Resistance to Columbia root-knot nematode inSolanum ssp. and in hybrids ofS. hougasii with tetraploid cultivated potato

Several tuber-bearingSolanum species were screened for resistance to races 1 and 2 of Columbia root-knot nematode,Meloidogyne chitwoodi. Reproductive factor (R_f = P_f/P_i) was assessed and used to infer host resistance. Several clonal selections in one Plant Introduction accession ofSolanum hougasii showed high levels of resistance to both races. Crosses ofS. hougasii with tetraploid breeding clones yielded 14 vigorous F₁ hybrids. Three of these interspecific hybrids expressed non-host (R_f<0.1) responses to race 1 and poor host (0.1f<1.0) responses to race 2. Six hybrids were nonhosts to race 1 but good hosts (R_f>1.0) to race 2. Three hybrids were good hosts to both races, and one was a poor host to race 1 and non-host to race 2. The occurrence of resistance to both races in the F₁ hybrids indicates success in the first step of introducing resistance to races 1 and 2 ofM. chitwoodi to the cultivated potato gene pool.     

Epidemiology of potato leaf roll virus in the Fraser River delta of British Columbia

The spread of potato leaf roll virus (PLRV) and the resulting phloem necrosis in the tubers has been a limiting factor in the production of the Netted Gem (Russet Burbank) variety in southwestern British Columbia, Canada. Steckling sugar beets are an important overwintering host for nymphs and adults of the green peach aphid,Myzus persicae (Sulzer). By June, aphids are widely disseminated to the florets and new leaves of sugar beet and other hosts, including potato. Colonies with alates are produced on summer hosts which are not treated with insecticides. The peak of the flight of alates from the colonies on the summer hosts occurs during the first half of August. Those alates that were produced on PLRV-infected potato and which travel to other potatoes transmit the virus as soon as they feed. By the end of August, the sugar beet seed crops and the early and mid-season potato crops have been defoliated or harvested. Thus many colonies ofM. persicae are destroyed and the threat of PLRV spread diminishes. Crop and weed plants which serve as overwintering hosts become infested during late summer with alate aphids. Their offspring survive the winter in numbers which are determined by the weather and survival of host plants.     

Albinism of potato spindle tuber viroid-infected Rutgers tomato in continuous light

Potato spindle tuber viroid (PSTV) caused albinism in the new growth of Rutgers tomato under continuous light. The albinism response was obtained with 4 isolates causing severe symptoms in tomato and with 1 mild isolate which is normally symptomless. Rutgers tomato inoculated with the 4 severe strains of PSTV developed typical ‘bunchy top’ symptoms in 12 to 16-hr day lengths. Symptoms developed slower and much less distinctly on plants incubated in 6-hr days. New growth in PSTV-infected plants was green in short days (12-hr illumination) and chlorotic to white in continuous light. Infectivities of extracts from white portions of plants were from 3 to 10 times greater than those from green portions of infected plants. Albino symptoms also developed in other tomato varieties when inoculated with PSTV and kept under continuous light. Albinism was most pronounced under continuous light at 30°C, somewhat less intense at 24°, and at 16°, new growth of infected leaves and stems remained green. Albinism did not develop in otherSolanum species which are symptomless hosts of PSTV nor in seedling potato plants grown in continuous light.     

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.     

Population dynamics of arthropods associated with early-season tomato plants as influenced by soil surface microenvironment

