Helen E. Stewart

An illustrated assessment key for foliage blight of potatoes

SummaryA series of drawings illustrates Malcolmsons (1976) 1–9 scoring scale of categories of foliage blight.

A cysteine protease gene is expressed early in resistant potato interactions with Phytophthora infestans.

A potato cysteine protease (cyp) cDNA expressed at an early stage of an incompatible interaction with Phytophthora infestans was isolated. Both the nucleotide and deduced amino acid sequences are highly homologous to those of a tomato cysteine protease, CYP1. Striking protein similarity to all known cathepsins in animals, particularly cathepsin K, was also observed. However, unlike cathepsins, a granulin binding domain is located near the carboxyl terminus of the putative CYP protein. In animals, granulins bind to receptors in the plasma membrane and signal cell growth and division. A ribonuclease protection assay demonstrated that the cyp gene is tightly regulated and is induced 15 h post inoculation with P. infestans in potato leaves either with high field resistance or in which a resistance (R) gene is activated. We conclude that a common signaling pathway is activated in each form of resistance.

Effect of plant age and inoculum concentration on expression of major gene resistance to Phytophthora infestans in detached potato leaflets

Ten genotypes of a differential host series, each possessing different major (R) genes effective against races of Phytophthora infestans, were grown in a glasshouse to produce plants ranging between 3- and 10-wk-old at the time of inoculation. Detached leaflets were inoculated with a suspension of 2·5, 5, 10, 20 or 50 × 103 zoospores ml−1 applied as a single drop or a covering of fine droplets. Plants expected to show susceptibility uniformly gave spreading sporulating lesions, but resistance was variably expressed, with symptoms ranging from hypersensitive or symptomless reactions indicating immunity to spreading sporulating lesions covering all the leaf surface. Immune reactions were most prevalent on leaflets of flowering plants (8–10-wk-old). The incidence of immune reactions was greater with the drop than with the spray method of inoculation and decreased as spore concentration increased.

Potato oxysterol binding protein and cathepsin B are rapidly up-regulated in independent defence pathways that distinguish R gene-mediated and field resistances to Phytophthora infestans

SUMMARY Suppression subtractive hybridization was used to isolate the genes which are specifically up-regulated in the biotrophic phase of the incompatible interaction between a potato genotype, 1512 c(16), containing the resistance gene R2, and a Phytophthora infestans isolate containing the avirulence gene Avr2. Eight cDNAs were up-regulated in the biotrophic phase of the incompatible interaction. Seven of these were also up-regulated in the compatible interaction, but not until late in the necrotrophic phase. Amongst the sequences to be isolated were genes encoding the cysteine protease cathepsin B, StCathB, and an oxysterol binding protein, StOBP1; equivalent genes are involved in programmed cell death (PCD) processes in animals, but have yet to be implicated in such processes in plants. Whereas StOBP1 was up-regulated early in potato plants containing either R gene-mediated or moderate to high levels of field resistance, the highest levels of up-regulation of StCathB were observed early in R gene-mediated resistance but gradually increased from the early to late stages of field resistance, revealing these genes to be components of independent defence pathways and providing a means of distinguishing between these forms of resistance. StOBP1 was up-regulated by oligogalacturonides (plant cell wall breakdown products generated by pectinase activities), indicating that it is also a component of a general, non-specific defence pathway and is unlikely to play a role in PCD. In contrast, the expression of StCathB was unaffected by oligogalacturonide treatment, further associating its up-regulation specifically with the gene-for-gene interaction.

Correlation between resistance to late blight in foliage and tubers in potato clones from parents of contrasting resistance

SummaryThe relationship between resistance to late blight in foliage and tuber was examined on 50 clones of each of five progenies from crosses where one parent was resistant in both foliage and tuber and the other susceptible. Foliage resistance was assessed in a field trial and tuber resistance in a laboratory test on glasshouse-grown tubers. The genetical and environmental components of variance and the phenotypic and genetical correlations between foliage and tuber blight scores were determined for each progeny. Foliage and tuber resistance were correlated, indicating either that both are determined by the same genes or by different linked genes.The environmental component of variation was greater for tuber blight than foliage blight. We suggest that the most effective way of selecting for resistance to both aspects of the disease in a breeding programme is to select those resistant in the foliage and then screen them for tuber resistance.

