| Potato spindle tuber viroid | |
|---|---|
| Scientific classification | |
| (unranked): | Subviral agents |
| (unranked): | Viroid |
| Family: | Pospiviroidae |
| Genus: | Pospiviroid |
| Species: | PSTVd |
| Species | |
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Potato spindle tuber viroid |
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The Potato spindle tuber viroid ("PSTVd") was the first viroid to be identified. PSTVd is a small, circular RNA molecule closely related to the Chrysanthemum stunt viroid. Present within the viroidal RNA is the Pospiviroid RY motif stem loop. The natural hosts are potatoes (Solanum tuberosum) and tomatoes (Lycopersicon esculentum). All potatoes and tomatoes are susceptible to PSTVd and there is no form of natural resistance. Natural infections have also been seen in avocados and infections in other solanaceous crops have been induced in the laboratory.
Different strains of PSTVd exist and symptoms range from mild to severe. Mild strains produce no obvious symptoms. Symptoms in severe strains are dependent on environmental conditions and are most severe in hot conditions. Symptoms may be mild in initial infections but become progressively worse in the following generations. Common symptoms of severe infections include colour changes in the foliage, smaller leaves and spindle like elongation. Sprouting also occurs at a slower rate than in unaffected potatoes. Infected tomatoes are slower to show symptoms which include stunted growth with a ‘bunchy top’ caused by shortened internodes. Leaves become yellow or purple and often become curled and twisted. Necrosis eventually occurs in the veins of the bottom and middle leaves and the top leaves don’t die but are reduced in size. Fruit ripening is also affected leading to hard, small, dark green tomatoes.
Long distance spread of PSTVd usually occurs via infected seeds but transmission via aphids (Myzus persicae) also occurs but only in the presence of PLRV (potato leaf roll virus). Mechanical transmission also occurs once it has been introduced to an area.
The PSTVd viroid has 359 nucleotides.
| Primary Structure |
1 CGGAACUAAA CUCGUGGUUC CUGUGGUUCA CACCUGACCU CCUGAGCAGA AAAGAAAAAA 61 GAAGGCGGCU CGGAGGAGCG CUUCAGGGAU CCCCGGGGAA ACCUGGAGCG AACUGGCAAA 121 AAAGGACGGU GGGGAGUGCC CAGCGGCCGA CAGGAGUAAU UCCCGCCGAA ACAGGGUUUU 181 CACCCUUCCU UUCUUCGGGU GUCCUUCCUC GCGCCCGCAG GACCACCCCU CGCCCCCUUU 241 GCGCUGUCGC UUCGGCUACU ACCCGGUGGA AACAACUGAA GCUCCCGAGA ACCGCUUUUU 301 CUCUAUCUUA CUUGCUUCGG GGCGAGGGUG UUUAGCCCUU GGAACCGCAG UUGGUUCCU |
| Secondary Structure |
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The nucleotides which are highlighted are found in most other viroids.
The potato spindle tuber disease occurs in the United States, Canada, Russia and South Africa. It causes quite severe losses and in some regions, it is one of the most destructive diseases of potatoes (solanum tuberosum). The Potato spindle tuber viroid (PSTVd) was the first viroid to be identified. Natural infections have also been seen in avocados and other solanaceous crops. It attacks all varieties, spreads rapidly, and often occurs in combination with virus diseases. It also attacks tomato but seems to be of little economic importance in that crop. So far PSTVd has not been detected in New Zealand potato crops, but it is possible that infection may occur if growers do not take care. With this in mind this broadsheet describes how they spread and can be controlled and damage and symptoms they cause in potatoes.
Showing stiff upright growth habit
Showing spindle shaped tubers (right)
As reported by George N. Agrios infected potato plants appear erect, spindly, and dwarfed. The leaves are small and erect and the leaflets are darker green and sometimes show rolling and twisting in figure 1. Once potato plants are mature, symptoms can be more obvious on tubers as shown in (figure 2). Affected tubers can be small, narrow or spindle-like and may develop knobs and swellings. In some varieties, eyes are numerous, shallow and more prominent and tubers are often cracked as shown in (figure 3). Yield losses of up to 64% have been recorded in potato crops infected by PSTVd.
The pathogen Potato Spindle Tuber Viroid(PSTV) is the first recognized viroid. PSTV is infectious RNA of low molecular weight, approximately 100,000 Daltons. The RNA is single stranded molecule of 359 nucleotides with extensive regions of base pairing. Under the electron microscope, purified but denatured PSTV appears as short strands about 50 nm long and thickness of double stranded DNA.
PSTV is mechanically transmissible and is spread primarily by knives used to cut healthy and infected potato “seed” tubers and during handling and planting of the crop. PSTV seems to also be transmitted by pollen and seed and by several insects including some aphids, grasshoppers, flea beetles and true bugs. The transmission by insects is apparently nonspecific and incidental, that is on contaminated mouth parts and feet of insects visiting the plants.
Potato spindle tuber can be controlled effectively by planting PSTV-free potato tubers in fields free of diseased tubers that may have survived from the previous year’s crop. Plant whole seed tubers rather than cut pieces. Avoid contact with leaves (either by staff or equipment) during field operations. Control insects, especially aphids and chewing species. Carefully remove the infected plants from the healthier plants, bag them and burn plants that are suspected of being infected well away from production areas. Monitor the crop carefully.
Apply hygiene procedures if there has been an outbreak of PSTVd. Wash equipment (knives, cutters, sprayers, cultivators) with sanitizers before moving to another field or sowing another seed-line. Viroids are resistant to heat sterilization so sometimes freezing or chemical treatments are more effective. So equipment, surfaces and tools can be disinfected with sodium or calcium hypochlorite (bleach) and 2% sodium hydroxide. Chemicals such as Virkon and quaternary compounds such as Dermasan can be used.