Two field studies were conducted in the San Joaquin Valley (SJV) of California in order to describe the association of arthropods and possible virus diseases with the plant canopy of early-season, bush-trained, fresh-market tomato (Lycopersicon esculentum L. cv. Shady Lady) over three soil surface microenvironments. The experimental treatments were based on alterations to planting bed surfaces, as follow: 1) soil covered with silver metalized, light-reflective plastic mulch; 2) soil covered with biological mulch consisting of killed, chopped, cool-season cover crop residues [mixture of triticale (X Triticosecale Wittm.), rye (Secale cereale L. cv. Merced), and common vetch (Vicia sativa L.)]; or 3) bare soil (not covered) – the SJV standard tomato production practice. In both studies, the predominant arthropods encountered in all treatments were the aphid species Aphis craccivora Koch, Macrosiphum euphorbiae Thomas, and Myzus persicae Sulzer; the western flower thrips (Frankliniella occidentalis Pergande), and the false chinch bug (Nysius raphanus Howard). These are all considered to be pests of cultivated tomato plants. Spiders were the most prevalent arthropod predator group recovered from foliage samples. All monitored arthropods, except spiders, were most abundant in tomato plants grown over the conventional, bare soil surface, and least abundant (P < 0.05) in plants grown over the silver plastic mulch. Spider numbers were highest in plant foliage over the biological mulch. Other arthropods commonly found on tomato plants in the SJV, including leafhoppers, whiteflies, various caterpillars, and predaceous taxa, were not commonly nor consistently encountered. This was likely due to the early-season timeframe of these studies. Similarly, foliar virus disease symptoms, which were originally targeted for monitoring, were virtually nonexistent. The only arthropod pest species encountered which consistently caused economically important damage to tomato fruit was N. raphanus. In both experiments, tomato fruits grown over the reflective plastic mulch had significantly less (69.5% and 39.7%, respectively) damage due to this pest than those over bare soil. The biological mulch gave inconsistent, intermediate damage reductions. Results from these two field studies showed that reflective plastic bed mulch suppressed populations of a variety of tomato canopy-associated arthropod taxa in early-season plantings, as compared to conventional, bare-soil beds. The biological bed mulch results were more inconsistent, but gave indications that killed cover crop residues, as well as the light-reflective plastic, may be of value in integrated pest management programs for fresh-market tomato production.     

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.     

Response of potato cultivars to North American isolates of PVYNTN

Potato cultivars were assessed for their sensitivity to potato tuber necrotic ringspot disease (PTNRD) induced by two North American isolates (Tu619 and Tu660) of the tobacco veinal necrosis strain of potato virus Y (PVY^N) and belonging to the tuber necrosis (PVY^NTN) sub-group. Several widely grown cultivars (e.g. Norchip and Ranger Russet) were found to be highly sensitive to PTNRD, while others (e.g. Russet Burbank, Russet Norkotah and Shepody) were not. The foliage symptoms induced by these isolates were distinct from those reported to be produced by European isolates of PVY^NTN, in that systemic necrotic rings and chlorophyll figures on the leaves were not produced. Instead, the symptoms were similar to those of normal strains of PVY^N. The apparent correlation between resistance to the PVY^O (common) strain group and cultivar sensitivity to PTNRD is discussed.     

Host range study of Argentine Nacobbus aberrans sensu Sher populations and comments on the differential host test

The existence of races within the species Nacobbus aberrans sensu Sher has been confirmed; however, there is no consensus on a consistent system for race determination. Four plant species previously used as differential hosts in some N. aberrans race tests (tomato, pepper, sugarbeet and potato) were tested for susceptibility to seven nematode populations from Argentina. Plants were inoculated with second-stage juveniles and kept under glasshouse conditions at 21 °C for 90 days. The parameters evaluated (root gall index, egg mass index, and reproduction factor) in different nematode populations and plants showed significant differences. The reproductive fitness of the nematode populations differed among the plants that showed some degree of susceptibility. The results showed that N. aberrans comprises populations differing in host preference. Based on the nematode populations’ behaviour on these hosts, two groups were distinguished: i) populations that multiplied on all hosts, ii) populations that did not multiply on potato. The present work contributes to the analysis of criteria for developing a differential host test.     

