The first study on alternative weed and cultivated hosts of Spongospora subterraneaf. sp. subterranea (Sss) in southern Africa identifies two species as ideal trap crops for rotation with potatoes. The study also introduces three new families to the known hosts of Sss in the region. 

A study by researchers in the Potato Pathology Programme at the University of Pretoria’s Forestry and Agricultural Biotechnology Institute identifies Avena strigosa and Glycine max as “ideal to be included as trap crops in rotations with potatoes”. Crop rotation is one of the management strategies used to manage powdery scab, a tuber disease caused by Sss, in potato growing regions around the world. Outcomes from this study appear in an article in the European Journal of Plant Pathology. Researchers in the programme conducted greenhouse trials to determine the host status of 14 weed species and 12 cultivated crop species commonly found in potato growing regions in southern Africa. The researchers used qPCR and light microscope examination to study the development of Sss in the roots of weeds and crops species grown in an inoculated growth medium. 

 

Spongospora subterraneaf. sp. subterranea is an obligate parasite that threatens the economic viability of potato production in southern Africa. It is the causal agent of powdery scab on potato tubers and galls on the root of potato plants. The pathogen has proven difficult to manage, partly because it is adaptable and can survive under different environmental conditions. The pathogen’s resting spores can remain in the soil for more than 50 years, even in the absence of a host. This contributes to the accumulation of inoculum in the soil and the contamination of fields. Crop rotation reduces the amount of inoculum in the soil and is one of the strategies used by potato producers to manage Sss. 

The researchers distinguish between “true hosts”, “trapping hosts” and “non-hosts” among the 26 species in their trial. They describe true hosts as those that “allow the development of sporosori in roots” and therefore “provide a means of survival for Sss over long periods of time”. Trapping hosts “are zoosporangial hosts only and thus prevent the completion of the life cycle of Sss, consequently leading to a reduction in the inoculum level in the soil.” Non-hosts displayed neither sporosori nor zoosporangia. 

 

“Trapping crops and non-hosts could be used in rotation with potatoes, as they may reduce or leave inoculum levels in the soil unchanged,” wrote the authors. 

 

Of the 16 species the researchers identified to be true hosts, 12 are weed species. Four cultivated crop species, namely Allium cepa (onion), Avena sativa (white oat), Triticum aestivum(wheat) and Zea mays(maize) are true hosts. Avena strigosa (black oat) and Glycine max (soybean) were categorised as trapping hosts. The authors describe A. strigosa and G. maxas “ideal to be included as trap crops in rotations with potatoes.” The researchers identified Brassica oleracea(cabbage), Daucus carota (carrot), Galinsoga parviflora (gallant soldier), Ipomoea plebia (Sabi morning glory), Raphanus raphanistrum (oilseed radish), Secale cereale (rye), Seteria verticillata (burgrass) and Sorghum bicolor (grazing sorghum) as non-hosts. 

 

The researchers added three families to the hosts of Sss, namely Oxalidaceae, Euphorbiaceae and Cappardaceae, bringing the number of host families to 26. Results from this study will contribute to the effective management of Sss in potato growing regions, wrote the authors. 

 

Professor Jacquie van der Waals leads the Potato Pathology Programme, whose research focus is the epidemiology, diagnosis and control of soil- and seed-borne diseases of potatoes. Diseases of interest include powdery scab, black scurf and stem canker caused by Rhizoctonia solanias well as blackleg and soft rot caused by Pectobacterium and Dickeya species respectively.

---