... Description of Onion Yellows phytoplasma . ... Genus Candidatus Phytoplasma . Species Onion yellows . ... Phytoplasmas are a group of microscopic organisms that cause over 700 diseases in plants. Phytoplasmas grow and multiply within host plants and insect vectors. ... Onion yellows phytoplasma causes a yellowing disease in onions . ... Onion yellows phytoplasma is a parasite that lives inside leafhopper insects and spreads to plants through the saliva of feeding insects. ...