Ash Yellows in Minnesota

C.L. Ash and D.W. French

Ash yellows is a plant disease induced by a mycoplasmalike organism (MLO). It has all the traits of animal and human mycoplasmas except that it cannot be grown on artificial media. Consequently, testing for the presence of this organism is considerably more sophisticated and costly than most disease cultures.

 

Ash yellows has been confirmed on a number of ash trees in the Minneapolis/St. Paul and Rochester areas. Nationally, the disease has been reported in a roughly east-west band extending from Iowa to Ontario, southern Quebec and eastern Massachusetts. The southern edge of the band extends along west-central Missouri and east to southeastern New York State. The disease occurs in scattered localities adjacent to this band, including several areas in Minnesota.

Figure 2
Approximate distribution of ash yellows

White ash, Fraxinus americana, is the most susceptible; green ash, F. pennsylvanica, is intermediate in susceptibility; and black ash, F. nigra, is thought to be the least susceptible at this time. Green ash is the dominant ash species in landscape and natural areas in Minnesota.

 

Symptoms


The only reliable diagnostic field symptom is the presence of witchs’ brooms on the trunk and major limbs. Witchs’ brooms are a proliferation of small branches which originate at a single point; each of these branches sprouts at the nodes giving the shoots a bushy appearance (Figure 1). Leaves on brooms tend to be small, simple and chlorotic (pale green to yellow) (Figure 2). Witchs’ brooms are positive proof of ash yellows. However, healthy ash trees commonly form sprouts on the trunk called epicormic sprouts which might be mistaken for witchs’ brooms.

Figure 1
Figure 1. A proliferation of pale green to yellow shoots, called witches brooms, are common on the lower trunk during the last stages of decline.
 Figure 2
Figure 2. Simple, yellowed leaves characteristic of witches brooms on a young sapling in a natural stand.



Figure 3. A decline in growth after infection is evident by the closeness of the outer annual growth rings.
Figure 3

The first visible symptom of ash yellows to develop after infection is a permanent reduction in growth (Figure 3). After several years of minimal growth, leaves often appear in tufts at the ends of branches. Leaves may fail to attain normal size and appear chlorotic. Branch dieback occurs following dormant periods. Small roots collected from symptomatic trees can be tested for the presence of the ash yellows mycoplasma.

Poor growth or decline symptoms can also result from poor site conditions or other mechanical, environmental, chemical, insect or disease influences. It is important to determine the cause of the symptoms on each tree. For example, bushy growth is common on small branches along highways where salt spray often kills the tips of the branches.


Disease Cycle


At this time the mechanism by which ash yellows is spread in the field is unknown. Certain leafhoppers and the meadow spittlebug are highly suspect in northeastern United States.

Trees of all ages can become infected. Once the organism is in the plant, it inhabits and destroys the phloem tissues. Symptoms develop on young ash one year after infection, while an incubation period longer than one year is likely in large trees. In white ash, root damage precedes other symptoms, suggesting that top symptoms are at least partially due to root mortality.

Ash yellows is not known to be transmitted through seeds, but it can be transmitted by grafting.


Control


Proper maintenance (fertilization, irrigation and pest control) of infected trees is suggested, as chemicals will not eradicate the disease from infected trees.


Conclusions


Infected trees do not recover. However, trees intermediate in susceptibility may live for many years. Landscape management practices which reduce plant stress, principally watering, may extend the life of infected trees.

Until further information is available on the epidemiology of ash yellows, the use of ash should be reduced.

 

Cynthia L. Ash
Assistant Extension Plant Pathologist
 David W. French
Professor
Department of Plant Pathology

Source: University of Minnesota.