Inheritance of resistance to plum pox potyvirus (PPV) in apricot, Prunus armeniaca

The results from the evaluation of apricot seedlings for resistance to the plum pox potyvirus (sharka) are presented. The susceptibility of 291 seedlings from 19 different crosses, most of them between resistant and susceptible cultivars, was determined. The results obtained seem to indicate that the resistance to plum pox potyvirus in apricot is controlled by a single gene, where resistance would be a dominant trait and the resistant parents used would be heterozygous for this trait. Since the resistance appeared to behave as a quantitative trait during the evaluation process, the hypothesis of a monogenic control is discussed.     

Inheritance of resistance to plum pox potyvirus (PPV) in 'Stella' apricot seedlings

Resistance to sharka (plum pox potyvirus) was studied in 74 seedlings resulting from open pollination of the resistant apricot cultivar ‘Stella’. Each seedling was inoculated at the base by a chip-bud from a diseased GF305 peach tree. To follow tlie spread of virus into the scedlings. a Chip-bud of a healthy GF305 and another from a healthy sensitive Manicot apricot were grafted above the inoculation point in the order described. Six observations of symptoms were made in the leaves of the GF305 used For inoculation, the GF305-control, the Manicot-control and the ‘Stella’ seedlings during four growth cycles. An enzyme-linked immunosorbent assay (ELISA) was applied to the leaves of the ‘Stella’ seedlings. No ‘Stella’ seedling showed symptoms of sharka. Only three seedlings reacted positively to the ELISA. Substantial differences were observed in the speed of virus propagation through the ‘Stella’ seedlings, some of them showing a strong resistance to virus translocation. The possible use of the ‘Stella’ cultivar in apricot breeding programmes to obtain resistant cultivars is discussed.     

Evaluation of resistance to sharka (Plum pox virus) of several Prunus rootstocks

Sharka (Plum pox virus, PPV) severely affects the production of most Prunus species in the areas affected by the disease. In this study, the resistance of 15 Prunus rootstocks to a Dideron type isolate of PPV was evaluated under controlled conditions in an insect‐proof greenhouse. After four cycles of study,‘GF677’ almond x peach hybrid,‘Myrobolan 29C plum and ‘L2’ cherry did not show any symptoms and were ELISA‐DASI and RT‐PCR negative. These were considered resistant to PPV. The rest of the rootstocks assayed showed symptoms of sharka (confirmed by ELISA‐DASI or RT‐PCR), although the level of susceptibility was different for each rootstock.‘GF305’ peach, ‘Puebla de Soto’ plum and ‘Real Fino’ apricot, were highly susceptible to PPV, showing strong sharka symptoms and being ELISA and RT‐PCR positive. ‘Marianna 2624’ plum,‘AC 9921‐07’ hybrid and ‘CP‐2’ plum showed susceptibility to PPV confirmed by ELISA‐DASI and RT‐PCR positives.‘Nemaguard’ and ‘Nemared’ hybrids, ‘Torinel’ plum and ‘STN2’ hybrid showed an intermediate susceptibility to PPV with slight sharka symptoms and were ELISA‐DASI and RT‐PCR positive. Whereas, ‘Montclar’ peach and ‘Evrica’ hybrid showed moderate resistance to PPV with slight sharka symptoms and were ELISA positive but RT‐PCR negative. The results open new possibilities in the search for different sources of resistance to PPV within Prunus.     

Resistance of almond cultivars to Plum pox virus (sharka)

Sharka, a disease caused by Plum pox virus (PPV), mainly affects some Prunus species, including apricot, peach and plum, and to a lesser degree, sweet cherry and sour cherry. In almond, different PPV isolates have been transmitted experimentally to the ‘Aï’ cultivar. In this study, the resistance of 10 almond cultivars to a Dideron PPV isolate was evaluated in controlled conditions by grafting the cultivars on to inoculated GF305 peach rootstocks. The results demonstrated a high level of resistance to PPV in all the almond cultivars assayed. They did not show any symptoms and were ELISA and RT‐PCR negative, despite the strong symptoms observed in their GF305 rootstocks. The implications of these results for the dispersion of PPV, and the potential role of almond as a source of resistance to PPV in other Prunus species such as peach, are also discussed.     

