Potato Spindel Tuber Viroïd (PSTVd)
PSTVd = Potato Spindel Tuber Viroid
Causative agent
In contrast to the viruses, viroids are microorganisms made up of a "particle" (nucleic acid of RNA type) unprotected by an envelope or capsid coat.
Description of symptoms on crops
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Viroids disrupt the cell division and metabolism in the host plant for their reproduction and consequently they can have strong effects on the plant growth. The severity of the symptoms varies with the type of strain, the cultivar and the environment.
Potato plants severely infected by PSTVd are upright, stunted and much thinner than normal plants. Leaves are smaller, sometimes twisted or distorted and may be either dark green or pale yellow. Dwarfed plants may have an erect growth habit as the stems are often more divided and petioles may form very sharp angles where they join the stem. Milder PSTVd infections may be symptomless on foliage.
Symptoms can be more obvious on tubers. Affected tubers of some cultivars are small, narrow and elongated giving the tubers a spindle-shaped appearance. In other cultivars, tubers develop knobs and swelling. Eyes may be shallow, more prominent and conspicuous and tubers are often cracked.
Yield losses of up to 64% have been recorded in potato crops infected by PSTVd.
Certain PSTVd isolates can cause severe leaf deformations and dwarfing on tomato plants grown at temperatures above 25°C.
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Transmission
PSTV can be transmitted through contact and can therefore be spread during cultivation activities. It can also be transmitted by aphids, but to a lesser degree.
Detection
Due to the lack of capsid proteins, we cannot use serological techniques (E.L.I.S.A., in particular) suitable for detecting the virus S. The detection method makes use of molecular techniques or electrophoresis.
Control
PSTVd is a quarantine organism in most areas, e.g. the European Union, and is therefore subject to mandatory control measures.
The control of PSTVd basically consists of planting certified seed potatoes which are derived from seed tubers that have been tested and are free of PSTVd. Most certification schemes include strict controls on the introduction of plant material (general trade and genetic resources). As ornamentals may be symptomless reservoirs, it is also important to take care with the introduction of ornamental solanaceae coming from infected areas (South America, Asia, Central Europe, etc.) and to avoid movement of personnel, equipment or plant materials into potato fields from susceptible host plants (ornamentals or horticultural crops such as tomato).
In case of an outbreak of PSTVd, it is important to apply stringent hygiene procedures:
- wash and disinfect equipment, surfaces and tools before moving to another field or planting another seed lot;
- plant whole seed tubers rather than cut pieces;
- control insects, especially aphids and chewing species;
- carefully inspect, remove and destroy infected as well as suspicious plants;
- apply a strict and thorough viroid testing programme.
