3 strain, wilting symptoms developed from 18/20°C, internal symptoms on progeny tubers from 20/22°C and the disease was transmitted by progeny tubers from plants grown at 16/18°C or higher.     

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.     

Detection ofCorynebacterium sepedonicum in potato cultivars with different propensities to express ring rot symptoms

Various potato cultivars were grown in field plots from seed pieces inoculated withCoryneacterium sepedanicum by vacuum infiltration. Foliage symptoms were recorded during the growing season and tuber symptoms at harvest. Cultivars were ranked according to the frequency with which the progeny tubers expressed external bacterial ring rot symptoms. Those that yielded 50–100% symptomatic tubers were placed in symptom expression class (SEC) 1; those that produced 25–49%, 10–24%, and ≤9% symptomatic tubers were placed, respectively, in SEC 2,3, and 4. The incidence of latent ring rot infections in stems and tubers were similar among cultivars in the different SEC classes. Bacterial population densities, quantified by immunofluorescence, were as high as 10¹⁰ cells/g in stems from cultivars of all SEC classes. Mean populations in tubers ranged from 10⁶ – 10⁷ cells/g and did not differ significantly among cultivars.