Uromyces ciceris-arietini, the Cause of Chickpea Rust: New Hosts in the Trifolieae, Fabaceae.

Uromyces ciceris-arietini has been reported on Cicer arietinum (chickpea) and Medicago polyceratia. Plants of Medicago polymorpha in Riverside and San Diego, CA were collected with severe rust caused by U. ciceris-arietini. To confirm the identification and potential new host range, a monouredinial isolate of U. ciceris-arietini from M. polymorpha was inoculated on eight accessions each of C. arietinum and M. polyceratia. All plants showed symptoms of the disease. Consequently, a range of fabaceous hosts were evaluated for their reaction to U. ciceris-arietini. New hosts for U. ciceris-arietini included 29 species of Medicago, specifically M. arabica, M. blancheana, M. ciliaris, M. constricta, M. coronata, M. doliata, M. granadensis, M. intertexta, M. italica, M. laciniata, M. lanigera, M. lesinsii, M. lupulina, M. minima, M. murex, M. muricoleptis, M. orbicularis, M. praecox, M. radiata, M. rigidula, M. rotata, M. rugosa, M. sativa, M. sauvagei, M. scutellata, M. soleirolii, M. tenoreana, M. truncatula, and M. varia, and three species of Melilotus, specifically M. italicus, M. speciosus, and M. spicatus. This isolate of U. ciceris-arietini produced no symptoms on plants in the 33 accessions tested in the genera Anthyllis, Astragalus, Lotus, and Lupinus. DNA sequences are provided to aid in the identification of this pathogen.

First Report of Uromyces striatus on White Sweetclover in Kansas.

Rust was observed on leaves and stems of Melilotus alba Medik. plants in Riley County, Kansas, in November 1999. Rust reoccurred during the summers of 2000 and 2001. Uredinia were orange to reddish brown and predominately hypophyllous. Infected leaves wilted and abscised prematurely. Urediniospores were one-celled, globoid or broadly ellipsoid, and measured 20 to 25 × 18 to 22 µm. Walls were 1.0 to 1.2 µm thick, echinulate, and with three to five (mostly four) pores. Pores were equatorial, or nearly so, in approximately one half of the spores examined and scattered in the other half. Telia occurred in the fall, were sparse, amphigenous, exposed, and much darker than uredinia. Teliospores were single-celled, ovoid, subglobose or ellipsoidal, and measured 18 to 25 × 15 to 21 µm. Wall thickness was 1.3 to 2 µm. Walls had ridges approximately 1 µm wide, 2 µm apart, and arranged in a longitudinal pattern as shown in photographs of Uromyces striatus (2). Pedicels were hyaline, fragile, and short. Pathogenicity tests were conducted in growth chambers (4) with monouredinial isolate K-SC-R1 from M. alba. Pots containing plants of 80 USDA Plant Introduction (PI) core accessions representing 18 Melilotus spp. were evaluated. Total accessions tested per species were: 22 M. alba, 2 M. altissimus, 3 M. dentata, 2 M. elegans, 1 M. hirsutus, 4 M. indica, 2 M. infestus, 2 M. italica, 25 M. officinalis, 2 M. polonicus, 2 M. segetalis, 2 M. siculus, 2 M. speciosus, 2 M. spicatus, 1 M. suaveolens,2 M. sulcatus, 2 M. tauricus, and 2 M. wolgicus. Various amounts of rust developed on plants of 79 accessions; no plants of M. indica PI 234674 exhibited signs of rust. A host range study of an isolate of U. striatus from alfalfa (4) included these 18 Melilotus species; 10 were hosts. Therefore, the reactions of Medicago accessions to isolate K-SC-R1 were evaluated. These included nine alfalfa germ plasm sources representing the diversity in North American alfalfas and the cvs. Saranac and Moapa 69, which are commonly used susceptible controls in alfalfa rust evaluations. No rust resulted on any entry of Medicago sativa subsp. sativa. A few scattered, small, open uredinia occurred on plants of Medicago sativa subsp. falcata ('WISFAL' PI 560533). Of 11 Medicago lupulina PI accessions inoculated, rust resulted only on a few plants of PI 269926. All M. alba plants included as susceptible controls in all tests became heavily rusted. The causal fungus fits U. striatus Schroet. as described by uredinologists who disregard urediniospore pore position in this species. For example, illustrations of U. striatus urediniospores by Savulescu (3) show scattered pores, however, descriptions of U. striatus Schroet. in North America (1,2) specify equatorial pores. To my knowledge, this is the first report of scattered pores in U. striatus urediniospores in the United States. However, an examination of urediniospores from heavily rusted alfalfa plants collected in 2001 near Manhattan, KS and from isolate KR1 (4) also revealed urediniospores with variously distributed pores. U. striatus was reported on M. alba in Florida in 2001. References: (1) G. B Cummins. Rust Fungi on Legumes and Composites in North America. University of Arizona Press, Tucson, 1978. (2) J. A. Parmelee. Can. J. Bot. 40:491, 1962. (3) T. Savulescu. Monografia Uredinalelor din Republica Populara Romana. II. Acad. Repub. Pop. Rom. 1953. (4) D. Z. Skinner and D. L. Stuteville. Plant Dis. 79:456, 1995.

