G. S. Santo

RFLP analysis of resistance to Columbia root-knot nematode derived from Solanum bulbocastanum in a BC2 population

The mapping of resistance toMeloidogyne chitwoodi derived from Solarium bulbocastanum is reported. A population suitable for mapping was developed as follows. A somatic hybrid of nematode-resistant S. bulbocastanum and cultivated tetraploid potato was produced. This was backcrossed to tetraploid potato, and a single resistant BC1 was selected and backcrossed again to the same recurrent tetraploid parent. The mapping population consisted of 64 BC2 progeny scored for restriction fragment length polymorphic (RFLP) markers and 62 of these were evaluated for the reproductive efficiency of race 1 of M. chitwoodi. Forty-eight polymorphic RFLP markers, originally derived from tomato and mapped in diploid cultivated potato, were assigned to 12 chromosomes of S. bulbocastanum. Of the 62 progeny screened for nematode resistance, 18 were non-hosts and four were poor hosts. The rest were highly susceptible (good hosts). Analysis of the resistance (including non-hosts and poor hosts) as both a qualitative trait and as a meristic trait on which QTL analysis was applied supported the same genetic hypothesis. Genetic control was localized solely to factor(s) lying at one end of chromosome 11. The level of expression of resistance in the S. bulbocastanum parent and the resistant portion of the BC2 was essentially the same. This fact, together with the highly significant LOD scores for one end of the chromosome-11 marker array, supports a genetic model equivalent to monogenic dominant control.

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 (Rf = Pf/Pi) 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 (Rf<0.1) responses to race 1 and poor host (0.11.0) 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.CompendioSe evaluaron y seleccionaron varias especies deSolanum tuberizantes para resistencia a las razas 1 y 2 del nematodo del nódulo de la raíz de Columbia,Meloidogyne chitwoodi. Se determinó el factor de reproducción (Rf = Pf/Pi) para inferir la resistencia del hospedante. Varias selecciones clonales en una entrada de la Introductión de Plantas desolanum bougassi mostraron altos niveles de resistencia a ambas razas. Cruzamientos con clones tetraploides de mejoramiento rindieron 14 vigorosos híbridos F1. Tres de estos híbridos interespecíficos no expresaron respuesta del hospedante (Rf<0,1) a la raza 1 y respondieron pobremente (0,11,0). Tres híbridos fueron buenos hospedantes para ambas razas y uno fue un pobre hospedante para la raza 1 y no fue hospedante de la raza 2. La presencia de resistencia a ambas razas en los híbridos F1 indica el éxito del primer paso de la introducción de la resistencia a las razas 1 y 2 deM. chitwoodi al pul de genes de la papa cultivada.

Ecology ofParatrichodorus allius and its relationship to the corky ring-spot disease of potato in the Pacific Northwest

Paratrichodorus allius, a vector of the tobacco rattle virus which causes the corky ringspot disease (CRS) in potato, was identified in soil samples from Oregon and Washington. Two populations from potato fields in Pasco, WA and Umatilla, OR reproduced successfully on alfalfa, wheat, and corn which are commonly rotated with potato, and most weeds associated with this crop in the Pacific Northwest. Greenhouse studies showed thatP. allius was sensitive to low soil moisture, and increased 10 and 100 fold on Samsun NN tobacco in soil above field capacity compared to 1/2 and 1/3 field capacity, respectively. Population dynamic studies for 1996 and 1997 on a potato-wheat cropping sequence showed thatP. allius declined after the winter wheat cover crop was disked in the spring of 1996, and remained low on potato at the 0–90 cm soil profile, with no evidence of downward migration. In 1997,P. allius declined early in the season on wheat, and remained low until June–July when it peaked before declining again. In soil columns,P. allius placed 30 cm below a confined tobacco root system was able to reach and transmit tobacco rattle virus. The nematodes below 30 cm transmitted the virus only when plant roots were allowed to grow to that depth.

Weed Hosts of Paratrichodorus 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.ResumenLa capacidad de diversas especies de malezas para servir como hospedante del virus rattle del tabaco (TRV), agente causal de la enfermedad de papa conocida como anillo corchoso (CRS) y de su vector, el nematodoParatrichodorus allius ha sido investigada en estudios de invernadero. ElP. allius se multiplicó en 24 de las 37 especies de malezas probadas, lo cual indica que son hospedantes apropiados del vector. Sin embargo, solamente 11 de estas malezas se infectaron con TRV de acuerdo a la determinación por el método ELISA. La condición de no hospedante de una especie dada de maleza no cambió aún en el caso de que la población del vector virulífero se hubiera originado en campos con problema de CRS de WA, OR, o ID. Diferentes malezas sirvieron como hospedante del vector y del virus incluyendo cochia-alfalfa, escarola, henbit, especies de hierba mora (negra, pilosa y de hoja cortada), hierba pajarera común y lechuguilla silvestre anual.P. allius libre de virus adquirió el TRV de las tres especies de hierba mora, de las plantas voluntarias de papa provenientes de tubérculos infectados y de la escarola y subsequentemente transmitió el virus a las plantas indicadoras de tabaco Sansum NN’. Es así como algunas malezas pueden jugar un rol en la epidemiología del CRS, perpetuando el TRV y su vector en un campo problema.

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.

Impact of wheat and corn as rotational crops on corky ringspot disease of Russet Norkotah potato

Tobacco rattle virus (TRV), the causal agent of corky ringspot disease of potato (CRS), was detected by reverse transcription-polymerase chain reaction (RT-PCR) in root tissues of field- and greenhouse-grown wheat (two) and corn (10) cultivars. TRV was also detected in some of the same samples by enzyme-linked immunosorbent assay (ELISA), but in a low frequency. Compared to ‘Samsun NN’ tobacco, wheat and corn appeared to be poorer hosts of TRV. However, these crops served as inoculum reservoirs of TRV. A nonviruliferous population of the nematodeParatrichodorus allius, the vector of TRV, acquired the virus from infected wheat and corn roots, and transmitted it to potato. ‘Russet Norkotah’ potatoes grown following TRV-infected corn and wheat, were blemished with arcs, concentric rings and diffuse browning, characteristic symptoms of CRS. The observations presented here suggest that wheat and corn as rotational crops will sustain TRV in a CRS problem field and the virus may cause severe damage on subsequent potato crops.ResumenEl virus “rattle” del tabaco (TRV en inglés), agente causal de la enfermedad de la papa denominada mancha de anillo corchosa (CRS), fue detectado por transcriptión inversa de la reacción en cadena de la polimerasa (RT-PCR) en el tejido de las raíces de dos cultivares de trigo y diez cultivares de maíz cultivados en campo y en invernadero. El TRV también fue detectado en algunas de las mismas muestras mediante el ensayo de inmunoabsorción con conjugados enzimáticos (ELISA), pero en baja frecuencia. Comparado con el tabaco de la variedad “Samsun NN”, el trigo y el maíz parecen ser hospederos más precarios de TRV. Sin embargo, esos cultivos sirvieron como reservorios de inóculos de TRV. Una población no virulenta del nematodoParatrichodorus allius, el vector del TRV, adquirió el virus de las raíces infectadas de trigo y maiz, y lo transmitió a la papa. Las papas de la variedad “Russet Norkotah” que crecieron a partir de la infestatión del TRV al maíz y trigo, estuvieron manchadas con arcos, anillos concéntricos y presentaron color marrón difuso, sintomas característicos de CRS. Las observaciones que aquí se presentan sugieren que el trigo y el maíz como cultivos de rotación mantendrán el TRV como un problema de campo, pudiendo este virus causar daños severos en los subsiguientes cultivos de papa.