Methods for screening maize against downy mildew Peronosclerospora sorghi

Methods of inoculum delivery and timing of test-row planting were assessed for efficacy in promoting development of downy mildew (Peronosclerospora sorghi) in susceptible maize, and for cost of implementation in terms of personnel and labour. Direct inoculation of pre-germinated seed of spreader rows resulted in consistent and high incidence of downy mildew infected plants and required substantially less labour and inoculum than the spray inoculation of spreader rows. The optimum time to plant test rows was around 15 days after inoculation of spreader rows. Eight breeding lines were screened for resistance to downy mildew using the revised inoculation method. After one cycle of screening with the new method, resistance levels improved by a range of 0–44% over the previous cycle.     

Improvement of maize populations for resistance to downy mildew

Upgrading levels of disease resistance are a primary objective of maize breeding programmes. Efficacy of S₁ recurrent selection in improving levels of resistance to downy mildew (DM) infection was assessed in Nigeria from 1997 to 2000 in six maize populations. Improvement procedures consisted of evaluating S₁ progenies under artificial infection with DM spores and in disease‐free environments and using a selection index to combine selection for reduced DM infection with appropriate agronomic characters from more than one environment. Three to four cycles of selection were completed in each of the populations. Products from the different cycles of selection were evaluated and data collected on DM infection parameters and agronomic traits. Result obtained showed 3–4 cycles of selection were adequate to reduce DM infection levels significantly and increase grain yield. Downy mildew infection decreased by between 58 and 100% while grain yield increases ranged from 10 to 98% for the 2‐4 cycles of selection relative to the C0 (original). Selection increased grain yield with acceptable changes in plant height while maintaining maturity in disease‐free environments.     

Genetic variability for resistance to sorghum downy mildew (Peronosclerospora sorghi) and Rajasthan downy mildew (P. heteropogoni) in the tropical/sub-tropical Asian maize germplasm

Downy mildews cause considerable damage to maize (Zea mays L.) worldwide, particularly in the tropical Asia. We have evaluated a set of 42 tropical/sub-tropical maize inbred lines developed in different countries in Asia (India, Thailand and Philippines), and Mexico, for analysing the genetic variability for resistance to sorghum downy mildew [Peronosclerospora sorghi; SDM] and Rajasthan downy mildew [P. heteropogoni; RDM]. Experiments were carried out in replicated trials under artificial infection in field conditions against SDM and RDM at Mandya in Karnataka, India, and Udaipur in Rajasthan, India, respectively, during 1999 and 2000. The study resulted in identification of five inbred lines offering consistent and strong resistance to both SDM and RDM, while several inbred lines revealed resistance only to RDM. It was also revealed that the SDM-resistant inbreds are invariably resistant to RDM, while the RDM-resistant inbreds might show differential responses to the SDM. The maize inbred lines identified in this study with broad-spectrum resistance to downy mildews could be potentially useful for basic and applied research work on downy mildews in tropical Asia. This revised version was published online in August 2006 with corrections to the Cover Date.     

Selection for partial resistance to downy mildew in peas by means of greenhouse tests

Summary A partially downy mildew resistant pea line was back-crossed to a susceptible cultivar with more pods per node and lower seed weight. Breeding lines with different degrees of infection in a greenhouse test were selected. These lines and the two parental lines were investigated in field trials and tested in the greenhouse for four generations. Significant genetic variation among lines was found for infection of seedlings in greenhouse tests, and infection of pods, pod set and seed weight in field-trials. Infection of seedlings in the greenhouse was correlated with infection of pods in the field. In greenhouse tests, the non-genetic variance component was large in comparison with the genetic component and a significant genotype trial interaction was found. Significant repeatability was obtained for downy mildew on seedlings and pods, number of pods per node and seed weight. An unfavourable correlation between susceptibility to downy mildew and number of pods per node was found. No single breeding line showed the ideal combination of good resistance, high number of pods per node and small seeds. However, one line showing better resistance than the susceptible parent, with smaller seeds and more pods per node than the resistant parent was found. The susceptible parent also carries some resistance factor that is not present in the resistant parent.     

