Vulnerability of Barley to African Pathotypes of Puccinia graminis f. sp. tritici and Sources of Resistance.

The emergence of widely virulent pathotypes (e.g., TTKSK in the Ug99 race group) of the stem rust pathogen (Puccinia graminis f. sp. tritici) in Africa threatens wheat production on a global scale. Although intensive research efforts have been advanced to address this threat in wheat, few studies have been conducted on barley, even though pathotypes such as TTKSK are known to attack the crop. The main objectives of this study were to assess the vulnerability of barley to pathotype TTKSK and identify possible sources of resistance. From seedling evaluations of more than 1,924 diverse cultivated barley accessions to pathotype TTKSK, more than 95% (1,844) were found susceptible. A similar high frequency (910 of 934 = 97.4%) of susceptibility was found for the wild progenitor (Hordeum vulgare subsp. spontaneum) of cultivated barley. Additionally, 55 barley lines with characterized or putative introgressions from various wild Hordeum spp. were also tested against pathotype TTKSK but none was found resistant. In total, more than 96% of the 2,913 Hordeum accessions tested were susceptible as seedlings, indicating the extreme vulnerability of the crop to the African pathotypes of P. graminis f. sp. tritici. In total, 32 (1.7% of accessions evaluated) and 13 (1.4%) cultivated and wild barley accessions, respectively, exhibited consistently highly resistant to moderately resistant reactions across all experiments. Molecular assays were conducted on these resistant accessions to determine whether they carried rpg4/Rpg5, the only gene complex known to be highly effective against pathotype TTKSK in barley. Twelve of the 32 (37.5%) resistant cultivated accessions and 11 of the 13 (84.6%) resistant wild barley accessions tested positive for a functional Rpg5 gene, highlighting the narrow genetic base of resistance in Hordeum spp. Other resistant accessions lacking the rpg4/Rpg5 complex were discovered in the evaluated germplasm and may possess useful resistance genes. Combining rpg4/Rpg5 with resistance genes from these other sources should provide more durable resistance against the array of different virulence types in the Ug99 race group.

First Report of Soybean Sudden Death Syndrome Caused by Fusarium virguliforme in South Africa.

Soybean (Glycine max (L.) Merr.) is an important crop in many countries and production is currently increasing (from 311,450 ha in 2010 to 516,500 ha in 2013) in South Africa. On 27 February 2013 in the Lydenburg/Badfontein area, Mpumalanga Province, on a no-till commercial farm planted to soybean cultivar PAN 737 (Roundup Ready, maturity group 7) under irrigation for a second consecutive season, leaf symptoms typical of soybean sudden death syndrome were observed and reported by a farmer (3). The symptoms developed at the R6 growth stage (near physiological maturity) of the soybean plants. Leaf symptoms were interveinal chlorotic blotches that became necrotic while the veins remained green. These symptoms appeared throughout the plant but were most severe on the top leaves. Some of the severely affected leaflets dropped off with the petioles remaining attached to the plant. The vascular tissue in the upper taproot and lower stem turned gray-brown, but the pith remained white. Roots of the affected plants had decayed lateral roots. Surface disinfested root pieces with rot symptoms and spores directly from blue sporodochia on the rotten root were plated on potato dextrose agar amended with novostreptomycin 0.04 g/L (PDA+). Slow growing Fusarium isolates with blue to purple masses of sporodochia were consistently obtained from diseased plants. Cultures were single-spored and plated on PDA+. Growth rate of cultures on PDA+ was on average 6 to 9 mm after 5 days at 20°C. The morphology of the isolates fit the description of Fusarium virguliforme in Aoki et al. (1). Sequence analyses of the nuclear ribosomal internal transcribed spacer (ITS) and partial translation elongation factor (EF-1a) gene of the recovered eight isolates revealed that these isolates matched 99.6% with F. virguliforme O'Donnell & T. Aoki (Accession Nos. KF648835 to KF648850), one of the soybean sudden death syndrome causing species found in North and South America (1). All isolates are identical in each loci except that three isolates had one nucleotide deletion and two insertions at the EF-1a loci. The isolates are deposited at the national culture collection in Pretoria (PPRI13434 to PPRI13441). A glasshouse bioassay was conducted to test the pathogenicity of eight single-spored isolates by inoculating pasteurized planting medium (1:1:1 ratio of sand, perlite, and soil) with a layer of infested sand-bran medium (2) to each pot (13 cm in diameter) and covered with 2 cm of planting medium (4) after planting 20 seeds of soybean cultivar PAN 737. There were three pots per isolate randomized in a complete block design trial. All the South African F. virguliforme isolates tested induced leaf and root rot symptoms of sudden death syndrome on the soybean seedlings under glasshouse conditions after 4 weeks of inoculation. The fungus was re-isolated on PDA+ from diseased roots of the soybean seedlings to fulfill Koch's postulates. This is the first record of F. virguliforme in South Africa, and as an important component of soilborne diseases of soybean it may pose a major threat to the South African soybean industry. References: (1) T. Aoki et al. Mycoscience 46:162, 2005. (2) S. C. Lamprecht et al. Plant Dis. 95:1153, 2011. (3) J. C. Rupe and G. L. Hartman. Compendium of Soybean Diseases, 4th ed. G. L. Hartman et al., eds. American Phytopathological Society, St. Paul, MN, 1999. (4) M. M. Scandiani et al. Trop. Plant Pathol. 36:133, 2011.

