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Ash yellows: morphological and histological symptoms. This description applies primarily to Fraxinus americana, which is relatively intolerant of infection. The information is distilled primarily from Hibben and Wolanski (1971), Matteoni and Sinclair (1985), Dyer and Sinclair (1991), Sinclair and Griffiths (1994) and Walla (1998). Symptoms in other hosts are less severe or less well known. Witches'-brooms, although not consistently present, are diagnostic for ash yellows in all hosts. They most often develop on declining and moribund trees, regardless of the cause of decline. Brooms may also develop at wounds and on stumps. Other symptoms are only indicative. Dieback is strongly associated with phytoplasma infection in F. americana but not in F. pennsylvanica or F. velutina, which are the only other species observed extensively. Cryptic infection is common, especially in tolerant species.
Seedlings and saplings have the most dramatic symptoms, although diseased seedlings are not often noticed. Stunted growth of both roots and stems begins abruptly. Roots up to 3-4 mm in diameter may die. The inner phloem and cambial region of surviving roots sometimes appears yellowish. (Root necrosis and phloem discolouration have been noted only in F. americana and apparently do not occur in F. pennsylvanica or F. velutina.) Shoots and leaves may be undersized. Dwarf shoots or witches'-brooms may form at the root collar and witches'-brooms may form on the main stems of saplings and uncommonly on trunks of larger trees. Brooms sometimes originate below the soil surface. Diseased seedlings and saplings sometimes wilt and die. Saplings often develop deliquescent branching and abnormally large malformed leaves on shaded low branches. Saplings that become suppressed by other vegetation die after a few years.
Symptoms in larger F. americana vary according to the degree of tolerance of the trees and the presence of other stressing factors, notably water shortage. Some infected trees have normal appearance and moderate growth rate, but the majority have permanently suppressed apical and radial growth. Trees undergoing progressive decline display slow twig growth and short internodes, which causes the foliage to appear bunched at twig tips and increases crown transparency. Lateral branches may lose apical dominance and assume the deliquescent habit in which no leader is apparent among several slowly growing twigs near the branch end. If sprouts develop along the main stem, these also often become deliquescent. Witches'-brooms develop with further loss of growth regulation. Branch dieback may occur, usually during dormancy. Root ends may also die. Roots produced by diseased white ash may be abnormally short and tapered, resulting in the aggregation of feeder roots in clusters.
Foliar colour of diseased white ash varies from normal dark green to light green in trees larger than saplings. Some saplings become chlorotic. Leaves of slowly growing trees are usually undersized and the edges of leaflets are often curled upward. Severely diseased plants may produce epicormic sprouts low on the bole, with simple leaves or dwarfed compound leaves with fewer than the normal five to nine leaflets.
Witches'-brooms form most often on trees with severe dieback, usually at the root collar but sometimes on the trunk. The brooms, usually <30 cm tall, consist of clusters of spindly upright twigs, many of which arise in leaf axils while leaves are present. Diffuse interveinal chlorosis, more pronounced toward leaf margins than near midveins, is common in leaves on brooms. Many twigs in brooms die back during winter. Brooms on relatively vigorous trees are rare but may grow for decades and attain heights >2 m. Inconspicuous, short-lived, dwarf shoots 1-10 cm tall, leafless or with tiny simple, chlorotic leaves sometimes form at ground level on small dying trees in woodlots.
Cambial damage by frost is more common in phytoplasma-infected than healthy F. americana. The damage is evident as longitudinal cracks in bark, not extending into the xylem, and sometimes as cankers near ground level.
Histological symptoms, studied mainly in F. americana roots, include mild hyperplasia, reduced starch accumulation, premature collapse of sieve tubes and companion cells, and the development of thick lignified walls in phloem parenchyma adjacent to affected sieve tubes. The altered parenchyma cells sometimes form a tissue called pathological sclerenchyma, located between conductive phloem and the inner band of phloem fibres. Many sieve tubes colonized by phytoplasmas accumulate substances in their walls that fluoresce bluish-white to white, or pinkish to tan with age, under UV illumination. Autofluorescent sieve tubes and pathological sclerenchyma have also been noted in F. pennsylvanica roots. These symptoms persist after phytoplasmas can no longer be detected microscopically in old phloem. Autofluorescent sieve tubes also occur in twigs.