Evaluation of greenhouse inoculation techniques to screen sorghum for resistance to downy mildew

Summary Six inoculation techniques were compared for the artificial promotion of downy mildew (Peronosclerospora sorghi) in sorghum. These were (1) sprouted seeds incubated between sporulating infected leaves, (2) sprouted seeds depped in conidial suspension, (3) sprouted seeds sprayed with conidial suspension, (4) seedlings at plumule stage inoculated with drops of a conidial suspension, (5) seedlings at plumule stage sprayed with a conidial suspension, and (6) seedling showered with conidia falling from infected leaves. Seedlings at the one-leaf stage sprayed with a conidial suspension (6 × 10⁵ ml^-1) showed the highest systemic infection (100%) in the susceptible lines IS 643 and IS 18433. This technique is effective, repeatable, and allows the deposition of a conidial suspension as a fine mist on the entire seedling surface. In the greenhouse, the technique was used to test the downy mildew reaction of genotypes previously reported as resistant (< 5% incidence) in 3–4 years of field screenings. Of the 61 genotypes tested, 21 were free from downy mildew, 14 had less than 5% incidence, and the rest showed variable susceptible reactions. Therefore, the technique can be reliably and effectively used in the greenhouse to detect disease escapes and to indentify resistance.     

Inheritance of downy mildew resistance in Indian cauliflower (group III)

Summary Inheritance of downy mildew (Peronospora parasitica) resistance in three resistant x susceptible crosses, one susceptible x susceptible and one resistant x resistant cross were studied in Indian cauliflower (Group III) over the two years (1990 and 1991). No significant difference was observed between the years for various estimates and hence pooled data are presented. Downy mildew resistance in crosses cc×HR 5-4 and 3-5-1-1×244 (R×S) is governed by single dominant gene PPA3 but in cross cc×244 (R×S), recessive epistasis was observed. The resistance level was not improved in both the cc×3-5-1-1 (R×R) and 244×267-6-9 (S×S) crosses. Exploitation of downy mildew resistance from cc and 3-5-1-1 in F₁ hybrid is explained in detail.     

RAPD markers linked to resistance to downy mildew disease in soybean

To determine and utilize RAPD markers linked to resistance to downymildew incited by Peronospora manshurica in soybean, a resistantcultivar `AGS129' was crossed to a susceptible cultivar `Nakhon Sawan 1'(NS1). F₂ and BC₁ populations were advanced from the F₁ and evaluatedfor resistance to the disease. ²-test demonstrated that the resistancewas controlled by a single dominant gene (Rpmx). Near-isogenic lines(NILs) and bulked segregant analysis (BSA) were used to identify RAPDmarkers linked to the gene. Six DNA bulks namely F₅(R), F₅(S),BC₆F₃(R), BC₆F₃(S), F₂(R) and F₂(S) were set up by pooling equalamount of DNA from 8 randomly selected plants of each disease responsetype. A total of 180 random sequence decamer oligonucleotide primerswere used for RAPD analysis. Primer OPH-02 (5 TCGGACGTGA 3 andOPP-10 (5 TCCCGCCTAC 3) generated OPH-02₁₂₅₀ and OPP-10₈₃₁fragments in donor parent and resistant bulks, but not in the recurrentparent and susceptible ones. Co-segregation analysis using 102 segregatingF₂ progenies confirmed that both markers were linked to the Rpmxgene controlling downy mildew disease resistance with a genetic distance of4.9 cm and 23.1 cm, respectively. Marker OPH-02₁₂₅₀ was presentin 13 of 16 resistant soybean cultivars and absent in susceptible cultivars,thus confirming a potential for MAS outside the mapping population.     

