Concentration and distribution of mild and severe strains of potato spindle tuber viroid in cross-protected tomato plants

Analysis by return of polyacrylamide gel electrophoresis (R-PAGE) of a mild strain of potato spindle tuber viroid (MA-PSTV) and a severe strain (S-PSTV) showed that both strains replicated in the plant at a similar rate and could be distinguished from each other by different electrophoretic mobilities. In singly infected plants, both strains were detected 10 days after inoculation; in doubly infected plants they were detected 8 days after inoculation. MA-PSTV was detected in all the leaflets of tomato plants 14 days postinoculation, a duration often used before challenge inoculation. In MA-PSTV-protected plants, the challenge strain (S-PSTV) was first detected 21 days after inoculation, and its concentration increased with time. Symptoms of S-PSTV appeared 48 days after challenge inoculation. In unprotected plants, S-PSTV was detected 10 days and the symptoms appeared 21-28 days after inoculation. In the top leaves of the MA-PSTV-protected plants, S-PSTV totally replaced the protecting strain in the later stages of infection. Both strains multiplied in the middle and bottom leaves of MA-PSTV-protected plants. In S-PSTV-protected plants, MA-PSTV as a challenge strain was detected only in the later stages of infection but was present in top, middle, and bottom leaves of doubly infected plants. Both strains were found to be present in sepals, petals, anthers, and pistils in MA-PSTV-protected and S-PSTV-challenged plants. Fruit pulp contained only S-PSTV in MA-PSTV-p rotected and S-PSTV-challenged plants but contained both strains in S-PSTV-protected and MA-PSTV-challenged plants. Only low percentages of seeds were infected with MA-PSTV, and none were infected with S-PSTV