Importance of Fungicide Seed Treatment and Environment on Seedling Diseases of Cotton.

The importance of fungicide seed treatments on cotton was examined using a series of standardized fungicide trials from 1993 to 2004. Fungicide seed treatments increased stands over those from seed not treated with fungicides in 119 of 211 trials. Metalaxyl increased stands compared to nontreated seed in 40 of 119 trials having significant fungicide responses, demonstrating the importance of Pythium spp. on stand establishment. Similarly, PCNB seed treatment increased stands compared to nontreated seed for 44 of 119 trials with a significant response, indicating the importance of Rhizoctonia solani in stand losses. Benefits from the use of newer seed treatment chemistries, azoxystrobin and triazoles, were demonstrated by comparison with a historic standard seed treatment, carboxin + PCNB + metalaxyl. Little to no stand improvement was found when minimal soil temperatures averaged 25°C the first 3 days after planting. Stand losses due to seedling pathogens increased dramatically as minimal soil temperatures decreased to 12°C and rainfall increased. The importance of Pythium increased dramatically as minimal soil temperature decreased and rainfall increased, while the importance of R. solani was not affected greatly by planting environment. These multi-year data support the widespread use of seed treatment fungicides for the control of the seedling disease complex on cotton.

Evidence that soilborne wheat mosaic virus moves long distance through the xylem in wheat.

Soilborne wheat mosaic virus (SBWMV) is a member of the genus Furovirus of plant viruses. SBWMV is transmitted to wheat roots by the plasmodiophorid vector Polymyxa graminis. Experiments were conducted to determine the path for SBWMV transport from roots to leaves. The results of immunogold labeling suggest that SBWMV enters and moves long distance through the xylem. SBWMV may enter primary xylem elements before cell death occurs and then move upward in the plant after the xylem has matured into hollow vessels. There is also evidence for lateral movement between adjacent xylem vessels.

First Report of Musk Thistle Rust (Puccinia carduorum) in Oklahoma.

Musk thistle, Carduus thoermeri (Carduus nutans subsp. leiophyllus), is an important, introduced pasture weed in central and northeastern Oklahoma. Puccinia carduorum was introduced into the United States from Turkey as a potential biological control for musk thistle. P. carduorum has not been reported previously in Oklahoma, thus precluding its field release for biological control research without APHIS approval. There is evidence the organism has moved westward since the initial field studies that began in 1987 in Virginia. In 1994 it was found in Missouri (1). In early November 1997, in Rogers County, Oklahoma, scattered populations of C. thoermeri were found that had moderate to heavy levels of infection with a rust fungus. The pustules contained mostly teliospores; based on teliospore and urediniospore morphology, the fungus was identified as P. carduorum. The morphology and dimensions of urediniospores (21 × 21 µm, avg.) and teliospores (35 × 21 µm, avg.), and the restriction of echinulations to the upper two-thirds to three-fourths of urediniospores, were consistent with P. carduorum. Infection studies with field inoculum were conducted at both Oklahoma State University and USDA-FDWSRU. Rust-infected leaves collected in Oklahoma were air dried and maintained at room temperature for 2 months prior to use as inoculum. Small, symptomless, first-year rosettes of musk thistle were transplanted from the field into a mixture of soil, sand, and peat moss in pots and placed into a growth chamber maintained at 20°C. Seeds of C. thoermeri planted into pots containing the same mixture were maintained in the same chambers. After approximately 6 to 8 weeks, when seedlings and transplants were growing vigorously, both groups of plants were dusted with urediniospores and teliospores from the dried leaves. Inoculated plants were placed either into a 20°C dew chamber for 24 h or were atomized with distilled water, placed into sealed, transparent, polyethylene bags and returned to the 20°C growth chamber for 24 h, after which time the bags were removed. Both sets of plants were then maintained at 20 to 25°C. Chlorotic flecks developed on inoculated leaves after 7 to 8 days; uredinia and urediniospores were present within 10 days after inoculation. Urediniospores from those leaves had the same dimensions and ornamentation pattern as those originally obtained from field collections. A DNA sequence analysis was conducted on the rRNA ITS2 region, which was polymerase chain reaction-amplified from genomic DNA (2) extracted from urediniospores of the Oklahoma isolate grown at FDWSRU. The sequence of the ITS2 region from those urediniospores was identical to the sequence (GenBank accession no. U57351) obtained from the isolate 7803 of P. carduorum from Turkey, used in the Virginia field studies. The confirmed presence of P. carduorum in Oklahoma will enable field research with this rust for management of musk thistle in the state. References: (1) A. B. A. M. Baudoin and W. L. Bruckart. Plant Dis. 80:1193, 1996. (2) Y. T. Berthier et al. Appl. Environ. Microbiol. 62:3037, 1996.