Correlation between late blight resistance and foliage maturity type in potato

The genetics of race-non-specific foliage resistance against Phytophthora infestans, of foliage maturity type, and of their association in potato (Solanum tuberosum) were studied. Six progenies were derived from a half-diallel set of crosses between diploid potato clones that represented a broad pool within the genus Solanum and were free of any of the 11 known R genes for late blight resistance. The progenies were evaluated for resistance to late blight and for foliage maturity type, and five of them showed a significant correlation between the two traits. The correlation did not account for all variation that was present for both traits, as reflected in the analysis in which the relative AUDPC values were adjusted for foliage maturity type. The present study adds to previous results: resistance against P. infestans always coincides with late foliage maturity. However, the results also indicate that some selection for late blight resistance without affecting the foliage maturity type should be possible.     

Early generation selection for potato tuber quality in progenies of late blight resistant parents

Developing disease resistant cultivars is one of the major objectives for a potato (Solanum tuberosum L.) breeding program, but many resistant clones have not achieved commercial acceptance because of late maturity and non-marketable tuber characteristics. Selection for tuber quality should have greater emphasis inbreeding disease resistant cultivars. The objectives of this study were 1) to evaluate the ability of late blight (Phytophthora infestans (Mont.) de Bary) resistant parents to transmit chip-processing (tuber appearance, specificgravity, and chip-color) or tablestock (tuber appearance) quality to the offspring and 2) to compare selecting for tuber quality in single-hill versus eight-hill generations. We made crosses among eight unadapted potato cultivars (B0718-3,Bertita, Bzura, Greta, Libertas, Stobrawa, Tollocan, and Zarevo) with reported late blight resistance with adapted susceptible cultivars/breeding clones to generate 95populations (4,750 seedlings). Approximately 10% of the progeny from each cross were selected from single-hill plots based on tuber appearance, number, shape, and internal defects. These selected clones (408) were evaluated for tuber appearance, specific gravity, and chip-color. The same evaluations in the following year were made on tuber samples from eight-hill plots. Libertas and Tollocan were the best parents for transmitting chip-color; B0718-3, Zarevo, and Tollocan for transmitting tuber appearance; and Bzura, Libertas, and Zarevo for transmitting high specific gravity to the highest percentage of the offspring. Overall, 50% and 56% of the clones based on single- and eight-hill generation, respectively, were considered to possess chip-processing quality; over 90% of the clones had acceptable tablestock quality. A total of 71% of the clones possessing acceptable chip-processing and 95% of the clones possessing acceptable tablestock quality selected in both generations were identified in single-hill plots. The evaluation of tuber quality characteristics in single-hill generation not only permitted the identification of clones with acceptable chip-processing and tablestock, but also increased the amount of clonal information for the following generation of selection. In crosses between late blight resistant and susceptible clones, selection for tuber quality traits can be initiated in single-hill generation using a moderate selection intensity and precede late blight testing. This revised version was published online in August 2006 with corrections to the Cover Date.     

Inheritance of resistance to late blight (Phytophthora infestans) in potato

Under epiphytotic conditions for late blight in spring seasons, data were recorded on its intensity four times at 4 days intervals from the start of the disease in the field, in 114 (19 females × 6 males) progenies and their parents planted in randomized complete block design in the years 2005 and 2006. Regression and stepwise regression analysis showed that observations during the rapidly increasing phase of disease between initial and last phase of disease are more important than the observations at initial and last phase of the disease. Combining ability analysis on a sub-set of 68 progenies showed that the additive component of genetic variance was more important than the non-additive component of variance in inheritance of quantitative resistance to late blight. The per se performance of the parents does give an idea about their general combining ability (GCA). However, selection of parents based on their GCA will be very useful for breeding for quantitative resistance to late blight. Parents JX 90, JF 4841, CP 3356, CP 1358, CP 3290, JN 1197 and CP 3125 were found to have good GCA for quantitative resistance for late blight and the best six crosses for late bilght resistance based on mean performance involved parents with good combining ability only.     

