Evaluation of resistance sources and inheritance of resistance in kidney bean to Indian virulences of Colletotrichum lindemuthianum

Summary Forty nine common bean lines comprising of exotic accessions and locally grown cultivars evaluated against Colletotrichum lindemuthianum exhibited differential resistance to its races in Himachal Pradesh, a north-western Himalayan state of India. Some exotic accessions like G 2333, Cornell 49242, PI 207262, Mexique 222, TO, Perry Marrow, Kaboon and Widusa were resistant to more than five Indian races, whereas two Indian accessions KRC-5 and Hans showed resistance to six and four races, respectively. However, nine accessions KRC-8, KR-40, KR-43, KR-81, KR-62-2, KR-90, KR-142, KR-148, and KR-216 were resistant to three races. Race specific resistance has been observed in different bean cultivars. Studies on inheritance of resistance in exotic accession G 2333 and Indian accession, KRC-5 showed that two independent dominant genes conferred resistance in G 2333 to race 3 and 515 and a single dominant gene controlled resistance in KRC-5 to race 775, indicating resistance from these sources is easily transferable to the locally adapted susceptible cultivars.     

An allelic series at the Co-1 locus conditioning resistance to anthracnose in common bean of Andean origin

In this study, we characterized the genetic resistance of the Andean bean cultivars Kaboon and Perry Marrow and their relation to other sources of anthracnose resistance in common bean. Based on the segregation ratio (3R:1S) observed in two F₂ populations we demonstrated that Kaboon carries one major dominant gene conferring resistance to races 7 and 73 of Colletotrichum lindemuthianum. This gene in Kaboon is independent from the Co-2 gene and is an allele of the Co-1 gene present in Michigan Dark Red Kidney (MDRK) cultivar. Therefore, we propose the symbol CO-1 ² for the major dominant gene in Kaboon. The Co-1 is the only gene of Andean origin among the Co anthracnose resistance genes characterized in common bean. When inoculated with the less virulent Andean race 5, the segregation ratio in the F₂ progeny of Cardinal and Kaboon was 57R:7S (p = 0.38). These data indicate that Kaboon must possess other weaker dominant resistance genes with a complementary mode of action, since Cardinal is not known to possess genes for anthracnose resistance. Perry Marrow, a second Andean cultivar with resistance to a different group of races, was shown to possess another resistant allele at the Co-1 locus and the gene symbol Co-1 ³ was assigned. In R × R crosses between Perry Marrow and MDRK or Kaboon, no susceptible F₂ plants were found when inoculated with race 73. These findings support the presence of a multiple allelic series at the Andean Co-1 locus, and have major implications in breeding for durable anthracnose resistance in common bean. This revised version was published online in July 2006 with corrections to the Cover Date.     

Genetic analysis of resistance to soybean cyst nematode in PI 438489B

Soybean (Glycine max L. Merr.) plant introduction PI 438489B is a unique source that has resistance to all known populations of soybean cyst nematode (Heterodera glycines Ichinohe, SCN). This PI line also has many desirable agronomic characteristics, which makes it an attractive source of SCN resistance for use in a soybean breeding program. However, characterization of SCN resistance genes in this PI line have not been fully researched. In this study, we investigated the inheritance of resistance to populations of SCN races 1, 2, 3, 5, and 14 in PI 438489B. PI 438489B was crossed to the susceptible cultivar ‘Hamilton’ to generate F₁ hybrids. The random F₂ plants and F₃ lines were evaluated in the greenhouse for reaction to these five populations of SCN races. Resistance to SCN races 1, 3, and 5 were mostly conditioned by three genes (Rhg Rhg rhg). Resistance to race 2 was controlled by four genes (Rhg rhg rgh rgh). Three recessive genes were most likely involved in giving resistance to race 14. We further concluded that resistance to different populations of SCN races may share some common genes or pleiotropic effects may be involved. This revised version was published online in July 2006 with corrections to the Cover Date.     

