Races Of Exserohilum Turcicum Research Articles

Occurrence and distribution of physiological races of Exserohilum turcicum in Ontario, Canada

Occurrence and distribution of physiological races of Exserohilum turcicum in Ontario, Canada

Exserohilum turcicum Race Population Distribution in the North Central United States.

Northern leaf blight (NLB) of corn, caused by Exserohilum turcicum, is a foliar disease common across corn production regions of the world, including those in the north central United States. Previous race population distribution studies identified five physiological races present in the United States, prior to 1995. For this study, 156 E. turcicum isolates were screened on corn differential lines containing Ht1, Ht2, Ht3, Htm1, and Htn1 resistance genes. Isolates were collected from fields in Iowa, Illinois, Indiana, Minnesota, North Carolina, Ohio, and Wisconsin, which included 143 isolates collected between 2007 and 2014 and 13 isolates collected between 1979 and 1985. Twenty different physiological races were observed based on the symptom response of the differential corn lines. E. turcicum race 0, 1, and 1mn were the most prevalent races, comprising 21, 27, and 13% of the 156 isolates, respectively. Race populations were diverse within states and years. Virulence to multiple Ht resistance genes within individual isolates was observed in 47% of those tested, with 3% of the isolates conferring virulence to all Ht resistance genes. Virulence to the Ht1, Ht2, Ht3, Htm1, and Htn1 resistance genes was present in 64, 20, 18, 32, and 27% of the E. turcicum isolates, respectively. Virulence to Ht resistance genes was fairly evenly distributed across states, in isolates collected after 2008. Virulence to Ht2, Ht3, Htm1, and Htn1 decreased after 2010. Variations in race population diversity are difficult to explain without knowing the level of selection pressure present in fields, and information regarding Ht resistance gene deployment in commercial varieties is not publicly available. Although virulence was observed against all Ht resistance genes, qualitative Ht resistance genes could be used in conjunction with quantitative resistance to increase NLB control.

Resistance Conferred by the Ht1 Gene in Sweet Corn Infected by Mixtures of Virulent and Avirulent Exserohilum turcicum.

The Ht1 gene conveys a chlorotic-lesion resistant reaction in corn infected by avirulent races of Exserohilum turcicum, the causal agent of northern corn leaf blight (NCLB). The widespread deployment of the Ht1 gene in field corn grown in North America since the 1960s resulted in an increased frequency of E. turcicum race 1, which is virulent against the Ht1 gene. The objective of this study was to assess the value of resistance conveyed by the Ht1 gene when initial inoculum consisted of different ratios of virulent and avirulent E. turcicum. Forty-two sweet corn hybrids with the Ht1 gene and 42 sweet corn hybrids without Ht1 were grown in five trials each in 2003 and 2004. In each trial, plants were inoculated with culture suspensions consisting of different percentages of E. turcicum race 0 and race 1, including: 100:0, 90:10, 75:25, 50:50, and 0:100. Severity of NCLB was rated visually from 0 to 100% leaf area infected when plants were about 3 to 4 weeks past the mid-silk growth stage. The Ht1 gene reduced severity of NCLB by as much as one-third when virulent isolates comprised 25% or less of the initial inoculum. Reduction in NCLB severity due to the Ht1 gene was more substantial on hybrids with susceptible backgrounds than on those with general resistance. When virulent isolates comprised 50% of the initial inoculum, NCLB severity was similar for hybrids with and without the Ht1 gene if hybrids had equivalent levels of general resistance (measured as NCLB severity from trials inoculated entirely with race 1). To accurately classify NCLB reactions of maize lines relative to their most probable performance in the United States, inoculum should consist of at least 50% E. turcicum race 1.

Frequency of the Ht1 Gene in Populations of Sweet Corn Selected for Resistance to Exserohilum turcicum Race 1

ABSTRACT The possibility that the Ht1 gene or genes tightly linked to Ht1 convey general resistance to races of Exserohilum turcicum that are virulent against Ht1 (i.e., residual resistance) could be useful in sweet corn where the Ht1 gene is present in many commercial hybrids and breeding populations. The objective of this study was to determine if the frequency of the Ht1 gene changed in populations of sweet corn selected for general resistance to E. turcicum race 1, thus conveying residual resistance. Four populations were developed with theoretical initial frequencies of the Ht1 gene of 0, 0.25, 0.25, and 0.5. The populations were advanced by recurrent mass selection with parental control through four or five cycles of selection following inoculation with an Ht-virulent race of E. turcicum (i.e., race 1). Plants from each cycle of each population were evaluated for severity of northern corn leaf blight (NCLB) and chlorotic lesion reactions following inoculation in field and greenhouse trials with either race 0 or 1 of E. turcicum. Recurrent mass selection for general resistance to E. turcicum race 1 reduced the severity of NCLB in all four populations of sweet corn, although the change in the most susceptible population was minimal. Percent gain per cycle was 14.5, 12.3, 14, and 3.7% for populations I, II, III, and IV, respectively. The Ht1 gene did not convey levels of general resistance to E. turcicum race 1 that were substantial enough to be selected for in this population improvement program. There was no apparent selection advantage for resistance to E. turcicum race 1 in the populations that contained the Ht1 gene. The frequency of the Ht1 gene did not differ among cycles of selection within any of the populations in response to improved levels of general resistance to NCLB. The lack of change in frequency of Ht1 in these populations and the similarity in gain per cycle among populations with and without Ht1 lead us to conclude that the Ht1 gene itself did not have a residual effect on resistance.