A single dominant gene for downy mildew resistance in broccoli

Downy mildew, incited by Peronospora parasitica (Pers.: Fr.) Fr., is a destructive disease of broccoli (Brassica oleraceaL., Italica Group). Resistant cultivars represent a desirable control method to provide a practical, environmentally benign, and long-term means of limiting damage from this disease. Doubled-haploid (DH) lines developed by us exhibit a high level of downy mildew resistance at the cotyledon stage. To determine the mode of inheritance for this resistance, a resistant DH line was crossed to a susceptible DH line to make an F₁, from which F₂ and backcross (BC) populations were developed. All populations were evaluated for response to artificial inoculation with P. parasitica at the cotyledon stage. All F₁ plants (including reciprocals) were as resistant as the resistant parent, indicating no maternal effect for this trait. F₂ populations segregated approximately 3resistant to 1 susceptible, BC populations using the resistant parent as the recurrent parent contained all resistant plants, and the BC to the susceptible parent segregated 1 resistant to 1 susceptible. These results indicate that resistance is controlled by a single dominant gene. This gene should be easily incorporated into F₁ hybrids and used commercially to prevent downy mildew at the cotyledon stage. This revised version was published online in August 2006 with corrections to the Cover Date.     

Linkage in Spinacia oleracea L. of two race-specific genes for resistance to downy mildew Peronospora farinosa f. sp. spinaciaeByford

Summary The genetics of resistance to races 1 and 2 of downy mildew Peronospora farinosa f. sp. spinaciae in the spinach variety Nores was investigated in crosses and backcrosses with the susceptible variety Eerste Oogst. Resistance appears to be governed by two closely linked genes in coupling phase with a recombination percentage of 4.6.     

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.     

Participatory plant breeding with maize in Mexico and Honduras

Maize is a staple food crop in many developing countries. However, if seven major maize producing countries are excluded from this group, data indicate that only 34% of the maize area is planted with improved seed despite considerable effort invested in maize breeding. This has led researchers to investigate other options, such as farmer-participatory plant breeding, for delivering the benefits of plant breeding knowledge and technology to farmers in developing countries. This paper describes short-term results from participatory maize breeding studies in Mexico and Honduras. Results from three selection cycles in Mexico suggest that stratified mass selection without pollination control, with selections carried out by researchers in farmers' fields, may be effective at improving yield in farmers' local varieties. In Honduras, mass selection with pollination control, where selections were done by collaborating farmers in their own fields on their own varieties, showed trends (non-significant) towards yield improvement. Farmer selection seemed to offer the greatest yield benefit over experiment station selection on the farm with the lowest yield potential, suggesting that farmer-participatory approaches may be most advantageous in marginal environments where experiment station conditions differ most dramatically from farmers' conditions. These studies highlighted the importance of seed systems knowledge in designing participatory plant breeding programs. For cross-pollinated crops, they also highlighted the need to balance progress from selection and demands on farmers' time and labor in choosing breeding methods. Further work is needed to investigate farmer-participatory breeding approaches that can address post-harvest traits. This revised version was published online in August 2006 with corrections to the Cover Date.     

The role of resistance breeding in the intergrated control of downy mildew (Bremia lactucae) in protected lettuce

Summary Over the last 30 years, six resistance alleles (Dm2, Dm3, Dm6, Dm7, Dm11 andDm16) located in two linkage groups, have contributed to the control of downy mildew in lettuce crops grown under protection (glass or polythene) in northern Europe. More recently, an as yet genetically uncharacterised resistance factor, R18, has also begun to assume importance. The occurrence of the various combinations of these resistance alleles that exist in commercial cultivars has been dictated by the pathotypes ofBremia lactucae used in their selection but also restricted by linkage in repulsion. In the UK, a pathotype ofB. lactucae insensitive to phenylamide fungicides, such as metalaxyl, emerged in 1978 and became prevalent throughout lettuce production areas in subsequent years. The specific virulence of this pathotype was identical to the previously described phenylamide sensitive pathotype NL10 and cultivars carryingDm11, Dm16 or R18 were resistant. Consequently, an integrated control strategy based on the utilisation of metalaxyl on cultivars carryingDm11 provided effective control in UK until 1987 when a new phenylamide insensitive pathotype began to cause problems. The specific virulence of this second pathotype, which was first reported in the Netherlands and France, was identical to the previously described phenylamide sensitive pathotype NL15. Cultivars carryingDm6, Dm16 or R18, but notDm11, were resistant to NL15; consequently an appropriate change in the cultivar recommendations for use in the integrated control strategy was successfully promulgated. It is predicted that variations of this integrated control strategy involving the use of appropriately selectedDm gene combinations may prove effective for some time. This prediction is based on studies of the status of the avirulence loci in the two phenylamide insensitive pathotypes and of the specific virulence characteristics of phenylamide sensitive components of the pathogen population.     