Evaluation of tuber-bearingSolanum species for symptomology,as diagnostic hosts,and sources of immunityto potato virus Y necrotic strain (PVYN)

A total of 258 Plant Introductions (PI) belonging to 69Solarium species were evaluated in the greenhouse for their reaction to the tobacco veinal necrosis strain of potato virus Y (PVY N). One hundred and thirty-one (50.7%) of the PI accessions produced mosaic symptoms ranging from mild to severe. Local lesion and veinal necrosis symptoms were observed in 19 PI accessions (7.3%) and a variety of other symptoms were observed in another 11 PI accessions (4.2%). Only 97 PI accessions (37.5%) were symptomless carriers of PVY^N. PI accession 473505 ofS. sparsipilum and PI accession 498021 ofS. brachycarpum developed local lesions and veinal necrosis with PVY^N, but necrotic spots and mosaic with PVY^o. Common mechanically-transmitted potato viruses A, S, M, and X did not interfere with PVY symptom development inS. sparsipilum andS. brachycarpum. Thus, PI 473505 and PI 498021 can be used as indicator plants for specific identification of PVY^N. PI accession 472819 ofS. chacoense developed local lesions with systemic spread in PVY^o, but without systemic spread in PVY^N. Thus, this can be used as a differential host plant for PVY strains. Two PI accessions ofS. stoloniferum, PI 160372 and 161171 were immune to PVY^N.     

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.     

Infection of aerial parts of potato plants byColletotrichum coccodes and its effects on premature vine death and yield

Collectotrichum coccodes, the pathogen causing black dot disease of potato, was investigated to determine its potential for causing symptoms on the aerial parts of potato plant, premature vine death, and reduction in yield under greenhouse and field conditions. Spray inoculations with conidial suspensions (10⁶ colony forming units/ml) ofC. coccodes caused sunken, dark, necrotic lesions on stems, leaves and petioles, vein and leaf necrosis, dieback and premature vine death under growth chamber and greenhouse conditions. Field inoculations in 1986 and 1987 provided evidence for an increased incidence of premature vine death in seven potato genotypes (Russet Burbank, Norgold Russet, Superior, Butte, NDA 8694-3, A68113-4 and A66107-51). Late season application of metribuzin at 0.84 kg/ha significantly increased the incidence of stems with black dot symptoms in cv. Russet Burbank at one location. Field inoculations in 1987 at two locations resulted in yield reduction in the late-maturing cv. Russet Burbank (6.3 and 6.5 t/ha) and clone A68113-4 (5.0 t/ha), but not in the early maturing cv. Norgold Russet. Similar results were obtained in greenhouse studies following soil inoculations withC. coccodes: yield reduction was observed in cv. Russet Burbank but not in cvs. Norgold Russet, Superior and Katahdin. Colonization of stem ends of tubers byC. coccodes from spray inoculated field plots was higher than in tubers from uninoculated plots at one of the two locations. These results point to a much broader potential ofC. coccodes as a potato pathogen, than hitherto documented. Colletotrichum coccodes (Wallr.) Hughes (syn.C. atramentarium (Berk. & Br. Taub.) was described in 1825 (21) and has a wide host range (5). Potato and tomato are the more economically important hosts. This fungus has been variously referred to as an unspecialized, minor pathogen or secondary invader (18), weak pathogen (22), specialized parasite (5), or superficial colonizer and widespread epiphyte (19). The symptoms produced by this pathogen on potato have been described as black dot (10, 14) on stems, stolons, tubers and roots, sometimes associated with leaf scorch or wilt (24), rubbery tuber wilt (1), and tuber skin necrosis (17). The fungus is commonly associated with senescent, decaying root and stem tissue, especially at the end of the season. In Idaho, instances of premature death of potato without typical symptoms of Verticillium wilt, but accompanied by a high incidence of stems with black dot symptoms, have been observed in potato crops grown on fields fumigated with metam sodium. Additional observations by growers have also implicated the possible interaction of the herbicide metribuzin with the incidence of black dot. Although it was observed by some workers to accelerate leaf senescence (12, 22), Schmiedeknecht (21) concluded thatC. coccodes was unable to infect the green leaves of potato plant. Even thoughVerticillium dahliae is generally considered the major factor in potato early dying (20), several workers (7, 11, 13, 19, 22) have investigated the possible role ofC. coccodes in causing premature death of potato (8). However, these studies examinedC. coccodes only as a soilborne, root and tuber pathogen of potato; its effects on shoot portions (aerial parts) of the plant were not considered. In addition to causing anthracnose of fruits of tomato, pepper and squash (18), recent reports indicate thatC. coccodes causes foliage blights on eastern black nightshade (2) and velvetleaf (25). The objectives of this study were: (1) to examine the potential ofC. coccodes to cause symptoms on aerial parts of potato plant, (2) to investigate the effect of spray inoculation in the field withC. coccodes on incidence of premature vine death, tuber colonization and yield of potato, and (3) to document the effect of soil inoculation withC. coccodes alone or in combination withV. dahliae on potato varieties under greenhouse conditions.     