Inheritance of resistance to the chlorosis component of tan spot of wheat caused by Pyrenophora tritici-repentis, races 1 and 3

The fungus Pyrenophora tritici-repentis, causal agent of tan spot of wheat, produces two phenotypically distinct symptoms, tan necrosis and extensive chlorosis. The inheritance of resistance to chlorosis induced by P. tritici-repentis races 1 and 3 was studied in crosses between common wheat resistant genotypes Erik, Hadden, Red Chief, Glenlea, and 86ISMN 2137 and susceptible genotype 6B-365. Plants were inoculated under controlled environmental conditions at the two-leaf stage and disease rating was based on presence or absence of chlorosis. In all the resistant × susceptible crosses, F₁ plants were resistant and the segregation of the F₂ generation and F₃ families indicated that a single dominant gene controlled resistance. Lack of segregation in a partial diallel series of crosses among the resistant genotypes tested with race 3␣indicated that the resistant genotypes possessed␣the same resistance gene. This resistance gene was effective against chlorosis induced by P.␣tritici-repentis races 1 and 3.     

Inheritance of resistance to two races of sunflower downy mildew (Plasmopara halstedii) in two Helianthus annuus L. lines

Sunflower downy mildew caused by Plasmopara halstedii is an important disease of sunflower capable of causing losses of more than 80% of production. Races 100, 300, 310, 330, 710, 703, 730 and770 of the fungus have been identified in Spain. Race 703, of high virulence, has been identified frequently in the northeast, while race 310 seems to occur over the south, the main sunflower growing region of the country. Oil sunflower lines RHA-274 and DM4 were studied for their resistance to races 310(RHA-274 and DM4) and 703 (DM4). In each cross, only one plant of the resistant parent was crossed to the inbred susceptible line HA-89 (or cmsHA-89).Plants from F₂ and backcross(BC₁F₁ to susceptible parent)generations were evaluated for fungal sporulation on true leaves and/or cotyledons. The resistant-to-susceptible ratios obtained in the F₂ and BC₁F₁ progenies from the crosses cmsHA-89 × RHA-274 and HA-89 × DM4suggested that one major gene in each line is responsible for resistance to race 703.The segregations of the progenies of the cross HA-89 × DM4 inoculated with race 703also fitted the ratios 1:1 and 3:1 (for BC₁F₁ and F₂, respectively)corresponding to control of resistance by a single dominant gene. In RHA-274, the gene for resistance to race 310 was designated Pl ₉, whereas Pl _v is tentatively proposed to designate the gene in DM4 responsible for resistance to races310 and 703. This revised version was published online in August 2006 with corrections to the Cover Date.     

Apium graveolens PI 181714 is a source of resistance to Fusarium oxysporum f. sp. apii race 4 in celery (A. graveolens var. dulce)

Celery has little genetic diversity and is highly susceptible to the new fungal pathogen Fusarium oxysporum f. sp. apii (Foa) race 4. After screening an Apium graveolens germplasm collection for resistance to Foa race 4, we crossed celery cv. 'Challenger', which is Foa race 2-resistant but Foa race 4-susceptible and A. graveolens PI 181714, which is Foa races 2- and 4-resistant but non-celery type. After selfing F₁s, we screened the F₁S₁ for race 4-resistance and celery-type and then selfed selected F₁S₁. Greenhouse and field trials indicate that three selected F₁S₂ families (76–8-4, 76–8-27 and 76–8-36) are suitable as germplasm for celery breeders for resistance to Foa race 4. A F1S3 76–8–36-124 is either fixed or nearly so for resistance to Foa races 4 and 2. Furthermore, quantitative PCR indicates that PI 181714 is resistant, rather than tolerant, to Foa races 4 and 2, and that this resistance has been introgressed into F1S3 76–8–36-124.     

