PSTVd is a small, unencapsidated, covalently closed, circular RNA of circa 359 nucleotides. Variants consisting of 356-363 nucleotides have been described (Gross et al., 1978;Puchta et al., 1990;Lakshman and Tavantzis, 1993;Behjatnia et al., 1996;Verhoeven et al., 2010b). Electron micrographs reveal a rod-like conformation of 37+/-6 nm in length of the renatured state. In the denatured state, rod-like molecules as well as completely open circles are found (Riesner et al., 1979).
In crops like tomato and potato, symptoms may indicate the presence of a pospiviroid. After mechanical inoculation to potato cultivar Nicola all pospiviroids except Iresine viroid 1 (IrVd-1) evoked similar tuber symptoms although the intensity varied with viroid and with isolate. Pospiviroid infections in commercial tomato crops also incite symptoms independent of the viroid species (Verhoeven et al., 2004;EFSA Panel on Plant Health, 2011). However, mild strains may not evoke symptoms, and symptom development is affected by temperature and light (Diener, 1979;Harris and Browning, 1980). In addition, true seeds of potato (TPS) and tomato and plants for planting of ornamental species, the primary means of shipment, may not show symptoms. Therefore, diagnosis on the basis of symptoms alone is not acceptable for quarantine purposes. Laboratory tests are the most reliable method of detection (see Diagnosis). Kahn (1989) and Salazar (1989) give comprehensive reviews of plant protection measures and quarantine implications for viroids in general, and PSTVd in particular, respectively. Furthermore, an comprehensive list of management options for pospiviroids has been evaluated by the EFSA Panel on Plant Health (2011).
Related invasive species
- Potato spindle tuber viroid
Related Farm Practice
- Canning
- Development
- Conditioning
In potato, PSTVd can induce severe growth reduction;however, reduction may also be hardly visible. Vines of infected plants may be smaller, more upright, and produce smaller leaves than their healthy counterparts. Infected tubers may be small, elongated (from which the disease derives its name), misshapen and cracked. Their eyes may be more pronounced than normal and may be borne on knob-like protuberances that may even develop into small tubers. Symptom expression is influenced by the potato cultivar, strain of PSTVd, environmental conditions and method of inoculation (Pfannenstiel and Slack, 1980;Diener, 1987;Owens and Verhoeven, 2009).
The first symptoms of PSTVd infection in tomato are growth reduction and chlorosis in the top of the plant. Subsequently, this growth reduction may develop into stunting, and the chlorosis may become more severe, turning into reddening and/or purpling. In this stage, leaves may become brittle. Generally, this stunting is permanent;occasionally, however, plants may either die or partially recover. As stunting begins, flower and fruit initiation stop. Generally, the disease spreads along the rows (Mackie et al., 2002;Owens and Verhoeven, 2009).
Peppers display only very mild symptoms in response to PSTVd infection. The only visible symptom is a certain 'waviness' or distortion of the leaf margins near the top of infected plants (Lebas et al., 2005).
Infections of solanaceous ornamentals are usually symptomless (Verhoeven et al., 2008a, b, 2010b;Luigi et al., 2011).
Due to serious symptoms and large scale outbreaks, potato is considered the main host of PSTVd. However, many more hosts are known. The viroid also causes symptoms in tomato and pepper (Capsicum annuum) (Mackie et al., 2002;Lebas et al., 2005). In addition symptomless infections have been reported from avocado (Persea americana), Brugmansia spp., Chrysanthemum sp., Calibrachoa sp., Cestrum spp., Dahlia sp., Datura sp. Lycianthes rantonnei, Petunia sp., Physalis peruviana, Solanum pseudocapsicum, Streptosolen jamesonii, Solanum jasminoides, Solanum muricatum, sweet potato (Ipomoea batatas) and wild Solanum spp. (Salazar, 1989;Owens et al., 1992;Querci et al., 1995;Behjatnia et al., 1996;Di Serio, 2007;Verhoeven et al., 2008a, b, 2009, 2010b;Lemmetty et al., 2011;Luigi et al., 2011;Mertelik et al., 2010;Verhoeven, 2010;Tsushima et al., 2011). The experimental host range of PSTVd includes a wide range of Solanaceous species, as well as species from other families (Singh, 1973;Diener, 1979).