Phytophthora erythroseptica in Peru: Its identification and pathogenesis

EightPhytophthora isolates cultured from diseased potato plants and tubers in Peru were identified asPhytophthora erythroseptica Pethyb. var.erythroseptica. This pathogen was not previously reported from South America. The isolates were pathogenic to potato plants but they differed in virulence. Injuring roots prior to inoculation enhanced infection and disease development but all isolates infected noninjured roots. Continuous high soil moisture during incubation favored disease development. Under these conditions, disease developed at soil temperatures from 10 C to 30 C, but most rapidly at 25 C and 30 C. Plants grown 60 days before inoculation developed symptoms earlier and more rapidly than plants grown 15-, 30-, and 45 days. Four concentrations of mycelial-suspension inocula did not alter disease development, but zoospore inoculum induced symptoms and plant death more rapidly than mycelial suspensions at 25 C.     

Association of viral,bacterial and fungal pathogens with vascular discoloration of Russet Burbank potato tubers

Russet Burbank potatoes from the 1979 and 1980 crop years, collected from Chicago, IL repack warehouses and retail markets, were sampled for vascular discoloration. The amount of discoloration varied among sampling months and states of origin and decreased from 7.7% in 1979 to 1.6% in 1980. Highest levels of vascular discoloration were detected in December and January samples. Vascular discolored and non-discolored (control) tubers were assayed for the presence of potato leafroll, potato virus X, and beet western yellows viruses by the enzyme-linked immunosorbent assay (ELISA). Tubers were also assayed for 2 subspecies ofErwinia carotovora and forVerticillium albo-atrum andV. dahliae. Potato leafroll virus was detected in 31 of 831 vascular discolored tubers.V. albo-atrum was detected in 1 of 180 discolored tubers. Beet western yellows virus was not detected in discolored or non-discolored tubers. Two subspecies ofE. carotovora and potato virus X were equally common in discolored and non-discolored tubers.     

Effect of time of inoculation with potato leafroll virus on development of net necrosis and stem-end browning in potato tubers

The Green Mountain cultivar was used in field tests to determine the effects of inoculating potato plants at various times with the potato leafroll virus (PLRV) on development of internal necrosis of tuber tissue. Viruliferous apterae of the green peach aphid,Myzus persicae (Sulz.), were placed on each stem in all hills to be inoculated in each 3.0 m single-row plot. Planting and inoculation dates were varied in all field experiments and, in one, several vine-killing dates were also included. All harvested tubers were stored for approximately four months at 10°C to enhance development of internal necrosis prior to examination. Similar but smaller greenhouse studies involving both apterous and alate green peach aphids were also conducted using Green Mountain, Irish Cobbler, and Russet Burbank cultivars. All results showed that as inoculation was delayed relative to plant development, more net necrosis (NN) occurred. Conversely, when plants were inoculated early, stem-end browning (SEB) rather than NN predominated. A high percentage of naturally occurring SEB tubers (cv. Russet Burbank) were found by ELISA to contain PLRV. Plants produced by these tubers only rarely developed leafroll symptoms. These findings suggest a previously unsuspected causal relationship between SEB and PLRV. Implications of this apparent relationship on the epidemiology of potato leafroll in Maine are discussed.     

Enzyme-linked immunosorbent assay with single or combined antisera for viruses S and X in potato tubers and plants

Enzyme-linked immunosorbent assay (ELISA), with single or combined antisera was effective for diagnosing potato virus S (PVS), potato virus X (PVX) or both viruses in plants grown in the greenhouse or field. In dormant tubers, stolon, middle and apical end composite sampling with or without eyes and sprouted tubers produced reliable positive assays for PVX. Only tuber pieces with sprouts resulted in consistently reliable assays for PVS. Composite sampling of potato foliage was effective for detecting one PVX infected plant in a total of 100 Kennebec, Norland, or Russet Burbank plants. There were some false negative results and greater variability in composite PVS assays, but on average, one PVS infected plant can be detected in composites of 10 Kennebec, Norland, or Russet Burbank plants. Sodium diethyldithiocarbamate (0.01M NaDIECA) in phosphate buffered saline + 0.5% Tween (PBS-T) added to plant extracts enhanced specific reactions for either virus. Onceor twiceused enzyme conjugate was effective in ELISA of either virus from potato foliage.     

