Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
6/7/99
Contacts:
Mark Gleason, Plant Pathology, (515) 294-0579, mgleason@iastate.edu
Elaine Edwards, Extension Communication Systems, (515)
294-5168, eedwards@iastate.edu
Yard and Garden column for the week beginning June 11
Yellows Diseases Go With the Phloem
Mark L. Gleason
Extension Plant Pathologist
Iowa State University Extension
It's no surprise that many people have never heard of yellows diseases. They were "discovered" by plant pathologists only about 30 years ago, but the symptoms they cause probably have been around since we were living in the trees.
Ash yellows and aster yellows, the most common yellows diseases in Iowa's urban landscapes, were once thought to be caused by viruses. It turned out that yellows diseases are instead caused by specialized bacteria called phytoplasmas. Unique offshoots of the bacterial kingdom, phytoplasmas lack cell walls and live in the phloem (food- and hormone-conducting passages) of plants.
Ash yellows attacks not only green and white ash, but also lilac. A common but subtle symptom is a slowdown in growth. Caliper growth of infected ashes can decrease by 50 percent or more. Lateral branches increasingly tend to lack a central leader; instead, the twigs tend to fork at the ends, a symptom called "deliquescent branching." In severe cases, ash yellows can cause branch dieback and even death of the entire tree.
Witches'-brooms are the only reliable diagnostic symptom of ash yellows, however. A witches'-broom is a cluster of spindly, short (usually less than 1 foot tall), bushy, upright twigs originating from a single point on a trunk or large limb &endash; where you don't usually find foliage on a healthy ash. Leaves on witches'-brooms often turn yellowish green, or chlorotic, by midsummer. Witches'-brooms also occur on infected lilacs. The unreliable part of this reliable indicator of ash yellows is that most infected ashes don't produce witches'-brooms. So in most cases, it takes a laboratory test of the roots or leaves -- traditionally by a staining technique, but more recently by a molecular method called PCR, or polymerase chain reaction -- to confirm the diagnosis.
How does ash yellows cause these symptoms? The answer is in the way phytoplasmas attack their hosts. As they multiply in the phloem sieve tubes, they interfere with the movement of plant sugars. Since growth-regulating hormones also move through the phloem passageways, clogging of the phloem will disrupt normal growth patterns. Roots of infected plants accumulate less starch then normal, and may even die. When you have an underfed ash with mixed-up growth hormones, it's no wonder you get witches'-brooms, chlorosis and deliquescent branching, not to mention retarded growth.
Ash yellows was first studied in forest stands in the northeastern U.S., where it kills many white ash, especially in the understory. Studies in the Upper Midwest and Great Plains have extended the known range of the disease as far south as the Ohio River, as far west as Utah and north to Alberta. In some Great Plains cities and windbreaks, more than half the green ash tested positive for the pathogen. While about 15 percent of green ash tested positive in Iowa, Wisconsin and Minnesota cities, the percentages in individual cities varied from zero to more than 30 percent.
Mystery still surrounds the question of how ash yellows spreads from tree to tree. The phytoplasma is easy to transmit during the grafting that occurs during production of green and white ash landscape trees, and the disease has occasionally appeared in nurseries, but the level of risk of spreading yellows through nursery stock -- though probably tiny -- is anybody's guess. Insects in the leafhopper group are thought to transmit the phytoplasma from tree to tree, but hard evidence is still lacking.
Management options for ash yellows range from slim to none. Antibiotic injections have been tried, with inconsistent results. Among the few recommendations for infected urban trees are to provide added TLC (in the form of mulching, watering and fertilization if needed), and remove them if they're in decline.
Does the threat of ash yellows mean that we should stop planting ash? Not at all. An encouraging outcome of the recent survey work is that most urban green ash seem to coexist with ash yellows &endash; apparently for decades &endash; and show no ill effect other than slowed growth. So while ash yellows is a cloud in the sky for landscape ashes, it's by no means a tornado.
Don't look for aster yellows on trees. Among the many plant hosts are carrot, marigold, purple coneflower, rudbeckia, petunia, pansy, chrysanthemum, coreopsis, delphinium, gaillardia and salvia, and weeds like wild carrot, dandelion, field daisy, black-eyed Susan and wide-leaved plantain.
Aster yellow outbreaks are sporadic but memorable. Symptoms range from the expected for yellows diseases, yellowing and witches'-brooms, to the truly bizarre. Infected marigolds (one of the most common hosts) are easy to spot, with yellowed or purple foliage and stiff, upright branching. Even more eye-catching are purple coneflowers, which sprout tiny, distorted leaves from the centers of their flowers. ISU's Reiman Gardens experienced a spectacular outbreak of aster yellows on rudbeckia and purple coneflower in 1998, which delighted plant pathologists but nobody else. Like other yellows diseases, aster yellows focuses on the phloem, so growth distortions come from disruption of the normal flow of growth hormones.
Also like other yellows diseases, aster yellows can't be cured, so plants with symptoms should be pulled up and discarded. This sanitation step is helpful, because otherwise insect vectors such as the aster leafhopper can continue to spread the disease in their patented suck-and-spit manner. Insecticides can deter the leafhoppers, but are likely to be more effective if applied in the early part of the growing season to nearby weeds that may be sources of the aster yellows phytoplasma. Another tactic is to go after the weedy areas with herbicides, but again you need to get started early in the season to beat the leafhoppers.
While ash yellows and aster yellows are different in many ways -- host plants, symptoms, and consequences -- a few common elements are worth remembering. Like all yellows diseases, the point of phytoplasma attack, and the source of damage, is the phloem. As these parasites slurp up the plant's food supply and throw its hormones out of whack, the results are bizarre symptoms like witches'-brooms and leaves sprouting from flowers. Phloem-feeding insects like leafhoppers are the natural vectors, but grafting can transmit phytoplasmas just as effectively.
Finally and unfortunately, another common element with yellows diseases is that our current management efforts don't amount to a proverbial hill of beans. But since yellows diseases were recognized a scant 30 years ago, and research on them has picked up speed continually since then, it's reasonable to hope for more effective tactics to emerge in the future.
ml: isugarden
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