Resistance toPhytophthora porri in leek and some of its wild relatives

Summary Winter leek and related plant material was evaluated for resistance to white tip disease (Phytophthora porri) in the Netherlands. Significant differences in partial resistance were found between winter leek accessions in field tests with zoospore and natural inoculation. These differences were consistent over three years, and indicate that resistance can be improved through mass selection in winter leek cultivars. Some accessions of wild or cultivated relatives of winter leek with higher levels of resistance were found in a glasshouse test with zoospore inoculation. These sources of resistance were also highly resistant in field experiments over a two year period.     

Partial resistance in lettuce to downy mildew (Bremia lactucae). 2. Differential interactions between plant genotypes and fungus races and the relationship of latent period,infection frequency and number of infected leaves with the level of partial resistance

Summary In three experiments differential interactions between lettuce genotypes and Bremia lactucae races were investigated and also the influence of latent period, infection frequency and number of infected leaves on the level of resistance.In experiment 1 very significant differential interactions were observed mainly caused by two plant genotypes (PIVT 82 and PIVT 315) and race NL 7. In experiments 2 and 3 differential interactions were absent or of minor importance. Results suggest that partial resistance often was of a race-non-specific character. Latent period appeared to be a major component of resistance in the lettuce-Bremia lactucae relationship. It determines the onset of the next cycle of spore production and is also indicative of fungus growth in host tissue. Compared with infection frequency and number of infected leaves, latent period shows a small error variation and can efficiently be measured. The closeness of the relationship of the number of primary infections and the number of infected leaves with resistance level varied, depending on the experiment. The above three components appeared to be associated but the degree of the association varied. The variable behaviour of the three components suggests that they are governed by different genes.For the level as well as for the complexity and durability of the resistance, accumulation of as many components of resistance as possible is desirable. Therefore, in parents all such components should be analysed before crosses are carried out to improve resistance level. In the final products of crossing and selection the relative importance of components should also be investigated to determine whether an increased resistance level results from additional effects of the parents on one component (e.g. latent period) or from accumulation of different components in a plant genotype.Students at the Agricultural University Wageningen     

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.     

Effect of leaf age on components of partial resistance in rice to leaf blast

Summary Ten rice genotypes were inoculated with a virulent isolate of Magnaporthe grisea to study the effect of leaf age on components of partial resistance, and evaluate associations between these components. The number of sporulating lesions per cm² leaf declined with increase of leaf age in all genotypes. The number of lesions per cm² leaf area in one week old leaves was about 25% that of the number in very young leaves in the susceptible cultivar CO39, but less than 2% in the more resistant cultivars IR36, IR60 and IR62. Large differences between genotypes were found for the number of sporulating lesions that developed, and this factor was closely related to the period that leaves remained susceptible after appearance. The number of lesions in the most susceptible cultivar CO39 was about 7 times that in the cultivars IR60 and IR64. Differences between genotypes were also found for lesion size. The effect of aging on average lesion size was less pronounced than on lesion density. Lesion size and lesion density were positively correlated, but a rapid decline of density was not necessarily accompanied by a rapid decline of size. No apparent differences between genotypes were observed for latent period. Genotypes with leaves that became highly resistant soon after appearance expressed higher levels of partial resistance in the field.     

Genetics and population improvement of resistance to bacterial soft rot in Chinese cabbage

Resistance to bacterial soft rot caused by Erwinia carotovora subsp. carotovora is a quantitative trait. The narrow-sense heritability was from 42% to 60% in the studied populations. Griffing's diallel analysis and generation mean analysis indicated that additive genetic effects were most significant, and the resistant sources involved in this study appeared to have different genes or alleles. Recurrent phenotypic selection was used for improving the resistance level by combining different resistance genes from the selected genotypes of Chinese cabbage. After 3 cycles of recurrent selection, the level of resistance increased markedly. When the cycle 3 population was compared with the cycle 0 population, the mean disease severity rating was reduced 38% from 6.3 to 3.9, and the percentage of plant survival increased from 65% to 97% based on the mean of two years experiments using mist-chamber seedling inoculation methods. The improvement of resistance was also demonstrated in field assays. This revised version was published online in July 2006 with corrections to the Cover Date.     