Reactions of Accessions in the Annual Medicago Core Germ Plasm Collection to Erysiphe pisi.

Reaction to Erysiphe pisi was evaluated in the annual Medicago core germ plasm collection of 199 USDA Plant Introduction accessions from 33 species. Seedlings at the first trifoliolate leaf stage were inoculated with conidia and maintained in growth chambers for 21 days at 20°C, 70 to 90% relative humidity, and a 12-h photoperiod. Beginning 4 days after inoculation, plants with the first sign of powdery mildew were recorded and removed daily. Results for each accession were presented as the percentage of plants mildew-free 21 days after inoculation and also as area under the disease progress curve (AUDPC) values expressed as standardized proportion/days. AUDPC values were useful to account for length of latent period (number of days from inoculation until first sign of mildew). Thirty-two accessions from 14 species included plants mildew-free 21 days after inoculation. Mildew-free plants in those 32 accessions ranged from 100% in M. heyniana PI 537136 and M. truncatula PI 292436 down to 1% in six accessions. All plants in 87 accessions, which included all accessions of 11 species, had powdery mildew 4 days after inoculation and thus expressed no resistance. The remaining 80 accessions included slow-mildewing plants with latent periods of 5 to 14 days. This slow-mildewing trait may be a useful source of resistance to E. pisi.

Reactions in the Annual Medicago Core Germ Plasm Collection to Two Isolates of Peronospora trifoliorum from Alfalfa.

Peronospora trifoliorum causes downy mildew of alfalfa throughout the temperate regions of the world, but little is known about its host range within the annual Medicago species (medics). Several medics have characteristics potentially beneficial to sustainable agriculture programs and to the genetic improvement of alfalfa. Therefore, we evaluated resistance in 199 accessions representing 33 species of the annual Medicago core germ plasm collection to P. trifoliorum isolates I7 and I8 from alfalfa from Kansas and southern California, respectively. Seedlings at the cotyledonary growth stage were inoculated with conidia suspended in water. One week later, the plants were rated resistant (no conidium production) or susceptible (conidium production). High levels of resistance existed in all species. All plants in 142 (71%) of the accessions were resistant to both isolates, but plants in only 24 of those resistant accessions were free of chlorotic to necrotic hypersensitive reactions on the inoculated cotyledons. All but one of the medic accessions had a higher percentage of plants resistant to both isolates than did cv. Saranac alfalfa, the resistant control. Conidium production generally was much less intense on the susceptible medic plants than on the alfalfa controls. A greater percentage of plants in four medic accessions was more resistant to I8 than to I7, and 22 were more resistant to I7 than to I8.

Powdery Mildew Caused by an Oidium sp. on Twenty-One Annual Medicago spp. in California.

