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.

First Report of False Smut of Corn (Zea mays) in the Mississippi Delta.

False smut, caused by Ustilaginoidea virens, occurs on rice, corn, and other Gramineae, but is of little economic importance. False or green smut of corn resembles most closely false smut of rice (1). During surveys in 2000 at Mississippi State University, Delta Experiment Station, Stoneville, 32 (2.5%) of 1,280 commercial and inbred corn hybrids had false smut on the tassel. The percentage of plants infected with false smut ranged from 5 to 15% based on a sample of 20 corn plants from a single 4.9-m row plot. Galls (sclerotia), which are similar to ergot, replaced flowers on tassels of infected plants. Disease severity ranged from 2 to 50% infected flowers per inbred. Mature sclerotia were spherical, 6 to 17 mm in diameter, olive green to black, and velvety with white interiors. The fungus could not be isolated on potato dextrose agar. Only the tassels were infected, which resulted in little damage to the plant, except for a decrease in pollen production. Although the disease has been reported to infect other parts of the corn plant, the injury will likely be less on corn than rice. In 2001 and 2002, the disease was not observed. Disease incidence was more prevalent in 2000 because of hot, wet weather immediately after the crop was planted. The observation of the disease in one growing season suggests the pathogen may be common in the Mississippi Delta and the disease may occur under favorable environmental conditions and susceptible corn hybrids. Reference: (1) J. K. Pataky. False smut. Page 35 in: Compendium of Corn Diseases, 3rd ed. D. G. White, ed. American Phytopathological Society, St. Paul, MN, 1999.

Mycotoxin production by Fusarium proliferatum isolates from rice with Fusarium sheath rot disease.

Twenty samples of unpolished (rough) rice collected in Arkansas and Texas during the 1995 harvesting season from fields exhibiting Fusarium sheath rot disease or panicle blight were previously shown to include 8 samples positive for fumonisin B1 (FB1) in the range 2.2-5.2 ppm, and moniliformin (MON), but no beauvericin (BEA), deoxynivalenol, its derivatives or zearalenone were detected. Fifteen cultures of F. proliferatum were established from the 20 rough rice samples. Single spore isolates of each culture were grown on rice and tested for the production of fumonisins (FB1, FB2, FB3, etc.), MON and BEA. All 15 isolates produced FB1, FB2, MON and BEA in culture on rice. No deoxynivalenol, its derivatives or zearalenone were detected. Seven cultures produced FB1 at > 50 ppm (range 80-230 ppm), with the rest producing FB1 in the range 14-43 ppm. FB2 was produced in the range 5-47 ppm, and those cultures which produced the most FB1 also produced the most FB2. Of the 15 cultures producing MON, 11 produced it at > 100 ppm in the range 188-6018 ppm, with the rest producing in the range 7-64 ppm. BEA was produced in the range 109-1350 ppm. Other derivatives of fumonisins, including FA1, FA2 and partially hydrolyzed FB1, as well as several unknown metabolites including a compound with MW 414, were identified in culture extracts by continuous flow fast atom bombardment with ion spray mass spectrometry (CF/FAB/MS). Further study is needed to identify the factors that control production of FB1, MON and BEA by F. proliferatum in culture and in field samples.

Natural Occurrence of Fumonisins in Rice with Fusarium Sheath Rot Disease.

Twenty samples of rough rice (Oryza sativa) (unpolished kernels) collected during the 1995 harvest season from Arkansas (seven samples) and Texas (13 samples) were obtained from rice fields known to include plants with symptoms of Fusarium sheath rot putatively caused by Fusarium proliferatum. Samples were analyzed for fumonisin B1 (FB1) at three laboratories using three different extracting solvents by high-performance liquid chromatography (HPLC) or enzyme-linked immunosorbent assay (ELISA) methods. Forty percent of the samples were positive for FB1 at levels ≤4.3 µg/g by HPLC. The same samples contained FB1 at ≤3.6 µg/g when measured by an ELISA method. Most samples that were positive for FB1 were positive for fumonisin B2 (FB2) and fumonisin B3 (FB3) by HPLC at levels ≤1.2 µg/g. Very good agreement was obtained among the two laboratories using HPLC methods and the third using ELISA. Shelling of the unpolished rice results in hull and brown rice fractions. In a sample that contained 4.3 µg/g in whole kernels, the fumonisin level was very high in hulls (≤16.8 µg/g) and low in brown rice (≤0.9 µg/g). Milling of brown rice results in bran and white rice fractions. Fumonisins were found in bran at a level of ≤3.7 µg/g but were below the level of detection by HPLC in white rice. The presence of fumonisins (FB1, FB2, and FB3) was confirmed by fast atom bombardment/mass spectrometry. This is the first report of fumonisins in naturally contaminated rice in the United States.