Variability in the concentration of three heat stable proteinase inhibitor proteins in potato tubers

The variability of three well characterized proteinase inhibitors, Inhibitor I, molecular weight 39,000, Inhibitor II, molecular weight 21,000, and Carboxypeptidase Inhibitor, molecular weight 4,100, were determined in apical cortical tissues of individual potato tubers of the Russet Burbank variety. The three inhibitors varied within ± 20% among sixty-five tubers and cumulatively represented about 7% of the total soluble proteins. The inhibitors were highly variable among tubers of 106 clones from randomly chosen varieties. Inhibitor I varied about twelve-fold (60 to 745 μg/ml juice), and Inhibitor II varied about seven-fold (158 to 1,025 μg/ml juice). Carboxypeptidase Inhibitor varied from as low as zero (seven varieties) to over 850 μg/ml tuber juice. With 80 tubers from fourteen varieties of potatoes, a positivecorrelation was found between the concentrations of Inhibitor I and Inhibitor II and total soluble protein. Carboxypeptidase Inhibitor did not correlate well with total soluble protein. The positive correlations of Inhibitors I and II (a correlation coefficient of 0.70) with total soluble protein indicated that the proteinase inhibitors may be excellent markers for genetic studies for selecting high protein potato tuber varieties.     

A method for large scale laboratory inoculation of potato tubers with late blight fungus

As part of a project to investigate the possibility that teratogenic factors are formed in potato tubers infected with the late blight fungus,Phytophthora infestans, a method was devised which allowed quick, efficient inoculation of large numbers of potato tubers. Tubers were placed in specially constructed trays, wounded by puncturing the periderm with flower “pin frogs”, and sprayed with a zoospore suspensionof P. infestans prepared from cultures grown on rye agar at 20C (68F) for 10 days. Approximately 95% of Katahdin and 93% of Russet Sebago tubers inoculated by this method exhibited typical late blight lesions after two weeks incubation at 20C, 80–90% RH. Eight tons of potatoes were processed in this manner.     

Steroid glycoalkaloids and related compounds as potato quality factors

The steroid glycoalkaloids are triterpenoid derivatives which are found in all tissues of the potato plant including the tubers. The compounds are largely localized in the peel of tubers, but tissue beneath the peel rapidly accumulates the steroid glycoalkaloids to levels equal to or greater than those in the peel as a result of injury or environmental stress. The accumulation is restricted to the outer 1–2 mm of injured or stressed tuber. Potatoes containing over 0.02% steroid glycoalkaloids are considered toxic to man, and at this concentration they would impart a distinctly bitter flavor. The accumulation of steroid glycoalkaloids is suppressed and the accumulation of sesquiterpenoids is elicited in tubers infected by various pathogens and nonpathogens including the late blight pathogen,Phytophthora infestans. Arachidonic acid and eicosapentaenoic acids, two polyunsaturated fatty acids isolated fromP. infestans, are potent inhibitors of steroid glycoalkaloid accumulation. Both acids elicit the localized accumulation of sesquiterpenoids including rishitin, lubimin, phytuberin, phytuberol and solavetivone. Rishitin and lubimin generally comprise 85–90% of the total sesquiterpenoids which accumulate. The steroid glycoalkaloids and sesquiterpenoids appear to have a role in disease resistance to some fungal pathogens. Both groups of compounds are synthesized via the acetate-mevalonate pathway. Arachidonic and eicosapentaenoic acids appear to inhibit steroid glycoalkaloid accumulation at the level of the conversion of farnesyl pyrophosphate to squalene and they activate the biosynthesis of sesquiterpenoids. The reduction of steroid glycoalkaloids in potato foliage and tubers for health and flavor considerations should be considered relative to the ability of tubers and foliage to accumulate sesquiterpenoids in response to infection and its influence on disease and insect resistance.     

