Analysis of durable resistance to stem rust in barley

Summary Since the mid-1940's, barley cultivars grown in the northern Great Plains of the USA and Canada have been resistant to stem rust caused byPuccinia graminis f. sp.tritici. This durable resistance is largely conferred by a single gene,Rpg1, derived from a single plant selection of the cultivar Wisconsin 37 and an unimproved Swiss cultivar. At the seedling stage, barley genotypes withRpg1 generally exhibit low mesothetic reactions at 16–20° C and slightly higher mesothetic reactions at 24–28° C to many stem rust pathotypes. This resistance is manifested by a low level of rust infection and mostly incompatible type uredia on adult plants.Rpg1 reacts in a pathotype-specific manner since some genotypes ofP. g. f. sp.tritici are virulent on cultivars carrying this gene in the field. Several factors may have contributed to the longevity of stem rust resistance in barley, a) since barley is planted early and matures early, it can sometimes escape damage from stem rust inoculum carried from the south; b) one or more minor genes may augment the level of resistance already provided byRpg1; c) the cultivation of resistant wheat cultivars and eradication of barberry have reduced the effective population size and number of potential new pathotypes ofP. g. f. sp.tritici, respectively; and d) virulent pathotypes ofP. g. f. sp.tritici andP. g. f. sp.secalis have not become established. This situation changed in 1989 when a virulent pathotype (Pgt-QCC) ofP. g. f. sp.tritici became widely distributed over the Great Plains. However,Rpg1 may still confer some degree of resistance to pathotype QCC because stem rust severities have been low to moderate and yield losses light on barley cultivars carrying the gene during the last four seasons (1989–1992). Several sources of incomplete resistance to pathotype QCC have been identified in barley. To facilitate the transfer of resistance genes from these sources into advanced breeding lines, molecular marker assisted selection is being employed.     

Early harvest and ensilage of forage sorghum infected with ergot (Claviceps africana) reduces the risk of livestock poisoning

Sorghum ergot produces dihydroergosine (DHES) and related alkaloids, which cause hyperthermia in cattle. Proportions of infected panicles (grain heads), leaves and stems were determined in two forage sorghum crops extensively infected 2 to 4 weeks prior to sampling and the panicles were assayed for DHES. Composite samples from each crop, plus a third grain variety crop, were coarsely chopped and half of each sealed in plastic buckets for 6 weeks to simulate ensilation. The worst-infected panicles contained up to 55 mg DHES/kg, but dilution reduced average concentrations of DHES in crops to approximately 1 mg/kg, a relatively safe level for cattle. Ensilation significantly (P = 0.043) reduced mean DHES concentrations from 0.85 to 0.46 mg/kg.     

Acute onset of pulmonary necrotising arteritis in a dog with a left-to-right patent ductus arteriosus

A 12-week-old, clinically normal Chihuahua was referred for investigation for a continuous heart murmur. Cardiac evaluation revealed an anatomically and haemodynamically typical left-to-right shunting patent ductus arteriosus. The continuous wave Doppler measurement of peak ductal jet velocity of 5.6 m/s was suggestive of a normal pulmonary to systemic arterial pressure ratio. The dog returned 16 days later with right heart failure and severe pulmonary hypertension. Marked reduction in left-to-right shunting was demonstrated and the ductal jet velocity had decreased to 2.5 m/s. Immediate ductus ligation, oxygen therapy before and after the operation, and administration of hydralazine failed to reduce pulmonary hypertension, and the dog was euthanased. Histopathological examination of the lung showed pulmonary necrotising arteritis with acute and chronic arterial lesions. Chronic pulmonary vascular changes related to high flow have been associated with altered nitric oxide and endothelin responses. These changes may be responsible for the acute onset of pulmonary hypertension due to relatively minor vascular insults in some human and veterinary patients with left-to-right shunts. The potential for acute progression supports the recommendations for early ductus ligation and the prognostic importance of detecting pulmonary hypertension presurgically in patent ductus arteriosus patients.     

Novel septoria speckled leaf blotch resistance loci in a barley doubled-haploid population

The genetics of resistance to Septoria speckled leaf blotch (SSLB), caused by Septoria passerinii, was studied in the Leger × CIho9831 barley doubled-haploid population. The 140 lines in the population segregated as 102 resistant and 38 susceptible, approximating a 3:1 ratio. A recombination map was developed using diversity arrays technology and other molecular markers. Quantitative trait locus (QTL) analysis demonstrated that resistance is primarily conferred either by having the CIho9831 allele at a QTL on 6HS or by having the CIho9831 allele at both of two QTLs on 3H and 2HL. In addition, ≈1/16 of the lines were resistant for unidentified reasons. This model predicts a resistant/susceptible ratio of 11:5, which fits the phenotypic observations. Minor QTLs were detected on 2HS and 1H. DNA sequences of linked markers suggest that the 6HS, 3H, and 2HS QTLs are part of resistance gene clusters and that the 6HS and 3H QTLs share homology. The 6HS QTL is identical to or closely linked to the SSLB resistance locus Rsp4 and the 1H QTL to the Rsp2 or Rsp3 locus. The 3H and 2HS QTLs are unique and offer new opportunities for pyramiding resistance genes through marker-assisted breeding for resistance to S. passerinii.     

Genetics of multiple disease resistance in a doubled-haploid population of barley

The barley accession Q21861 possesses resistance to the stem-rust (Puccinia graminis f.sp. tritici), leaf-rust (P. hordei), and powdery-mildew (Blumeria graminis f.sp. hordei) pathogens. An anther-culture-derived doubled-haploid population was produced from F₁ plants from a cross of this accession and the susceptible breeding line SM89010 as a means of rapidly and efficiently determining the genetics of multiple disease resistance. The doubled-haploid population segregated 1:1 (resistant:susceptible) for resistance to the stem rust pathotype QCC indicating the involvement of a single resistance gene, rpg4. Two-gene (3:1) and one-gene (1:1) segregation ratios were observed for resistance to the stem-rust pathotype MCC at low (23–25°c) and high (27–29°C) temperature, respectively. These different segregation patterns were due to a pathotype × temperature interaction exhibited by rpg4 and Rpg1. another stem-rust-resistance gene present in Q21861. One-gene and two-gene segregation ratios were observed in reaction to the leaf rust and powdery mildew pathogens. These data demonstrate the utility of doubled haploid populations for determining the genetics of multiple disease resistance in barley.