Genetics of Cyst Nematode Resistance in Soybean PIs 467312 and 507354

Soybean Cyst nematode (SCN) Heterodera glycines Ichinohe is the most serious pest of soybean [Glycine max (L.) Merr.] in the world and genetic resistance in soybean cultivars have been the most effective means of control. Nematode populations, however, are variable and have adapted to reproduce on resistant cultivars over time due mainly to the narrow genetic base of SCN resistance in G. max. The majority of the resistant cultivars trace to two soybean accessions. It is hoped that new sources of resistance might provide durable resistance. Soybean plant introductions PI 467312 and PI 507354, are unique because they provide resistance to several nematode populations, i.e. SCN HG types 0, 2.7, and 1.3.6.7 (corresponding to races 3, 5, and 14) and HG types 2.5.7, 0, and 2.7 (corresponding to races 1, 3, and 5), respectively. The genetic basis of SCN resistance in these PIs is not yet known. We have investigated the inheritance of resistance to SCN HG types 0, 2.7, and 1.3.6.7 (races 3, 5, and14) in PI467312 and the SCN resistance to SCN HG types 2.5.7 and 2.7 (races 1 and 5) in PI 507354. PI 467312 was crossed to ‘Marcus’, a susceptible cultivar to generate F₁ hybrids, 196 random F₂ individuals, and 196 F_2:3 families (designated as Pop 467). PI 507354 and the cultivar Hutcheson, susceptible to all known SCN races, were crossed to generate F₁ hybrids, 225 random F₂ individuals and 225 F_2:3 families (designated as Pop 507). The F_2:3 families from each cross were evaluated for responses to the specific SCN HG types in the greenhouse. Chi-square (χ²) analyses showed resistance from PI 467312 to HG types 2.7, and 1.3.6.7 (races 5 and 14) in Pop 467 were conditioned by one dominant and two recessive genes (Rhg rhg rhg) and resistance to HG type 0 (race 3) was controlled by three recessive genes (rhg rhg rhg). The 225 F_2:3 progenies in Pop 507 showed a segregation of 2:223 (R:S) for response to both HG types 2.5.7 and 2.7 (corresponding to races 1 and 5). The Chi-square analysis showed SCN resistance from PI 507354 fit a one dominant and 3 recessive gene model (Rhg rhg rhg rhg). This information will be useful to soybean breeders who use these sources to develop SCN resistant cultivars. The complex inheritance patterns determined for the two PIs are similar to the three and four gene models for other SCN resistance sources known to date.     

Heritability and gain from selection for quantitative resistance to Xanthomonas campestris pv. vesicatoria in Capsicum annuum L.

Summary Breeding for disease resistance in peppers (Capsicum spp.) to the bacterial spot pathogen (Xanthomonas campestris pv. vesicatoria (Doidge) Dye) has been based on either qualitative or quantitative evaluation methods. Quantitative evaluation of components of resistance, lesion number and lesion diameter, has been useful for determining quantitative resistance, but few breeders have applied these methods in routine selection programs. This study was aimed at determining the heritability and gain from selection for resistance to the bacterial spot pathogen based on three components of resistance. Random selections from a diverse intermated population of Capsicum annuum L. were self-pollinated for two generations to create S₁ and S₂ families. Thirty S₁ families, corresponding S₂ families and four homozygous check lines were evaluated. At forty-two days after seeding, two different leaves of each plant were inoculated by leaf infiltration with low concentrations (5×10³ colony forming units) of Group 2 (XCV PT, race 1) and 4 (XCV P, race 1) of the bacterium, respectively. After 15 days, lesion number cm^-2 and lesion diameter were measured. Total lesion area was calculated. Narrow-sense heritabilities for lesion number, lesion diameter, and total lesion area were 0, 43, and 31%, respectively, with Group 2, and 26, 43, and 33%, respectively, with Group 4. Actual S₂ gain from 20% selection pressure in the S₁ was approximately 50%, when selection towards resistance was based on total lesion area.     

