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

Abstract

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.