Potato spindle tuber viroid (PSTVd) causes serious disease in tomato (Lycopersicon esculentum), potato (Solanum tuberosum) and eggplant (Solanum melongena).
Natural infections have also been reported on pepino (Solanum muricatum), avocado (Persea americana) and sweet potato (Ipomoea batatas) plus a wide range of wild solanaceous hosts.
PSTVd was first reported as a disease in potatoes in North America and first detected on tomatoes in South Africa.
The disease is reported to occur in Asia, Africa, North America, South America, Europe and New Zealand. There have been previous detections and eradications of PSTVd in tomatoes and potatoes in Australia, including Western Australia.
There are both mild and severe strains. Symptoms may be confused with those of nutrient imbalance, spray damage, insect damage or other plant diseases such as true viruses. Symptoms become more pronounced in warm conditions and under high light intensity.
Foliage symptoms are often difficult to recognise and are rarely distinguishable before maturing.
With severe strains, stems remain upright and internodes are longer and more slender than normal. Leaflets are slightly decreased in size with fluted margins. Leaves near ground level are held in an upright position in contrast to healthy plant leaves that often rest on the ground.
Twisted leaflets and wrinkled leaf surfaces are also seen with severe strains.
Tubers are elongated, often with pointed ends, but are round in cross-section. Eyes are deep, more prominent and surface cracking occurs.
Tubers of some cultivars develop knobs and swellings and are severely misshapen. With some strains, foliage and tuber symptoms are mild versions of the above, or may not be visible.
The foliage and tuber symptoms become progressively more severe with each generation.
In mature tomato plants, infection with severe PSTVd strains causes purpling and yellowing of the leaflets, shortening of leaflet internodes resulting in a ‘bunchy top’ effect, leaflet down-curling and twisting and general plant stunting.
Spindly shoot growth can occur, flowers may abort and fruit can be dark green, fail to ripen normally and have thicker outer walls. With some strains, symptoms may be mild versions of those caused by severe strains or plants and fruits may be infected without showing symptoms.