Disease Nursery Research Articles

Incidence of crown rust and virulence of puccinia coronata cda. f.sp. avenae eriks. and the effectiveness of pc genes for resistance in europe, middle east and north africa: Inzidenz von kronenrost und virulenz von puccinia coronata cda. f.sp. avenae eriks. und die wirksamkeit von pc-genen für resistenz in europa, nahost und nordafrika

Crown rust on oat incited by Puccinia coronata Cda f.sp. avenae is a wide-spread disease in Europe, Middle East (Israel) and North Africa (Morocco). High natural levels of the disease were recorded in Austria, Bulgaria, Czech Republic, Estonia, Israel, Italy, Poland, Russia and the former Yugoslavia. The severity of the disease in the European and Mediterranean Oat Disease Nursery (EMODN) between 1995 and 1999 showed that it occurred at damaging levels in a number of countries. Considerable differences in a disease resistance index (DRI) to crown rust among 67 oat lines in the period 1995 – 2000 were found. The values of the DRI ranged from18 (KR 8122) to over 290 (Pc 59, Pc 68). Detailed studies of the virulence combinations of P.coronata f.sp. avenae in Europe, Middle East and North Africa were carried out between 1995 – 2001. There were considerable differences in the average number of virulence among countries. Virulence to Pc 39 and 68 was found for the first time and is a significant finding. Nevertheless the major genes Pc 68, Pc 39, Pc 50-2, Pc Pc 59, Pc 60, Pc 61 proved to be highly effective.

Tomato Bacterial Spot Resistance Derived from PI 114490; Inheritance of Resistance to Race T2 and Relationship across Three Pathogen Races

Crosses were made between tomato (Lycopersicon esculentum Mill.) inbreds susceptible to races T2 and T3 of bacterial spot (Xanthomonas vesicatoria and Xanthomonas campestris pv. vesicatoria, respectively) and accession PI 114490 with resistance to races T1, T2, and T3. Resistance to race T2 was analyzed using the parents, F1, and F2 generations from one of the crosses. The F1 was intermediate between the parents for disease severity suggesting additive gene action. The segregation of F2 progeny fit a two-locus model (χ2 = 0.96, P = 0.9-0.5) where four resistance alleles are required for a high resistance level, two or three resistance alleles provide intermediate resistance, and zero or one resistance allele results in susceptibility. The narrow sense heritability of resistance to T2 strains was estimated to be 0.37 ± 0.1 based on F2 to F3 parent-offspring regression. A second cross was developed into an inbred backcross (IBC) population to facilitate multilocation replicated testing with multiple races. Segregation for T2 resistance in the inbred backcross population also suggested control was by two loci, lending support to the two-locus model hypothesized based on the F2 segregation. To determine if the same loci conferred resistance to the other races, selections for race T2 resistance were made in the F2 and F3 generations and for race T3 resistance in the F2 through F4 generations. Six T3 selections (F5), 13 T2 selections (F4's that diverged from seven F2 selections), and control lines were then evaluated for disease severity to races T1, T2, and T3 over two seasons. Linear correlations were used to estimate the efficiency of selecting for resistance to multiple races based on a disease nursery inoculated with a single race. Race T1 and race T2 disease severities were correlated (r ≥ 0.80, P< 0.001) within and between years while neither was correlated to race T3 either year. These results suggest that selecting for race T2 resistance in progeny derived from crosses to PI 114490 would be an effective strategy to obtain resistance to both race T1 and T2 in the populations tested. In contrast, selection for race T3 or T2 will be less likely to result in lines with resistance to the other race. PI 114490 had less resistance to T3 than to T2 or T1. Independent segregation of T2 and T3 resistance from the IBC population derived from PI 114490 suggests that T3 resistance is not controlled by the same genes as T2 resistance, supporting the linear correlation data.