Hosts of potato spindle tuber virus in suborder Solanineae

Additional hosts for potato spindle tuber virus (PSTV) found in the suborder Solanineae are:Nolana sp. (Nolanaceae);Antirrhinum sp.,Diascia barberae, Nemesia sp., N. foetens, Penstemon sp. (Scrophulariacceae); andBrowallia speciosa (B. major) andSolanum melongena in the Solanaceae. PSTV produced no local lesions upon inoculation of these hosts. Inoculated plants ofS. melongena (eggplant, ‘Black Beauty’) were dwarfed and epinastic. These rarely used genera in the Solanineae can be readily infected with the virus; leaf and floral parts are profuse and the tissues, upon grinding, leave very little residue.     

Male fertility and 2n pollen production in haploid-wild species hybrids

Hybrids (2n = 2x = 24) from crosses between Group Tuberosum haploids (2n = 2x = 24) and 24-chromosome, wild species must be fertile and produce 2n gametes if they are to be used in breeding schemes involving sexual polyploidization. Male fertility of the 2x hybrids depends primarily on the species parent. Hybrids involvingS. berthaultii, S. chacoense, S. kurtzianum, S. spegazzinii, andS. tarijense as parents are male fertile. The haploid parents are male sterile, so the species not only contribute genetic diversity and desirable traits, but also genes necessary for male fertility.S. infundibuliforme, S. raphanifolium, andS. sanctae-rosae parents produced male sterile hybrid progeny, while use ofS. boliviense, S. bukasovii, S. canasense, S. microdontum, andS. sparsipilum results in families containing both fertile and sterile plants. Sterility may be due to interactions between Tuberosum cytoplasm and dominant nuclear genes from the species. Genetic male sterility and environmental conditions may also be responsible for low pollen stainability in some hybrids. Many fertile, 2x hybrids produce 2n pollen. Selection for 2n pollen in the species is fast and efficient, but meiotic analysis is required in male sterile haploids. Fortunately, selection in the species parent alone results in a large number of 2n pollen-producing hybrids. These 2x hybrids have been used to generate tetraploids via 4x × 2x crosses.     

Host range,symptomology, and serology of isolates of potato virus Y (PVY) that share properties with both the PVYN and PVYO strain groups

In surveys for the tobacco veinal necrosis strain of potato virus Y (PVY^N) in potato, two isolates (I-136 and I-L56) were obtained that shared properties with both the PVY^N and the common (PVY^O) strain groups. The isolates produced veinal necrosis on tobacco (Nicotiana tabacum L.) and mild symptoms on potato (Solanum tuberosum L.) cv. Jemseg, typical of PVY^N but their symptoms on some other indicator species such asCapsicum frutescens L.,Chenopodium amaranticolor Coste & Reyn.),Physalis angulata L.,P. floridana Rybd. were more typical of PVY^O. Their serological properties were also more typical of PVY^O in that they reacted with a PVY^O-specific monoclonal antibody (MAb) in ELISA and they failed to react with four PVY^N-specific MAbs. The possible taxonomic position of these isolates is discussed.     