Stage of development of leaf and tuber tissue of the potato plant influences the titer of potato virus M

The enzyme-linked immunosorbent assay (ELISA) was adapted for the detection of potato virus M (PVM). Detectability of PVM was influenced by the stage of plant growth and that of the plant part assayed. In young potato plants, one week prior to flowering, PVM titer was relatively high (0.77 A_405nm units) in the basal leaves and barely detectable (0.09 A_{405 nm} units) in the top leaves. In mature plants, two weeks prior to harvesting, top leaves contained more virus (0.34 A_{405 nm} units) than middle (0.24 A_405nm units) or basal leaves (0.15 A_{405 nm} units). In rapidly growing tubers PVM content was higher (1.57 A_{405 nm} units) than that in fully grown tubers (0.71 A_405nm units) whereas the virus was not detectable in the mother tuber four weeks following planting. PVM was reliably detected when disks from intact leaves were substituted for leaf extracts as the test sample     

Potato leafroll virus purification and antiserum preparation for enzyme-linked immunosorbent assays

Potato leafroll virus (PLRV) was purified from potato foliage and stems with an average yield of 0.14 mg of PLRV/kg of potato. Modifications of an existing purification procedure are reported. Five low dosage (38-118 μg of PLRV) intravenous injections were used to produce a PLRV antiserum for use in enzyme-linked immunosorbent assays (ELISA) from tubers. PLRV was readily detected in ELISA testing of potato tubers and leaves and inPhysalis floridana Rybd. Non-specific reactions were low with all tissues. In parallel tests, a Canadian antiserum produced higher nonspecific reactions with tuber and leaf tissue. The results indicated that the use of low dosage-intravenous injections might be necessary methodology for producing PLRV antiserum for use in ELISA diagnostic tests with tuber tissue where high non-specific reactions have been reported.     

Fungi associated with Collembola and mites isolated from scabby potatoes

Collembola and mites from the lesions of tubers infected with the acid scab organism were tested for the presence of fungal potato pathogens.Streptomyces spp. were found on and in the bodies of these arthropods.Verticillium albo-atrum was present on the external surface and in the body of the Collembola. Various other soil-inhabiting fungi which are not known to be potato pathogens and which were found on and in the bodies of these arthropods are noted.     

Evaluation of immunofluorescence with monoclonal antibodies for detecting latent bacterial ring rot infections

Corynebacterium sepedonicum was detected in symptomless potato stems and tubers with immunofluorescence using monoclonal antibodies specific for the bacterial ring rot pathogen. The concentration of bacterial cells in potato tissue preparations ranged from >500 cells/microscope field to 1 cell per preparation. Symptomless tubers containing ring rot bacteria planted in field plots yielded plants with ring rot symptoms, plants with latent ring rot infections, or plants with no detectable levels of ring rot bacteria. Tubers with the greatest number of bacteria were most likely to develop plants expressing ring rot symptoms, but even some seed tubers with a low number of bacteria developed into plants with symptoms. Some seed tubers with high levels of ring rot bacteria produced plants with only low numbers ofC. sepedonicum.     

Purification of potato leaf roll virus and an evaluation of methods for its diagnosis

To evaluate four diagnostic methods for potato leaf roll virus (PLRV), an antiserum was prepared against a virus preparation purified from infectedDatura stramonium L. by an exudation method. The antiserum had a titer of 1:64 in microprecipitin tests. In a procedure developed subsequently PLRV was purified by a method that involved grinding liquid nitrogen frozen stems, petioles, and veins from different solanaceous hosts. A chloroform and n-butanol clarification was done followed by two cycles of differential centrifugation. On sucrose gradients, these preparations formed one band containing spherical particles 25 nm in diameter. In Ouchterlony double-diffusion plates, the antiserum reacted positively with virus preparations from frozenPhysalis, Datura or potato tissue partially purified by two cycles of differential centrifugation. No visible reaction was obtained between the antiserum and the virus preparation from density gradient centrifugation. Antiserum neutralized infectivity of 40–50% of the virions in a partially purified preparation. No virus peak was detected in the scanning patterns obtained after a mixture of antiserum and virus was centrifuged on a sucrose density gradient. A few virus particles were seen attached to serologically specific grids. Methods to diagnose PLRV in infected potato tubers were then compared. In the first, aphids were used to acquire the virus from tuber sprouts. They transmitted the virus to 100% of thePhysalis test plants. Results of the second method, visual inspection of symptoms on potato plants, were similar to those of the first method. Sarkar’s electron microscopic method and immuno-specific grids were less efficient than aphid transmission to diagnose PLRV in tuber sprouts. The low virus concentration in infected tissue made these last two methods unreliable.     