Molecular variation in Rhynchosporium secalis isolates obtained from hotspots

A unique sampling strategy was used to search for genetic similarity among isolates of Rhynchosporium secalis, the causal agent of leaf scald of barley. A field of barley cv. Skiff with discrete ‘hotspots’ of leaf scald was identified and infected leaves were collected for this study. The isolates were genotyped using amplified fragment length polymorphism (AFLP) analysis. In contrast to previous reports of high diversity of R. secalis isolates in areas of only 1 m2, we found close genetic similarity between isolates from the same hotspot as determined by AFLP marker alleles, with a higher level of variation between isolates from different hotspots. A UPGMA phenogram showed that most isolates clustered with members of the same hotspot. This pattern is consistent with the inoculum source for the hotspots being airborne ascospores or wind-dispersed conidia from a local founder population, borne in splash-created aerosols. AFLP variation within hotspots suggests a high rate of mutation within a few cycles of fungal infection. A collection of isolates from cv. Skiff plots in a straw-inoculated disease nursery 60 km from the hotspot field was fingerprinted with AFLPs for comparison and was found to have much greater genotypic diversity.

Heritability of Fusarium Head Blight Resistance and Deoxynivalenol Accumulation from Barley Accession CIho 4196

Fusarium head blight (FHB), incited by Fusarium graminearum Schwabe [telomorph Gibberella zea (Schwein)], has caused devastating losses to yield and quality of barley (Hordeum vulgare L.) produced in the upper U.S. Midwest from 1993 to 2000. Design of an efficient breeding strategy for developing FHB resistant cultivars is dependent on knowing (i) the heritability of FHB resistance and accumulation of deoxynivalenol (DON), a mycotoxin contaminant produced by F. graminearum and (ii) the correlated response of other traits during selection for reduced FHB. We conducted field studies in FHB disease nurseries using F4:5 and F4:6 families from the cross between the FHB susceptible six-rowed cultivar Foster and the resistant two-rowed accession CIho 4196 to gain knowledge in the areas listed above. Heritability of FHB severity and DON accumulation was 0.65 and 0.46, respectively. A moderately strong positive association between FHB severity and DON accumulation was observed (r = 0.62). FHB severity and DON accumulation were negatively associated with plant height, days to heading, spike angle, and spike density. The selection differentials calculated between the top F4:6 families selected for low FHB severity and the unselected F4:5 families were moderately high for FHB severity, DON accumulation, and days to heading. Less than 14% of the selected lines had six-rowed spikes. No difference in plant height was observed between the selected and unselected families. Thus, development of FHB resistant lines with acceptable DON accumulation and days to heading is obtainable. However, because no lines were as short as Foster, development of FHB resistant plants with acceptable plant height from a cross using CIho 4196 as a parent will be difficult.

Reaction of a range of Brassica species under Australian conditions to the fungus, Leptosphaeria maculans, the causal agent of blackleg

A range of Brassica species was screened for resistance to Leptosphaeria maculans, the causal agent of blackleg. The lines were assessed in 8 disease nurseries in 4 canola growing regions of Australia and in 1�glasshouse trial, with a view to identifying alternative sources of resistance to L. maculans for Australian breeding programs. Lines were screened for degree of internal and external blackleg symptoms during both the seedling and adult plant growth stages. Correlation for resistance with ranking between disease nurseries was very strong (0.41-0.98). Brassica carinata and B. nigra were the most resistant species in the disease nurseries, being even more resistant than B. juncea. The 7 European winter B. napus lines tested were significantly more resistant than the 7�Australian spring B. napus lines, with another crucifer, Sinapis alba, being intermediate in resistance between the European and Australian B. napus lines. The same ranking of lines from most to least resistant was also seen when cotyledons and stems were inoculated in the glasshouse with 2 well-characterised Australian isolates. With the exception of the B. napus susceptible control Westar, all lines had similar frequencies of seedling survival in the nurseries. However, mature plants of these lines varied significantly in their degree of resistance. This indicates that screening for seedling survival is not useful in selecting L. maculans resistant lines in Australia. The Brassica lines with the B genome, especially B. carinata, and the winter B. napus types are now being used as sources of resistance in Australian breeding programs.

