Symptoms

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.

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.

Certain PSTVd isolates can cause severe leaf deformations and dwarfing on tomato plants grown at temperatures above 25°C (photo 4).