Crop & Pest Report


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Understanding Aster Yellows (7/26/12)

Aster yellows has become a significant concern this year for growers in the state. This article provides detailed information on what causes aster yellows and why it is difficult to manage.

Aster yellowsAster yellows is caused by a phytoplasma (class = Mollicutes in Kingdom = Bacteria), which are a group of pathogens that plant pathologists don’t commonly deal with.  A phytoplasma is basically a bacterial pathogen without a cell wall.  The aster yellows pathogen is an obligate parasite, which means it can only live in a living host.  The aster yellows pathogen can cause disease in many different plants and crops, but in North Dakota we are most concerned about canola, wheat and potatoes (see Andy Robinson’s article).  Although the aster yellows pathogen is a bacterium, the disease is spread in a way that is more common to viruses: with a vector.

The Aster leafhopper (Figure 1) is the most common vector of the disease.  Aster leafhoppers (or six-spotted leafhoppers) are small (1/8 of an inch), wedge-shaped and green to yellow with three pairs of spots on its head.   Leafhoppers are active and mobile insects that have piercing-sucking mouthparts, and feed on plant sap.  When the aster leafhopper feeds on a plant with aster yellows it ingests the bacteria pathogen.  The bacteria will multiply in the insect, move to its salivary glands and after about two to three weeks the insect will transmit the disease to plants it feeds on; like our succulent canola in the NE part of the state.  Once the leaf hopper has the bacteria in its salivary glands it will transmit it for the rest of its life.

When a leaf hopper injects the pathogen into the sap of a canola plant it will develop aster yellows.  Often infection is limited to a single branch where the leaf hopper has fed, but entire plants can be infected.  Symptoms of the disease are most severe when the plant is infected early in the season.  In the NE corner of the state a full range of symptoms are common in canola, including purpling leaves and misshaped pods (short, bladder-like) (Figures 2 and 3).

Unfortunately, management of the pathogen is difficult.  Fungicides and bactericides are not effective against the pathogen.  Resistance to the disease is elusive, in part because the pathogen is very difficult to work with.  Because the pathogen is an obligate parasite plant pathologists cannot grow the pathogen in the laboratory to inoculate and screen plants for resistance like we do with many pathogens.   Similarly, the pathogen needs to be vectored by the leafhopper, so greenhouse and/or field screening would depend on controlling an infected colony of aster leafhoppers.

Lastly, there is no clear cut answer as to whether spraying one or more insecticide treatments for aster leafhoppers will reduce the incidence of aster yellows in canola. We have no established scouting protocol or action threshold for aster leafhoppers in canola. We do know that high numbers of vectors (leafhoppers) will increase the incidence and severity of aster yellows, especially when aster leafhoppers arrive early and a large proportion of the aster leafhoppers is carrying the phytoplasma (>20% infectivity rate). There is little research data documenting high populations of aster leafhoppers to direct yield losses in canola. Disease transmission of aster yellows occurs rapidly, usually within 30 minutes of feeding by leafhoppers. So, any insecticide could be sprayed too late to prevent phytoplasma transmission and result in more revenge spraying than pest management of aster leafhoppers. Since aster leafhoppers migrate into North Dakota and then in-season populations move around from crop to crop and field to field, there’s no guarantee that treated fields will not become re-infested with aster leafhoppers. In addition, treating with a broad spectrum insecticide will kill beneficial organisms and could lead to secondary pest problems, such as aphids in canola.

Sam Markell, Extension Plant Pathologist

Janet Knodel, Extension Entomologist

This site is supported in part by the Crop Protection and Pest Management Program [grant no. 2017-70006-27144/accession 1013592] from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed are those of the website author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

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