Phytoplasma Diseases and Their Relationships with Insect and Plant Hosts in Canadian Horticultural and Field Crops

Phytoplasmas are bacterial plant pathogens consisting of more than 50 phylogenetic groups that cause devastating diseases in various crops worldwide. They are obligate parasites restricted to the phloem tissue of the host plant and are transmitted from plant to plant mostly by leafhoppers (Hemiptera: Cicadellidae). They reproduce within the tissues of their insect vectors and are transferred in the salivary secretions to new host plants during feeding. Phytoplasma epidemiology involves a tritrophic relationship between the pathogen and usually several hosts and vectors. The host-plant range depends on the number of vectors, their feeding habits, and their dispersal pattern. Interactions between phytoplasmas and their vector hosts are complex and influenced by insects' vectoring abilities and the consequences of infection for vectors. In Canada, seven phytoplasma taxa have been detected in various crops. Aster yellows, the primary vector of which is the leafhopper Macrosteles quadrilineatus (Forbes), is the most common and widespread. X-disease, transmitted by at least eight leafhopper species, is economically damaging to all cultivated species of Prunus L. (Rosaceae). Clover proliferation, also transmitted by M. quadrilineatus, is the causal agent of important diseases such as clover proliferation and alfalfa witches’ broom. Ash yellows and pear decline have caused economic problems for several decades, while bois noir, a quarantinable disease in Canada, was detected in Ontario and British Columbia for the first time only recently. Because of their cryptic nature, phytoplasmas are difficult to manage; quarantine measures and insecticide sprays remain the most common control measures. However, integrated pest management techniques using beneficial insects, biotechnology, and plant resistance are emerging.