ملخص
Summary This study aimed to identify Potato Spindle Tuber Viroid (PSTVd) on potato crop in Libya. Five samples were collected from farms located in Al-Maamoura and Sabratha areas during autumn 2005. The apparent symptoms of infection were reduced leaf size and curling downside, while tubers were showing cracks and they were spindle shaped with sprouts surrounded by swollen tissue like protruding eyelids. The causative agent of these symptoms was identified as Potato Spindle Tuber Viroid (PSTVd) by applying molecular hybridization of nucleic acid and polymerase chain reaction (PCR) assays. The Libyan isolate of PSTVd was similar to Egyptian and American isolates because its nucleic acid contained 360 bp like them, and its band was appeared at the same level as the other two isolates in electrophoresis upon agar slices. The effect of Egyptian isolate of PSTVd on growth and production was studied on 5 common tomato varieties in Libya that grown under plastic greenhouse conditions. This isolate showed obvious effect on fruit productivity and plant growth. The reduction in fruit production in infected varieties (No.185, Jasmine, Lebda, Super Haleem and Kenza) reached 46,42,31,29, and 24%, respectively. This isolate affected also on the growth of these varieties, where the reduction in fresh weight of vegetative part reached percentages of 55,32,36,45 and 37%; and in dry weight of vegetative part 30,33,66,38 and 20, respectively. A reduction was noticed also in fresh weight of root part, which reached 60,20,50,25 and 20%; and in dry weight of root part 60,20,46,40 and 20%, respectively. In addition, a reduction in height of infected varieties was observed also, which reached 31,14,17,15 and 10%, respectively. The Libyan isolate was assayed biologically by infecting a Viroid-susceptible plant (Scopolia sinensis) , which showed symptoms of dwarfism after one month of infection , in contrast to what was known previously about its production of local spots only. Another experiment was carried out to study the susceptibility of number of common tomato varieties widely grown in Libya, for infection with Libyan and Egyptian isolates of PSTVd, and to discover the resistant varieties through inspection of apparent manifestations. Seven varieties (Fulcato, Thuraya, Hanaa, Lebda, Sankara, Kenza and Jasmine) were infected with the Libyan isolate, while 10 varieties (Thuraya, Fulcato, Lebda, Huda, Farwa, Nazeha, Zahra, Al-Karaz, Reem star and Kartika) were infected with the Egyptian isolate. Distinct symptoms were appeared on all the varieties infected with the Libyan and Egyptian isolates. The symptoms varied from curling , deformation , yellowing and existing of dark brown , yellow and white on foliate, except two varieties (Reem star and Kartika)that were infected with the Egyptian isolate which sowed no manifestations of infection.