Search for sources of early blight (Alternaria solani) field resistance not associated with vegetative late maturity in tetraploid potato germplasm

Summary A total of 934 potato cultivars and clones from different breeding programmes across the world were tested for their reaction to early blight (Alternaria solani) in Brasilia-DF, Brazil. This field trial was set up in order to identify tetraploid potato genotypes with useful levels of resistance to early blight (EB) that were not correlated with late maturity or lateness. Artificial inoculation (spread ofA. solani infected leaves over plant canopy) was employed even though EB occurs naturally at epidemic levels in the test area. Four disease assessments were made every 10 days, beginning 45 days after planting. Based on the evaluation made 65 days after planting, only 27 clones were identified that had less lesioned leaf area than the resistant standard (cv. Aracy). Area under disease progress curve (AUDPC) was also used as a criterion of resistance. Using AUDPC as a parameter, 22 genotypes were identified presenting values lower than or identical to cv. Aracy. Differences in the commercial yield were recorded in genotypes with identical response to EB, suggesting that distinct levels of tolerance to EB also exist. In spite of the strong correlation between EB resistance and late maturity, some genotypes derived from the parental materials NDD 277-2, Kufri Jyoti, CIP 377888-7, Maine-28, and the clones CFS 69.1 and I-853 were identified as having good levels of EB field resistance not linked with the negative phenotypic trait of lateness. These materials presented medium to medium-early maturity with a vegetative cycle of around 90–95 days. Some selected genotypes presented more acceptable agronomic characteristics including resistance toPhytophthora infestans, potato leaf roll luteovirus (PLRV) and potato virus Y (PVY). The EB resistance sources identified in this work can be recommended as parental materials for widening the genetic base for EB disease resistance in breeding programmes for highland tropical and subtropical areas.     

Field resistance to late blight (Phytophthora infestans (Mont.) De Bary) in potato (Solanum tuberosum L.) somaclones associated with instability and pleiotropic effects

Summary Adventitious regenerants (somaclones) of Bintje and their vegetative progeny were screened for field resistance to Phytophthora infestans as follows: the area under the disease progress curve was computed and correlated with resistance rating in Bintje and reference varieties. The resistance rating of the somaclones was determined from this relationship.Clones with stable improved field resistance in successive years' trials were detected, however, most of such clones were also maturation mutants. Variation in resistance rating in clone replicates and between years was detected in most clones.The possible basis of the field resistance and reasons for its instability are discussed.Abbreviations AUDPC area under the disease progress curve     

Field resistance to late blight and potato root eelworm in group tuberosum dihaploids

Summary An examination of the feasibility of obtaining dihaploids with high levels of field resistance to important potato pathogens, showed that dihaploids may be produced which are more resistant to late blight or to potato root eelworm pathotype E than are their tetraploid parents. The significance of this is discussed in relation to the possible types of gene action involved in conferring field resistance.     

Resistance to potato late blight (Phytophthora infestans (Mont.) de Bary) in Solanum nigrum,S. villosum and their sexual hybrids with S. tuberosum and S. demissum

Summary Solanum nigrum and S. villosum, and their sexual hybrids with S. tuberosum and S. demissum respectively, were inoculated with a complex race of Phytophthora infestans. No visible reaction was seen on S. villosum and one genotype of S. nigrum. Another genotype of S. nigrum and the hybrids showed a hypersensitive response on most inoculated leaves. In one experiment, some sporulation was observed on detached leaves of a hybrid derived from S. nigrum. Microscopical examination of infections in S. nigrum and in a hybrid from S. nigrum, showed that penetration of epidermal cells and subsequent intercellular growth of the pathogen into the spongy mesophyll occurred, but without the formation of haustoria, and that invaded and neighbouring cells became necrotic. Callose appositions were found in epidermis and mesophyll cells of all inoculated genotypes, and also in epidermal cells of the unrelated nonhost species Brassica campestris.     