Wheat Stripe Rust Epidemics and Races of Puccinia striiformis f. sp. tritici in the United States in 2000.

Wheat stripe rust, caused by Puccinia striiformis f. sp. tritici, is most destructive in the western United States and has become increasingly important in the south-central states. The disease has been monitored by collaborators through field surveys and in disease nurseries throughout the United States. In the year 2000, stripe rust occurred in more than 20 states throughout the country, which was the most widespread occurrence in recorded history. Although fungicide applications in many states reduced yield losses, the disease caused multimillion dollar losses in the United States, especially in Arkansas and California. One of the prevalent cultivars, RSI 5, had a yield loss of about 50% in the Sacramento-San Joaquin Delta region of California. In the Pacific Northwest, wheat losses due to stripe rust were minimal because cultivars with durable resistance were widely grown and the weather in May 2000 was not favorable for the disease. To identify races of the pathogen, stripe rust collections from 20 states across the United States were analyzed on 20 wheat differential cultivars, including Clement (Yr9, YrCle), Compair (Yr8, Yr19), and the Yr8 and Yr9 near-isogenic lines. In 2000, 21 previously identified races and 21 new races were identified. Of the 21 new races, 8 were pathotypes with combinations of virulences previously known to exist in the United States, and 13 had virulences to one or more of the lines Yr8, Yr9, Clement, or Compair. This is the first report of virulence to Yr8 and Yr9 in the United States. Most of the new races were also virulent on Express. Races that are virulent on Express have been identified in California since 1998. The races virulent on Yr8, Yr9, and Express were widely distributed in California and states east of the Rocky Mountains in 2000. The epidemic in 2000 demonstrates that increased efforts to breed for stripe rust resistance are needed in California, the south-central states, and some other states in the Great Plains. Diversification of resistance genes and use of durable resistance should prevent large-scale and severe epidemics.

Genotype × environment interactions in the expression of net blotch resistance in spring and winter barley varieties

The net blotch resistance of barley varieties widely grown in Denmark was studied using data obtained from naturally infected field plots and inoculated disease nurseries in multiple years and locations. Data were analysed by combining two statistical procedures, 1) joint regression (JR) and 2) additive main effect, multiplicative interactive effect analysis (AMMI). Of the total variation for net blotch disease severity, 61–81% could be explained by plant genotype (G) and environment (E) main effects. Of the remaining variation, 77–86% could be explained by significant G × E interactions that were due to genotype sensitivity to the mean environmental disease level and/or to specific reactions of individual genotypes in particular environments. G × E interactions led to a different ranking of varietal performance across environments. There was evidence for G × E interactions caused by the virulence characteristics of the initial inoculum sources. The spring barley varieties Alexis and Bartok and the winter barley varieties Jolante, Ludo, and Rafiki were identified as varieties with high levels of net blotch resistance in all environments. Their performance was little affected by G × E interactions, suggesting that they represent good sources for non-specific resistance. Combining JR and AMMI reduced the dimensionality of complex G × E problems greatly, identified systematic reaction patterns of varieties possibly pertaining to different resistance mechanisms, and described specific and non-specific resistance of varieties by means of few parameters while maintaining the possibility to reconstruct the original data with little loss of information. This is very useful for improving the evaluation of varietal resistance for breeding and disease management purposes.

Genetic Resistance of Pearl Millet Male-Sterile Lines to Diverse Indian Pathotypes of Sclerospora graminicola.

