Rhizoctonia Anastomosis Groups Associated with Diseased Rooibos Seedlings and the Potential of Compost as Soil Amendment for Disease Suppression.

Rhizoctonia spp. associated with rooibos in the Western Cape province of South Africa were recovered during the 2008 season by planting seedlings in rhizosphere soils collected from 14 rooibos nurseries. In all, 75 Rhizoctonia isolates were obtained, of which 67 were multinucleate and 8 were binucleate Rhizoctonia spp. The identity of these isolates to anastomosis group (AG) was determined through sequence analysis of the ribosomal DNA internal transcribed spacer region. The collection of multinucleate isolates included representatives of AG-2-2 (67%), AG-4 HGI (14%), AG-11 (5%), and R. zeae (3%). Binucleate AGs included AG-Bo (4%) and AG-K (4%) and an unidentified binucleate Rhizoctonia (3%). Rhizoctonia solani AG-2-2 was the most widely distributed species of Rhizoctonia detected among the 11 nurseries sampled. All AGs recovered from rooibos have been previously reported on crop plants in South Africa, with the exception of R. zeae. However, this is the first study to classify the Rhizoctonia AGs recovered from rooibos. In glasshouse bioassays, the most virulent Rhizoctonia AGs on rooibos and lupin were AG-2-2, AG-4 HGI, and AG-11. Although plant damage was less than that observed for lupin and rooibos, oat was significantly affected by AG-2-2 and AG-4 HGI. Two composts sourced from independent suppliers were evaluated for disease suppression under glasshouse conditions. Compost amendment suppressed damping-off by most R. solani AGs, except for AG-4 HGI. Furthermore, within AG-2-2, suppression by compost was isolate specific.

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.

Fusarium graminearum Species Complex Associated with Maize Crowns and Roots in the KwaZulu-Natal Province of South Africa.

Thirty-three isolates of the Fusarium graminearum species complex obtained from diseased maize (Zea mays) crowns and roots in the Winterton district, KwaZulu-Natal province of South Africa were identified to species level. Their pathogenicity and virulence to maize 'PHI 32D96B' seedlings were determined under glasshouse conditions, with seedling survival and growth and crown and root rot as criteria. Phylogenetic analyses using the 3-O-acetyltransferase (Tri101) gene region sequences revealed the presence of F. boothii (2 isolates), F. graminearum sensu stricto (26 isolates), and F. meridionale (5 isolates) in the F. graminearum species complex associated with diseased maize crowns and roots. Pathogenicity results showed that F. boothii was the most and F. meridionale the least virulent of the three species. F. boothii and F. graminearum sensu stricto significantly reduced survival of seedlings and all three species caused significant reduction in growth and significantly more crown and root rot than the control (uninoculated). This is the first report of F. boothii, F. graminearum sensu stricto, and F. meridionale associated with diseased maize crowns and roots and their pathogenicity and virulence as soilborne pathogens on maize seedlings in South Africa.

Characterization of Rhizoctonia spp. Recovered from Crop Plants Used in Rotational Cropping Systems in the Western Cape Province of South Africa.

Isolates of Rhizoctonia spp. associated with barley, canola, clover, lucerne, lupin, annual Medicago spp. (medic), and wheat were recovered during the conduct of a 4-year (2000 to 2003) crop rotation trial in the Western Cape province of South Africa. These isolates were characterized by determining their anastomosis group (AG), in vitro optimum growth temperature, and pathogenicity toward emerging and 14-day-old seedlings of all the aforementioned crops. During the 4-year rotational trial, 428 Rhizoctonia isolates, in all, were obtained. The most abundant multinucleate AG was AG-4 HG-II (69%), followed by AG-2-1 (19%), AG-3 (8%), AG-2-2 (2%), and AG-11 (2%). The population of binucleate Rhizoctonia spp. comprised AG-K (53%), AG-A (10%), AG-I (5%), and unidentified AGs (32%). The optimal time for isolating Rhizoctonia spp. was found to be at the flowering or seedpod stage (20 to 22 weeks after planting). Temperature studies showed that isolates belonging to AG-2-2, AG-4 HG-II, and AG-K had significantly higher optimum growth temperatures than those from other AGs. In pathogenicity assays conducted on emerging as well as 14-day-old seedlings, isolates of AG-2-2 and AG-4 HG-II were the most virulent on all crops. Rhizoctonia solani AG-2-1 was highly virulent on canola, moderately virulent on medic and lupin, weakly virulent on lucerne and barley, and nonpathogenic on wheat. AG-11 isolates were moderate to weakly virulent on all crops, with the exception of barley and wheat. AG-3 was weakly virulent on canola, lupin, and medic. AG-K was the only binucleate Rhizoctonia sp. capable of inciting disease in our assays. This is the first comprehensive study to elucidate the identity and potential importance of Rhizoctonia spp. as a yield limiting factor in crop production systems in the Western Cape province of South Africa.