Disease Severity and Yield of Sweet Corn Hybrids with Resistance to Northern Leaf Blight.

Reactions of supersweet (sh2) sweet corn to northern leaf blight (NLB) and associated yields were evaluated in Belle Glade, Florida and Urbana, Illinois in yield-loss trials, hybrid evaluations, and evaluations of breeding materials. Hybrids differed significantly for NLB in all trials. Severity of NLB ranged from 0 to 66% on 35 sh2 hybrids in yield-loss trials, and from 0 to 60% on 80 sh2 hybrids in hybrid evaluations. NLB ratings ranged from 1 to 9 (approximately 0 to 80% severity) on 375 hybrids and 186 inbred lines in evaluations of breeding materials. Various methods of rating NLB and ratings from multiple dates were highly correlated, with correlation coefficients ranging from 0.76 to 0.98. Yield, measured as weight of ears and number of marketable ears from inoculated plots as a percentage of that from control plots, decreased as disease severity increased. Linear or quadratic regression models explained 31 to 70% of the variation in percent yield as a function of disease severity at harvest. The effects of NLB on yield were limited by NLB-resistance in several hybrids, including CCO 3268, Chieftain, Crisp N Sweet 710A, Day Star, Envy, Forever, GSS 1526, Jupiter, Midship, Prime Plus, Sch 5005, and SummerSweet 7630. Although high levels of partial resistance to NLB were prevalent among 375 new experimental sh2 hybrids and 186 sh2 inbred lines evaluated in 1995, use of the gene HtN may increase in the near future as breeders are incorporating this resistance into new inbreds and hybrids. Breeders and plant pathologists would be wise to continue improving partial resistance to NLB without using the gene HtN in genotypes with adequate levels of partial resistance, because the widespread use of the gene HtN will select for virulent races of Exserohilum turcicum which occur in Florida, or for races with new combinations of virulence.

Interaction of Ht and Partial Resistance to Exserohilum turcicum in Maize.

Components of northern leaf blight resistance in maize due to race-specific resistance controlled by the Ht gene, partial resistance derived from inbred H99, and a combination of the two kinds of resistance, were studied subsequent to inoculation with Exserohilum turcicum race O. Lesion types, number of lesions (lesion number), percent leaf area affected (severity), and area under the disease progress curve (AUDPC) based on lesion number and severity were assessed in field studies conducted at two locations in Uganda and one location in Ohio in 1993. Lesion types observed were consistent for genotypes across locations. In general, significant differences among genotypes for data based on lesion number and severity were consistent for AUDPC based on lesion number and severity, respectively, at all locations. In Ohio, both Ht and partial resistance were effective in limiting disease development. In Uganda, susceptible inbreds (A619, A635, and B73) generally had higher severity than genotypes with partial resistance (H99, Mo17, and Babungo 3). However, there was a difference in response among genotypes depending on disease intensity at each location. Ht resistance and moderate partial resistance did not greatly affect lesion number at the higher disease intensity location, compared with the susceptible inbreds, but at the lower disease intensity location genotypes with partial resistance had fewer lesions than susceptible inbreds or the Ht conversions of the susceptible inbreds. At both plot locations, genotypes with partial resistance had lower severity than the susceptible inbreds or Ht conversions of the susceptible inbreds. Hybrids derived from crossing H99 with genotypes with moderate levels of partial resistance (Mo17 and Babungo 3) did not have significantly lower lesion numbers than hybrids of susceptible inbreds crossed with H99, but severity was significantly lower on these hybrids at the high disease intensity location. Results indicate that the level of partial resistance in H99 would be as effective in controlling northern leaf blight as using Ht resistance, or a combination of Ht resistance and moderate levels of partial resistance as found in Mo17.

Effect of Three Tillage Practices on Development of Northern Corn Leaf Blight (Exserohilum turcicum) Under Continuous Corn

Corn hybrid A632×A619 was planted in 1985 on 0.6 ha, and plants were inoculated 6 wk later with Exserohilum turcicum race 2. A severe epidemic of northern corn leaf blight developed. Three tillage treatments, ridge-till, mulch-till, and no-till, were established, and three corn hybrids, A632×A619, B73×Mo17, and B73×LH38, were planted in the same field in 1986. The experiment was repeated by planting and inoculating A632×A619 on 0.6 ha in 1986 and reevaluating the three tillage treatments and three corn hybrids in 1987 (.)