Symptoms of this disease, which is also caused by 'Ca. Phytoplasma fraxini', are highly similar to those of ash yellows. In brief, the lilac disease is characterized by slow apical and radial growth, diminished apical dominance, precocious flowering and/or shoot growth, witches'-brooms and dieback. Undersized and deformed leaves are common, and chlorosis occurs in occasional plants. Plants intolerant of infection die prematurely. Diseased plants are abnormally sensitive to freezing. Syringa species and cultivars vary widely in tolerance of infection. S. vulgaris, being rather tolerant, seldom has severe symptoms. S. x josiflexa, S. josikaea, S. x prestoniae, S. sweginzowii, and hybrids involving these species are intolerant and often undergo decline (Hibben et al., 1986; Hibben and Franzen, 1987, 1989).
Ash yellows phytoplasmas induce partial stomatal closure and elevated diffusive resistance in leaves of F. americana. Stomatal closure is in turn associated with abnormally high water potential (Matteoni and Sinclair, 1983).
In the experimental host Catharanthus roseus, ash yellows phytoplasmas induce symptoms in leaves that become colonized while developing. Leaves that are full grown before phytoplasmas enter them do not show morphological or physiological abnormalities until they senesce. Photosynthesis is reduced in diseased leaves, due to reductions in the carboxylation rate and noncyclic electron transport. Stomatal conductance is reduced, but this is not responsible for reduced photosynthesis (Tan and Whitlow, 2001). Ash yellows phytoplasma infection in C. roseus also causes an increase in soluble carbohydrate and starch in source leaves and a decrease in sugars and an increase in starch in sink leaves. Amino acid concentrations are not significantly altered by infection (Lepka et al., 1999).
No regulations that deal specifically with ash yellows are in effect.
Cultural Control and Sanitary Methods
The impact of ash yellows in Fraxinus americana in woodlots and forest stands where the disease occurs can be reduced by gradually replacing this species with others. Merchantable white ash that are growing slowly or showing dieback may be removed during partial harvests. This measured approach is possible because most large diseased trees live for at least 5-10 years. Many diseased individuals without severe dieback and with crowns exposed to the sky continue to grow at moderate rates for many years. Normal-appearing ash in yellows-affected stands may be left for future harvests and as seed sources (Smallidge et al., 1991b; WA Sinclair, Dept. Plant Pathology, Cornell University, Ithaca, NY 14850-5904, USA, unpublished data). Green ash, being moderately tolerant of infection, does not need to be managed to mitigate ash yellows impact. Experience with other species is insufficient to justify recommendations.
Many ash shade trees affected by ash yellows remain capable of responding to favourable environmental stimuli. Therefore, ash trees suspected to have yellows disease, but which do not show severe dieback, may benefit from fertilization and from watering during dry weather. Special care for ash with severe dieback is probably a bad investment. Curative treatments are not available. Ash yellows management prescriptions are not needed for species that tolerate the disease without undergoing decline.
Control methods for lilac witches'-broom, other than sanitation, have not been suggested, and the efficacy of rigorous sanitation is unproven.
The impact of ash yellows in ash shade trees can be lessened by using tolerant species, tolerant cultivars and tolerant rootstocks. High-level tolerance has been demonstrated in Fraxinus velutina and moderate tolerance in F. pennsylvanica (Sinclair et al., 1993, 1997a). The widespread use of F. pennsylvanica seedlings as rootstocks for ash cultivars has been fortuitous in this regard. Tolerance in F. pennsylvanica is heritable (Sinclair et al., 1997b). Ash yellows tolerance in several Fraxinus cultivars has been demonstrated (Sinclair et al., 2000).
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
The main impact of ash yellows in forests and woodlots is growth loss. The effect of the disease on radial growth of Fraxinus americana and F. pennsylvanica was measured in two stands of each species in New York State. Growth trends of trees that became diseased and those that remained healthy diverged when the stands were 12-21 year old, and the divergence increased progressively. In the fifth year after divergence, the average growth rate of diseased green ash was 70% of that of healthy trees. For white ash, the corresponding average was 61% (Sinclair et al., 1993). In contract, however, survey observations of yellows-affected F. pennsylvanica in the Midwest and Great Plains regions did not reveal significant growth impact of ash yellows (Luley et al., 1992; Walla et al., 2000). Velvet ash, assessed in southern Utah, did not sustain yellows-associated growth reduction (Sinclair et al., 1993). Economic impacts of ash yellows have not been estimated.