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.     

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     

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.     

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.     

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.     

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.     

The breeding potential of wild potato species resistant to the potato tuber moth,Phthorimaea operculella (Zeller)

Summary The potato tuber moth, Phthorimaea operculella (Zeller) is an important pest of potatoes in the field and in stores in warm environments throughout the world. In this study genetic resistance to potato tuber moth was identified in clones of Solanum sparsipilum (coded MBN) originally developed for resistance to bacterial wilt and root-knot nematode. Resistance to this pest in S. sucrense and S. tarijense as well as S. sparsipilum was exploited in wide crosses with diploid and tetraploid cultivated potatoes, and haploids derived from S. tuberosum; hybrid progenies were produced. Crosses between resistant S. pinnatisectum or S. commersonii and cultivated potatoes failed completely, although S. commersonii did hybridise with two bridging species S. lignicaule and S. capsicibaccatum which are slightly compatible with cultivated potatoes. Resistance to potato tuber moth was transferred to all progenies except those in which S. tarijense was the resistant parent. The development of potatoes resistant to potato tuber moth is discussed in the context of population breeding for the lowland tropics.     

Resistance to late blight in potato. A putative gene that suppresses R genes and is elicited by specific isolates

Eleven progenies, ten bi-parental and one selfed, of tetraploid potato were assessed for phenotypic segregation ratios of resistance to susceptible individuals against two isolates of P. infestans, one avirulent to all 11 known potato R genes (avirulent pathotype) and one virulent to eight R genes (virulent pathotype). Assuming Mendelian inheritance of independent R genes, the numbers of individuals segregating for resistance to both races could not be explained sufficiently for all progenies. However, the assumed presence of dominant, specific suppressors of the R gene function that were elicited by specific isolates of the fungus and that segregated in the host independently from R genes explained all frequencies of resistant and susceptible individuals in each of the progenies studied. This revised version was published online in July 2006 with corrections to the Cover Date.     

Genetic linkage of QTLs for late blight resistance and foliage maturity type in six related potato progenies

A set of test crosses of diploid potatoes was used to identify QTLs for foliage resistance against Phytophthora infestans and QTLs for foliage maturity type, and to assess their genetic relationship. The most important locus for both traits was found on chromosome 5 near marker GP21: the allele of marker GP21 that is associated with resistance to late blight is also associated with late foliage maturity. An additional QTL with a small effect on foliage maturity type was identified on chromosome 3, and additional QTLs for late blight resistance were found on chromosomes 3 and 10. Another QTL was detected on chromosome 7 when resistance was adjusted for the effect of foliage maturity type. The additional QTLs for resistance against P. infestans on chromosomes 3 and 10 seem to be independent of foliage maturity type and are not affected by epistatic effects of the locus on chromosome 5. The effects of the additional QTLs for resistance are small, but early maturing genotypes that necessarily have the allele for susceptibility for late blight on chromosome 5 may benefit from the resistance that is provided by these QTLs on chromosomes 3 and 10.     

Differentiation of the high molecular weight glutenin subunit D t x 2.1 of Aegilops tauschii indicated by partial sequences of its encoding gene and SSR markers

The devastating late blight pathogen Phytophthora infestans infects foliage as well as tubers of potato. To date, resistance breeding has often focused on foliage blight resistance, but tuber blight resistance is becoming more and more important in cultivated potatoes. In this study, a reliable tuber assay for resistance assessment was developed and foliage and tuber blight resistance (R) was compared in four mapping populations. In the RH4X-103 population, tuber blight resistance inherited independently from foliage blight resistance. Three specific R genes against P. infestans were segregating. The Rpi-abpt and R3a genes function as foliage-specific R genes, whereas the R1 gene acts on both foliage and tuber. In the segregating populations SHRH and RH94-076, tuber and foliage blight resistance correlated significantly, which suggests that resistance in foliage and tuber is conferred by the same gene (could be R3b) and quantitative trait loci (QTL), respectively. In the CE population neither tuber nor foliage resistance was observed.     

Mapping of a novel,major late blight resistance locus in the diploid (1EBN) Mexican Solanum pinnatisectum Dunal on chromosome VII

Late blight caused by the oomycete Phytophthora infestans (Mont.) de Bary (Pi) is the most important foliar disease of potato worldwide. An intraspecific hybrid between individuals of a resistant and a susceptible S. pinnatisectum accession was backcrossed to the susceptible parent to generate a segregating population for late blight resistance consisting of 84 plants. In detached‐leaflet assays, reaction to late blight segregated in a 1r:1s manner in BC₁ progeny indicating the presence of a single dominant resistance gene. A genetic map was constructed based on 1,583 DArT/SSR markers which were allocated to 12 linkage groups, covering 1,793.5 cM with an average marker distance of 1.1 cM. The late blight resistance locus derived from S. pinnatisectum was mapped on chromosome VII. In comparison with the previously reported resistance genes Rpi1 and Rpi2, the new target resistance locus most likely is located on the opposite arm of chromosome VII. Results of this study will serve as a basis for future fine mapping of the late blight resistance locus and the development of locus‐specific markers for marker‐assisted selection.     

