Effects of weather variables of mould development on sorghum grain were studied over three consecutive seasons in South Africa. Five sorghum hybrids planted at different dates ensured developing seeds were exposed to different weather conditions. Incidence of grain mould fungi was determined at harvest by incubating seeds on 2% malt extract agar. Averages of different weather variables (maximum and minimum temperatures, maximum relative humidity, total precipitation and frequency of precipitation) were determined for all permutations of weekly time intervals for a 2-month postflowering period to identify when these variables and pathogen incidence were significantly correlated. Significant correlations were used to develop models to quantify relationships between variables. Significant positive correlations were observed between the incidence of mould fungi and weather 4 -6 weeks after flowering in the shorter season hybrid cv. Buster, and 5-8 weeks after flowering in the remaining hybrids. In most hybrids, correlations between the incidence of grain mould pathogens, including Alternaria alternata , Curvularia spp. ( C. lunata and C. clavata ), Fusarium spp. ( F. proliferatum and F. graminearum ), and Drechslera sorghicola , and average minimum temperature, total rainfall and frequency of rainfall were significant ( P = 0·05). In four hybrids, models showing a linear relationship between the logarithm of pathogen incidence and minimum temperature, and in one hybrid, between pathogen incidence and rainfall frequency, were developed. Depending on the hybrid, models that used minimum temperature as predictor described 60 -82% of variation in the incidence of pathogens. Frequency of rainfall explained 93% of the variation in pathogen incidence in one sorghum hybrid genotype. Evaluation of the models using an independent data set yielded average prediction errors near zero, indicating that the models were acceptable.
The effect of seed quality and fungicide seed treatment on root rot of groundnut was studied at Babile, eastern Ethiopia. Seed lots of a commonly grown variety (Oldhale) were divided into two parts. The first part was cleaned (discoloured, shrivelled and broken seeds removed) and treated with the fungicides mancozeb, benomyl or copper hydroxide. The remaining seed lot was planted without cleaning but treated with each of the above fungicides. A split plot design was used, seed quality and fungicides being main and subplot treatments, respectively. Compared with the untreated control, significantly higher seedling emergence and lower plant mortality as well as higher dry pod yield and yield components were recorded from seeds treated with fungicides. Mancozeb-treated seeds had a higher seedling emergence, pod yield, pod number per plant, shelling percentage and seed weight but a lower plant mortality than the other treatments. Cleaned seeds showed lower plant mortality, higher seedling emergence and dry pod yield than uncleaned seeds. The highest pod yield (2344 kg/ha) was obtained from cleaned seeds treated with mancozeb and the lowest (1665 kg/ha) from uncleaned seeds that were not treated with fungicide. Fusarium oxysporum (80%) was the most dominant fungal species isolated from wilting plants observed in the experimental field. Our results indicated that effective management of root rot and significant yield benefit from groundnut can be obtained through integrated use of mancozeb treatment (3 g/kg seed) with properly cleaned seeds.
Kenaf (Malvaceae; Hibiscus cannabinus L.) is being commercially cultivated in Winterton, South Africa for its high-quality cellulose fibers with approximately 2,000 ha currently under cultivation. In 2004, 25% of 1-month-old kenaf plants grown from seed were observed in the field with severe wilting followed by lodging and mortality within 1 week. Isolations from diseased stem and root tissue on malt extract agar (MEA) consistently yielded Fusarium verticillioides (Sacc.) Nirenberg (2). Pathogenicity tests were conducted by inoculating kenaf seedlings with inoculum prepared from barley grains that had been colonized by the pathogen in vitro for 2 weeks prior to being finely ground in a laboratory mill. Fifty seeds from each of eight kenaf cultivars were incubated at 25°C on sterile filter paper to ensure germination and the absence of pathogens. Germinated seeds were sown in pots (400 cm3) containing steam sterilized loam soil (200 g) by placing 20 germinated seeds from each cultivar, with four replicates (5 seeds per pot), on the soil in each pot and covering them with 100 g of the same soil. Inoculum powder was sprinkled on the surface of the soil in each pot and covered by 100 g of soil. Pots were maintained in a glasshouse at an ambient temperature of 25°C. Sterile ground barley seeds served as the control treatment. Pots were watered daily with 20 ml of water and observed periodically for seedling emergence. The percentage of diseased seedlings was recorded after 3 weeks and the experiment was repeated. Wilting had occurred in 85% of seedlings when they were approximately 4 cm high and all diseased seedlings had died within 1 week thereafter. Subsequent examination revealed dark brown lesions girdling the stem and decayed roots in all instances. No symptoms developed on control plants. From means of combined data, the greatest seedling mortality was observed for cv. Gregg (65%) and the least for cv. Cuba108 (5%). Mean mortalities for the remaining six cultivars ranged from 30 to 55%. The pathogen was reisolated on MEA from all diseased seedlings. To our knowledge, this is the first report of F. verticillioides occurring on kenaf in South Africa. The only other report of Fusarium sp. causing serious damping-off of kenaf is from Iran (1). The potential impact of the pathogen on kenaf production in South Africa must be considered in the implementation of disease control measures. References: (1) J. M. Dempsey. Kenaf. In: Fiber Crops. The University Press of Florida, Gainesville, 1975. (2) J. F. Leslie and B. A. Summerell. The Fusarium Laboratory Manual. Blackwell Publishing, Ames, IA, 2006.
A greenhouse experiment was conducted to determine the influence of sorghum ergot on the incidence of grain mould pathogens and on damage to sorghum grains. Grain mould pathogens used were Curvularia lunata, Fusarium subglutinans and Phoma sorghina. Inoculation with the ergot pathogen (Claviceps africana) was performed at anthesis and with the grain mould pathogens at the soft dough growth stage. In all four sorghum cultivars tested, seeds from panicles inoculated with grain mould pathogens following inoculation with C. africana revealed a significantly higher (P*0.05) incidence of P sorghina than those inoculated with only the er~ot or grain mould pathogens. A significantly higher incidence of C. lunata (in PAN8564 and Gambelia1107) and F.subglutinans (in PAN8564) was recorded in seeds from panicles inoculated with ergot and grain mould than from the remaining treatments. Likewise, significantly higher grain discolouration was found where both ergot and grain mould pathogens were inoculated on panicles. Relatively lower 1000-grain weight and percentage germination were observed from ergot and grain mould inoculated panicles. There was a significant relationship between ergot severity and the incidence of P sorghina (r = 0.82 to 0.97; P*0.05) in all the cultivars and that of C. lunata (r = 0.92) in PAN8564. Ergot severity correlated positively (P*0.05) with grain discolouration except in PAN8446. Results indicate that ergot can significantly influence grain mould development and hence the yield and quality of sorghum grains. Management of ergot may thus help to reduce the severity of grain mould thereby decreasing the resulting damage to grains.
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