This is about systemic infections of phloem caused by prokaryotic microorganisms called mollicutes. Two broad groups of mollicutes that infect plants are known: those with helical, motile cells, called spiroplasmas, and those with variable shape called mycoplasma like organisms (MLOs). Some spiroplasmas can be grown in laboratory media, but the requirements of unicellular, pleomorphic microorganisms that have the general characteristics of bacteria but lack cell walls. In shape they range from spheres to branched filaments, and they appear to reproduce by fission. Details of their size, form, and structure can be seen only with the aid of an electron microscope. In strict usage, the term mycoplasma is reserved for certain mollicutes that can be cultivated and characterized apart from plant or animal hosts. Unicellular plant-infecting organisms that look like mycoplasmas but are not yet well characterized are called MLOs. Plant-pathogenic MLOs are obligate parasites that invade only phloem sieve cells. Sieve cells are organized into tubes that serve as conduits for translocation of photosynthetic products and growth regulators. MLOs multiply in sieve tubes and are translocated throughout the plant.

Elm yellows is a systemic, lethal disease formerly called phloem necrosis. It occurs in the eastern half of the United States and in southern Ontario, erupting in localized epidemics. It is caused by an unnamed MLO that is transmitted by leafhoppers. Highly susceptible plants die rapidly. Tolerant ones become stunted and may develop chlorosis and witches'-brooms. Diseased trees never recover. Natural infections are known only in five North American elm species, American or white, elm (Ulmus americana); cedar elm (U. crassiflolia; red, or slippery, elm (U. rubra); September elm (U. serotina); and winged elm (U. alata). Elm species of European or Asiatic origin, and hybrids between these and native species tolerant or immune. Such trees have survived in localities where epidemics killed all large native elms.

Symptoms usually develop in mid to late summer. External symptoms include yellowing, epinasty (drooping or downward bending of the petioles of turgid leaves), and premature casting of leaves, then death of branches. The sequence takes only a few weeks. All branches usually show symptoms at once, but occasionally in American elm and commonly in southern species, yellowing develops first in one branch system of a tree and spreads to other parts during 2 or more seasons. Bright yellow leaves may be interspersed with green ones on a single branch but often all leaves become yellowish green, then yellow. When these symptoms arise in late summer or early autumn, only the timing distinguishes them from normal leaf senescence. Diseased trees that survive a period of dormancy may open buds at the normal time but may grow only enough to produce chlorotic, dwarfed shoots and leaves that soon wilt or turn yellow and drop. Trees sometimes wilt and die quickly without prior external symptoms. Shriveled brown leaves may adhere to such trees for several weeks.

By the time foliar symptoms appear, root mortality and the degeneration of phloem in the roots and base of the tree are extensive. Fine roots die first, then successivley larger ones succumb. Within roots and basal parts of trunks and low branches, the innermost bark and cambial zone change color from nearly white to yellow, the cambial zone change color from nearly white to yellow, then butterscotch or tan, sometimes with darker flecks, and finally dark brown. Then other tissues die. The surface of the wood may also be discolored light butterscotch where pigment diffuses from degenerating phloem. On exposure to air, the inner phloem and cambial region turn brown much more rapidly in diseased elms than in healthy ones.

Diseased, living phloem of American, cedar, September, and winged elms produces methyl salicylate (oil of wintergreen). This chemical occurs in elms only as a symptom of yellows. It can be detected by sniffing at the surface of freshly exposed inner bark or at the mouth of a vial enclosing a sample of inner bark. Diseased red elms produce a characteristic aroma somewhat like that of maple syrup. The odor emanates from newly killed bark and leaves.

Red elms usually show symptoms in 2 seasons before death. Witches'-brooms form during the final season. The brooms, which begin as stunted, branching shoots, are ordinarily a few centimeters long but sometimes grow much larger. Red x Siberian elm hybrids and Chinese elm also produce brooms but remain alive. The response of Chinese elm is known from experiments in which the yellows agent was transmitted by grafting bark patches from diseased red elm.

Populations of the elm yellows agent become most dense in the petioles and stems of brooms. The MLOs are easier to find in tolerant species than in those that are rapidly killed. They occur both in living discolored phloem and in apparently normal phloem of diseased trees.

Water movement in xylem and translocation in phloem diminish before any visible symptoms of yellows develop. Stomata remain partially or completely closed, beginning some weeks before discolored phloem can be found. How the MLOs cause these malfunctions and kill elm tissues is unknown.

One vector of the elm yellows agent is the white-banded elm leafhopper, Scaphoideus luteolus. Adults of this species are wide-spread in some areas from early summer until frost in autumn. Additional vectors are probably, moreover, because epiderics of elm yellows sometimes develop and persist where S. luteolus is scarce. Two possible vectors are the meadow spittlebug and a leafhopper, Allygus atomarius, each of which after capture in the vicinity of naturally diseased trees transmitted yellows to test seedlings.

The yellows agent overwinters in its plant hosts: in roots of American elm and perhaps also in brooms on hosts that produce them. Vectors can acquire the pathogen from elm phloem beginning after the first flush of shoot growth. Few, if any, trees develop symptoms inoculation. The incubation period is at least 3 months in the year of inoculation. The incubation period is at least 3 months in very small trees and 9-10 months or more in large ones. Elm yellows spreads among closely spaced trees of the same species via root grafts.

Epidemics of elm yellows, although locally spectacular, do not spread rapidly. The edges of an area of outbreak in New York State observed for 15 years, advanced at rates up to 6 km per year. The disease can be endemic for many years between flareups in a given locale. Spot outbreaks and single-tree occurrences beyond the main range of the disease presumably develop after long-distance transport of vectors by wind.

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