Viroids are a nonliving pathogen that affects primarily plant cells. They are extremely small, much smaller than viruses. They also lack the protein coat most viruses seem to have, in which they house their viral DNA. Viroids only have about 220 nucleobases in their RNA, as opposed to the smallest of viruses, which have 2 kilobases in their DNA. The smallest vioid is the 220 nucleobase rice yellow mottle subemovins. Viroids were discovered in 1971 by Theodor Diener, in Maryland. The first identified viroid was a potato spindle tuber viroid, which afflicts the development of potatoes on the nodules of the potato plant roots. Viroid replication involves the use of RNA polymerase II. So, viroids seem to be the smallest of the pathogens you have listed, and target plants as their host receptors. They use RNA rather than DNA as their method of replication.