Yellows, or Phytoplasmas
Phytoplasmas, formerly called mycoplasmas, are minute, single-cell organisms that resemble bacteria. Phytoplasma diseases are often called yellows because chlorotic foliage is a common damage symptom.
Yellows of ash, elm, and lilacs and lethal yellowing of palm are important phytoplasma diseases that so far have been reported only in the eastern United States. Aster yellows complex is the most important phytoplasma infecting herbaceous ornamentals in California, but not woody plants.
Phytoplasmas commonly cause distorted, dwarfed, and yellowish leaves and shoots. Other symptoms include abnormal flower and leaf development, shortened internodes, and shoot proliferation (witches’ broom). The flowers of infected plants sometimes develop green, leaflike structures, called phyllody.
Phytoplasma diseases in California include peach yellow leafroll of almonds, nectarines, and peaches; pear decline of pear; and cherry buckskin, or X-disease of cherry. Phytoplasma infection gives fruit a bitter flavor and abnormal appearance and causes trees to decline.
The symptoms of phytoplasmas can be confused with those caused by viruses. Some phytoplasmas cause severe decline and eventually death of the plant. Many phytoplasmas are poorly known, and their importance as pests may be underestimated.
Phytoplasmas are often spread during feeding by certain species of leafhoppers. Pear decline is vectored by a psyllid, the pear psylla. The insects acquire phytoplasma during their sucking feeding on infected hosts, then spread the microorganism when they move to feed on other host plants.
No chemicals are effective against phytoplasmas. Control these pathogens primarily through proper sanitation, excluding and controlling insect vectors, and using only pathogen-free stock.
Remove infected plants that are a source of pathogens, including certain weeds. For example, burclover (Medicago polymorpha), clovers (Melilotus and Trifolium spp.), and dandelion (Taraxacum officinale) host the X-disease phytoplasma spread by certain leafhoppers.
Scion and rootstock cultivars often vary significantly in their susceptibility to phytoplasmas. Seek information on resistant cultivars and plant them where phytoplasmas are a problem.
Leaf yellowed by phytoplasma (right)
X-disease affected cherries (right)