Inheritance of the Powdery Mildew Resistance Mlk in Wheat

The inheritance of the Mlk powdery mildew resistance originating from ‘Heine 2174.50’ was analyzed by crossing the Mlk resistant cultivar ‘Ralle’× cv. ‘Amor’ (highly susceptible) and vice versa and by observing the reactions of F₁- and F₂-plants after inoculation with two different Mlk avirulent powdery mildew isolates. In all cases, a 3 (resistant): I (susceptible) segregation was found in F₂. The reactions of the F₂plants against the two powdery mildew isolates were identical in each case. Therefore, it is supposed that one dominant resistant gene is responsible for the resistant reactions against these two isolates. These results support the earlier assumption of Heun and Fischbeck (1987b) that the whole Mlk resistance pattern is controlled by a single gene.     

Resistance to Rhopalosiphum padi (Homoptera: Aphididae) in Hordeum vulgare subsp. spontaneum and in hybrids with H. vulgare subsp. vulgare

Summary Accessions of Hordeum vulgare subsp. spontaneum, the progenitor of cultivated barley, were screened in field and glasshouse trials for resistance to the aphid Rhopalosiphum padi. A few selected lines were furthermore hybridized with a modern barley variety and the resulting populations evaluated. High levels of resistance were found among some of the spontaneum accessions resulting in lower aphid growth rates (maximum reduction 57%). Segregation patterns among siblings in F2 populations were continuous, indicating the presence of several genes with possibly additive effects. The usefulness of H. vulgare subsp. spontaneum for breeding aphid resistant barley is discussed.     

Screening fruit tree genetic resources in Belgium for disease resistance and other desirable characters

Summary The wide diversity of old fruit-tree cultivars originating or introduced into Belgium during the 18 ^th and 19 ^th centuries was collected as far as feasible over the last fifteen years at the State Plant Pathology Station in Gembloux. Out of the 2400 accessions now collected, one quarter was recovered from old public collections, and three quarters came from farms or gardens. The initial intention was to screen the material for disease resistance and other characters of agronomic interest with a view to using the best cultivars as breeding parents. However, as the collection developed, genetic resources conservation also became an objectiveper se. The collection presently contains 1150 apple, 850 pear and 300 plum accessions, and smaller numbers of other fruit species. Each accession is evaluated in an experimental orchard for at least ten years. In view of the growing public interest in old fruit-tree cultivars, the Plant Pathology Station has for several years been releasing to the nursery trade the better cultivars emerging from the evaluation, namely nine apple and four plum cultivars, and one peach cultivar. The principal features of the apple cultivars are presented in this paper. Since 1988, old apple and plum cultivars have been being used at the Station as parents in a breeding programme, with both controlled and open pollination. In some instances, old apple cultivars have also been crossed with a modern parent carrying the Vf gene for scab resistance. The preliminary observations on some of these seedlings are presented.     

膜脂组成和膜结合酶活性与玉米幼苗抗冷性的关系

玉米黄化幼苗经4～5℃低温处理36小时后,膜脂组成在自交系间、处理和对照间没有差异;膜脂肪酸不饱和度发生明显变化,抗性强的自交系不饱和指数提高,抗性差的不饱和指数降低,膜结合酶(叶绿体K~+-Mg~(2+)ATPase和线粒体Na~+-K~+ATPase)活性降低,抗性差的自交系膜结合酶活性降低幅度大于抗性强的自交系。     

Inheritance of resistance to Didymella lycopersici Klebb. in tomato (Lycopersicon Mill.)

Summary The inheritance of the resistance to Didymella lycopersici was studied on F₃- and Bc₁-lines from interspecific crosses of L. esculentum with L. hirsutum and with L. hirsutum glabratum. The resistance is not monogenic and is inherited in dominant fashion. The high h² values based on line means offer possibilities to seleet efficiently for enhanced levels of resistance.     

Rapid survey for presence of a blast resistance gene Pi-ta in rice cultivars using the dominant DNA markers derived from portions of the Pi-ta gene

The Pi‐ta gene in rice confers resistance to strains of the blast pathogen Magnaporthe grisea (Herbert) Borr. (anamorph Pyricularia oryza Cav.) containing the corresponding avirulence gene AVR‐Pita in a gene‐for‐gene fashion. The Pi‐ta gene is a typical nucleotide‐binding site type resistance gene. Nucleotide sequences distinguishing the resistant Pi‐ta and susceptible pi‐ta alleles were previously identified and used for developing DNA markers for a resistant Pi‐ta haplotype and three susceptible pi‐ta haplotypes. In the present study, the existence of the Pi‐ta gene in 141 rice germplasm accessions was rapidly determined using these markers, and the results were confirmed by inoculating rice germplasm with an M. grisea strain containing AVR‐Pita. The Pi‐ta gene was found in accessions from several major rice producing countries, including China, Colombia, Japan, Vietnam, the Philippines, Iran and the United States. The usefulness of DNA markers for rapid determination of the genotype of rice germplasm was thus demonstrated. The Pi‐ta gene also was found in rice cultivar known to contain the Pi‐ta² gene, although the allelic relationship of these genes remains to be determined. The presence of the Pi‐ta gene in landrace cultivars in several different geographical locations, the Philippines and Vietnam, other indica rice cultivars in China and Colombia suggest that the Pi‐ta gene may have spontaneously originated in indica rice cultivars. These results are useful for incorporating the Pi‐ta gene into advanced breeding lines by marker‐assisted selection for rice breeding programmes worldwide.     