First Report of Soybean Rust in South Africa.

In February 2001, rust caused by Phakopsora pachyrhizi Syd. was detected for the first time on soybean (Glycine max (L.) Merr.) near Vryheid in northern KwaZulu-Natal, South Africa. As the season progressed, the disease was also observed in other parts of the province, and epidemic levels were reached in the Karkloof, Cedara, Howick, and Greytown production regions. In affected areas, infection foci gradually increased in size and caused premature yellowing and defoliation of soybean crops, usually after the flowering stage. Typical rust symptoms (3) were produced predominantly on the lower surface of soybean leaves. Soybean rust subsequently spread to Amsterdam and Ermelo in the Highveld region of South Africa. Following emergency registration of triazole compounds, fungicides were commonly used to control soybean rust, especially in the more humid eastern production areas. Available yield data suggested a reduction in kernel mass between 4 and 23%, depending on the cultivar and host growth stage at the time of infection. Urediniospores from the original collection (isolate PREM 57280, Plant Protection Research Institute, Pretoria, South Africa) were 23 to 33 × 15 to 22 µm, indicating that spore dimensions fell within the known range for P. pachyrhizi (3). To confirm pathogenicity, 10 to 15 plants of each of the South African soybean cvs. Pan 589, Pan 780, Pan 854, Octa, and Prima were inoculated with isolate PREM 57280. Primary leaves were sprayed with a suspension of spores in light mineral oil (approximately 1 mg of spores per ml) before incubating plants in the dark in a dew chamber for 16 h. Large, sporulating uredinia, producing typical soybean rust urediniospores, developed on all inoculated plants. Classical and real-time fluorescent polymerase chain reaction assays as well as sequence analysis of the internal transcribed spacer regions verified the identity of isolate PREM 57280 as P. pachyrhizi (2). Since the disease is known to occur in Zimbabwe, Mozambique, and several other African countries (1,3,4), inoculum was most likely introduced by air currents from countries to the north of South Africa. It is highly probable that soybean rust will successfully overwinter in South Africa based on experience in other southern African countries. References: (1) O. A. Akinsanmi and J. L. Ladipo. Plant Dis. 85:97, 2001. (2) R. D. Frederick et al. (Abstr.) Phytopathology 90 (suppl):S25, 2000. (3) G. L. Hartman et al. eds. Compendium of Soybean Diseases, 4th ed. The American Phytopathological Society, St. Paul, MN, 1999. (4) J. B. Sinclair and G. L. Hartman, eds. Soybean Rust Workshop, Publ. 1 College of Agricultural, Consumer, and Environmental Sciences, National Soybean Research Laboratory, Urbana, IL. 1996.

First Report of Botryotinia fuckeliana on Kerguelen Cabbage on the Sub-Antarctic Marion Island.

Pringlea antiscorbutica R. Br. (the Kerguelen cabbage) is a monotypic species endemic to five sub-Antarctic islands. It does not occur elsewhere and is the only brassica found in the sub-Antarctic region as a whole (1). On Marion Island (46° 54'S, 37° 45'E) the distribution and abundance of the cabbage has declined alarmingly over the past 20 years and the plants increasingly are showing symptoms of microbial pathogen attack. Leaves display brown, water-soaked lesions and sometimes whole plants collapse into a black, slimy residue. Small sections of lesions were sampled, surface sterilized, and placed on water agar and potato dextrose agar (PDA) to which streptomycin (0.1 g/liter) was added. Nonsterilized sections were placed in petri dishes on moist, sterilized, filter paper. Plates were incubated at 15°C in the dark. Botryotinia fuckeliana (de Bary) Whetzel (conidial state; Botrytis cinerea Pers.:Fr) was consistently isolated. Gray to white mycelium grew rapidly on PDA and after 10 days abundant black sclerotia (2 to 16 mm long) were observed on the medium. To confirm pathogenicity, four detached leaves of P. antiscorbutica were each inoculated with a single, 5-mm-diameter PDA plug of B. fuckeliana. Four leaves with noncolonized PDA plugs were used as controls. The leaves were placed with their petioles in sterile water in a transparent incubation chamber. Chamber temperature (minimum 3.9°C, maximum 7.7°C and mean 6.4°C) and light (100 µmol s-1 m-2 photosynthetic photon flux density) approximated field conditions quite closely but relative humidity (annual mean for the island, 81%) was on average 10% higher in the chamber. First symptoms were observed after about 48 h. After 5, 7, and 8 days, brown, water-soaked lesions averaged 14 × 7, 28 × 11, and 36 × 18 mm, respectively. Control leaves showed no symptoms. Reisolations from lesions produced B. fuckeliana. Identity of the pathogen was confirmed by the South African National Collection of Fungi Biosystematics Division, Pretoria. The sub-Antarctic climate is probably conducive to the rapid spread of B. fuckeliana and to its ability to infect P. antiscorbutica. Other factors, such as the recent introduction of the diamondback cabbage moth (Plutella xylostella) to the island and grazing damage by an introduced slug species (Deroceros caruanae) probably exacerbate the threat offered by the fungus to this plant species, one of the last and perhaps the only, remaining relic of an extensive circum-Antarctic flora. Reference: (1) A. J. Dorne and R. Bligny. Polar Biol. 13:55, 1993.