An analysis of genes for resistance against Indian stem rust races in two bread wheat cultivars

Summary The genetic constitution of two bread wheat accessions from the International Spring Wheat Rust Nurseries (E 5883 and E 6032) has been studied for reaction to four Indian races of stem rust. Analysis of E 5883 has revealed that for each of the races 15C, 21 and 40 a single dominant gene operates for resistance. The dominant gene against race 15C was identified as Sr6. The dominant genes for resistance against races 21 and 40 were found to be different from the genes described so far. Resistance against race 122 is controlled by a single recessive gene producing characteristically a 2 type of reaction. This gene was identified as Sr8.The resistance of E 6032 against each of the races 15C, 21 and 40 is controlled by two genes, one dominant and one recessive, which act independently. Dominant genes effective against 15C, 21 and 40 were conclusively identified as Sr6, Sr5 and Sr9b, respectively. From the correlated behaviour against races 15C and 40 as well as from the phenotypes of the resistance reactions rhe same recessive gene, undescribed so far, operates against the two races. The second recessive gene operating against race 21 was also observed to be different from those so far designated. E 6032 was, however, found to be susceptible to races 122.The presence of Sr6 both in E 5883 and E 6032 against race 15C was further confirmed through F₂ and F₃ segregation data.     

The inheritance of stem rust and leaf rust resistance in some Ethiopian wheat collections

Summary Common and durum wheat populations obtained from Sweden and originally collected in Ethiopia were screened for resistance to steum rust and leaf rust. Resistant selections of common wheat were crossed and backcrossed with either stem rust susceptible RL6071, or leaf rust susceptible Thatcher. Genetic studies, based largely on tests of backcross F₂ families, showed that four of the selections had in common a recessive gene SrA. Plants with this gene were resistant (1⁺ infection type) to all stem rust races tested. This gene was neither Sr26 nor Sr29. The resistance of other selections, based on tests with an array of rust isolates, was due to various combinations of Sr6, 8a, 9a, 9d, 9c, 11, 13, 30, and 36. One of the selections had linked genes, Lr19/Sr25. Another selection had a dominant gene for resistance (;1 infection type) to all the races of leaf rust. With the possible exception of this gene for leaf rust resistance and SrA, no obviously new resistance was found.     

Monogenic resistance in tomato to Fusarium oxysporum f. sp. lycopersici race 3

Summary Resistance to fusarium wilt, incited by Fusarium oxysporum (Schlecht.) f. sp. lycopersici (Sacc.) Snyder & Hansen race 3 in tomato (Lycopersicon esculentum Mill.) was discovered in LA 716, a L. pennellii accession. A resistant BC₁F₃ breeding line, E427, was developed from LA 716. E427 was crossed with the susceptible cv. Suncoast and F₁, BCP₁, BCP₂ (to Fla 7155, a susceptible parent) F₂, F₃, and BCP₂S₁ seeds were obtained. Segregation for resistance following root dip inoculation over three experiments indicated a single dominant gene controlled resistance. Five of the 12 BCP₁S₁'s segregated more susceptible plants, whereas one of the 12 segregated more resistant plants than expected (P<0.05). Three of 23 F₃ lines segregated more susceptible plants than expected while 1 of the 23 had more resistant plants than expected (P<0.05). Segregation in all other lines fit expected ratios. Five of the 23 F₃'s were homozygous resistant which was an acceptable fit to expectations (P=0.1–0.5). The gene symbol I ₃ is proposed for resistance to race 3 of the wilt pathogen. Deviations from expected ratios in data reported here and for other breeding lines indicate an effect of modifier genes and/or incomplete penetrance. Plant age at inoculation and seed dormancy did not affect results.Florida Agricultural Experiment Station Journal Series No. 8101.     

