Prepared by Laura Bostic
Class Project for PP 728, Soilborne Plant Pathogens, Fall 2012
Spongospora subterranea is the causal agent of powdery scab on potato. Though this disease can reduce yield somewhat, the true economic effect is due to the unmarketable, scabbed appearance (Figure 1) as well as rejection of tubers as seed to prevent infestation of clean fields. Though the blemishes can be cut away with no effect to the rest of the tuber, the appearance reduces the marketability as fresh product and processing centers often reject them on the basis of needing to remove more skin to remove the infected portions. Beyond the effects of powdery scab itself, infected tubers are also more susceptible to other diseases, such as secondary storage diseases or as a vector of the mop-top virus (1,2,3,4).
Figure 1: Scabbed appearance on potato (Photo: Melodie Putnam, Oregon State University Plant Clinic)
Host range and distribution
Spongospora subterranea can be found worldwide, wherever potatoes are consistently grown. Other than potato (Solanum tuberosum), few hosts have been reported. But it is assumed that solanaceous plants are susceptible including reports of tomato (Lycopersicon esculentum) and other Solanumsp. such as nightshade (S. demissum). Nasturtium sp. has also been reported as a host (1,2,3).
Spongospora subterranea is an obligate biotroph and therefore requires a living host and cannot be cultured in vitro. Detection in the soil and on tubers is based on the presence of sporosori, the resting spores of the pathogen. These spores can be detected through bioassays, PCR, and antibody methods. Though the pathogen cannot be grown in culture, spore suspensions can be made for inoculation purposes. Infected tubers can be peeled and the skin dried and ground, then passed through a 53 µm sieve. The spores can be quantified using a hemocytometer (2,5).
Spongospora subterranea is a protozoan-like organism of uncertain taxonomy in the family Plasmodiophoridae. It produces biflagellate zoospores which can result from zoosporangium or a resting spore produced in a spore ball or sporosori (Figure 2). Spore balls can be seen via a microscope in tissue where a lesion has already burst and has a white, powdery appearance (1,3).
Figure 2: Sporosori or spore balls of Spongospora subterranea (Photo: Sandra Jensen, Cornell University, Bugwood.org)
The most notable symptom associated with Spongospora subterranea is the blemish formed on the tuber. Initial infections begin with small, purple lesions, described as “pimple-like”. With time, the lesion will expand and eventually burst (Figure 3), exposing the white spore balls (Figure 4), lending the name “powdery scab”. Infected roots can also produce gall symptoms. With galling (Figure 5), reduced yield can also be observed, though this is not a major symptom (1,2,3,4).
Figure 3: Tuber exhibiting Powdery Scab (Spongospora subterranea) lesions (Photos: Sandra Jensen, Cornell University, Bugwood.org)
Figure 4: A burst lesion exhibiting powdery spore balls (Photo: Melodie Putnam, Oregon State University Plant Clinic)
Figure 5: Powdery scab galls on roots and stolons of potato caused by S. subterranea (Photo: W.T. Cobb, apsnet.org)
Ecology and life cycle
The details of all parts of the life cycle of Spongospora subterranea have not yet been fully proven, but the main aspects are known and assumptions based on other plasmodia have been applied to gain an understanding of the life cycle.
Starting with a biflagellate, uninucleate zoospore (n) there are two paths possible. The zoospore can directly infect the root, encyst, and form a uninucleate plasmodium which will multiply and develop into a multinucleate plasmodium, though all nuclei will remain identical (n). The plasmodium will develop into a thin-walled zoosporangium containing many new zoospores, all identical (n). Alternately, the zoospore (n) can fuse with another, differing zoospore (n) producing a zygote (2n). This zygote then infects the root, encysts, and creates a binucleate, single-celled plasmodium. The plasmodium will multiply and eventually undergo meiosis and form a spore ball or sporosori (2n), which contains the resting spores, each resting spore containing one zoospore with one nucleus.
Resting spores can persist in the soil for up to ten years. The cell walls contain three layers, aiding in the spore’s longevity. Zoospores swim through water films and therefore require free water in order to infect. After emergence, they swim to a host but only survive for about two hours. Infected seed tubers are a source of inoculum as well as infested soils where the organism persists for long periods of time. Symptomatic tubers will, therefore, not be accepted for seed (1,3).
The most effective method for disease control is to avoid the pathogen and use fields without infestations and prevent infestations through clean seed programs. Once the pathogen is present, resistant cultivars, pesticides, and cultural control may be necessary. Cultural control methods center around controlling the available water to prevent the zoospores from infecting; including irrigating appropriately, using appropriate soil types, and ensuring good drainage. Long rotations may reduce inoculum levels; however, due to the longevity of the sporosori in the soil, this may not be effective. Composting may also reduce inoculum, but since temperatures would need to be extremely high, some spores will still remain. Some chemical control has been shown with zinc oxide, fluazinam, mancozeb, and metam sodium (2,3,4).
Links to Other Sites
2. Fallooon, R. 2008. Control of powdery scab of potato: towards integrated disease management. Am. J. Pos. Res. 85:253-260.
3. Merz, U. 2008. Powder scab of potato—occurrence, life cycle, and epidemiology. Am. J. Pot. Res. 85:241-246.
5. van de Graaf, P., Lees, A., and Wale, S. 2005. Effect of soil inoculum level and environmental factors on potato powdery scab caused bySpongospora subterranea. Plant Pathology 51: 22-28.