Effect of Barberry Eradication on Changes in Populations of Puccinia graminis in Minnesota.

Proportions of formae speciales of Puccinia graminis in collections of aeciospores from barberry were determined from cereal rust survey records from 1912 to 2002 in Minnesota. The frequency of P. graminis f. sp. avenae in aeciospore collections fluctuated between 0 and 10% from 1920 to 2002, even though oat was the dominant small grain crop in Minnesota until 1970. In early years, P. graminis f. sp. tritici was common, but the frequency of P. graminis f. sp. tritici in aeciospore collections declined to a low of 4% in the 1980s, whereas P. graminis f. sp. secalis increased to 96%. After 1990, the frequency of P. graminis f. sp. tritici increased and P. graminis f. sp. secalis declined in aecial collections, possibly indicating a changing proportion of P. graminis f. sp. secalis and tritici on wild grasses near barberry bushes. Diversity of races among uredinial collections of P. graminis f. sp. tritici from wheat in Minnesota declined sharply from 1912 to 1930 and remained low to 2002. Although the races of P. graminis f. sp. tritici most common in uredinial collections also were most common in the aecial collections in five of nine decades from 1912 to 1999 as well in the years 2000 to 2002, the diversity of races was greater among aecial than uredinial collections. Diversity in aecial collections remained nearly constant for 90 years, indicating a continuing contribution of the sexual stage to diversity of P. graminis f. sp. tritici.

The Effect of Stand Density on the Development of Puccinia graminis f. sp. tritici in Barley.

The progress of stem rust epidemics caused by race Pgt-QCCJ of Puccinia graminis f. sp. tritici was examined in field plots of the six-rowed barley cvs. Robust and Steptoe at four stand densities, and in short rows and hill plots. Cv. Robust carries Rpg1, which confers resistance to most races of P. graminis f. sp. tritici, other than Pgt-QCCJ, while cv. Steptoe lacks any reported genes for resistance to P. graminis f. sp. tritici. The stand-density treatments established were approximately double, the equivalent of, half of, and one quarter of the planting rate recommended for commercial barley. Stem rust caused by race Pgt-QCCJ spread more rapidly in cv. Steptoe than in cv. Robust, irrespective of the stand-density treatment. The effect of stand density on rust severity was, however, greater than the effect of cultivar. Rust severity was 18 to 36% greater in sparse stands than in dense stands of both cultivars. Reductions in kernel size were most severe in sparse stands where rust development was greatest. Knowledge that reduced stand densities may promote the development of stem rust in barley may be useful in refining procedures for screening breeding material for resistance to stem rust, and in allaying concern over the high levels of stem rust occasionally reported in lightly seeded seed increases and in commercial fields where sparse stands are encountered.

Inheritance of Adult Plant Leaf Rust Resistance in the Brazilian Wheat Cultivar Toropi.

Adult plant resistance to leaf rust in the Brazilian wheat cultivar Toropi (Triticum aestivum) was studied in crosses with the susceptible cultivar IAC 13. Cvs. Toropi and IAC 13 are susceptible at the seedling stage to race LCG-RS of Puccinia triticina Erikss., and to all other known Brazilian leaf-rust races. Thus, the resistance observed in Toropi in the field was due to adult plant-resistance genes. In the greenhouse at the adult plant stage, resistance segregated in a 7:9 ratio for two complementary recessive genes. Additionally, two recessive genes for leaf-tip necrosis were identified in the greenhouse environment. Necrosis was expressed when the two homozygous recessive genes occurred together in the F2, independently of the response to leaf rust. The resistance and leaf-necrosis genes differ from those previously reported in wheat. Segregation for leaf-rust resistance in the field at Passo Fundo, Brazil, fit a 1:3 ratio for a single recessive gene. With a different pathogen race, and in crosses of cvs. Toropi and ThatcherLr34, two recessive genes and a dominant gene for resistance were detected in the field in Mexico. The dominant gene was likely Lr34 from cv. ThatcherLr34 and the two recessive genes were likely those detected in the greenhouse adult plants tests at Passo Fundo.

Rust Disease of Oregano and Sweet Marjoram in California.

