Aug 3, 2018
Veggie diagnostics: The latest on nematodes, tillage and herbicide injury

Identifying alternate hosts for nematodes, using shallow tillage to reduce weed pressure and correctly identifying herbicide injury are important topics for vegetable growers.

“The northern root knot nematode is very common in Michigan, but we can manage it successfully by rotating with non-host crops,” said Marisol Quintanilla, nematologist in Michigan State University’s (MSU) Department of Entomology.

Look for patterns when trying to diagnose herbicide injury on vegetable crops, said Erin Hill, Michigan State University weed science diagnostician with MSU Diagnostics Services. Photo: Dean Peterson

However, a clear trend emerging from MSU’s recent survey of vegetable growers about their nematode populations is not everyone knows what plants are non-host crops. Sometimes, cover crops recommended as effective at controlling nematodes instead can be very good hosts for some nematodes and even increase their numbers.

As one example from the survey, some vegetable growers alternated their vegetable production with two years of corn as a rotation and used rye as a cover crop. These growers had very low pressure from northern root knot nematode because the nematode can’t feed on grasses. Growers who did not rotate with a grass, like corn or wheat, had greater northern root knot nematode numbers.

As another example, some vegetable growers were very interested in soil health and planted multi-species cover crops. The cover crops significantly improved soil health and increased the number of beneficial organisms – which was good. However, some of the cover crop species were alternate hosts for root knot nematode and nematode numbers were high.

Root knot nematode numbers do drop quickly if you rotate to non-host crops, which are corn, rye, wheat or other grasses.  Two years without a susceptible crop will significantly reduce the nematode’s numbers. “Be careful with your cover crop choice,” Quintanilla said. “Some are very good hosts for root knot nematode. It’s really important to get good information about what you plant.”

These and other research findings were recently presented at the 2018 Bay Thumb Vegetable Meeting in Burton, Michigan.

Other pointers on managing nematodes include:

  • Know what nematodes you have. “Take a soil test for nematodes,” Quintanilla said. If the numbers aren’t high, try to exclude them from your fields by not moving soil into them with equipment. MSU Diagnostics Services at pestid.msu.edu offers nematode testing.
  • Know what the nematodes feed on and see if you can find a rotation that can help. Some varieties of pearl millets, for example, are poor hosts for root lesion nematode.
  • Recognize some nematodes that are beneficial. Bacteriavores feed on bacteria and help cycle soil nutrients, carnivores feed on other nematodes, fungivores feed on soil fungi, and omnivores have many food sources and are an indicator of overall, soil health.
  • Stay informed. Soil solarization – using sunlight to reduce nematode numbers, cover crops and trap crops, manure applications – particularly chicken manure, nematicides and seed treatments are all being studied as nematode management tools.

Reducing weed pressure

Finger weeders, flex tine harrows, torsion weeders, and other new technologies offer mechanical weed control options for vegetable growers.

There are two key principles to remember. You must reduce weed densities before you plant, and you must maintain a difference in height or rooting depth between the crop and the weeds.

“These tools are not magic,” said Sam Hitchcock Tilton, MSU graduate student and a columnist for the Vegetable Growers News. The tools work when the crop is better rooted or taller than the weeds.

A key principle is that 90 percent of the weeds in vegetable crops are germinating in the top one-inch to one and a half inches of soil. Cultivation limited to that germination zone won’t bring up new weed seed.

Using a stale seedbed concept involves letting the weed seeds in the top one and a half inches of soil germinate and emerge. The weeds are then controlled with shallow cultivation or propane burning before planting the crop.

“There’s a lot going on when it’s field season, and it may be hard to find time for another trip across the field,” Hitchcock Tilton said. The trip does, however, reduce the amount of weed pressure on the crop after planting and helps the crop get a head start on the weeds.

Postemergence cultivations must not be too deep. “You only want to scratch the surface to an inch or so deep,” Hitchcock Tilton said.

Cover crops that winterkill can also help reduce weed pressure.

Consider the amount of weed seeds at different soil depths. Do you need to moldboard plow and bury a lot of shallow-germinating, weed seeds?

What are the three top weeds on your farm? What are their characteristics? Do they germinate at different depths? Do they set seed at different times? “These factors determine when the weeds are going to be germinating,” Hitchcock Tilton said.

Diagnosing herbicide injury

Separating herbicide injury from disease, insect or nematode damage can be a challenge in vegetable crops. One key is to look for patterns in the injury, said Erin Hill, MSU weed science diagnostician with MSU Diagnostics Services.

  • Uniform injury across the whole field or crop can be the result of a misread label, misapplication or herbicide carryover. “Consider the application area and the product choice in that area,” Hill said. Uniform injury could also be from not cleaning out the spray tank or mixing system. Some herbicides adhere to plastic tanks and can leach into future loads.
  • Crop injury across similar soil types or topographies within a field could also be the result of herbicide carryover. This is most likely the result of not following rotation restrictions or soil or environmental conditions that slowed herbicide degradation.
  • Crop injury in strips across the field is likely from spray boom overlap and subsequent overapplication either in the current season or seasons past.
  • Spotting or other injury concentrated on the field edge near another field that was treated with a herbicide could be the result of spray drift. Off-target particle movement could occur when applications are made under windy conditions or when an incorrect nozzle selection made the spray particles too small.
  • Injury that’s random in a field could be from herbicide volatilization. Volatilization injury is more likely under low wind conditions, low cloud cover and applications made in the early morning or evening. “Volatilization is not drift,” Hill said. It’s the off-target movement of a volatile herbicide.
  • A temperature inversion can also cause random injury. An inversion occurs when a layer of warm air exists above cooler air at ground level. “The herbicide can get trapped under the warm air layer and move onto other fields,” Hill said. Inversions are most likely when winds are calm, and there are clear skies and long nights. Inversions are most common in the summer just before dusk.

MSU Diagnostics Services at www.pestid.msu.edu can help diagnose the cause of herbicide injury. The assessments are based on field history, surveys completed by the grower and any other available sources of information.

Diagnostic Services also specializes in diagnosing plant problems associated with arthropods, pathogens and nematodes.

Dean Peterson, VGN Correspondent

Above: Mechanical weed control options are increasing for vegetable growers. Photo: Dean Peterson


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