TerraRossa: A Mid-Season Specialty Potato with Red Flesh and Skin and Resistance to Common Scab and Golden Cyst Nematode

TerraRossa (POR01PG20–12) is a mid-season specialty potato, released by Oregon State University, and is a product of the Northwest Potato Variety (Tri-State) Development Program. This cultivar is unique among commercially available potato cultivars in that plants produce small- to medium-sized smooth, oblong- to long-shaped tubers with red skin and red flesh. Total tuber yields of TerraRossa are similar to Dark Red Norland and less than Red La Soda. Average tuber size (136 g) is less than both of the comparison cultivars, reflecting inherent differences in tuber size distribution. TerraRossa tubers have total antioxidant levels higher than traditional white fresh varieties and comparable to the All Blue purple potato, known for its high antioxidant levels. Sensory evaluations of TerraRossa tubers indicated that it has good culinary attributes following boiling, baking, and microwaving. Potato chips made from TerraRossa tubers retained their unique red color, which represents a novelty in the chipping industry. TerraRossa could be a good candidate for the organic sector due to its tolerance to common scab (Streptomyces scabies) and tuber late blight (Phytophthora infestans) and its resistance to golden cyst nematode (Globodera rostochiensis). Due to its high yields, high chipping quality, good culinary properties, high antioxidant content and disease resistance, TerraRossa is a good candidate for opening new specialty type markets, adding diversity to the marketplace.     

A method for estimating the decrease in marketable tubers caused by potato late blight

A regression analysis was used to develop a method to estimate the decrease in marketable tubers caused by late blight of potato (Phytophthora infestans), based on estimates of percentage loss in total tuber yield computed from a published method. The economic loss due to late blight in table stocks is the relative percentage marketable yield loss, calculated from the expression: {fx19-01} when total yield losses (in percentage) did not exceed 40%. A graphical method is presented as an alternative. Yield results from seven field experiments conducted in the Maritime Provinces of Canada during the period 1953 to 1971 were used in the study, where marketable tubers were defined as 2 1/4 inch or more in diameter.     

Anatomical evidence for the existence of roots on potato tubers and stolons

Roots on potato tubers and stolons displayed the normal root anatomy which consisted of a central vascular cylinder surrounded by endodermis with Casparian strips, the cortex and epidermis. Tuber roots appear to initiate from the parenchyma cells adjacent to the vascular tissue. Shoot tips were similar to normal apical meristems. These observations support our research demonstrating the growth of functional roots from potato tubers and stolons.     

Carrot Pathogen ‘<Emphasis Type="Italic">Candidatus</Emphasis> Liberibacter solanacearum’ Haplotype C Detected in Symptomless Potato Plants in Finland

‘Candidatus Liberibacter solanacearum’ (CLso) haplotype C, a bacterial pathogen transmitted by the carrot psyllid Trioza apicalis, causes yield losses in carrot production. Due to concerns that this pathogen might also threaten potato (Solanum tuberosum) production, the occurrence of CLso in cultivated and volunteer potatoes in Tavastia Proper and Satakunta regions of Finland was studied. Volunteer potato plants were found in 13 of the 27 inspected carrot fields. Of the 148 potato samples tested by PCR, eight volunteer potato plants and one cultivated potato grown at the edge of a carrot field were found to be CLso positive. The PCR products obtained from these potatoes with primers OA2/OI2c, LpFrag4-1611F/LpFrag4-480R and CL514F/CL514R all showed 100% sequence identity to CLso haplotype C. This is the first observation of CLso haplotype C in field-grown potatoes. In addition, transmission experiments were performed. Attempts to transmit CLso into potato with carrot psyllids were not successful; however, CLso haplotype C was transmitted from infected carrots to potato plants by leaf grafting and by phloem connection formed by dodder, a parasitic plant, and found to survive in the potato plants for several weeks after transmission. However, the bacterial colonisation progressed slowly in the potato phloem and the amount of bacteria detected was low. The plants produced from the daughter tubers of the CLso-positive potato plants were all CLso negative, suggesting that CLso haplotype C was not able to pass to the daughter plants. None of the CLso-positive potatoes inoculated in greenhouse or collected from fields showed symptoms characteristic of zebra chip disease, associated with CLso haplotypes A and B.     