Physiological races ofExserohilum turcicum in Israel

A set of differentials of corn plants(Zea mays L.) containing Ht1, Ht2, Ht3 or HtN genes was used to identify races ofExserohilum turcicum in Israel. Plants were inoculated with 14 isolates ofE. turcicum collected from various regions in Israel (from Ayyelet HaShahar in the north to Sa’ad in the south). Differentials containing Ht1, Ht2, Ht3 or HtN genes were resistant to the 14 isolates tested, whereas the inbred lines without Ht genes were highly sensitive. Resistance was characterized by the formation of non-sporulating chlorotic lesions. When plants containing Ht1, Ht2 or Ht3 genes were inoculated with relatively high inoculum concentrations (over 50 conidia/drop), chlorotic lesions were associated with necrosis in the center of the lesions. Sporulation of the fungus in the necrotic parts of the lesions was significantly less than on plants without Ht genes. No necrosis was observed in plants with the HtN gene. Our results indicate that the physiological race ofE. turcicum in Israel is race 1.

A Dominant Inhibitor Gene Inhibits the Expression of Ht2 against Exserohilum turcicum Race 2 in Corn Inbred Lines Related to B14

AbstractThe purpose of this study was to elucidate the genetic cause for the lack of express ion of the gene Ht2 against Exserohilum turcicum race 2 on certain genetic backgrounds related to the inbred line B14. Two such inbreds (A635Rp and B59), susceptible to this pathogen were crossed onto the inbred Oh43—‐Ht2Ht2. The following generations: F1, F2, F3, F4, backcrosses to both parents, selfings of the backcrosses, in addition to the parent lines, were evaluated for Ht2 expression in this study. Plants of each generation were inoculated with E. turcicum race 2 and evaluated for the expression of the chlorotic‐lesion resistance determined by the gene Ht2. In spite of the reported dominance of Ht2 on some genetic backgrounds, the F1 generations studied here did not show Ht2 resistant lesions. The data presented herein suggest that B14 and related inbred lines carry a dominant gene(I) that inhibits the expression of the Ht2 gene. Chi‐square analyses of the reactions of 108 progeny families studied supported this hypothesis.

Effects of theHt, Ht2, and/orHt3 Genes in Three Maize Inbreds on Quantitative Resistance toExserohilum turcicumRace 2

Effects of the<i>Ht, Ht</i>2, and/or<i>Ht</i>3 Genes in Three Maize Inbreds on Quantitative Resistance to<i>Exserohilum turcicum</i>Race 2

Occurrence of Race 2 ofExserohilum turcicumon Corn in the Central and Eastern United States

also tested. Jordan, E. G., Perkins, J. M., Schall, R. A., and Pedersen, W. L. 1983. Occurrence of race 2 of Differences in virulence of E. turcicum Exserohilum turcicum on corn in the central and eastern United States. Plant Disease 67:1163-1165. isolates were determined using the following set of corn differentials: B37, One hundred sixty samples of northern corn leaf blight (NCLB)-infected tissue collected from B37Ht, B37Ht2, and Oh43Ht3. Corn inbreds and hybrids of Zea mays in seven states during the 1979, 1980, and 1981 growing seasons inbreds were grown in 16-cm clay pots were tested to determine which races of Exserohilum turcicum were present. Races were with one set of inbred differentials per determined by the reaction of corn inbreds B37, B37Ht, B37H2, and Oh43Ht3 after inoculation pot. NCLB lesions were cut from leaves with spore suspensions from infected tissue. Race I was virulent only on inbred B37, whereas race 2 and induced to sporulate on moist filter was virulent on both B37 and B37Ht. None of the isolates were virulent on B37Ht2 or Oh43Ht3; apd in g la te on m m). therefore, race 3 was not identified. Race 2 was identified from NCLB tissue collected in Florida, paper in glass petri dishes (1 50 X 15 mm). Illinois, Indiana, Iowa, Pennsylvania, Minnesota, and New York. Race 2 was widespread in Illinois A spore suspension of each isolate was (53 counties) and Indiana (24 counties). prepared by washing conidia from infected leaf tissue and adjusting the concentration to about 20,000 spores per Northern corn leaf blight (NCLB), year at several locations in Ohio (11). The milliliter. Each spore suspension was caused by Exserohilum turcicum (Pass.) purpose of this paper is to report the sprayed on two replicates (two or three Leonard & Suggs (imperfect state of distribution of race 2 in Illinois and plants per replicate) of the inbred Setosphaeria turcica (Luttrell) Leonard Indiana and its occurrence in other states. differentials. Inoculations were made & Suggs), was first observed by Passerini