You don’t need me to tell you that it has been a hot and dry growing season. The season’s weather has increased the chances that we will run into certain pests. One heat-loving pest whose numbers have been increasing in field crops is leafhoppers.
Scouting data from Southwest Minnesota reported an unusually high number of leafhopper nymphs, and scouting has shown a large number of adults. While their populations in some part of the state seem to be on the decline, for growers of carrots, celery, garlic, lettuce, and onions, it is worth being look out for leafhoppers and aster yellows, a disease they sometimes transmit.
Aster Leafhoppers Can Move Aster Yellows Around
Aster leafhoppers are small, quick-moving, and innocuous in appearance. They have piercing-sucking mouthparts that they use to feed on plant juices.
Aster leafhoppers are small and yellow-green. Photo: Whitney Cranshaw, Colorado State University, Bugwood.org
While leafhoppers are often present in many crops, their direct feeding damage (referred to as hopperburn) doesn’t typically rise to economic levels. When they do become a problem, it is because they are travelling with a disease called aster yellows.
Aster yellows infects and impacts many plant species, with crops of concern being carrots, celery, garlic, lettuce, and onions. The pathogen is picked up by aster leafhopper from infected plants (be it crop or weed) and carried with and deposited by the insects as it feeds (much like how aphids vector many cucurbit viruses). The amount of aster yellows circulating in the environment and in aster leafhopper populations varies by year, making it hard to predict when aster yellows will be a major issue.
Aster yellows is most commonly seen in carrots, celery, and lettuce, but is occasionally economically damaging in onions and garlic (some might remember a large outbreak in 2012). The symptoms appear differently in different crops, but generally appear as distorted growth.
Recognizing Aster Yellows
The population of aster yellows varies each year. We know there are large leafhopper populations this year, but how many of these leafhoppers are carrying aster yellows is unknown. Complicating this equation is that this year’s weather means drought stress and frost damage are both out there, which can look a lot like aster yellows in some crops.
In carrots, things to look out for include:
Distorted, yellowed, and shrunken new growth
Bronze or red coloration on older leaves
Shortened carrots with dense, hair like roots
Aster yellow symptoms in new growth of carrots. Photo: Whitney Cranshaw, Colorado State University, Bugwood.org
In garlic, things to look for include:
Early yellowing and dieback of garlic plants
Unusual smelling bulbs that are small and soft
Dark streaking and discoloration of wrapper (most diagnostic symptom)
Aster yellows infected garlic prematurely yellowing. Photo: M. Grabowski.
Garlic bulbs with discoloration caused by aster yellows. Photo: Michelle Grabowski.
Estimating Leafhopper Population
Leafhoppers are quick moving, which makes estimating their population difficult. Walking fields can result in missing leafhoppers. Instead, sweep nets are the best way to estimate the size of aster leafhopper populations. Use a 15-inch diameter sweep net in an 180 arc through the top six to eight inches of the crop 20 times in five different locations in the field (100 sweeps) and counting the aster leafhoppers captured. AY susceptible carrot varieties have a threshold of 20 aster leafhoppers per 100 sweeps. There are not thresholds available for garlic or other impacted crops.
The rough motions of sweep netting in carrots. Diagram: Marissa Schuh using Biorender.
Managing leafhoppers and aster yellows
If high numbers of aster leafhoppers are found and you are growing a vulnerable crop, management action may be necessary. The window has passed for cultural controls like reflective mulches and row covers.
Pulling infected plants and removing perennial weeds will help reduce the reservoirs of aster yellows on your farm.
Aster yellows is caused by a phytoplasma (specialized bacteria). It is spread like a plant virus, and like a virus, you cannot use fungicides or bactericides to prevent or slow the spread of aster yellows. Chemical control instead has to be focused on the vector, aster leafhopper.
At this point in the season, it may be worth using pesticides to manage aster leafhoppers. Because of their piercing-sucking mouthparts and quick movements, pyrethroid insecticides are the best option for control. Make sure the product you are looking at is registered in your crop, and make repeat applications according to label directions. Repeat until 2 to 3 weeks before harvest, so the window for leafhopper control has likely passed for some crops (such as garlic).
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