Colocation of downy mildew (Plasmopara halstedii) resistance genes in sunflower (Helianthus annuus L.)

Summary The Pl6 locus in the inbred sunflower (Helianthus annuus L.) line HA335 giving resistance to French races of downy mildew (Plasmopara halstedii (Farl.) Berl. & de Toni. was localized by molecular techniques. A bulked segregant analysis was made on the F2 progeny from a cross between this line and H52, a downy mildew susceptible line. The resistance gene in HA335 was found to have the same linked RFLP marker loci as those determined for Pl1 (resistance to race 1 in the line RHA266) on linkage group 1 of the consensus RFLP map of the cultivated sunflower. Pl1 and Pl6 thus appear either to be allelic or closely linked. The implications for sunflower breeding are discussed.     

Partial resistance in lettuce to downy mildew (Bremia lactucae). 3. Correspondence between resistance levels of cotyledons and leaf discs and resistance of adult plants

Summary The correlation between resistance levels of seedlings with cotyledons only and leaf discs taken from young and old leaves on the one hand and the resistance of adult plants in bicyclic field tests on the other was investigated. Components of resistance were: latent period (LP) the percentage of seedlings (SEED) or the number of cotyledons (COT) infected, the number of sporangiophores (SPOR) per cotyledon or leaf dise and the % of the surface (SURF) per cotyledon or disc covered with sporangiophores.For both cotyledons and leaf discs, significant differences occurred between plant genotypes for LP, SPOR and SURF. The correspondence between results of cotyledon testing and field testing was generally negligible. There was a rather good correlation between the resistance levels of leaf discs, expressed in SPOR and SURF and the resistance of adult plants. On the basis of a smaller coefficient of variation for SPOR this parameter should be preferred to predict the resistance of adult plants by testing leaf discs.     

Pedigree selection for Gibberella ear rot resistance in maize

The pedigree method is often used for developing inbred lines in maize (Zea mays L.). This study was conducted to assess the effectiveness of pedigree selection for improving resistance to Gibberella ear rot in four maize populations. Selection was based on the severity of ear rot symptoms after inoculation with macroconidial suspensions of Fusarium graminearum (Schwabe) into the silk channel (for two populations) and into the developing kernels (for two other populations). Samples of the selfed families (S₁ to S₅), recovered from remnant seed from the selection programs, were evaluated for disease resistance during three years, using inoculation and evaluation protocols similar to those used during selection. Among-family selection was effective in both of the populations selected after silk inoculation and in one of the populations selected after kernel inoculation. Responses to selection were more evident in later than in earlier generations for both types of inoculation. Changes in the estimated genetic gain over generations were consistent with changes in the variances among families, which tended to increase in early generations and to decrease in later generations. Selection after kernel inoculation seemed to have been more effective than selection after silk inoculation in developing families with more stable resistance. Based on the results obtained here, it seems that responses to family selection could be accelerated without increasing operational costs by increasing selection intensity in later generations and inoculating fewer plants per family.     

Effects of recurrent selection for grain yield on plant and ear traits of five maize populations

Summary Maize (Zea mays L.) breeders are interested in the effects of recurrent selection for grain yield on other traits. Changes in plant traits could alter agronomic acceptability of the populations under selection, and observed improvements in grain yield could be explained by changes in ear traits. We evaluated changes in combining ability for plant and ear traits of BS10(FR), BS11(FR), BSSS(R), BSCB1(R), and Lancaster Surecrop associated with recurrent selection for grain yield.Recurrent selection procedures generally did not change plant and ear heights or date of silking of testcrosses of populations or of the population crosses, BS10(FR)×BSS11(FR) and BSSS(R)×BSCB1(R). Grain yield improvements, however, were associated with increases in ear-sink size.Journal Paper No. J 9517 of the Iowa Agriculture and Home Economics Exp. Stn., Ames, Iowa. Project 2152.     