Pathogenicity of the Rhynchosporium secalis population in the Western Cape province of South Africa

The virulence spectra of 50 Rhynchosporium secalis isolates from a population in the Western Cape province of South Africa were determined, and 21 races were detected when evaluated against 17 differential cultivars. The virulence spectrum of the R. secalis population shows considerable variation, and carries unnecessary virulence genes which is quite unexpected, since chiefly susceptible barley cultivars are grown in the south Western Cape. The two most prevalent races, namely races 4 and 7 had three and four virulence genes respectively. Both race 4 and 7 were virulent on the most susceptible cultivars, West China, Steudelli, C.I.8618 and C.I.2226. Considering the resistance genes reported for the cultivars Atlas 46, Turk, and C.I.3515 which showed no susceptible cultivar-pathogen interaction, it would appear that the Rh-Rh3-Rh4 complex is primarily involved in conferring resistance to the local R. secalis isolates This revised version was published online in July 2006 with corrections to the Cover Date.     

Inheritance of resistance to wildfire and angular leaf spot derived from Nicotiana rustica var. Brasilea

Summary The introgression of wildfire (races 0 and 1) and angular leaf spot (ALS) resistance from N. rustica var. Brasilea into N. tabacum has proved economically useful in Zimbabwe although the mode of inheritance of, and genetic relationships between the resistance are unknown. This study was undertaken to (1) examine the mode of inheritance of the resistance to races 0 and 1 of wildfire, and ALS, (2) determine the genetic relationship between the resistances and (3) establish whether the N. rustica-derived wildfire race 0 resistance is allelic to that obtained from N. longiflora. Inheritance was examined under greenhouse and field conditions by studying disease reactions in the parental, F1, F2 and backcross generations derived from crosses of three susceptible lines to a resistant line Nr-7. Three-point backcrosses to the susceptible parent were examined for linkage and segregating generations from a cross of Nr-7 to Burley 21 which carries the N. longiflora race 0 resistance were used to test for allelism. In general, we observed that all resistances are determined by a single dominant gene although some incosistent ratios were obtained likely due to misclassification of disease reactions and erratic transmission. All resistances showed linkage although pleiotropism cannot be ruled out. Allelism tests demonstrated that the N. rustica race 0 resistance is not allelic to that obtained from N. longiflora. Our findings are examined in relation to the efficacy of indirect selection for resistance.     

Resistance and partial resistance to Phytophthora sojae in early maturity group soybean plant introductions

Phytophthora sojae, an important yield limiting pathogen of soybean, causes seed, seedling, root, and stem rots. Losses caused by P. sojae can be controlled by both major gene and partial resistance. Early maturity group (MG) soybeans are an increasingly important crop in northwestern Minnesota and eastern North Dakota. Early MG plant introductions (PIs) from the USDA Soybean Germplasm Collection and early MG public and private cultivars were evaluated for resistance and partial resistance to P. sojae. Of the 113 PIs, PI438445, and PI438454 exhibited resistance to P. sojae races 4, 7, 17, and 28 indicating they may possess either Rps1c, Rps1k, previously unidentified or multiple resistance gene to Phytophthora sojae (Rps) genes. Because they exhibited partial resistance equal to or greater than the standard check cultivar Conrad, three early MG soybean cultivars (MN0902, MN0302, and 91B53) were selected as standard checks to evaluate early MG PIs for partial resistance. Sixty-nine PIs were evaluated for partial resistance to P. sojae races 7 and 25 using the inoculum layer method. Of this group of PIs, 22 had the same level of partial resistance as Conrad to P. sojae race 7 while 19 had the same degree of partial resistance to race 25. Twelve PIs had same level of partial resistance as Conrad to both P. sojae races 7 and 25. The PIs and cultivars identified in this study will be of great value in developing early MG soybean cultivars suitable for planting in Canada and the northern United States.     