In 1996 and 1997, a rust disease was detected on commercial, fieldgrown oregano (Origanum vulgare) and sweet marjoram (Origanum majorana) in coastal California. Symptoms on both plants were similar and mostly consisted of small (2 to 5 mm in diameter), circular, brown, necrotic leaf spots that developed cinnamon brown pustules in the center of the spot or in concentric groups on the spot periphery. Pustules sometimes developed without spots. On sweet marjoram, leaf spots were sometimes surrounded by a chlorotic halo. Teliospores were not detected on either host. Ellipsoidal urediniospores measured 22 to 25 µm by 19 to 22µm and contained 2 to 3 germ pores in an equatorial configuration. The rust was identified as Puccinia menthae. Pathogenicity was tested by depositing urediniospores onto leaves of healthy plants and then incubating plants in a humidity chamber for 48 h. Urediniospores from oregano infected Italian (Origanum× majoricum), Sicilian (Origanum × marjorana), trailing (O. prostrata), and Greek (O. heracleoticum) oregano, and sweet marjoram. Urediniospores from sweet marjoram infected sweet marjoram and the one oregano tested, Italian oregano. With all inoculations, both symptoms and fungal fruiting bodies were similar to those observed in the field. Neither the oregano nor the sweet marjoram isolates infected spearmint (Mentha spicata), which is consistent with previous studies (1,2). This is the first report of a rust disease of oregano and sweet marjoram in California. Rust significantly reduced the quality and yield of both crops. References: (1) J. T. Fletcher. Trans. Br. Mycol. Soc. 46:345, 1963. (2) M. Wilson and D. M. Henderson. 1966. British Rust Fungi, Cambridge University Press, Cambridge.

Responses of Barley Cultivars and Lines to Isolates of Pyrenophora teres.

Net blotch, caused by Pyrenophora teres, is among the most damaging foliar diseases of barley worldwide. A knowledge of the reaction of local cultivars, putative resistant lines, and variability in the net blotch pathogen is necessary to develop a successful resistance breeding program. Disease responses of 38 barley lines to 15 P. teres isolates were studied at the seedling and adult plant stages in the glasshouse, and field responses to net blotch were evaluated at three Moroccan locations. No tested barley was resistant to all isolates, and resistance was apparently of the specific type. Pathogenic variability was great, as none of the 15 isolates were identical. For each isolate tested, a specific high level of resistance was found in one or more host lines. Seedling and adults plants often differed in response to the same isolate. Adult plant resistance was commonly observed in response to isolate I-1, and seedling resistance was more common to isolate I-14. Adult plant resistance of nine lines was undetected in seedling evaluations using isolate I-1. The seedling glasshouse and field responses of the barley lines varied considerably, limiting the value of seedling testing for resistance. Field reactions of resistant and moderately resistant were consistent across the three locations for the lines Heartland, Minn 7, CI 2333, and CI 2549. The variability observed in P. teres and failure to find lines with resistance to all isolates suggests that breeding for resistance should emphasize pyramiding of resistance genes.

The Effect of Race QCCJ of Puccinia graminis f. sp. tritici on Yield and Quality in Barley.

The progress of epidemics of race QCCJ of Puccinia graminis f. sp. tritici was examined in field plots of the barley cultivars Robust, Excel, Stander, Azure, Stark, Larker, Bonanza, and Steptoe. In addition, a potential source of resistance (cv. Diamond), and a susceptible line (ND11075) were included. Rust-free treatments were maintained to provide comparisons of grain yield and quality. Barleys with Rpg1, which confers resistance to races of P. graminis f. sp. tritici other than Pgt-QCCJ, were generally more resistant than cultivars or lines that lacked this gene. However the severity of losses indicated that under high disease pressure the resistance conferred by Rpg1 would not be effective. Yield reductions of up to 29% were recorded and were generally associated with reduced grain size and weight. The continued presence of Pgt-QCCJ as a component of the P. graminis population in the United States, since it was first detected in 1989, indicates that Pgt-QCCJ remains a threat to commercial barley production.

Comparison of Virulence and Isozyme Phenotypes of Pgt-QCCJ and Great Plains Races of Puccinia graminis f. sp. tritici.

ABSTRACT Stem rust race Pgt-QCCJ was first found in the Great Plains of the United States in 1989, collected primarily from barley. This race became a major part of the Puccinia graminis f. sp. tritici population, even though it is virulent to only a few hard red winter wheat cultivars in the central Great Plains and to barley in the northern Great Plains. It threatens barley production in the northern Great Plains of the United States and Canada due to virulence to Rpg-1. Six differences in virulence and two in isozyme banding patterns from the most similar stem rust races make it unlikely that QCCJ arose as a mutant. Thus, QCCJ likely arose through sexual or parasexual recombination. Sexual recombination in the Great Plains is unlikely, as it has not been detected in many years. Avirulence to 'McNair 70l' is only known from the Pacific Northwest of the United States and adjacent Canada. The rust population in this area is of sexual origin, and the pattern of virulence/avirulence and isozyme banding for QCCJ occurs there. Pgt-QCCJ likely originated in the Pacific Northwest during or before 1989 and was wind-transported into the Great Plains.