Combination of resistance to potato late blight in foliage and tubers by intraspecific dihaploid protoplast fusion

Five dihaploid Solanum tuberosum genotypes, encompassing different levels of polygenically inherited resistance to potato late blight disease (Phytophthora infestans (Mont.) de Bary) in foliage and tubers, were used in four intraspecific protoplast fusion combinations. Vigorous growing putative hybrid calli were selected four weeks after electrofusion. Intact plants were regenerated from 6% of the selected calli. Verification of hybridity was accomplished by use of RAPD analysis which revealed that 53% of the regenerated plants were true somatic hybrids. The score of true somatic hybrids in the different fusion combinations ranged within 21% – 100%. The hybrid plants were analysed for resistance to foliage blight in a field trial and assessed for tuber blight resistance by use of a laboratory test. Resistance to late blight in foliage and tubers varied between the hybrids. Very high levels of resistance to both foliage and tuber blight were obtained in some hybrids. However, loss of resistance in some hybrids as compared to the parental plants were also observed. Possible reasons for the phenotypic disappearance of resistance to either foliage or tuber blight are suggested. This revised version was published online in July 2006 with corrections to the Cover Date.     

Race specific resistance against Phytophthora infestans in potato is controlled by more genetic factors than only R-genes

Summary For RFLP mapping of R-genes, determining resistance to specific races of Phytophthora infestans in tetraploid potato, it is necessary to develop well segregating populations at the 2x level. During mapping studies, evidence was obtained that more genetic factor(s) are involved in the expression of R-genes than conventionally believed. Two experiments are described in which such an additional genetic factor was suppressing or enhancing the expression of unknown R _nand R _ifactors. R _nand R _iappeared to be present in the investigated plant material, containing R4 and R10, or in one of the susceptible crossing parents. In a third experiment, the expression and the segregation of the well known R1 gene was influenced by an additional genetic factor. In that case there were indications for a dominant suppressor. This was established by the selection of susceptible plants carrying a RFLP allele of probe GP21 closely linked to R1. In three of the four F₁ populations, resulting from crosses between such susceptible plants and susceptible tester plants, resistnat progenies were found. The resistance appeared to be R1-specific. This clearly indicates that in three of the four investigated susceptible plants, the R1 gene was still present but not expressed.     

Linkage and quantitative trait locus mapping of foliage late blight resistance in the wild species Solanum vernei

The global cultivation of potato (Solanum tuberosum) is threatened by epidemics caused by new variants of the late blight pathogen, Phytophthora infestans. New sources of durable late blight resistance are urgently needed and these may be found in wild Solanum species. The diploid wild species, S. vernei, has not previously been subjected to mapping of quantitative trait loci (QTLs) for late blight resistance. Two populations designated HGIHJS and HGG, originating from a cross between a clone of S. vernei and two different S. tuberosum clones were evaluated in field trials for late blight infestation. The relative area under the disease progress curve (RAUDPC) was estimated and used for QTL mapping. A linkage map of S. vernei, comprising 11 linkage groups, nine of which could be assigned to chromosomes, was constructed. Results indicated that the resistance in S. vernei was quantitatively inherited. Significant QTLs for late blight resistance were identified on chromosomes VIII (HGG), VI and IX (HGIHJS). In addition, potential QTLs were detected on chromosomes VII (HGIHJS) and IX (HGG). A putative and a significant QTL for tuber yield were found on chromosomes VI and VII in HGG, but no linkage between yield and resistance was indicated. The QTL for late blight resistance, which mapped to chromosome IX, could be useful for late blight resistance breeding as it was located close to the microsatellite marker STM1051 in both populations.     

Vertical resistance to late blight in wild potato species from Argentina

The possible presence of vertical resistance to late blight conferred by R genes in Argentine wild Solanum species, which presumably have not evolved under the pressure of the fungus, was investigated. Solanum microdontum, S. commersonii and S. chacoense clones were tested in the greenhouse and with detached leaves in the laboratory after inoculation with a complex race and a non-virulent race of Phytophthora infestans. Two progeny tests were carried out to assess the performance of contrasting parents in regards to their resistance. A varying frequency of incompatible clones was detected among the species, with S. microdontum having the higher proportion of clones yielding incompatible reactions both in the laboratory and in the greenhouse. S. chacoense and S. commersonii showed a small but still considerable frequency of incompatible clones. The progeny tests confirmed the presence of R genes in a S. commersonii cross and their absence in a S. chacoense cross. The unexpected presence of R genes, its evolutionary significance and the consequences on the use of these species in breeding is discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

The breeding values of potato parents for field resistance to late blight measured by whole seedlings

Summary Parents used in a potato breeding programme were intercrossed in a multiple mating scheme. The resulting progenies were assessed for resistance to late blight, using as criteria the size and frequencies of lesions on leaves, on petioles and on stems, and an overall score. Analysis of variance showed that all the statistically significant genetic variation was attributable to general combining ability (GCA) differences. The seven variates were highly correlated.     