Susceptibility to sharka (Plum pox virus) in Prunus mandshurica×P. armeniaca seedlings

Prunus mandshurica [(Maxim.) Koehne] from Central Asia is a species related to apricot and grown in China and Mongolia. This species has been used in apricot breeding as a source of frost resistance. In addition, P. mandshurica has been suggested as the possible origin of some North American apricot cultivars resistant to sharka (Plum pox virus, PPV). The aim of this work was to transmit the resistance to PPV from P. mandshurica to the Spanish apricot cultivar ‘Currot’ by traditional crossing. The resistance to a Dideron PPV isolate of the descendants of P. mandshuricaבCurrot’ and their progenitors was evaluated under controlled conditions in a greenhouse. The results showed the susceptibility of both, the progenitors and the offspring to PPV, as being much higher than in other apricot seedlings. The results showed that the P. mandshurica accession studied is not a good progenitor in breeding apricot for PPV resistance, but just the opposite. The possible role of P. mandshurica as a source of resistance in apricot resistant cultivars is questioned.     

Genetic diversity in apricot, Prunus armeniaca, aimed at improving resistance to plum pox virus

Plum Pox Virus, a non-persistent virus transmitted by aphids, causes serious damage to stone fruits. The apricot tree is very sensitive and in order to breed apricot cultivars resistant to Plum Pox Virus and establish breeding strategies, genetic diversity based on 10 enzymatic systems, six of which were polymorphic, has been studied. The plant material studied, 94 accessions, included the most important apricot cultivars grown in PPV-affected areas. Genetic diversity is high and showed important differences between the three geographical groups studied (North African, European and North American). The North American group was very diverse and allozymes can be used to identify three subgroups. Some North American PPV-resistant cultivars were very distant from the rest of the cultivars, mainly due to the presence of rare alleles found in an Asian apricot related species. These results support the hypothesis that Asian-related species might be the origin of PPV resistance within the North American cultivars. Three North American cultivars have been considered as putative donors of PPV resistance to the European cultivars because of their agronomic behaviour, chilling requirements and distance from European cultivars. However, to increase the genetic variability of the European group and thereby to favour recombination, the study of Asian apricot resources is also recommended.     

Coat protein-mediated protection against plum pox virus in herbaceous model plants and transformation of apricot and plum

Summary The expression of the viral coat protein gene in transgenic plants has been shown to induce tolerance against virus infection (Beachy et al., 1990). Transgenic plants ofNicotiana clevelandii andNicotiana benthamiana- herbaceous host plants for PPV - transformed withAgrobacterium strain LBA 4404 containing the plasmid pBinPPVm, regenerated on selection media containing kanamycin were tested for the expression of the PPV coat protein gene by ELISA and immuno western blot. After rooting and acclimatisation plants were tested for the protection against PPV Following the inoculation plants were investigated for symptom development and virus accumulation. Different lines were identified, according to the different reaction to the mechanical inoculation, ranging from a complete absence to a strong reduction of symptoms. There have not been many reports on transformation of trees in general, and in fruit trees particularly. It is obvious that the major obstacle is the regeneration of transformed plantlets. Attempts to improve crop plants by genetic engineering techniques will always depend very strongly on the availability of reliable protocols for transformation, selection and regeneration (Laimer et al., 1989, 1990). Different systems involving juvenile and adult plant material have been developed allowing the transfer of foreign genes into apricot and plum cultivars. We report the transformation and regeneration ofPrunus armeniaca andPrunus domestica plants withAgrobacterium tumefaciens strain LBA 4404 containing various binary plasmids, pBinGUSint, carrying the marker geneβ-glucuronidase (GUS) and pBinPPVm, carrying the coat protein gene of Plum Pox Virus (PPV), the causal agent of Sharka disease. The marker geneGUS was used for the optical evaluation of the efficiency of different transformation systems involving cotyledons of immature embryos as well as somatic embryos and leaf discs. The coat protein gene of PPV was used to introduce the coat protein mediated resistance against one of the most important pathogens of stone fruit trees in Europe and the whole Mediterranean area.     

