Potato Spindle Tuber Viroid - Pathogen and Plant Damage

Pathogen and Plant Damage

As reported by George N. Agrios infected potato plants appear erect, spindly, and dwarfed. The leaves are small and erect and the leaflets are darker green and sometimes show rolling and twisting in figure 1. Once potato plants are mature, symptoms can be more obvious on tubers as shown in (figure 2). Affected tubers can be small, narrow or spindle-like and may develop knobs and swellings. In some varieties, eyes are numerous, shallow and more prominent and tubers are often cracked as shown in (figure 3). Yield losses of up to 64% have been recorded in potato crops infected by PSTVd.

The pathogen Potato Spindle Tuber Viroid(PSTV) is the first recognized viroid. PSTV is infectious RNA of low molecular weight, approximately 100,000 Daltons. The RNA is single stranded molecule of 359 nucleotides with extensive regions of base pairing. Under the electron microscope, purified but denatured PSTV appears as short strands about 50 nm long and thickness of double stranded DNA.

PSTV is mechanically transmissible and is spread primarily by knives used to cut healthy and infected potato “seed” tubers and during handling and planting of the crop. PSTV seems to also be transmitted by pollen and seed and by several insects including some aphids, grasshoppers, flea beetles and true bugs. The transmission by insects is apparently nonspecific and incidental, that is on contaminated mouth parts and feet of insects visiting the plants.