Aphid vectors of Papaya ringspot virus and their weed hosts in orchards in the major papaya producing and exporting region of Brazil

Papaya ringspot, one of the most important diseases of papaya, is caused by Papaya ringspot virus type P (PRSV-P), which is transmitted by various aphid species. Objectives of this research were to identify the aphid species and their weed hosts of papaya orchards in the major papaya producing and exporting region of Brazil in northern Espírito Santo State. Aphids were collected from weed hosts, and with Möericke-type traps in papaya orchards of twenty farms in Linhares, Sooretama and Aracruz municipalities of Espírito Santo, Brazil. A total of forty aphid species were collected, of which 22 are new records in the State. Thirty-eight aphid species were identified from traps, including Aphis craccivora Koch, A. gossypii Glover, A. spiraecola Patch, Myzus persicae (Sulzer), and Toxoptera citricidus (Kirkaldy), known vectors of PRSV-P. Cryptomyzus Oestlund (Aphididae) is recorded for the first time in Brazil. Thirteen aphid species associated with 22 weed species of 14 families were collected. Aphis gossypii, which has been reported to be one of the most efficient vectors of PRSV-P, infested the greatest number of weed hosts. Chamaesyce hyssopifolia (L.) Small, Euphorbia hirta L. (Euphorbiaceae), Commelina benghalensis L. (Commelinaceae), Lepidium virginicum L. (Brassicaceae) and Sida sp. (Malvaceae) were the most frequently infested weeds. Commelina benghalensis was infested by the greatest number of aphid species during this study. Integrated management of weed hosts of aphids to reduce aphid populations in papaya orchards may be useful for control of PRSV-P. Based on accurate knowledge of the aphid and weed species present in papaya orchards obtained in this study, integrated pest management efforts may be more efficiently directed toward eliminating the most frequently infested weed species that serve as hosts for aphid vectors of PRSV-P.     

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.     

A detached-leaf method to evaluate late blight resistance in potato and tomato

We developed a detached leaf method for evaluating potato (Solanum tuberosum L.) and tomato (Lycopersicon esculentum Mill.) germplasm for reactions toPhytophthora infestans, the causal organism of late blight. Primary leaves from the third to the sixth node of potato plants, and the fourth to sixth node of tomato plants were excised at the stem. Their petioles were inserted into 14 mm × 100 mm floral aqua tubes containing 9 ml of sterilized distilled water. The leaves in the aqua tubes were placed with abaxial sides down on galvanized metal hardware cloth (12.5 × 12.5 mm mesh). A 12.5 mm sensi-disc containing 50μl of 2 × 10⁴ zoospores was placed in the centers of the terminal leaflet and the second leaflet pair of the potato leaf. A single disc was placed on the center of the tomato leaf. The supporting hardware cloth was placed 2.5 cm above distilled water (2.5 cm deep) in 31 cm × 17 cm × 8 cm clear plastic boxes with tight fitting lids. Leaves on intact tomato and potato plants were inoculated in a similar manner and placed in a mist chamber. Lesion growth was determined 4, 5, and 6 days following inoculation. There were no significant differences in reactions to isolates ofP. infestans on detached and intact leaves of potato cultivars Green Mountain and Kennebec and the tomato cultivar Bonnie Best.     

Potato virus A lesions onPhysalis species

PVA produces distinct local lesions on open leaves ofPhysalis floridana, as does PVY. But, whereas PVA induced such lesions on detached leaves, PVY did not. PVX induced similar lesions, thus the test for PVA will be most useful in work with isolated cultures of PVA or with virus tested seed lines or in screening for resistance, where PVX is absent.P. floridana plants and leaves were best held at 15 to 18 C with low light intensity (4–5 K lux). From 4–6 week old potato plants, PVA was best obtained from lower leaves, ground in 0.05 M glycine — 0.03 M phosphate buffer, pH 9.2. One leaf disc infected with PVA could be detected in a composite sample with 9 discs from healthy leaves.P. angulata andP. pubescens may also be used for diagnosis of PVA. PVY becomes systemic in these species without inducing local lesions.