Genotyping apricot cultivars for self-(in)compatibility by means of RNases associated with S alleles

In previous work the existence of proteins with RNase activity associated with S alleles in apricot was demonstrated. These proteins were inherited as described previously for the inheritance of self‐compatibility in this species. In this study, new cultivars have been genotyped for self‐compatibility using this method and it has been demonstrated that in all self‐compatible cultivars examined, the self‐compatibility allele is the same and is associated with an RNase with high activity. Homozygous self‐compatible individuals have been detected among established cultivars as well as among seedlings following breeding activity. This germplasm is of great value within the breeding programme because only self‐compatible seedlings will be produced. The number of S alleles in apricot appears to be low and only eight different alleles have been found in the large number of different cultivars screened. Furthermore, there are alleles present in the Spanish population that are also found in the genetic pool of North American cultivars. The screening of a progeny from the cross between the American cultivar ‘Goldrich’ and the Spanish cultivar ‘Pepito’ demonstrated the existence of the common allele S₂ (detected previously by examining RNases), which was confirmed by the segregation of self‐compatibility in the progeny.     

Evaluation of two screening methods for resistance of apricot,plum and peach toMonilinia laxa

Summary Monilinia laxa causes important pre- and post-harvest losses in stonefruit. In order to initiate a breeding programme for increased resistance toM. laxa, two screening tests were used. In the “uninjured fruit inoculation” test, 30 mature fruits of each variety were inoculated on their surface by depositing 20 μl of conidial suspension. In the “artificially injured fruit inoculation” test, 10 mature fruits were inoculated on both sides by injecting 20 μl of conidial suspension. Genotypic differences were found in both tests, within the three species studied. Marked differences were observed in the uninjured fruit inoculation test. Differences between genotypes were slighter in the artificially injured fruit inoculation test. Within each species, the rankings of the genotypes according to the two tests were not correlated. Both testing procedures would be usefully applied in a breeding programme to obtain genotypes with combined resistance toMonilinia laxa.     

Evidence of a new gene for high resistance to Meloidogyne spp. in Myrobalan plum, Prunus cerasifera

The Myrobalan plum Prunus cerasifera clones ‘P,2175’ and ‘P,1079’ carry single major genes (Ma1 and Ma2. respectively) for resistance to the predominant root–knot nematode (RKN) species Meloidogyne arenaria (MA), Meloidogyne incognita (MI) and Meloidogyne jaranica(MJ). The Myrobalan plum clone ‘P.2980’ is another complete–spec–trum source bearing favourable agronomic features. The genetics of its resistance to MA, MI, MJ and to the population Meloidogyne sp. Florida has been investigated from G1 crosses with the host Myrobalan plum clone ‘P.16.5’ (recessive for both Ma genes), the resistant clone ‘P.2175’ (heterozygous for Mal) and the ‘Nemared’ peach (homozygous for resistance to MA, MI and MJ but a host for M. sp. Florida). The segregation of the Gl hybrids from the intraspecific crosses into two significantly separated resistant and host classes, independent of the RKN species, indicates that resistance to all four species in ‘P.2980’ is also controlled by a single major dominant (heterozygous) gene. This gene is designated Ma3 because it shares the same spectrum and efficiency as Ma1 and Ma2, and its relationship with these last two genes is discussed. The first hybrids between ‘P.2980’ and ‘Nemared’ tested only segregated for resistance to M. sp. Florida. These results illustrate the possibility of cumulating (pyramiding) Ma3 and the ‘Nemared’ peach resistance gene(s) into new interspecific rootstocks.     

Evaluation of common wheat cultivars for tan spot resistance and chromosomal location of a resistance gene in the cultivar 'Salamouni'

A total of 50 wheat (Triticum aestivum L.) cultivars were evaluated for resistance to tan spot, using Pyrenophora tritici‐repentis race 1 and race 5 isolates. The cultivars ‘Salamouni’, ‘Red Chief’, ‘Dashen’, ‘Empire’ and ‘Armada’ were resistant to isolate ASC1a (race 1), whereas 76% of the cultivars were susceptible. Chi‐squared analysis of the F₂ segregation data of hybrids between 20 monosomic lines of the wheat cultivar ‘Chinese Spring’ and the resistant cultivar ‘Salamouni’ revealed that tan spot resistance in ‘Salamouni’ was controlled by a single recessive gene located on chromosome 3A. This gene is designated tsn4. The resistant cultivars identified in this study are recommended for use in breeding programmes to improve tan spot resistance in common wheat.     