Powdery mildew was observed in plots of USDA Plant Introduction accessions of medic plants grown for seed increase at Riverside, CA, from 1995 to 1998. White, diffuse to dense, amphigenous mycelia bearing Oidium conidia appeared on shoots in April each year and remained active until the last plants matured in June. Invaded leaflets became necrotic and dropped prematurely, sometimes leaving a green petiole. However, mildew incidence was low in most plots, and symptomless plants occurred in all species. Conidia were ellipsoid-cylindrical and measured 15 to17 × 39 to 47 µm. A sexual state of the fungus was not observed. Morphological characteristics of the anamorph matched those of Erysiphe pisi (1). In growth chambers, conidia from medic plants caused severe mildew of garden pea (Pisum sativum L. 'Early Perfection'). Powdery mildew occurred in plots of Medicago ciliaris, M. constricta, M. coronata, M. disciformis, M. doliata, M. granadensis, M. intertexta, M. italica, M. lesinsii, M. littoralis, M. lupulina, M. minima, M. murex, M. noeana, M. orbicularis, M. rigidula, M. rotata, M. rugosa, M. scutellata, M. truncatula, and M. turbinata. We believe this is the first report of naturally occurring powdery mildew on these species in the United States. Reference: (1) H. J. Boesewinkel. Bot. Rev. 46:167, 1980.

Powdery Mildew Caused by an Oidium sp. in Alfalfa in Wisconsin.

In July 1996 and August 1997, dense amphigenous mycelia bearing Oidium conidia infected the lower stems and leaves of alfalfa (Medicago sativa L.) plants in research nurseries in south-central Wisconsin, causing the defoliation of the affected leaves. In the greenhouse, the disease spread rapidly from transplanted field plants to adjacent flats of alfalfa seedlings. A teleomorphic state of the fungus was not observed. Morphological characteristics of the oidia were similar to those of Erysiphe pisi (1). Conidia were ellipsoid-cylindrical and closely resembled those shown from Pisum sativum L. (2). Dry (2) conidia averaged 14.6 × 41.4 µm (range: 13.0 to 15.6 × 39 to 46.8 µm). Conidia shaken from mildewed alfalfa plants onto plants of garden pea (Pisum sativum) cv. Early Perfection and red clover (Trifolium pratense L.) cv. Lakeland infected both hosts. However, powdery mildew developed sooner and was much more severe in pea than in red clover. Noninoculated control plants remained free of powdery mildew. This is the first report of powdery mildew in alfalfa in Wisconsin fields. In extensive pathogenicity studies in greenhouses in Wisconsin (2), conidia from at least one isolate from red clover infected Medicago falcata L. and M. sativa L. var. glutinosa Urban, but not alfalfa cv. Vernal. References: (1) H. J. Boesewinkel. Bot. Rev. 46:167, 1980. (2) J. R. Stavely and E. W. Hanson. Phytopathology 56:309, 1966.

First Report of Powdery Mildew Caused by an Oidium sp. on White Sweetclover in Kansas.

Powdery mildew was observed on plants of white sweetclover (Melilotus alba Medik.) in Riley County, Kansas, in 1995, 1996, and 1997 but was not examined closely until 1997. White, effuse to dense, amphigenous mycelia bearing Oidium conidia were obvious by mid June. Effuse growth occurred on petioles and stems. Invaded leaflets wilted and dropped, sometimes leaving a green petiole. Signs and symptoms diminished during the warmer periods of July and August, reoccurred in September, and remained prominent until the foliage froze in November. However, on 1 December, viable, newly produced conidia were collected from scant hyphal growth on protected leaves at the base of a few plants. A sexual state was not found. Characteristics of the conidial state fit those of Erysiphe pisi (1). Powdery mildew was never observed on plants of yellow sweetclover (M. officinalis (L.) Lam.) growing among the mildewed plants of white sweetclover. This is the first report of powdery mildew on M. alba in Kansas. Reference: (1) H. J. Boesewinkel. Bot. Rev. 46:167, 1980.