The Potato spindle tuber viroid ("PSTVd") was the first viroid to be identified. PSTVd is a small, single stranded circular RNA molecule closely related to the chrysanthemum stunt viroid. Present within the viroidal RNA is the Pospiviroid RY motif stem loop. The natural hosts are potatoes (Solanum tuberosum) and tomatoes (Solanum lycopersicum). All potatoes and tomatoes are susceptible to PSTVd and there is no form of natural resistance. Natural infections have also been seen in avocados and infections in other solanaceous crops have been induced in the laboratory.