Alternative hosts of Spongospora subterranea f. sp. subterranea in southern Africa

Spongospora subterranea f. sp. subterranea (Sss) is an obligate parasite that can only multiply within the living tissues of a host plant. Greenhouse experiments were carried out to determine the host status of Sss in some crops and weeds commonly found in southern African fields. These experiments used both qPCR and light microscope examination to study the development of Sss in the roots of weeds and crops grown in inoculated Merz solution. Zoosporangia and sporosori were observed microscopically in weed species Acanthospermum hispidum, Ageratum conyzoides, Bidens pilosa, Cleome monophylla, Datura stramonium, Euphorbia heterophylla, Oxalis latifolia, Physalis angulata, Setaria pumila and Tagetes minuta, whilst Ipomea plebia, Setaria verticillata and Galinsoga parviflora were non-hosts of Sss. Root galls were only observed on D. stramonium roots. The crops Allium cepa, Solanum lycopersicum, Zea mays, Avena sativa and Triticum aestivum were both zoosporangial and sporosorial hosts, whilst Phaseolus vulgaris and Brassica juncea were sporosorial hosts only. This study has added three new families to the hosts of Sss, namely Oxalidaceae, Euphorbiaceae and Cappardaceae. Two zoosporangial hosts were identified, namely Avena strigosa and Glycine max. These crops would be ideal to be included as trap crops in rotations with potatoes. The highest quantity of Sss DNA in weed species was recorded in the roots of P. angulata, N. physalodes and A. conyzoides, whilst in cultivated crops the highest quantity of Sss DNA was recorded in Brassica juncea followed by Avena strigosa. The study has shown the importance of weeds and some crops used in rotation with potatoes as alternative hosts of Sss and their potential to increase Sss inoculum level in the soil.