Identification of growth stage dependent expression of partial resistance of barley to powdery mildew

Summary Genotypic differences for growth stage dependent expression of partial resistance to barley powdery mildew have been identified on the basis of two components of resistance in the glasshouse and the pattern of epidemic development in the field.Differences for infection frequency and proportion of sporulating colonies were highly significant between the 16 genotypes investigated at four stages of plant development in the glasshouse. Both resistance components were significantly correlated mutually (r=0.73 to 0.86) and with the infection level on leaves developed at comparable growth stages in the field (r=0.52 to 0.73). The infection level of seedlings in the glasshouse was significantly correlated with the infection level of the leaves of the first until the third node in the field (r=0.70 to 0.73). Adult plant resistance was predominantly expressed at the uppermost leaf. A clear genotype × growth stage interaction was apparent for infection level; some genotypes showed partial resistance predominantly at the seedling stage and others predominantly at the adult plant stage.Abbreviations IF Infection Frequency, number of colonies per cm² leaf area, glasshouse experiment - IL Infection Level, number of colonies per cm² leaf area, field experiment - PSC Proportion Sporulating Colonies from total number of colonies, glasshouse experiment     

Interactions between plant and aphid genotypes in resistance of lettuce to Myzus persicae and Macrosiphum euphorbiae

Summary Six lettuce lines, representing two types of resistance to the green peach aphid, Myzus persicae, and a control line with high susceptibility to M. persicae were tested for resistance to six different clones of Myzus persicae and two clones of the potato aphid, Macrosiphum euphorbiae.The clones of M. persicae showed very different levels of aggressiveness on lettuce: two had a high level of reproduction, two had an intermediate level and two were poorly adapted to lettuce as a host. Differences between lettuce lines in aphid reproduction increased with increasing aggressiveness of the aphid clone, which means that aggressive clones are most effective for selection purposes. No evidence was found for clone-specific plant genotype reactions, meaning that lines resistant to one clone will also be resistant to other clones of M. persicae, allthough not neccessarily at the same level. The lettuce lines selected for partial resistance to the aggressive clone WMp1 were completely or almost completely resistant to less aggresive clones.No differences in level of reproduction were found between the two clones of M. euphorbiae and no relation was observed between resistance to M. persicae and M. euphorbiae, indicating the species-specific character of resistance to leaf aphids in lettuce.     

Qualitative and quantitative evaluation of cassava bacterial blight resistance in F1 progeny of a cross between elite cassava clones

Deployment of resistant varieties is one major approach to controlling cassava bacterial blight (CBB), caused by Xanthomonas axonopodis pv. manihotis (Xam). To understand the genetic determinism of resistance to CBB, the use of reliable parameters measuring resistance is necessary. In order to test a relevant method for evaluation of quantitative resistance for mapping QTL (quantitative trait loci), the response of 150 F₁ individuals, inoculated with four different Xam strains (CIO-84, CIO-1, CIO-136 and CIO-295), was assessed under controlled conditions. We used two types of evaluations at different intervals after inoculation, one based on a scale of 0 to 5 and the second based on the determination of the bacterial population in the vascular system. Both evaluation types revealed interaction between strains and F₁ genotypes. Population values at 3 and 6 cm from the point of inoculation showed a high level of correlation. By performing an association analysis, at 7 and 15 days after inoculation, a significant positive correlation between both evaluation types was obtained. However, the disease rating at 30 days did not correlate with bacterial populations at either 7 or 15 days after inoculation, except for one strain, CIO-84. Evaluation of the bacterial population in stem tissues is time and labour consuming, consequently, for a rapid and reliable assessment of CBB resistance for QTL analysis, we strongly recommend evaluation based on the use of a symptom scale.     

Screening for resistance to Nectria galligena Bres. in cut shoots of apple

Summary To find a fast and reliable test to assess resistance to Nectria galligena in apple, different methods of inoculation were compared using macroconidia of N. galligena and one-year-old cut shoots from mature trees of Cox's Orange Pippin, IVT 69078-19, James Grieve and Jonathan.With the best inoculation method 11 genotypes were screened for resistance. Elstar, Golden Delicious, Jonathan and Lombart's Calville were highly resistant and the level of resistance of Ingrid marie, Gloster, Melrose, IVT 69078-19, Cox's Orange Pippin, James Grieve and Idared decreased in this order.The best inoculation method proved to be simple, giving results within four to nine weeks after inoculation.     

