Potato Leafroll Virus‐assisted Aphid Transmission of Potato Spindle Tuber Viroid to Potato Leafroll Virus‐resistant Potato

Potato leafroll virus (PLRV) and potato spindle tuber viroid (PSTVd) are important pathogens affecting the potato (Solanum tuberosum L.). PLRV is transmitted only by aphids, Myzus persicae being its principal vector. In contrast, PSTVd is transmitted by foliar contact and true seeds, but is not aphid‐transmissible. However, in doubly infected plants, PSTVd can be encapsidated by the capsid protein of PLRV and transmitted by M. persicae. In the present experiments, the aphids successfully transmitted PSTVd from potato plants infected with this viroid and PLRV to potato and tomato cv. Rutgers plants. The efficiency of PSTVd transmission depended on the potato cultivar used as inoculum source or as test plant, and ranged from 0 to 55%. Interestingly, PSTVd was the sole pathogen detected in some potato or tomato plants inoculated by aphids fed on the plants infected with the viroid and virus. Sole infections with PSTVd were found in as many as 23% of plants of the potato cv. Irga which is highly resistant to PLRV. This finding indicates that PLRV served as a carrier of PSTVd, but the plants escaped virus infection due to a high level of resistance. The response of potato plants to mixed infections with PSTVd and PLRV was strikingly more severe than those to infection with either pathogen alone. Furthermore, serious disturbances in sprout emergence were indicative of the double infection.