A demonstration of the inheritance of field resistance to lettuce downy mildew (Bremia lactucae Regel.) in progeny derived from cv. Grand Rapids

Summary The inheritance of field resistance to downy mildew in lettuce derived from cv. Grand Rapids was studied in F₂, F₃ and F₄ progeny from a cross with the more susceptible cv. Lobjoits Green Cos. The results for 32 randomly produced F₃ families and the F₂ population corresponded closely although the absolute level of resistance varied from season to season according to disease pressure and environmental conditions. The field resistance character showed a quantitative pattern of inheritance but the rapid response to selection at F₃ implied that only a few genes may be involved. F₄ families raised from families selected from the two extremes of the F₃ distribution fell into distinct categories with resistance levels corresponding to that of their F₃ parents. An association between resistance and morphological features of the resistant parent was present at F₂ but not in subsequent generations when individuals and families which were as resistant as cv. Grand Rapids and yet bore little morphological resemblance to this cultivar were readily recovered.     

The location and characteristics of novel sources of resistance to Bremia Lactucae Regel (downy mildew) in wild Lactuca L. Species

Summary Twenty-one accessions of 3 wild Lactuca species which could be hybridised with L. sativa, the cultivated lettuce, were inoculated at different stages of plant development with 3 multivirulent isolates of Bremia lactucae. Nineteen sources of resistance to B. lactucae, not attributable to the previously recognised resistance factors 1–11 were identified. Two lines of L. serriola showed similar resistance patterns as lines carrying R11. The resistance of some accession was incomplete particularly at the seedling stage and this phenomenon may be race specific.Tests on segregating F₂ populations of crosses between 2 different L. serriola accessions and L. sativa cultivars showed that the resistance in one line (LSE/18) appears to be inherited as a single dominant gene, which is sometimes incomplete in expression and allelic to either Dm6 or R7. The segregation patterns for resistance in PI 281876 did not give readily interpretable ratios.To assess the frequency of occurrence in B. lactucae populations of virulence factors to overcome this novel resistance, 11 of the novel sources of resistance were inoculated with numerous collections of the pathogen from the UK, Czechoslovakia and elsewhere and found to show a high level of resistance.     

An association between resistance to root aphid (Pemphigus bursarius L.) and downy mildew (Bremia lactucae Regel) in lettuce

Summary Many lettuce cultivars (Lactuca sativa L.) with high resistance to lettuce root aphid (Pemphigus bursarius L.) also carried the gene Dm-6 for specific resistance to downy mildew (Bremia lactucae Regel). This suggests the possibility of linkage between this gene and root aphid resistance. The origin of this association is discussed.     

Selection of sites for preliminary maize yield trials in the rainforest zone of South-Western Nigeria

Summary Grain-yield data obtained from 49 varieties of maize (Zea mays L.), grown at each of five sites for two years, were analyzed with the objective of identifying two or three sites where mean performance adequately predicted the mean performance of all five sites combined. Regression of overall varietal mean on site or site-combination means was done and the predictive potential of a site or combination of sites was judged by four criteria: (i) r²0.9, (ii) b1.0, (iii) high yield (i.e., yield greater than or equal to overall mean yield) and (iv) consistent yield (i.e., yield did not fall below the mean yield by more than 10%). In addition, a trial differentiation index, D, was used to compare the site combinations selected on the basis of the above criteria with the differentiation potential of all five sites combined. None of the five individual sites and the ten two-site combinations satisfactorily predicted overall mean yield. However, five three-sites combinations had values of D similar to that of all the five sites. Rank-correlation coefficients of the three-site combinations with all five sites ranged from 0.90 to 0.79. It was concluded that certain three-site combinations could be satisfactorily used for preliminary maize yield trials in the rainforest ecological zone of S.W. Nigeria.