Single-cross F1 hybrid cultivars based on cytoplasmic-nuclear male-sterility (CMS) system have contributed significantly to increasing productivity of pearl millet (Pennisetum glaucum). Genetic resistance to downy mildew (Sclerospora graminicola) in parental lines is critical for successful commercial cultivation of a hybrid cultivar. In this study, 46 genetically diverse male-sterile lines (A-lines), including 42 test A-lines, four control A-lines, a commercial hybrid, and a highly susceptible line, were evaluated in disease nurseries at four diverse locations in India and compared with pathotype isolates from the same locations under greenhouse environments. Variability in downy mildew incidence (0 to 100%) due to genetic differences among lines, among pathotypes, and that due to line × pathotype interaction were all highly significant (P < 0.001). In the field experiment, eight of the 42 test A-lines, including 841A (control), that recorded ≤10% disease incidence, were identified as resistant compared with 84 to 100% incidence on the control susceptible line 7042S. Resistance in eight of these test A-lines (863A, ICMA 88004, -94333, -98222, -98111, -92777, and -96666) and 841A was confirmed against the four pathotypes in greenhouse experiments. Cluster analysis of downy mildew incidence data from field and greenhouse experiments, using the Euclidian distance, classified the 48 lines into four distinct groups with the above eight A-lines in the resistant group. These resistant A-lines would be useful in the development of F1 hybrids with stable resistance to diverse pathotypes of downy mildew in India.

Resistance of New Zealand barley cultivars to scald, caused by &lt;i&gt;Rhynchosporium secalis&lt;/i&gt;

The resistance of New Zealand barley cultivars to leaf scald, caused by Rhynchosporium secalis, was assessed under conditions of high disease pressure in two autumn-sown disease nurseries. Assessments of disease severity were made at several growth stages and area under the disease progress curves calculated. Cultivars ranged from highly resistant to highly susceptible, with several cultivars showing intermediate levels of resistance. Relative disease levels between cultivars were largely consistent between assessment dates. Subsets of these cultivars were spring-sown in three trials situated within commercial crops, where disease pressures ranged from low to high. Two trials, one in Canterbury and one in the Wanganui district, consisted of four cultivars, with or without fungicide treatments. The other trial, in Southland, consisted of eight cultivars. Scald severity differed between cultivars. Partial resistance to scald, such as that found in Regatta, is likely to be more affected by environment than high level resistance. Such cultivars will probably require fungicide control of scald when inoculum pressure is high.

Decrease in Disease Severity in Sugarbeet in an Established Rhizoctonia Crown and Root Rot Nursery

A disease nursery used for more than 20 years to evaluate resistance of sugarbeet (Beta vulgaris) to crown and root rot caused by Rhizoctonia solani (AG 2-2) has failed in recent years to produce disease suitably severe to discriminate between resistant, partially resistant, and susceptible lines. Experiments were done to determine if biological control of the pathogen was responsible for the reduced disease severity. Highly susceptible to highly resistant sugarbeet lines were planted in the established nursery and in adjacent fields with no prior history as sugarbeet root rot disease nurseries. In each of 3 years, the disease was more severe at the new sites than in the established nursery. The weight of non-inoculated roots was similar at the two sites. Microbiological assays of soil from the sites revealed no quantitative differences in populations of microorganisms antagonistic to R. solani that were consistent with biological control of disease. Greenhouse experiments revealed no difference in seedling growth as a result of the site of soil origin, andautoclaving of soil did not produce the increase in disease severity that would be anticipated with biological control of the pathogen. Soil analyses of likely disease suppressing edaphic factors revealed no consistent differences between sites. Thus, artificial epiphytotics of crown and root rot caused by R. solani were more severe in new sites than in an established disease nursery, but the reasons for this difference remain obscure.

Comparative Slow-Blasting in Rice Grown Under Upland and Flooded Blast Nursery Culture.