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.     

Identification of levels of resistance to cassava root rot disease (<Emphasis Type="Italic">Botryodiplodia theobromae</Emphasis>) in African landraces and improved germplasm using <Emphasis Type="Italic">in vitro</Emphasis> inoculation method

Cassava root rot disease is an increasing problem in Africa where yield losses of about 80% have been recorded. We evaluated 290 African landraces and 306 improved genotypes from the germplasm collections of the International Institute of Tropical Agriculture (IITA), for sources of resistance using root slice laboratory assay. Disease severity was assessed quantitatively by direct percentage estimation (PS) and by use of a rating scale (RS). Both methods of assessment were compared for identification of variability in the germplasm, and genotypes were classified into response groups using an enlarged rank-sum method that combined the PS and RS assessments. The two scoring methods revealed continuous variation (P < 0.001) for resistance in the sets of germplasm. Disease assessments based on PS and RS were highly correlated in both the improved germplasm (r = 0.75) and the landraces (r = 0.72). Based on PS assessment, 50 improved genotypes (16.3%) and 53 landraces (18.3%) showed significantly lower disease scores than the resistant control. The rank-sum method separated each set of collections into highly resistant, resistant, moderately resistant, moderately susceptible, susceptible and highly susceptible groups. Fifty-nine improved genotypes (16.4%) and 61 African landraces (16.9%) were identified as either highly resistant or resistant. Generally, these genotypes exhibited resistance by limiting the growth of the pathogen (reduced amount of invaded surface area). This type of rate-reducing resistance is highly heritable and a quantitative trait which can be harnessed in breeding. Genotypes subsets were identified for further studies into the genetic basis of resistance to root rot disease.     

Resistance of wheat to Fusarium spp. in an in vitro seed germination assay and preliminary investigations into the relationship with Fusarium head blight resistance

Resistance of wheat cultivars from the UK, 2003 recommended list were evaluated against Fusarium culmorum, F. graminearum, Microdochium nivale var. majus and var. nivale, F. avenaceum and F. poae using a seed germination assay and related to previously published data on detached leaf assays, using M. nivale, and UK recommended list Fusarium head blight (FHB) resistance ratings. Significant cultivar differences were found after inoculation with all fungal species with the exception of F. poae where the decline in the percentage of seed germinating relative to the controls was small. Correlations of the percentage seed germinating inoculated with the remaining Fusarium spp. and M. nivale were high (r = 0.68; P < 0.01 to r = 0.94; P < 0.001). Overall, F. graminearum caused the greatest reduction with a mean seed germination of 61.7% relative to the controls followed by F. avenaceum (65.5%), M. nivale var. majus (67.2%), F. culmorum (76.6%), M. nivale var. nivale (89.2%) and was least for F. poae (92.5%). The resistance detected in the germination assay was significantly correlated to whole plant FHB resistance ratings (r_s = 0.45; P < 0.05) but was not correlated to partial disease resistance (PDR) components detected using the detached leaf assay, namely, incubation period, latent period and lesion length. The results suggest that while resistances detected in the seed germination and detached leaf assays may in part share a common genetic basis to FHB resistance, resistances detected in both in vitro assays are independent of each other. Resistances detected in the seed germination assay had a lower correlation with FHB resistance ratings against F. culmorum, than those detected by latent period in the detached leaf assay.     

Environmental factors influence the expression of resistance to sorghum midge, Stenodiplosis sorghicola

Host plant resistance is an effective means of controlling sorghum midge (Stenodiplosis sorghicola). We studied the influence of environmental factors on expression of resistance to sorghum midge in three midge-resistant and two midge-susceptible genotypes. Midge-resistant lines AF 28, ICSV 197, and TAM 2566 suffered 8.8 to 17.3% damage across seven so wings compared to 25.6%damage in ICSV 112, and 69.4% damage in CSH 5. Susceptibility of the midge-resistant lines (AF 28, ICSV 197, and TAM 2566) decreased with an increase in open pan evaporation, maximum and minimum temperatures, and solar radiation; while the midge-susceptible lines (ICSV 112 and CSH 5) showed a poor interaction with these factors. Midge damage in ICSV 197 showed a negative correlation with minimum temperature and relative humidity and positive correlation with sunshine hours,while the reverse was true for CSH 5. Grain growth rate between 0 and 3 days after anthesis was lower in crops sown on 1st October, when AF 28 and ICSV 197 suffered maximum midge damage. Maximum and minimum temperatures and maximum relative humidity influenced the moisture content of the grain, grain growth rate, and sorghum midge damage. There was considerable variation in genotype × environment interaction for expression of resistance to sorghum midge,and the implications of these results have been discussed in relation to development of sorghum cultivars with resistance to this insect. This revised version was published online in August 2006 with corrections to the Cover Date.