SFB‐based S‐haplotyping of apricot (Prunus armeniaca) with DHPLC

Most cultivars that belong to the Rosaceae are self‐incompatible and depend on cross‐pollination. The pollen donor and pollen recipient have to flower synchronously and must be genetically compatible. Genetic compatibility is governed by the S‐locus, which holds the S‐RNase and S‐haplotype‐specific F‐Box (SFB) genes. Thus, the S‐genotype of cultivars is an important feature and is characterized molecularly by the S‐RNase and SFB alleles which are distinctive for each S‐haplotype. Here, we report the usage of DNA chromatography (denaturing high‐performance liquid chromatography – DHPLC) for identifying the S‐genotypes of European apricots on the basis of their SFB alleles. DHPLC is amenable to high‐throughput automation, and therefore is valuable for breeding and for high‐quality plant typing in the nursery.     

Evaluation of field resistance of Daucus carota cultivars to Cercospora carotae (carrot leaf spot)

Summary A world collection of 142 cultivars of carrot was tested for resistance to Cercospora carotae in the field. C. carotae infection was observed in all the cultivars. Field resistance was observed in only about 30% of the cultivars, the remainder being highly susceptible.     

A new resistance gene to PepYMV (Pepper yellow mosaic virus) in Capsicum annuum L.

Pvr4 locus, commonly associated with a co-dominant CAPS marker, confers monogenic dominant resistance to potyvirus complex in Capsicum spp. Aiming to investigate whether the resistance found in resistant genotypes not bearing Pvr4 marker is due to allelism in the Pvr4 locus, or due to a new locus of the pvr series, segregation analyses of an F₂ population obtained from a cross between two pepper lines “Myr-29-10” (P₁) (resistant to PepYMV, showing a single band of 444 bp, Pvr4/Pvr4) and “PIM-025” (P₂) (resistant to PepYMV, showing a single band of 458 bp, Pvr4⁺/Pvr4⁺) were performed. According to the results, there is strong evidence that the locus controlling PepYMV resistance in PIM-025 (P₂) is not Pvr4. These results provide evidence that those resistant genotypes, bearing a susceptible band pattern in the Pvr4 locus (458 bp, Pvr4⁺/Pvr4⁺), carry a different gene from those described in the literature up to the present time.     

Seasonal variation in resistance of chrysanthemum cultivars to Frankliniella occidentalis (Thysanoptera: Thripidae)

Seasonal variation in the level of host plant resistance can have important consequences for the repeatability of tests to measure host plant resistance to insect pests. In the present study, the levels of resistance to Frankliniella occidentalis of a susceptible and a partially resistant cultivar of chrysanthemum, Dendranthema grandiflora, were determined throughout the year. Thrips damage, reproduction and adult female survival were determined on excised leaves in Petri dishes under uniform conditions. Strong seasonal fluctuations were observed in these three characteristics. On leaves from plants grown in winter, damage, reproduction, and survival were higher than on leaves from plants grown in summer. Clear differences in resistance were observed between the susceptible and the partially resistant cultivar on leaves from plants grown in winter, while differences disappeared in summer. Damage on both cultivars and survival on the susceptible cultivar were negatively correlated with mean daily solar radiation during plant growth, suggesting that the level of resistance depends on light intensity during plant growth. This was confirmed in an experiment carried out in summertime with shaded and unshaded plants. Leaves from control plants, grown under high light intensity had a higher level of resistance than leaves from shaded plants grown under reduced light intensity. There were clear differences in resistance between the cultivars under shaded conditions (low light intensity), but not under unshaded control conditions (high light intensity). This revised version was published online in July 2006 with corrections to the Cover Date.     

Molecular and phenotypic characterization of Chinese wheat (Triticum aestivum) cultivars for resistance to Fusarium head blight

To identify excellent cultivars resistant to Fusarium head blight (FHB), 104 wheat cultivars were tested by single-flower inoculation using two prevalent pathogens from 2018 to 2020. Agronomic traits were also investigated. Six FHB-resistance quantitative trait loci (QTL), Fhb1, Fhb2, Fhb4, Fhb5, Fhb7 and Qfhb.crc-2D, have been assessed using previously reported DNA markers. A diagnostic marker has been used for Fhb1, and indicative markers linked to the other QTL were used. Results showed that (i) 12 (11.5%) cultivars were resistant to two pathogens in 3 years; among them, ‘Shengxuan 6’, ‘Wanhongbian 759’, ‘Yunong 903’ and ‘Yunong 901’ had good agronomic traits. (ii) Among cultivars with one resistance QTL, the severities of cultivars carrying Fhb1 and Qfhb.crc-2D were 2.2 and 2.8, respectively, whereas those of cultivars with Fhb2 or Fhb7 were 3.6. Among cultivars with two resistance QTL, the severities of cultivars with Fhb1 + Fhb4, Fhb1 + Fhb7 and Fhb4 + Fhb5 were 2.2, 3.0 and 3.6, respectively. The severity of five cultivars possessing three or four resistance QTL was below 2.5. Fhb1 and Qfhb.crc-2D showed better resistance effects than other resistance QTL.     

