Aspergillus tubingensis is a pre-emergent pathogen of date palm seedlings

Abstract

Many diseases of date palm are known. However, pathogens that might affect seed germination and seedling emergence from soil are poorly studied, perhaps because date palm cultivars are propagated vegetatively. Here, we first determined the effects of date seed fungi on the germination and emergence of 600 seeds overall (i.e., 200 of each of three cultivars: ‘Thoory’, ‘Halawi’, and ‘Barhi’). In each cultivar, 100 seeds were from Saudi Arabia (part of the native range), and 100 were from the southwestern USA (where the date palm was introduced around 1765). Just four fungal genera (i.e., Alternaria, Aspergillus, Chaetomium, and Penicillium) were isolated from the surface-sterilized date seeds. Aspergillus isolates all belonged to Aspergillus sect. Nigri; collectively they were in the highest relative abundance at 39%, and significantly more common in Saudi Arabian seeds than in American seeds. Aspergillus reduced seed germination and also reduced emergence when germinated and non-germinated seeds were planted in potting mix in a greenhouse. In contrast, Penicillium species were more common in American than in Saudi seeds; Penicillium did not affect germination, although it did have a positive effect on seedling emergence. In a second experiment with 17 seeds of the ‘Halawi’ cultivar, fungus-free seeds were either inoculated with isolates of Aspergillus sect. Nigri or not, and then planted. Controls emerged whereas Aspergillus-inoculated seeds did not. Finally, a third experiment was conducted with Aspergillus tubingensis Mosseray, a sect. Nigri member, as sole inoculum of 100 ‘Halawi’ seeds versus 100 uninoculated controls. Aspergillus tubingensis exerted the same pathogenic effects on germinating and emerging seedlings as the isolates identified only to Aspergillus sect. Nigri. Aspergillus tubingensis is thus a previously unreported, seedborne pathogen affecting date palm seedlings. Our findings also suggest that A. tubingensis may be more common in seeds in the host’s native range than in its introduced range.