Causal agent(s) and transmission
Viroids are small organisms known to cause serious diseases in plants and consisting of a single-stranded particle of genetic material (i.e.ribonucleic acid RNA), that is not protected by coat protein as opposed to viral RNA.
Potato spindle tuber viroid is easily transmissible by contact and hence may be mechanically disseminated from plant to plant or with contaminated equipement, tools, skin or clothes during field operations. Cutting seed potatoes before planting increase the risk of disseminating the viroids. The transmission of PSTVd by chewing and sucking insects has been reported but it has been shown that in potato crops, aphids can spread the disease efficiently only if they are already carrying Potato leaf roll virus (PLRV).
Besides infected seed-tubers, PSTVd is also transmitted by the pollen of potato and tomato flowers. True botanical potato seeds produced by an infected plant may be up to 100% infected with PSTVd.
PSTVd is a serious threat for host crops such as the potato and the tomato but natural infection by PSTVd has also been recorded in avocado, aubergines, pepinos, peppers and wild Solanum species. Natural infection of ornamentals such as Solanum jasminoides or weed hosts in the Solanum family (such as nightshades) has also been recorded. On potato, yield losses vary with the strain, the cultivar and the growth conditions but may reach 65 to 80% for severe PSTVd strains.
The only viroid known to infect naturally cultivated species of potato is the Pospiviroid Potato spindle tuber viroid (PSTVd) but other viroid species in the genus Pospiviroid, e.g. Citrus exocortis viroid (CEVd), Columnea latent viroid (CLVd), Chrysanthemum stunt viroid (CSVd) and Tomato chlorotic dwarf viroid (TCDVd) may infect tomato and/or ornementals naturally. Experimentally, they have been shown to infect cultivated potato species. Such experimental infection with other pospiviroids may result in severe symptoms with strong tuber distorsion and elongation, similar to PSTVd.
Symptoms on foliage
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 (photos 2 and 3). 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.
Certain PSTVd isolates can cause severe leaf deformations and dwarfing on tomato plants grown at temperatures above 25°C (photo 4).
Symptoms on tubers
Symptoms can be more obvious on tubers. Affected tubers of some cultivars are small, narrow and elongated giving the tubers a spindle-shaped appearance (photo 1). 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.
In a healthy area, an outbreak of PSTVd or other viroids results from the introduction of infected host plants, including ornamentals, coming from infected areas and introduced for plant reproduction or as genetic resources without an efficient certification/quarantine scheme.
Mechanical contact from plant to plant, with operators or equipment, can easily disseminate PSTVd either in the field with farm equipment – such as sprayers or tractors – passing through the crop but also in greenhouses or during micropropagation process.
PSTVd is a quarantine organism in most areas, e.g. the European Union, and is therefore subject to mandatory control measures.
Due to the lack of capsid proteins, serological techniques otherwise suitable for virus detection (Elisa in particular) are not suitable for the detection of viroids. Current detection methods rely on molecular techniques (hybridization, RT-PCR, etc.), R-PAGE electrophoresis or indexing.
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.