The expression of resistance of subterranean clover cultivars to races of Phytophthora clandestina

Infection and sporulation of races of Phytophthora clandestina in susceptible and resistant cultivars of subterranean clover and the effect of the races on growth of the cultivars in pots were studied using a total of five races of the pathogen and eight host cultivars. The pathogen caused severe disease where races and cultivars were compatible, although a significant reduction in root and shoot dry weight occurred only on Woogenellup inoculated with race 2. There was little increase in disease severity after 1 wk from the time of inoculation. Sporangia and oospores were formed at high frequency in tap and lateral roots in all compatible combinations, except in Woogenellup in which sporangia were produced inconsistently. The pathogen caused negligible disease and produced very few sporangia in incompatible combinations. Plant age had a strong effect on disease development. Disease severity caused by virulent races on cultivars Gosse, Trikkala, Meteora and Woogenellup was less on older than on younger plants. A further experiment using aggressive isolates of races 0, 1, 2, 3 and 4 revealed that cultivar Gosse showed no reduction in root and shoot weight, despite it being susceptible to races 1, 3 and 4. This provided evidence of the existence of race-non-specific or horizontal resistance to P. clandestina in cultivar Gosse. This revised version was published online in July 2006 with corrections to the Cover Date.     

Inheritance of resistance to whitebacked planthopper,Sogatella furcifera (Horvath) in some rice cultivars

Summary The genetics of resistance to whitebacked planthopper, Sogatella furcifera (Horvath) in ten resistant cultivars was studied. The reactions of the F₁, F₂ and F₃ populations of resistant varieties with Taichung Native 1, a suspectible check, showed that WBPH resistance is monogenic in nature and governed by dominant gene(s) in Ptb 19 and IET 6288 and recessive gene in eight cultivars viz. ARC 5838, ARC 6579, ARC 6624, ARC 10464, ACR 11321, ARC 11320, Balamawee and IR 2415-90-4-3. Allelic relationship of resistance gene(s) in the test cultivars revealed recessive gene in IR 2415-90-4-3, ARC 5838 and ARC 11324 to be allelic but it was non allelic to the resistance gene in ARC 6624. Cultivars ARC 6579, ARC 11321 and Balamawee have identical gene among themselves but their relationship with IR 2415-90-4-3, ARC 5838, ARC 11324 and ARC 6624 is unknown. The recessive gene in ARC 10464 is non-identical to all other cultivars having the recessive gene except ARC 6624 with which its relationship needs further investigation.     

Genetic control of resistance to soilborne wheat mosaic virus in Brazilian cultivars of Triticum aestivum L. Thell.

In Southern Brazil, under ideal conditions Soilborne Wheat Mosaic Virus (SBWMV) induces yield reductions to the wheat crop of over 50%. The only effective way of controlling the disease is through resistance. However, the inheritance of resistance is not been fully understood. The purpose of this work was to study the genetic control of resistance to SBWMV. Crosses were carried out between the resistant wheat cultivar Embrapa 16 and the susceptible cultivars BR 23 and IAC 5-Maringá, at the National Wheat Research Centre, Passo Fundo, RS, Brazil. The parents, F₁, F₂, and backcrosses were sown in plots in a field where soil was naturally infested with the vector of the virus, the fungus Polymyxa graminis, in order to promote natural infection. All plants were individually evaluated for severity and type of lesions, according to a scale of 0 to 5, where, 0 = absence of symptoms and 5 = plants severely affected by mosaic plus dwarfing and rosetting. The statistical analyses of the data showed broad sense heritability values between 43% and 55%. The data suggested the presence of two genes controlling resistance to SBWMV in the segregating population of both crosses. This revised version was published online in August 2006 with corrections to the Cover Date.