Intraspecific somatic and sexual hybridisation between dihaploid lines of Solanum tuberosum L.: evaluation of morphological traits and resistance to late blight Phytophthora infestans (Mont.) de Bary in the foliage

Intraspecific tetraploid somatic and sexual hybrid plants have been resynthesised following protoplast fusion and by sexual crosses between two dihaploid potato (Solanum tuberosum) lines each possessing complementary agronomic traits. The dihaploid PDH 40 possesses good tuber shape and yield but has foliage susceptibility to late blight (Phytophthora infestans). On the other hand, the dihaploid PDH 727 possesses resistance to blight in the foliage but has a low yield of small and irregular shaped tubers. Since it was only possible to use a partial selection strategy based on culture media to facilitate recovery of somatic hybrid plants-further morphological and esterase isozyme based characterisations were performed to identify somatic hybrid plants from amongst the non-hybrid plant material. When the blight resistance of both the intraspecific somatic and sexual hybrid plants was assessed there was no significant difference in the mean resistance value and it was intermediate between those of their parents. However, the range of resistance was much wider among the sexual hybrids than among the plants derived from somatic fusion. An assessment of tuber yield between tetraploid sexual and somatic hybrids showed no significant difference and it was higher than that of either parent value. The implication of these results in the context of potato genetics and breeding is discussed.     

Chilean potato germplasm with resistance to potato viruses Y and S

Summary Four clones of the Chilota Potato Collection from the germplasm bank of the Universidad Austral de Chile, with potential combined resistance to PVX, PVY and PVS, were mechanically inoculated with these viruses in order to demonstrate wether they were truly resistant or not. Virus coat protein in the inoculated clones was determined by means of NCM-ELISA, DAS-ELISA, Western Blot and NASH.Two of the four inoculated clones were resistant to both PVY and PVS. This information will allow the utilization of these two clones in plant breeding programs.Abbreviations NCM-ELISA Nitrocellulose membrane enzyme-linked immunosorbent assay - DAS-ELISA double antibody sandwich enzyme-linked immunosorbent assay - NASH nucleic acid spot hybridization     

Inheritance of early blight resistance in diploid potatoes

Early blight (Alternatia solani) is a fungal disease in hot and humid environments, which causes leaf, stem and tuber lesions. Early blight resistance should be incorporated into potato cultivars because the fungicide spraying is an expensive solution for developing countries. The diploid cultivated species Solanum tuberosum group Phureja and group Stenotomum are sources of resistance alleles. The elucidation of the inheritance for early blight resistance must help to decide what could be the best breeding procedure to improve this diploid germplasm and transfer the resistance to the tetraploid level. Three experiments were carried out under controlled and field conditions to determine the heritability of this trait using nested and diallel mating designs with haploid, species and haploid-species hybrids. The narrow-sense heritability estimates were relatively high (0.64–0.78). This means that additivity was the most important type of gene action for determining resistance to early blight at the diploid level. The results suggested that diploid parents showing highest levels of resistance, throughout the cycle of disease development, can be used in 4x×2x crosses to obtain resistant tetraploid progenies to this fungal disease.     

Mutation breeding for resistance to blackspot bruise and low temperature sweetening in the potato cultivar Lemhi Russet

A mutation breeding method was developed to select clones of Lemhi Russet that have resistance to blackspot bruise and low temperature sweetening. Following irradiation with gamma rays from a Cobalt⁶⁰ source, over 2000 tuber eye pieces were planted directly to the field and tubers from the resulting crop were individually evaluated for blackspot bruise potential. Selection for blackspot bruise resistance continued for five clonal cycles. Selection for low temperature sweetening began in the M₃ and continued for three cycles. Ten clones were selected, eight with significantly (p=0.05) better blackspot bruise resistance, and two with increased resistance to low temperature sweetening. The results confirm the possibility for the development of a system to improve single selectable quality characteristics in potatoes.     