Variation in resistance level of potato genotypes and virulence level of potato cyst nematode populations

Summary In two experiments, using different testing methods, the number of newly formed cysts was determined on nine potato genotypes with resistance from various sources. Ten potato cyst nematode (PCN) populations were used in these experiments. Rank correlation between numbers of cysts over potato genotype-PCN population combinations for both experiments was high (r_s = 0.90). Dendrograms for PCN populations and potato genotypes were constructed, based on a simultaneous hierarchical clustering procedure for potato genotype-PCN population interaction terms. Several virulence groups could be identified within Globodera rostochiensis as well as within G. pallida. Host genotypes, derived from the same sources of resistance, were clustered in different resistance groups.     

A study of genotype-isolate interaction in cacao (Theobroma cacao L.): resistance of cacao genotypes to isolates of Phytophthora palmivora

The resistance of cacao (Theobroma cacao. L)leaves to 10 isolates of Phytophthora palmivora(Butler) Butler was investigated in 18 genotypes of cacao in three experiments. Tissue-paper-mount and punch-inoculation methods were used to distinguish between resistance at the penetration and post-penetration levels based on lesion frequency and size, respectively. There was a 5–6 fold difference in aggressiveness among isolates collected from different locations in the islands of Trinidad and Tobago for both types of resistance. However, host genotype ×isolate interaction effects were not significant for resistance at both the penetration and post-penetration levels. A strong positive relationship (r = 0.8) was observed in the abilities of isolates to effectively breach the two types of resistance. This suggests that the same isolate can be used in screening for both types of resistance. No significant relationship was found between resistance at the penetration and post-penetration levels suggesting that the resistance mechanisms were different. Subsequent experiments using a wider range of cacao genotypes from the International Germplasm Collection, Trinidad, confirmed the initial results. These experiments indicate that resistance found using one isolate would be equally valid using other isolates of Phytophthora palmivora. The implications of the findings are discussed. 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.     

Assessment of quantitative resistance of rice cultivars to Xanthomonas campestris pv. oryzae: A comparison of assessment methods

Summary Lesion length, leaf length, and leaf width were measured on infected leaves two weeks after clip inoculation of 64 rice cultivars with two virulent isolates of Xanthomonas campestris pv. oryzae (X.c. pv. oryzae). No significant correlation was found between the lesion length and the leaf dimensions, indicating that physical leaf size does not affect the spread of the bacteria once these have entered the leaf. Lesion length is therefore an acceptable parameter for assessing resistance to (X.c. pv. oryzae), and is to be preferred above the parameter percentage diseased leaf area (% DLA), especially when small differences between genotypes are to be assessed. The confounding influence of differences in leaf length can cause large changes in the ranking order of cultivars when assessed by the % DLA. For this reason lesion length is a better assessor of the value of a quantitative resistance for breeding and research purposes than % DLA.     

The mechanism of preferential fertilization in tomatoes carrying the I-Allele for fusarium resistance

Summary Preferential fertilization of ovules by male gametes carrying the dominant I allele for Fusarium resistance occurred when heterozygous Ii plants served as male parent. Pollination with a mechanical l:l mixture of I and i pollen gave the normally expected Mendelian ratio.No significant differences were found in vivo in growth rate or in final length of pollen tubes of genotypes I and i. The experiments indicated that preferential fertilization was mainly caused by prepollination events leading to production of unequal amounts of fertile pollen grains of the two types.     

The identification and characteristics of field resistance to lettuce downy mildew (Bremia lactucae Regel)

Summary Eighty-one accessions of three Lactuca species which showed no recognisable race specific resistance to Bremia lactucae when tested in the laboratory as seedlings, exhibited different degrees of susceptibility when exposed to natural field infection. As a group, crisp genotypes had less mildew and a slower rate of disease development than other types of lettuce. Wild forms of Lactuca sativa and Lactuca serriola were particularly susceptible. In a further trial, the low field susceptibility of three lettuce cultivars (Iceberg, Batavia blonde de Paris and Grand Rapids) was confirmed. Disease development on cv. Iceberg was compared to that on the highly susceptible cv. Hilde in experiments where the two cultivars were grown either in close proximity or in isolation. The absolute level of attack on cv. Iceberg depended upon disease pressure and differences between the two types only became apparent approximately 8 wk after sowing.     