Evaluation of rice for resistance to rice blast disease caused by Pyricularia grisea usually is conducted in upland (nonflooded) disease nurseries, although all commercial U.S. rice is produced under flood irrigation. Upland rice is more susceptible to leaf blast than is flooded rice, and the magnitude of this differential susceptibility can vary among cultivars. This 2-year study was undertaken to determine (i) the relationship between rates of disease development (slow-blasting) in upland and flooded rice and (ii) the value to a rice breeding program of establishing a flooded blast nursery, a facility far more difficult to manage than an upland nursery. Among 200 rice lines compared for leaf blast susceptibility under upland and flooded cultures, only 7 were rated as slightly more resistant under upland culture and 136 rated more resistant under flooded culture. Disease ratings under upland and flooded cultures were highly correlated (R = 0.819). Among 14 preselected cultivars over 2 years, disease development curves under upland and flooded cultures were highly correlated (R = 0.990). The cultivars with intermediate susceptibility under upland culture appeared to benefit most from flooded culture. Upland culture provided more opportunities to assess slow-blasting than did flooded culture, since many lines that produced susceptible-type lesions in upland culture failed to do so under flooded culture. It was concluded that adequate information on comparative leaf blast resistance among rice lines was attainable from upland blast nurseries and that routine evaluation of rice breeding lines for blast resistance in flooded nurseries was not necessary.

Genetic Structure and Temporal Dynamics of a Colletotrichum graminicola Population in a Sorghum Disease Nursery

ABSTRACT Restriction fragment length polymorphisms (RFLPs) were used to study the population genetics of Colletotrichum graminicola (= C. sublineolum), the causal agent of sorghum anthracnose. Screening of 80 anonymous probes from a genomic library detected polymorphisms in 81% of 299 probe-enzyme combinations among nine international isolates. Seven single- or low-copy probes were used to study a collection of 411 isolates sampled during 1991 to 1993 from a sorghum disease nursery in Georgia. Nei's gene diversity was moderately high, with = 0.215 on average, while genotypic diversity was extremely low with an average genotypic diversity value of G = 1.513. Only nine multilocus haplotypes were identified, with one haplotype being present at a frequency of approximately 80% each year. Two other haplotypes were found at significant frequencies (4 to 10%). Allele and haplotype frequencies did not differ over the 3 years, indicating that this population was stable. Our findings suggest that genetic drift and gene flow were not major contributors to genetic structure, while asexual reproduction had a significant effect.

Changes in the Racial Composition of Phytophthora sojae in Australia Between 1979 and 1996.

Surveys of commercial soybean fields, disease nurseries, and trial plots of soybean were conducted throughout eastern Australia between 1979 and 1996, and 694 isolates of Phytophthora sojae were collected and classified into races. Fourteen races, 1, 2, 4, 10, 15, and 25, and eight new races, 46 to 53, were identified, but only races 1, 4, 15, 25, 46, and 53 were found in commercial fields. Races 1 and 15 were the only races found in commercial fields in the soybean-growing areas of Australia up until 1989, with race 1 being the dominant race. Race 4 was found in central New South Wales in 1989 on cultivars with the Rps1a gene, and it is now the dominant race in central and southern New South Wales. Races 46 and 53 have only been found once, in southern New South Wales, and race 25 was identified in the same region in 1994 on a cultivar with the Rps1k gene. Only races 1 and 15 have been found in the northern soybean-growing regions, with the latter dominating, which coincides with the widespread use of cultivars with the Rps2 gene. Changes in the race structure of the P. sojae population from commercial fields in Australia follow the deployment of specific resistance genes.

Incidence of oat stem rust and virulence of Puccinia graminis pers. f. sp. avenae Erikss. et Henn. on oat and the effectiveness of resistances in Europe during 1988 to 1996*