Correlation of phytotoxin production with pathogenicity ofStreptomyces scabies isolates from scab infected potato tubers

A total of 23 isolates obtained from scab infected potato tubers representative of six sampling areas in Eastern and Central Canada and five ATCCStreptomyces strains were screened for pathogenicity on the basis of their ability to initiate scab development on aseptically cultured minitubers and plant generated tubers. The results were then correlated with any associated generation of the scab inducing phytotoxin, Thaxtomin A. In all cases a positive correlation was demonstrated between the pathogenicity of variousStreptomyces scabies isolates and their ability to produce the phytotoxin.     

Use of enzyme-linked immunosorbent assay to detect potato leafroll virus in field grown potato,cv. Russet Burbank

Use of enzyme-linked immunosorbent assay (ELISA) in detecting potato leafroll infections in field grown potato, cv. Russet Burbank, was studied from 1986 to 1988 at Rosemount, Minnesota. The objective was to determine relative reliability of current season foliage ELISA, tuber tissue ELISA, and tuber progeny foliage ELISA. Serological tests were most accurate when foliage of tuber progenies was tested. ELISA underestimated total leafroll infection when current season foliage from the inoculated plant was used, in those plants inoculated during late tuber bulking stage. Current season foliage ELISA tests using newly expanded terminal leaflets were more reliable than were tests using older leaflets. Leafroll infection was detected in the current season foliage and tuber progenies (tuber tissue as well as tuber progeny foliage) of some plants seven days after inoculation. Most current season foliage infections were detected by day 14–28 depending on year. Differences among years were most likely caused by variation in quality of virus source plants and numbers of vectors used in inoculation. ELISA tests on tuber tissue were almost as effective as ELISA tests on tuber progeny foliage in detecting potato leafroll 20 days after inoculation, but ELISA on tuber tissue substantially underestimated infection if plants were sampled earlier. Maximum percent tuber infection occurred 20 days or more after inoculation. Movement of the virus from the inoculated stem to other stems decreased with increased plant age at inoculation. Percent infected tubers declined with increased plant age at inoculation. Action thresholds developed for aphids in managing potato leafroll virus should take into account the temporal change in percent infected tubers.     

Role of ‘<Emphasis Type="Italic">Candidatus</Emphasis> Liberibacter solanacearum’ and <Emphasis Type="Italic">Bactericera cockerelli</Emphasis> Haplotypes in Zebra Chip Incidence and Symptom Severity

Two haplotypes of the pathogen, ‘Candidatus Liberibacter solanacearum,’ (Lso) and four haplotypes of the insect vector, Bactericera cockerelli, are associated with zebra chip disease of potato. Whether disease severity or incidence is influenced by pathogen or insect haplotype is poorly understood. The role of Lso ‘A’ and ‘B,’ transmitted by three haplotypes of B. cockerelli, on disease severity and incidence in eight potato cultivars was analyzed. Both haplotypes of Lso induced tuber symptoms. In general, Lso B caused higher incidence of symptoms, and greater reduction in tubers compared with Lso A. Lso B was associated with more severe tuber symptoms, producing fewer mild or moderate tuber symptoms. Lso A was associated with less severe tuber symptoms, despite being able to induce severe symptoms. Disease incidence, tuber yield, and symptom severity ratings were not dependent upon the psyllid haplotype transmitting the pathogen, suggesting that pathogen, not insect haplotype affects Lso transmission.