Molecular markers for late blight resistance breeding of potato: an update

Late blight is the most devastating disease of the potato crop that can be effectively managed by growing resistant cultivars. Introgression of resistance (R) genes/quantitative trait loci (QTLs) from the Solanum germplasm into common potato is one of the plausible approaches to breed resistant cultivars. Although the conventional method of breeding will continue to play a primary role in potato improvement, molecular marker technology is becoming one of its integral components. To achieve rapid success, from the past to recent years, several R genes/QTLs that originated from wild/cultivated Solanum species were mapped on the potato genome and a few genes were cloned using molecular approaches. As a result, molecular markers closely linked to resistance genes or QTLs offer a quicker potato breeding option through marker‐assisted selection (MAS). However, limited progress has been achieved so far through MAS in potato breeding. In near future, new resistance genes/QTLs are expected to be discovered from wild Solanum gene pools and linked molecular markers would be available for MAS. This article presents an update on the development of molecular markers linked to late blight resistance genes or QTLs by utilization of Solanum species for MAS in potato.     

Comparative performance of diploid and tetraploid progenies from 2x.2x crosses in potato

Summary Selection criteria for agronomic characters in a potato breeding program at the diploid (2x) level may differ from selection criteria used when selecting breeding lines at the tetraploid (4x) level. Differences between selection criteria are expected, (1) when expression of the characters is different at both ploidy levels and/or (2) when the effect of diploid breeding lines on agronomic characters of tetraploid progenies is different from the effect of tetraploid breeding lines. In this investigation sets of diploid and tetraploid progenies, each set derived from the same 2x.2x cross, were compared as to the expression of six agronomic characters. Diploid progenies had significantly lower yields (due to smaller tubers) and significantly higher under water weights than tetraploid progenies. Vine maturity and chip colour were similarly expressed at both ploidy levels. Correlations between yield and yield components, and between under water weight and chip colour were similar at both ploidy levels. The lower yields and higher under water weights found in diploids point to the need of different selection criteria for selecting diploid and tetraploid breeding lines.     

Genetic base of Indian potato selections as revealed by pedigree analysis

Pedigree analysis of the 77 advanced Indian potato selections showed that their origin could be traced to only 49 ancestors out of which 29 were exotic, which accounted for 69.52% of the total genomic constitution and maximum contribution (40.65%) was by 10 ancestors from U.K. Breeders tendency was to involve adapted advanced clones as immediate parents. Coefficient of relationship between pair of selections ranged from 0 to 0.68. The success of the parents used was not related to region-specific choice of parents. The findings are discussed in the context of genetic uniformity and the relevance of having separate breeding programmes for the three major areas of potato cultivation in India. Attempt has also been made to give some directions for choosing selections for use as parents and release as cultivars based on their coefficient of relationship so as to broaden the genetic base of the future potato cultivars.     

Production of somatic hybrids between S. tuberosum L. and late blight resistant Mexican wild potato species

Interspecific somatic hybrids between dihaploid breeding clones of potato, S. tuberosum and two accessions of wild Mexican species S. pinnatisectum and the hybrid line S. pinnatisectum × S. bulbocastanum were regenerated following electrofusion of mesophyll protoplasts to combine important agricultural traits of S. tuberosum and a high level of late blight resistance from selected wild accessions. In two fusion combinations 239 calli were regenerated; 162 from 195 calli analysed were identified as hybrids by means of isozyme analysis of peroxidases and, for some hybrid clones, by RAPD analysis. Depending on the fusion combination, 47–89 percent of the somatic hybrids had the expected ploidy level and 7–16 percent were mixoploids. Somatic hybrids were phenotypically intermediate as compared to their parents and some of them were able to be backcrossed sexually with potato. Fertility and crossability depended on combination and ploidy level of the somatic hybrids. In tests with detached leaves the wild partner clones had a high late blight resistance score of 8,6 and 8,9; the susceptible tuberosum-partners of 2,8 and 3,5, respectively. Nearly 25 percent of somatic hybrids had a resistance level of 6 or higher in the first year of assessment. The average resistance value of most somatic hybrids was lower than the average parental level. The reasons for variation in resistance values are discussed in connection with the practical application of fusion hybrids. This revised version was published online in July 2006 with corrections to the Cover Date.