Phytoplasma Diseases (Yellows)
Phytoplasma spp. (Candidatus Phytoplasma asteris and others)
Overview
Phytoplasma diseases, often called yellows diseases, are caused by unculturable, wall-less bacteria that inhabit plant phloem. They affect hundreds of plant species, from ornamentals and vegetables to fruit trees and palms. Leafhoppers, planthoppers, and psyllids are the usual vectors. Infection is systemic and permanent in most hosts. Symptoms overlap among different phytoplasmas, so management centers on removing infected plants, reducing vectors, and eliminating weed reservoirs rather than spraying a curative chemical in the garden.
Symptoms
Typical signs include yellowing of leaves, stunting, witches-broom shoot proliferation, green flowers (virescence), leafy flowers (phyllody), and decline. Fruit set may fail. Palms can show lethal yellowing-type decline. Woody plants may have sparse canopies and dieback over seasons. Because phytoplasmas move in phloem, symptoms often appear first in new growth weeks after insect feeding. Multiple plants in a bed may show similar distortions when leafhopper flights are heavy.
Environmental Conditions
Variable
Prevention
Buy clean planting stock and quarantine unusual, yellowed transplants. Maintain habitat for predators of leafhoppers without letting weedy reservoirs dominate. Avoid continuous plantings of highly susceptible crops in the same beds year after year during outbreak seasons. Mow or manage wild hosts near production areas. Scout for witches-broom and green flowers so you can remove sources before the next vector generation.
Management
Roguing symptomatic plants early reduces the chance that vectors will acquire the pathogen and spread it. Control broadleaf weeds that host phytoplasmas and leafhoppers along fence lines and ditch banks. Use row covers on susceptible vegetables during peak vector periods. For high-value trees, consult specialists; antibiotic injections are sometimes used commercially but are temporary and not a home-garden routine. Replace chronically infected ornamentals with less susceptible species.