Aster Yellows in Wheat

Crop scientists in northwest Minnesota are taking a closer look at this  problem spread by aster leafhoppers, which has resulted in sporadic  problems in some wheat fields, and may threaten other crops

By Charla Hollingsworth and Ian MacRae

During the summer of 1998, a Minnesota agronomist noted scattered wheat fields with red to purplish-colored foliage. Since then, there have been reports of oddly-colored plants nearly every year in northwest Minnesota. 

Until recently, the problem was assumed to be barley yellow dwarf virus (BYDV), a disease spread exclusively by aphids.  However, these symptoms have been noted during years when aphid populations were very low or absent altogether.  But, there is another disease that produces symptoms similar to BYDV called aster yellows, which is spread by leafhoppers; a common insect in the area. 

Aster yellows can easily be misidentified as BYDV, because the diseases produce symptoms that are nearly identical.  As a result, it’s possible that aster yellows may have been responsible, at least in part, for previous yield losses that were routinely attributed to BYDV.

During 2002, plant symptoms similar to BYDV and aster yellows were noted in the University of Minnesota’s wheat varietal trial located near Strathcona.  The trial, consisting of 24 wheat varieties, had a wide range of disease responses. Varietal symptoms varied from healthy-appearing plants (Granite) to severely diseased (Russ). Plants with severe symptoms showed stunting, head sterility, reduced tillering, and red to purplish leaves. Plant material was submitted to the U of M’s Plant Disease Clinic in St. Paul, which confirmed the disease was indeed aster yellows.

Aster yellows is caused by a microscopic pathogen called a phytoplasm, which is similar to a bacteria although it has no cell wall. Aster yellows phyto-plasma live, and multiply, inside several different species of insects and many different species of plants.  In fact, more than 300 different plant species are susceptible to the disease. 

In susceptible plants, aster yellows causes tissue discoloration, stunting, plant feature distortion, flower sterility, abnormal shoot production, and yield loss.  Some other hosts of aster yellows phytoplasma include canola, barley, sunflower, flax, oat, potato, soybean as well as ryegrass, bluegrass, wild mustard, lambsquarter, and other weeds.

Aster yellows has caused significant loss in a number of vegetable crops, notably carrots, in Minnesota and Manitoba.  Disease symptoms of carrots begin as yellowing of younger leaves, but as the disease progresses, the plant produces multiple shoots until it looks bushy, a symptom known as a “witches broom.” In 1999, a serious outbreak of aster yellows in canola caused seeds still in the pod to germinate, resulting in significant yield loss for some Minnesota growers.  Another closely-related group of phytoplasma are known to cause elm yellows and ash yellows, tree diseases that can occur in this region. 

The only way aster yellows phyto-plasma can be transmitted, or vectored, from plant to plant is by the aster leafhopper.  This insect feeds by sucking sap, much like an aphid does.  Sucking insects frequently inject saliva into the plant to facilitate the flow of sap. Aster leafhoppers acquire, or pick up, the phytoplasma when they feed on the sap of infected plants.  When an infected leafhopper feeds on an uninfected plant, it injects the aster yellows phytoplasma with its saliva, and the plant becomes infected and begins to show symptoms in 2-3 weeks (depending on weather conditions).  Consequently, plants that are infected earlier in their life histories are more likely to suffer an economic loss in yield. 

Leafhoppers are active, mobile insects.  Thus, the source of infection in a field is probably going to be leafhoppers arriving already infected from another source. This region typically has high populations of these insects in small grains fairly early in the year. As you walk through fields around 3-5 leaf stage, you’ll notice small, gray insects flying away in front of your feet; these are aster leafhopper (see photo below).

As with other insects, only adults have wings and they readily fly from field to field.  Although aster leafhoppers do overwinter as eggs in our region, we also get adult leafhoppers arriving from southern states from late April through late May on northern gulf streams. Many of these leafhoppers have been overwintering in locations that support aster yellows and may be already infected when they arrive.

Because our overwintering populations do not become adults until mid- to-late June and plants are most susceptible to economic damage when they are infected early in their life history, it is the spring immigrant leafhoppers that have the greatest economic impact.  The ready dispersal of these adults from field to field also means it is unlikely that we can control this disease by spraying early season leafhoppers. Planting resistant varieties as well as monitoring aster leafhoppers and alternative hosts in the area will likely be the most economic and potentially successful management techniques available.

To determine the extent of this problem in Minnesota, field surveys will be conducted over the next two years. Plants and leafhoppers collected from around the region will be tested for the presence of the aster yellows phytoplasma. In addition, wheat varieties commonly grown in Minnesota will be exposed to the disease in the field and greenhouse to determine their levels of aster yellows resistance.  Once the data is in, recommendations can be made for growers in affected areas. This research is being funded in part by the Minnesota wheat checkoff.

Small grains producers should be aware of the insect and disease symptoms and the potential for yield loss caused by aster yellows.  When infected at the seedling stage, susceptible plants may fail to produce kernels due to stunting and head distortion.  Disease severity tends to vary widely due to plant resistance, plant age and vigor when infected, aster leafhopper population, and environmental conditions.

Hollingsworth, holli030@umn.edu is an extension plant pathologist and MacRae,imacrae@umn.edu an entomologist, both at the Northwest Research and Outreach Center, Crookston, Minn.