The incidence of oat stem rust in Europe between 1988 and 1996, as recorded in the European Oat Disease Nursery, varied from high (Austria, Yugoslavia, Italy), moderate (Bulgaria, Czech Republic, Slovakia) to a low level (Great Britain). The occurrence of oat stem rust in the U.K. in 1992 was a rare event. A disease resistance index (DRI) was calculated for each of the cultivars or lines in the EODN and this ranged from 139 from Rodney M to 4 for the susceptible cv. Pan. A high DRI value is mainly an indication of major gene resistance. The virulence of Puccinia graminis f. sp. avenae samples collected between 1989 and 1996 was studied in populations from Austria, Bulgaria, Czechoslovakia, Czech Republic, Estonia, Finland, Germany, Great Britain, Italy, Poland, Russia, Slovakia, Spain, Sweden and Yugoslavia. In addition, a few samples from Israel were analyzed. The effectiveness of the Pg resistance genes indicated that Pg 13 was the most effective followed by Pg a, Pg 16, Pg 4, Pg 15, Pg 9, Pg 12, Pg 2, Pg 8, Pg 1 and Pg 3. The effectiveness of unknown resistance genes in cv. Saia (Avena strigosa), A. sterilis accessions VIR 343–1 and VIR 343–2 was 99.0, 85.7 and 89.2, respectively.

Pythium spp. associated with cavity spot of carrots in Western Australia

Cavity spot lesions on mature carrots from growers’ properties, and mature and maturing carrots from a disease nursery, were sampled for Pythium spp. Over 98% of the isolates were P. sulcatum; other species isolated were P. irregulare and Pythium ‘D’. P. coloratum was not found. In vitro pathogenicity tests on carrots showed that isolates of P. sulcaturn caused lesions while P. irregulare and Pythium ‘D’ did not. We conclude that P. sulcatum is the most common and most important species associated with cavity spot of carrots in Western Australia.

Reaction of Brassica juncea (Indian mustard) lines to Australian isolates of Leptosphaeria maculans under glasshouse and field conditions

Brassica juncea (Indian mustard) lines from diverse geographical locations around the world and from Australian breeding programs were screened for resistance to the blackleg fungus, Leptosphaeria maculans, in both glasshouse and field trials. The five Australian L. maculans isolates used in glasshouse trials could be classified into two groups; those that attacked all B. juncea lines, and those that attacked none. All these isolates caused lesions on cotyledons of B. napus cultivars including Westar, Glacier and Quinta, suggesting that they are in Pathogenicity Group 4 as described by Koch et al. (1991). The two isolates that attacked B. juncea also attacked B. napus lines to a similar extent, but did not attack the two B. carinata lines tested. Brassica lines were sown in a blackleg disease nursery at Lake Bolac, Victoria, Australia, and five indicators of blackleg disease were measured (survival rate, disease rating, disease incidence, external and internal lesion length). All 92 B. juncea lines developed blackleg symptoms. Although they displayed a high disease incidence in the field, almost all of the B. juncea lines were more blackleg-resistant than a B. napus cultivar, Dunkeld, which is amongst the most resistant cultivars in commercial production in Australia. Four B. carinata lines were more resistant than any of the B. juncea lines, suggesting that this species may be a useful source of blackleg resistance in B. napus breeding programs.

Molecular mapping of a major gene conferring resistance to maize mosaic virus

The objective of this study was to determine the genetic basis of resistance to maize mosaic virus (MMV). Molecular markers were used to map resistance loci to MMV in a set of 91 maize (Zea mays L.) recombinant inbred lines (RILs), derived from the cross between Hi31 (a B68 conversion resistant to MMV) and Kil4 (a Thai inbred susceptible to MMV). The RILs were evaluated for MMV resistance in disease nurseries in Hawaii in the winter of 1993 and the summer of 1994. Twenty-eight highly susceptible RILs were chosen for gene mapping using the pooled-sampling approach. Initial evidence from the pooled DNA indicated that restriction fragment length polymorphism (RFLP) probes on chromosome 3 near the centromere were biased to the susceptible parent allele. Analysis of 91 RILs at 103 RFLP loci confirmed the presence of a major MMV resistance gene on chromosome 3. The resistant allele at this locus, previously named Mv1, is present in the resistant parent Hi31 and traces back to the Argentine parent used in conferring common rust resistance to B68. We conclude that resistance to MMV in B68 and Caribbean flints involves a major gene mv1 on chromosome 3 located between RFLP markers umc102 and php20508.