New sources of major gene resistance in Lactuca to Bremia lactucae

Summary A total of 1789 accessions of several lettuce collections was screened to find new major gene resistance to the downy mildew fungus Bremia lactucae Regel. The accessions belonged to the species Lactuca sativa (N=1288), L. serriola (N=399), L. saligna (N=52) and L. virosa (N=50). A total of 20 races of B. lactucae were used, 14 of which were NL-races, isolated from cultivated lettuce in the Netherlands. The other six races were isolated from wild L. serriola in Czechoslovakia. The accessions were initially screened with two races: NL1 and NL3. Accessions with resistance to one or both of these races were tested with the other races. Phenotypes with new resistance were found in accessions of all four Lactuca species. Of L. sativa, four accessions were found with resistance phenotypes that could not be explained by combinations of known major genes. Many accessions of L. serriola had resistance phenotypes that indicated the presence of unknown resistance genes. All interactions between accessions of L. saligna and races of B. lactucae were incompatible in leaf disc tests, except for four accessions, which showed some sporulation with race NL6. Several accessions of L. virosa were resistant to all races used. Other accessions of L. virosa gave a race-specific interaction with B. lactucae.     

Use of molecular markers to accelerate the breeding of common bean lines resistant to rust and anthracnose

The main goal of this work was to introduce resistance genes for rust, caused by Uromyces appendiculatus, and anthracnose, caused by Colletotrichum lindemuthianum, in an adapted common bean cultivar through marker-assisted backcrossing. DNA fingerprinting was used to select plants genetically closer to the recurrent parent which were also resistant to rust and to race 89 of C. lindemuthianum. DNA samples extracted from the resistant parent (cv. Ouro Negro), the recurrent parent (cv. Rudá), and from BC₁, BC₂ and BC₃ resistant plants were amplified by the RAPD technique. The relative genetic distances in relation to the recurrent parent varied between 9 and 59% for BC₁, 7 and 33% for BC₂, and 0 and 7% for BC₃ resistant plants. After only three backcrosses, five lines resistant to rust and anthracnose with, approximately, 0% genetic distance in relation to the recurrent parent were obtained. These lines underwent field yield tests in two consecutive growing seasons and three of them presented a good yield performance, surpassing in that sense their parents and most of the reference cultivars tested.     

Evaluation of the virulence of races 1, 2 and 4 of Fusarium oxysporum f.sp. dianthi in carnation

Summary A simultaneous analysis of the virulence of races 1, 2 and 4 of Fusarium oxysporum f.sp. dianthi to a series of nine carnation cultivars revealed the presence of different interactions between races and cultivars, as well as differences in pathogenesis between race 1 on the one hand and race 2 and 4 on the other.The most common race 2 induced typical symptoms of Fusarium wilt in all susceptible cultivars. The cultivars showed considerable variation in resistance to race 2. Only Novada remained free of external symptoms throughout the experiment. In diseased plants of all cultivars studied, infected vascular tissue was white with dark brown margins, and heavy degradation of the cell walls and xylem parenchyma cells had occurred. All Dutch isolates corresponded with race 2.Race 4 induced wilt symptoms similar to those induced by race 2, and there was a similar variation in resistance to race 2 and 4 in the cultivars. On average, the race 4 isolates were less aggressive than those of race 2. Compared with race 2, there was evidence of some genotype × race interactions: Pallas proved to be considerably more susceptible, and Lena more resistant to race 4 than to race 2. The isolates of race 4 induced a nistopathology similar to that induced by race 2, but with less vascular browning.Race 1 induced atypical but severe wilt symptoms and unusual vascular discoloration in Elsy, Niky and Sam's Pride only. The vascular tissue in these cultivars turned pale brown; in spite of heavy colonization of these tissues virtually no degradation of cell walls was observed. All other cultivars tested proved virtually resistant to race 1, providing further evidence for genotype × race interactions.Within races, limited but statistically significant genotype × isolate interactions were found as well, in particular within race 4. These are tentatively attributed to independent variation of two (or more) resistance components.     

