Biological and chemical control of fungal seedling diseases of cowpea

Cowpea is a worldwide-distributed crop, and is important to the livelihood of poor people in developing countries. Cowpea is also susceptible to a wide range of pests and pathogens, which can cause damage to the crop at all stages. Seedling diseases caused by pathogens such as Rhizoctonia solani Fusarium solani and Pythium spp. affect cowpea and result in low yields especially in rural areas where there are few or no control measures against these pathogens. This research aimed at evaluating the efficacy of a biological control agent and fungicides against fungal seedling diseases of cowpea. The bacterium, Bacillus cereus1, and the fungicides, Apron®, Subdue® and Celest® were screened for the control of cowpea seedling diseases, after obtaining promising in vitro results on their effectivity against Rhizoctonia solani, Pythium ultimum and Fusarium solani. The experiment was conducted in a greenhouse using seedling trays with 128 cells, each filled with pasteurised growing medium (Braaks lawn dressing). Seedling trays were placed randomly on greenhouse tables with four replication per treatment, each replication consisting of 56 plants. Cowpea seeds (Cultivar-Pietersburg blue) were obtained from the Dry Bean Seeds Producers Organisation. The pasteurised growing medium was artificially inoculated with the three fungi. Two plugs of actively growing fungal mycelium of the three pathogens were inoculated in each cell of the polystyrene seedling trays. Trays were drenched with Bacillus cereus1 at 106 cells/ml (3 ml per tray cell) at planting and fungicides were applied on the 14th and 28th days at the recommended rate. The experiment was conducted at temperatures ranging from 22-25 oC. Plants were harvested on the 35th day after planting and percentage germination, diseased height of the plants and dry mass of roots and shoots were determined. Results indicated that the biological control agent (B. cereus1) was able to significantly reduce the damage done by the pathogens Rhizoctonia solani, Pythium ultimum and Fusarium solani in all trials. It was also confirmed that the application of the biological control agent during planting could reduce disease incidence. The biological control agent increased seed emergence rate and shoot length. All three fungicides significantly reduced the disease incidence caused by all pathogens. All fungicides treatments applied increased emergence rate and shoot length. Seedling diseases should be given too much attention, as they cause severe losses to many crops. There is a need for future research on the effectivity of B. cereus1 as relatively little work has been published on its antagonistic behaviour against seedling diseases. There are also few registered fungicides available for the control of these seedling diseases on cowpea, therefore research on these and other potential products is required as seedling diseases play a major role in reducing yield of many crops.Dissertation (MInstAgrar)--University of Pretoria, 2008.Microbiology and Plant Pathologyunrestricte

Distribution of Alternaria leaf blight of sunflowers caused by Alternaria alternata in South Africa

Alternaria leaf blight (ALB) has been shown in recent years to be one of the major potential disease threats of sunflower capable of causing yield losses in all major production areas. The aim of this study was to determine the causal agent, prevalence and geographical distribution of ALB in the major sunflower production areas of South Africa. Surveys were conducted during 2012/13, 2013/14 and 2014/15 growing seasons at commercial sunflower production fields and at commercial cultivar trials. In the three growing seasons, twenty-nine sunflower commercial production sites were surveyed for ALB disease severity. Furthermore, four cultivars (AGSUN8251, PHB65A25, SY4200 and PAN7049) were surveyed for ALB during cultivar trials in a total of 25 localities during the three growing seasons. The plants were surveyed between 90 to 120 days after planting and leaves showing ALB symptoms were collected. Alternaria alternata was identified as the primary disease-causing organism of ALB in all the fields. Wesselsbron consistently had the lowest ALB disease severity during the 2013/14 and 2014/15 growing seasons, whereas Potchefstroom had the highest disease severity in all three growing seasons. Pearson’s correlation coefficient was greatest for temperature (r=0.6 in 2012/13, r=0.71 in 2013/14 and r=0.84 in 2014/15) and disease severity in all the growing seasons. Information about the distribution of sunflower diseases is important and this survey demonstrated that A. alternata is widespread across sunflower production areas in South Africa and may result in potential yield losses

Dating the origins of the maize-adapted strain of maize streak virus, MSV-A

Maize streak virus (MSV), which causes maize streak disease (MSD), is one of the most serious biotic threats to African food security. Here, we use whole MSV genomes sampled over 30 years to estimate the dates of key evolutionary events in the 500 year association of MSV and maize. The substitution rates implied by our analyses agree closely with those estimated previously in controlled MSV evolution experiments, and we use them to infer the date when the maize-adapted strain, MSV-A, was generated by recombination between two grass-adapted MSV strains. Our results indicate that this recombination event occurred in the mid-1800s, similar to 20 years before the first credible reports of MSD in South Africa and centuries after the introduction of maize to the continent in the early 1500s. This suggests a causal link between MSV recombination and the emergence of MSV-A as a serious pathogen of maize

Reconstructing the history of maize streak virus strain : a dispersal to reveal diversification hot spots and its origin in Southern Africa

Maize streak virus strain A (MSV-A), the causal agent of maize streak disease, is today one of the most serious biotic threats to African food security. Determining where MSV-A originated and how it spread transcontinentally could yield valuable insights into its historical emergence as a crop pathogen. Similarly, determining where the major extant MSV-A lineages arose could identify geographical hot spots of MSV evolution. Here, we use model-based phylogeographic analyses of 353 fully sequenced MSV-A isolates to reconstruct a plausible history of MSV-A movements over the past 150 years. We show that since the probable emergence of MSV-A in southern Africa around 1863, the virus spread transcontinentally at an average rate of 32.5 km/year (95% highest probability density interval, 15.6 to 51.6 km/year). Using distinctive patterns of nucleotide variation caused by 20 unique intra-MSV-A recombination events, we tentatively classified the MSV-A isolates into 24 easily discernible lineages. Despite many of these lineages displaying distinct geographical distributions, it is apparent that almost all have emerged within the past 4 decades from either southern or east-central Africa. Collectively, our results suggest that regular analysis of MSV-A genomes within these diversification hot spots could be used to monitor the emergence of future MSV-A lineages that could affect maize cultivation in Africa