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

Alternaria alternata, the causal agent of leaf blight of sunflower in South Africa

Sunflower (Helianthus annuus L.) is an important oilseed crop in South Africa, and is grown in rotation with maize in some parts of North West, Limpopo, Free State, Mpumalanga and Gauteng provinces. Alternaria leaf blight is currently one of the major potential disease threats of sunflower and is capable of causing yield losses in all production regions. Alternaria helianthi was reported as the main cause of Alternaria leaf blight of sunflower in South Africa; however small-spored Alternaria species have been consistently isolated from leaf blight symptoms during recent surveys. The aim of this study was to use morphological and molecular techniques to identify the causal agent(s) of Alternaria blight isolated from South African sunflower production areas. Alternaria helianthi was not recovered from any of the sunflower lesions or seeds, with only Alternaria alternata retrieved from the symptomatic tissue. Molecular identification based on a combined phylogenetic dataset using the partial internal transcribed spacer regions, RNA polymerase second largest subunit, glyceraldehyde-3-phosphate dehydrogenase, translation elongation factor and Alternaria allergen gene regions was done to support the morphological identification based on the three-dimensional sporulation patterns of Alternaria. Furthermore, this study aimed at evaluating the pathogenicity of the recovered Alternaria isolates and their potential as causal agents of Alternaria leaf blight of sunflower. Pathogenicity tests showed that all the Alternaria alternata isolates tested were capable of causing Alternaria leaf blight of sunflower as seen in the field. This is the first report of A. alternata causing leaf blight of sunflower in South Africa.http://link.springer.com/journal/106582019-07-01hj2018Forestry and Agricultural Biotechnology Institute (FABI)Plant Production and Soil Scienc

Pathogenic seed-borne, small-spored Alternaria spp. on sunflower (Helianthus annuus L)

Sunflower (Helianthus annuus L.) is the most important oilseed crop in South Africa at present and is grown in all summer rainfall areas. Sunflower seed oil is preferred over soybean and rapeseed oil because of its high quality and high poly-unsaturated fatty acids content that helps avoid the accumulation of cholesterol in the blood (Ward et al., 1985). A field trip was taken to sunflower fields in Greytown, Northern Kwa-Zulu Natal in early March 2010 during the warm and rainy summer season. Alternaria helianthicola Rao and Rajagopalan was consistently isolated from diseased plant material. Alternaria helianthicola has not previously been recorded on sunflower in South Africa. The pathogenicity of was A. helianthicola confirmed on sunflower plants using Koch’s postulates. Standard germination and seed health tests were conducted for thirteen sunflower seed lots from various sunflowers farms and companies of South Africa. Germination percentages ranged from 60 to 94% and germinated seedlings of the thirteen seed lots often showed seedling blight. Seed infection ranged from 18 to 98% caused by various small-spored Alternariaspecies. Seed infection did not severely influence seed germination and the Alternaria species may either cause a quiescent infection of the seeds or theAlternaria species may be mere saprobes and contaminants of the seed coats that do not cause disease. Seed component plating tests showed that the Alternaria species were more prevalent in the embryo and cotyledon than on the seed coats. Morphological characterization of these small-spored Alternaria species has been found to be unreliable due to the overlap in cultural characteristics between the various species. Molecular characterization using the rDNA ITS operon, β-tubulin gene and the EF-1α gene was done to support the morphological characterization. The rDNA ITS operon showed extensive length polymorphism among the Alternaria species that did not allow proper molecular identification of the isolates. The in vitro test showed that A. helianthicola had an optimum growth temperature of 25°C and maximum temperature of 35°C. Light was observed to promote hyphal growth increasing the radial growth rate of the fungus. In in vivo tests, approximately 12 hours of continuous high RH was required for infection to progress at optimal temperatures. Temperature had a significant effect on infection, with lesion development and enlargement observed to increase from 20 to 30°C, declining at 35°C.Dissertation (MSc)--University of Pretoria, 2013.Microbiology and Plant PathologyMScUnrestricte