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.     

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.     

Manipulation of the 2n-gametes frequencies in Solanum pollen

Besides the normal gametes, some 2x potato genotypes produce 2n-gametes as well. In interploidy crosses (2x×4x, 4x×2x) 2n-gametes are necessary, whilst in intercrossing at the diploid level they are undesirable.By means of a set of simply constructed sieves, pollen of several 2n-pollen producing progenitors have been sifted. It was possible to increase the 2n-pollen content of low 2n graded pollen to interesting levels. No decrease in in vitro pollen vitality due to the treatment could be demonstrated. The in vivo pollen vitality of the aqueous pollen suspension was satisfactory to very good under cool weather conditions.     

Resistance to Ascochyta blight in chickpea - Genetic basis

Summary Genetic regulation of host resistance in chickpea-Ascochyta rabiei interaction system is governed by two dominant complementary genes each in the genotypes GLG 84038 and GL 84099, whereas the resistance in a black seeded genotype ICC 1468 was controlled by one dominant and one recessive independent gene. In all the genotypes, resistance is operated by inter-allelic interactions. The genes conferring resistance in GLG 84038 were found to be different to those operating in GL 84099 and ICC 1468. Among the five dominant genes dispersed in 3 genotypes under study, at least one has been reported for the first time, as to date, only three dominant genes have been reported in the literature.The four identified dominant genes in GLG 84038 and GL 84099 have been named as Arc₁, Arc₂ (in GLG 84038) and Arc₃, Arc₄ (in GL 84099). The undistinguished dominant gene in ICC 1468 has been named as Arc_5(3,4) as it could not be equated or differentiated from Arc₃ or Arc₄. The recessive gene in ICC 1468 has been named as Arc₁.Generation mean analysis of the 6 resistant × susceptible crosses involving the same genotypes, revealed that the genes conferring resistance in any of the 3 genotypes did not follow simple Mendelian inheritance but were influenced by inter allelic interactions. Additive gene effect along with dominance were operative in all the 3 genotypes under study in conferring resistance. However, the mechanism of resistance in GLG 84038 and GL 84099 were primarily additive in nature while that in ICC 1468, dominance as well as dominance × dominance interactions were more important than additive gene action.     

Resistance to bacterial blight (Pseudomonas syringae pv. pisi) in Spanish pea (Pisum sativum) landraces

Pea bacterial blight (Pseudomonas syringae pv. pisi) has long been known to be present in pea growing areas of Spain and to cause serious crop losses, although there is no published record of its occurrence. A collection of 16 isolates from a winter pea trial in Valladolid in 1991 which were shown in this study to be P.s. pv. pisi races 4 and 6 would appear to be the first published record of the disease in Spain. This occurrence of races 4 and 6 is the same as reported for winter-sown peas in the South of France. Ten Pisum sativum landraces from different geographical areas of Spain and considered to be representative of the traditional pea crop, were tested for resistance to seven races of P.s. pv. pisi. Seedlings of each landrace were stem inoculated with the type strain of each race in a glasshouse. Resistance exhibited by the different landraces mainly conformed to those previously described in pea cultivars indicating various combinations of the main resistance genes: R3, R2+4, R3+4 and R2+3+4. R3 was the most frequent R gene, being present in all landraces. R4 was present in four and R2 in three of the landraces tested. Variation for resistance within landraces was limited except for landrace accessions ZP-0102, ZP-0109 and ZP-0137 which also showed variation for morphological traits. The resistance responses of landrace ZP-0109 were difficult to interpret, but suggested a genetic mixture with some evidence of less well documented R genes, R5 and/or R6, and possibly some unknown resistance to race 6. This revised version was published online in July 2006 with corrections to the Cover Date.     

The importance of wild potato species resistant to the potato cyst nematode,Globodera pallida,pathotypes P4A and P5A,in potato breeding. I. Resistance studies

Summary Seven wild diploid potato species, Series Tuberosa, representing 1023 clones were screened for resistance to the potato cyst nematode, Globodera pallida. Over 25% of the clones were resistant to pathotype P₄A and almost 30% were resistant to pathotype P₅A. The resistance in hybrid progenies of these and other resistant species with cultivated potatoes was evaluated, and over 2200 seedlings were screened. High frequencies of resistance (>50%) to P₄A were found in progenies with Solanum leptophyes, S. vernei, S. gourlayi and S. capsicibaccatum, whereas resistance to P₅A was found in these species as well as S. sparsipilum. The importance of nematode resistant wild species for potato breeding is discussed.     

Gene action in the resistance of peppers (Capsicum annuum) to Phytophthora stem blight (Phytophthora capsici L.)

Summary The variation among single and three-way crosses derived from seven lines of pepper (Capsicum annuum) selected for resistance to Phytophthora capsici was analyzed into statistical-genetic components due to general and specific combining abilities and additive-by-additive epistasis. Indirect evidence of higher order epistasis was also estimated. Epistasis was a principal source of variation. It was correlated with the level of aggressiveness of the pathogen. A breeding method such as recurrent selection seems to be the most appropriate to improve resistance to the fungus.