Breeding Great Northern and Pinto Dry Beans with Multiple Disease Resistance Combined with Improved Seed Quality, Adaptation, Yield, and Plant Type

Common bacterial blight (CBB), rust (RU), and white mold (WM) are serious diseases of great northern (GN) and pinto (P) beans in Nebraska and Colorado. The bacterial diseases halo blight (HB) and brown spot (BS) are sporadic. Severe Fe-induced leaf chlorosis (Fe ILC) occurs on calcareous sites. Separate inoculated disease nurseries are used to screen for resistance to the pathogens causing the above diseases. Yields and seed quality of lines are also determined in non-disease trials. Sources of exotic resistance to the above pathogens and to Fe ILD have been identified and their inheritance determined. A non-structured recurrent selection scheme has mainly been used, occasionally with a backcross program, to combine high levels of the desired traits. Selection for highly heritable traits such as seed size, shape and color, maturity, plant architecture, and RU resistance occurs in early generations while traits of low heritability, such as CBB resistance, WM avoidance, yield, seed coat cracking resistance, and canning quality, are evaluated in separate replicated tests over several years and finally for yield in on-farm-trials. A number of multiple disease resistant, high-yielding, well-adapted GN and P lines are or will be released; P `Chase' (on about 30,000 acres in 1996) and GN WM 3-94-9 (for possible release).

Linked morphological and molecular markers associated with common root rot reaction in barley

Selection of common root rot [Cochliobolus sativus (Ito &amp; Kurib.) Drechsl. ex Dastur] resistant barley germplasm is difficult due to the influence of environment on disease expression and the quantitative nature of resistance. Morphological and molecular markers associated with common root rot resistance could be used to increase the efficiency of selection of resistant germplasm and facilitate transfer to desirable barley genotypes Forty-five morphological marker sets consisting of four to six backcross-derived lines for each marker in a common background (cultivar Bowman) were planted in disease nurseries at two locations in each of 3 yr. Disease reactions of the back-cross-derived lines were tested for a difference from the disease reaction of Bowman, which indicated a putative association between the marker and common root rot reaction. An association was obtained between common root rot reaction and one set of backcross-derived lines that contained two linked markers: glossy-sheath (gs4) and orange lemma (o). A RAPD marker closely linked to gs4 and o was also associated with common root rot reaction. The RAPD marker was associated with the allele for resistance, while gs4 and o were associated with the allele for susceptibility. The RAPD marker would be most useful in the selection of common root rot resistant germplasm and the transfer of this resistance to other genotypes. Key words: Common root rot, Cochliobolus sativus, barley, morphological markers, molecular markers

Ergot reaction of pearl millet hybrids affected by fertility restoration and genetic resistance of parental lines

High ergot (Claviceps fusiformis Loveless) susceptibility of pearl millet (Pennisetum glaucum (L.) R. Br.) hybrids has often been associated with the A1 cytoplasm of male-sterile lines (A-lines). To understand the underlying basis of this association and to examine the prospects of breeding ergot-resistant hybrids, we evaluated 56 hybrids and their 15 parental lines for ergot reaction and selfed seedset for 2 years in disease nurseries at ICRISAT Asia Center. Hybrids were made by crossing seven pollen parents (2 susceptible and 5 resistant) onto two resistant and two susceptible A-lines, and their four corresponding maintainer lines (B-lines). A-lines had no selfed seedset while B-lines had 32–75% selfed seedset. Hybrids of A-lines had significantly less selfed seedset than the hybrids of the corresponding B-lines. The reduced seedset of A-lines and their hybrids, however, was not always accompanied by significantly higher ergot susceptibility. Highly resistant hybrids were obtained where both A-lines and pollen parents were highly resistant, regardless of male fertility levels of the hybrids. Thus, although the A1 cytoplasm, by its reduction of male fertility, had a large and significant effect in increasing ergot severity of hybrids, the contribution of nuclear genetic factors of female parents was about 1.8 times larger than that of the cytoplasm.