Inheritance of resistance to late blight in foliage and tubers of progenies from parents differing in resistance

SummaryTenSolanum tuberosum genotypes differing in resistance to late blight (Phytophthora infestans (Mont.) de Bary) in foliage and tubers were intercrossed to determine the inheritance of disease resistance in their progenies. Plots of 10–15 clones per progeny were established in each of 2 years and resistance assessed by field or laboratory tests. The parental genotypes were similarly tested each year.The parents differed in general combining ability (GCA) for both foliage blight (FB) and tuber blight (TB). The parental and GCA scores were significantly correlated for both aspects of the disease, but the correlations between foliage and tuber scores for parents and for GCAs were not significant.Three parental genotypes were highly resistant in both foliage and tubers, and the genotype with the highest GCA for resistance to both FB and TB (cv. stirling) is recommended as the best parent. There was no evidence of strong genetic correlation between both aspects of resistance, and it is suggested that both be selected for in a breeding programme.

Resistance to late blight in foliage (Phytophthora infestans) of potatoes assessed as true seedlings and as adult plants in the glasshouse

SummaryTrue seedlings from crosses designed to produce progenies having high levels of field resistance to late blight (Phytophthora infestans (Mont.) de Bary) were scored in lots of 25 in a glasshouse. The results agreed with assessments subsequently obtained from plants grown from tubers from the same progenies when screened as individual whole plants in the glasshouse.The resistance of seedlings of both resistant and susceptible progenies was assessed by inoculating them 3–8 weeks after sowing. All were highly susceptible when inoculated at 3 weeks but resistance increased from 3 to 6 weeks and differed little between 6 and 8 weeks. The seedling test appears to provide a valid screen for resistance at an early stage in a breeding programme.

Disease resistance inSolanum phureja and diploid and tetraploidS. tuberosum × S. phureja hybrids

SummaryDiploidSolanum phureja, diploid and tetraploidS. tuberosum × S. phureja hybrid, and tetraploidS. tuberosum backcross clones were assessed for resistance to commercially important diseases. There was a reduction in the expression ofS. phureja characteristics with increasing dosage ofS. tuberosum genetic material. The generally high resistance of theS. phureja group to common scab, potato leafroll virus and potato virus Y decreased, while resistance to gangrene, foliage and tuber blight tended to increase.Resistance genes were probably well dispersed throughout theS. phureja genotype so that many were lost on hybridisation, butS. phureja could be a useful source of scab and virus resistance in the production ofS. tuberosum cultivars.

Assessing the resistance to gangrene of progenies of potato (Solanum tuberosum L.) from parents differing in susceptibility

SummaryThe resistance to gangrene (caused byPhoma exigua var.foveata) of progenies from two hybridisation schedules involving cultivars resistant, susceptible, or of intermediate reaction to gangrene was assessed. Two methods of sampling tubers from field-grown single-plant plots of 18 clones representing each progeny were used: one tuber was taken from each plant, and a bulk sample of 18 tubers was selected at random from the whole progeny. Gangrene resistance was assessed after incubating tubers which had been rolled in cornmeal-sand inoculum ofP. exigua var.foveata, and was expressed as a percentage of tubers infected. Results from the two methods of sampling correlated closely, and agreed well with tests in a subsequent year on 10-tuber samples of each individual clone.It is concluded that an 18-clone sample adequately represented each progeny, and that the mid-parent gangrene score satisfactorily predicted the resistance of the resulting progeny. There are indications that susceptibility tended to be dominant. The implication of these findings to breeding for resistance to gangrene is discussed.

Dissection and analysis of quantitative disease resistance in tetraploid potato

The theory of linkage and QTL analysis in autotetraploids is summarised. Applications are presented to the analysis of quantitative disease resistance in the European cultivated potato, Solanum tuberosum subsp. tuberosum. The implications are discussed for practical potato breeding, practical QTL analysis and theory.