Genetics of host reactions to three races of the bacterial pustule pathogen in cowpea

Summary Inheritance of the brown hypersensitive resistant (BHR), non-hypersensitive resistant (R) and susceptible (S) host reactions produced by three races of the bacterial pustule pathogen (Xanthomonas campestris pv. vignaeunguiculatae) was studied in 45 F₁, F₂ and testcross progenies, using the infiltration inoculation method BHR reaction was dominant over R and S reactions, and R was recessive to S reaction. Two genes appeared to be involved in BHR reaction; one governing BHR reaction to race 1 and the other to races 1 and 2. Both were ineffective against race 3. R reaction, effective against all the races, appeared to be controlled by one, two or three recessive genes. One cowpea line had one BHR gene and two duplicate recessive R genes. Reaction expression in the segregants was clear and as expected with races 2 and 3 but was modified with race 1, possibly due to interactions between dominant or recessive alleles of the BHR genes and the homozygous recessive allele of the R genes. Gene symbols Bp-1 and Bp-2 are proposed for the BHR genes and bp-3, bp-4 and bp-5 for the recessive R genes. The genes present in each of the differential cowpea line are suggested.Contribution from the International Institute of Tropical Agriculture, Ibadan, Nigeria and Crop Development Division, Ministry of Agriculture, P.O. Box 9071, Dar es Salaam Tanzania.     

Resistance of nineteen cultivars of subterranean clover to four races of Phytophthora clandestina

Summary The resistance of 19 cultivars of subterranean clover was screened against 4 races of P. clandestina by mycelial inoculation of roots of 10-day-old seedlings growing in water agar and by growing seedlings in pasteurised potting mix containing infested vermiculite in controlled conditions. The cultivars showed differential resistance (vertical resistance) to races of the pathogen and can be divided into 4 resistance groups. Cultivars Clare, Esperance, Green Range, Junee, Mount Barker, Rosedale, Woogenellup and Yarloop were susceptible to all races. Cultivars Bacchus Marsh, Denmark, Enfield, Gosse, Goulburn, Karridale, Larisa, Leura and Trikkala were susceptible to races 1 and 3, but resistant to races 0 and 2. Cultivar Meteora was susceptible to races 2 and 3, but resistant to races 0 and 1. Cultivar Seaton Park (LF) was resistant to all races. Cultivars also varied in their race-non-specific (horizontal) resistance: cultivars that were susceptible to particular races usually varied in their degree of susceptibility to those races. In particular, Junee was more resistant to all four races than the other cultivars within its group. Similarly, cultivars Gosse, Larisa, Denmark and Trikkala were more resistant to races 1 and 3 than the other cultivars in their group. Races of the pathogen varied in their aggressiveness as well as in their virulence, as shown by the variation in aggressiveness of different isolates of race 0.     

Quantitative resistance to Botrytis cinerea from Solanum neorickii

Tomato (Solanum lycopersicum) is susceptible to gray mold (Botrytis cinerea). Quantitative resistance to B. cinerea was previously identified in a wild relative, S. neorickii G1.1601. The 122 F₃ families derived from a cross between the susceptible S. lycopersicum cv. Moneymaker and the partially resistant S. neorickii G1.1601 were tested for susceptibility to B. cinerea using a stem bioassay. Three putative quantitative trait loci (pQTL) were detected: pQTL3 and pQTL9 reducing lesion growth (LG) and pQTL4 reducing disease incidence (DI). For each pQTL, a putative homologous locus was identified recently in another wild tomato relative, S. habrochaites LYC4. pQTL3 was confirmed by assessing disease resistance in BC₃S₁ and BC₃S₂ progenies of S. neorickii G1.1601. pQTL4 was not statistically confirmed but the presence of the S. neorickii resistance allele reduced DI in all three tested populations. The reduction in LG of pQTL9 was not confirmed but rather, this locus conferred a reduced DI, similar to observations in the QTL study using S. habrochaites. The results are discussed in relation to other disease resistance loci identified in studies with other wild tomato relatives.