Potato spindle tuber viroid

Potato spindle tuber viroid is a Pospoviroid whose natural hosts are potatoes and tomatoes as well as other Solanum spp..

Symptoms
Mild strains generally cause no obvious symptoms in either potato or tomato, but can be detected experimentally in potato by cross-protection methods. Severe strains in sensitive cultivars cause the following symptoms:

Potato
In the field, a clockwise phyllotaxy of the foliage may be apparent when the plants are viewed from directly above. Foliage is spindly and very upright, often being a darker green than normal and slightly rugose. There may be an accumulation of pigment at the top of the stems, usually accompanied by an upward rolling of the terminal leaflets. Axillary buds may proliferate to give symptoms similar to those of witches' broom. Plants are stunted. Tubers are small, elongated, cylindrical, spindle or dumb-bell-shaped, with prominent eyes evenly distributed over the tuber. Sprouting is slower than in healthy tubers.

Tomato
Epinasty and rugosity of the apical leaves is followed by a necrosis of the leaflet midribs and a yellowing of leaflets in the central region of the plant. In the severe chronic stage, the entire plant is stunted, the apical leaves are small and compacted and the central leaves die.

Control
Control is essentially by production of healthy planting material and good crop sanitation. Cold treatment, followed by meristem-tip culture, can be used to eliminate PSTVd from potato stocks. Resistance is looked for in potato cultivars in infested countries.