Elm Yellows - a Sporadic Yet Lethal Disease of Elm

Elm Yellows, a lethal systemic disease of elm, was confirmed on two American elm (Ulmus americana) specimens in Franklin County in August 2013.  The disease can occur in isolated areas across the eastern portion of the US and can quickly devastate large plantings of native elm.  Elm yellows occurs only occasionally in Kentucky.  In fact, only one other incidence has been reported in the commonwealth during the past 30 years (Jefferson Co., 1990).

Symptoms of elm yellows usually appear during summer months and include bright yellowing that resembles early senescence (Figure 1).  Leaves can change hues with a few weeks, with petioles turning downward (epinasty) (Figure 2).  Leaves eventually turn brown and can remain attached to branches for several weeks (Figure 3). 



Figure 1.  Foliar symptoms of elm yellows disease include bright yellowing of leaves during summer.



Mature trees develop disease symptoms approximately nine months following infection, while young trees may show symptoms in as little as three months.  Trees usually die within a year or two after symptoms develop.  There is no cure. 

The causal agent of elm yellows is a phytoplasma (bacterium-like prokaryote) called ‘Candidatus Phytoplasma ulmi’. The pathogen inhabits phloem tissue of elm, and as the pathogen builds up in tissue, it becomes a metabolic sink for photosynthetic products.  Phloem then degenerates downstream from these sinks, causing root mortality in fine roots and subsequently in larger ones.  As this process ensues, tree canopies begin to show yellowing symptoms as described above. 

Figure 2.  Petioles droop and turn downward as elm yellows disease advances.

 

Hosts of the elm yellows bacterium are limited to elm species, particularly native elm, including the American elm (U. americana) and winged elm (U. alata).  Chinese elm (U. parvifolia) is more tolerant of infection and often remains unaffected in areas where disease has killed native elm. 

Spread of the bacterium is believed to be caused by several species of leafhoppers and possibly spittlebugs, although the white-banded elm leaf hopper has been confirmed as the primary vector.  These insects inoculate trees during summer or early autumn as they feed. 

Figure 3.  Within a few weeks of symptom development, elm yellows causes leaves to turn brown.  Leaves may fall or remain attached to trees for several weeks.

 

Control of elm yellows is not possible and control of insects is not practical.  Infected trees should be removed as soon as possible to prevent spread of disease.  Confirmation of elm yellows requires a molecular diagnostic test.  Non-elm or tolerant elm species, including Asian species and hybrids, should be used as replacement plants.

Blueberry Mosaic Virus Detected in Kentucky

Blueberry mosaic virus was confirmed in a blueberry orchard in central Kentucky this summer.  This is the first report of blueberry mosaic in the state.  The virus has not previously been found in southern states, but it has been reported in Michigan, Indiana, New Jersey, New York, Oregon, Washington, and British Columbia. 

Formerly considered a “disorder,” blueberry mosaic has only recently been classified as a virus.  Not much else is known about it.  Highbush cultivars ‘Bluecrop,’ ‘Pioneer,’  ‘Concord,’  ‘Earlibule,’ and ‘Jersey’ are among the susceptible cultivars.  Virus symptoms have not been reported in rabbiteye blueberry.  Limited research has been conducted thus far, but it is under evaluation by a team of researchers at the USDA-ARS Marucci Center in New Jersey.

 

Symptoms

Foliar symptoms include mottling and mosaic-patterned characteristics on leaves.  Patterns range from mild to brilliant with yellow, orange, and/or red colorations (see photos).  Mosaic symptoms are not always produced each year, and environmental conditions may affect symptom development.  It is possible that during cool seasons, symptoms are more pronounced.  Thus, under ideal conditions, symptoms may appear suddenly, and it may appear that spread is rapid.

Infected plants often result in reduced fruit load.  Yield losses of 15% have been reported on ‘Bluecrop’ in Michigan.  Fruit on infected plants have been shown to ripen later than noninfected fruit, and fruit quality is low.

 

Spread

The vector (carrier) of the virus is not known.  Researchers report that blueberry mosaic is not transmissible by mechanical means (i.e. pruners).  However, the virus is graft transmitted.  Because virus particles are systemic (dispersed throughout the vascular system), once infected, all plant parts are infected, even when symptoms are absent.  Thus, cuttings should never be taken from a virus-infected plant.

As mentioned above, the vector of blueberry mosaic virus is not known.  Therefore, growers are encouraged to destroy infected plants until it is clear on how the virus spreads.  Whether an insect vector is involved is yet to be determined.

Current research indicates that the virus is slow-spreading (if at all) under field conditions.  However, in Michigan, spread is rapid. So far, the reason is unknown.

 

Treatment

There is no cure for plant viruses, including blueberry mosaic.  Growers should remove infected plants and destroy them by burning or burying.  Remove all roots within soil, as well.  Scout orchards, especially nearby plants, several times during the growing season.  Contact a local extension agent if a plant appears suspicious.

Most importantly, purchase plants from a reputable grower.  Visit nurseries before plant purchase and ask to inspect parent plants.  Furthermore, a virus-free certification is always worth the extra cost.