Assessment of quantitative resistance of rice cultivars to Xanthomonas campestris pv. Oryzae: a comparison of inoculation methods

Summary Lesion size and lesion number were measured on cultivars of rice inoculated by clipping or spraying with virulent isolates of Xanthomonas campestris pv. oryzae. Mean percentage diseased leaf area (%DLA) gave a similar ranking for the two inoculation methods but differences in lesion size among cultivars were much more evident after clip than after spray inoculation. Correlation between the methods was high (r=0.82^**), but some cultivars responded differently with the two techniques. Cultivars which had low scores following spray inoculation showed low disease progress during the first nine weeks after transplanting into a screen-house experiment. Assessment after clip inoculation measures resistance due to spread of bacteria within the leaf xylem system, an important component of quantitative resistance. Assessment after spray inoculation measures all resistance, including resistance to entrance of bacteria into the leaf. In order to select rice entries with improved quantitative resistance to X. c. oryzae based on both components, a screening based on lesion length after clip inoculation, followed by a test for lesion number after spray inoculation, is advised.     

Field assessment of partial resistance to powdery mildew in spring barley

Summary Partial resistance to powdery mildew in spring barley was evaluated in three plot types: large isolation plots, in 1.4 m² plots in chessboard design with guard plots of spring wheat and in single rows. Percentage leaf area covered by powdery mildew was scored four to six times during the season and partial resistance was characterized by the area under the disease progress curve. Varietal differences were revealed in al three plot designs, differences between the most resistant and susceptible genotypes being of a factor five. Differences between varieties decreased with decreasing plot size. The relationship between single scores of amount of powdery mildew on the upper four leaves and the area under the disease progress curve was high in all plot designs during the first two to three weeks after heading, allowing selection for the trait by one or two scorings. Differential ranking of varieties between different plot designs was observed, and is assumed to be due to increasing plot interference with reduced plot size and reduced distance between plots. A reliable selection for partial resistance could be made in large isolation plots and in 1.4 m² plots, but hardly in single rows.     

Inheritance of resistance to Phytophthora fragariae Hickman in strawberry

Summary It has been found that complete resistance of the strawberry cultivars and selections Earliglow, Guardian, MdUS 2700, MdUS 2929, MdUS 3816 and Redchief to Phytophthora fragariae is inherited in a one-to-one ratio, suggesting that these genotypes have one major resistance gene effective to the races present in the test field.The high level of partial resistance of Cambridge Favourite has clearly been recovered in its progenies. However, from results presented here, no conclusions could be drawn with regard to the number of genes involved in the partial resistance of Cambridge Favourite. It is, however, suggestive that about half the susceptible selections derived from crosses with Cambridge Favourite have shown a relatively high level of partial resistance. The selections concerned were obtained by selection on horticultural characters in seedling populations planted on an uninfested field.     

Genetic variation for resistance to Fusarium head blight in bread wheat

Summary During a four year period, a total of 258 winter and spring wheat genotypes were evaluated for resistance to head blight after inoculation with Fusarium culmorum strain IPO 39-01. It was concluded that genetic variation for resistance is very large. Spring wheat genotypes which had been reported to be resistant to head blight caused by Fusarium graminearum were also resistant to F. culmorum. The resistant germplasm was divided into three gene pools: winter wheats from Eastern Europe, spring wheats from China/Japan and spring wheats from Brazil. In 32 winter wheat genotypes in 1987, and 54 winter wheat genotypes in 1989, the percentage yield reduction depended on the square root of percentage head blight with an average regression coefficient of 6.6. Heritability estimates indicated that for selection for Fusarium head blight resistance, visually assessed head blight was a better selection criterion than yield reduction.     

Variation in resistance to different winter stress factors within a full-sib family of perennial ryegrass

Wintering ability in the field and resistance to different winter-stress factors under controlled environmental conditions were studied in a full-sib family of perennial ryegrass (Lolium perenne L.). Significant variation in tolerance to freezing and ice encasement, resistance to pink snow mould (Microdochium nivale) and also in winter survival and spring growth were found between the different genotypes. No strong correlations were found between the resistances to the different stress factors. These results indicate that resistance to different winter-stress factors is controlled by separate genes in perennial ryegrass. A low but significant positive correlation was found between spring growth of plants in the field after the first winter and both freezing tolerance and M. nivale resistance measured in controlled environments. Cold hardening seemed to influence freezing tolerance and M. nivale resistance differently in the different genotypes, since no distinct correlation in tolerance to freezing or resistance to M. nivale was found between unhardened and hardened plants. Tolerance or resistance to most of the winter stress factors measured was positively correlated with plant size. This revised version was published online in July 2006 with corrections to the Cover Date.