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Timing is crucial in controlling alfalfa disease and insects

Jeremy Hayward Published on 30 April 2015
Aphanomyces goes undiagnosed in many fields

While hay and forage prices remain elevated, and dairy-quality hay is in short supply

It is beneficial to actively scout for diseases and insects that may compromise your alfalfa crop – especially if the varieties you plant do not have disease or insect resistance traits built in.

All growers have to be ready to confront the diseases and insects that may challenge their crops. No matter where you farm, there are production problems that can develop very quickly and take a toll on yield potential, quality or both.

Alfalfa producers can choose from a number of disease- and insect-resistant alfalfa varieties on the market, yet an infestation and subsequent damage can still occur with these “resistant” varieties, albeit less virulently than if those traits were not present.

Below are several of the most common diseases and pests that can occur in alfalfa fields, along with ways to help minimize their impact.


Aphanomyces root rot
Once thought of as a wet-soil disease, aphanomyces is more widespread than many people realize. Found primarily in northern regions of the U.S., this disease weakens alfalfa seedlings and causes root pruning in established plants, reducing water and nutrient uptake.

Other alfalfa diseases, such as cotton root rot and sclerotinia crown and root rot, both found primarily in the South, are much more regionalized.

Though race 1 and race 2 aphanomyces-resistant alfalfa varieties are widely available, it is still important to scout for aphanomyces and other diseases by digging up plants and sending them in for analysis.

If aphanomyces is diagnosed, it should be quickly addressed with an appropriate fungicide treatment.

While the industry waits for race 3-resistant alfalfa varieties to become available, you can take steps to continue producing high-quality, high-yielding alfalfa despite the possibility of aphanomyces infection.

1. Start with the best genetics. Many alfalfa varieties have resistance to races 1 and 2, with several race 2-resistant varieties also providing some protection against race 3.

If you don’t already plant aphanomyces-resistant alfalfa, consider investing in it to realize the potential for better results.

2. Invest in seed treatment. Several fungicide seed treatments currently on the market offer reliable protection against aphanomyces races 1 and 2 during stand establishment.

Once plants mature, the crop must rely on genetic resistance for disease protection. Seed treatment technologies are perhaps even more important when used on alfalfa than on other crops, since alfalfa traditionally has a fairly high mortality rate during stand establishment. As a result, it’s important to keep as many of these seedlings alive and thriving as possible.

3. Consider field topography. Unlike phytophthora, which thrives in valleys, aphanomyces is prevalent in a wide variety of terrains.

When planting in a valley or slope, choose varieties with maximum aphanomyces resistance and include a fungicide seed treatment.


In addition to disease, it is critical to scout your alfalfa fields for insects that can affect your crop. Digging up plants or running a sweep net over portions of your field to determine the right economic thresholds at which you should treat for a particular insect is very important.

Either you, your agronomist or both of you in tandem should get out into the field and scout.

When it comes to alfalfa, there is resistance of varied levels to a variety of pests including potato leafhoppers, alfalfa weevils and aphids.

Especially with non-potato leafhopper-resistant alfalfas, you have to be in the field on top of the stand, looking at individual plants and determining the appropriate economic threshold at which to take action.

That action may be a rescue application or cutting the stand and starting over with the next cutting. Usually by the time you see major insect issues in your field, the damage has been done.

Potato leafhoppers are a huge issue in the Midwest. It’s almost as if there is a potato leafhopper “belt” that runs from Iowa through Illinois and Indiana on up to Pennsylvania and into New York.

We can also get them north and south of that line – even west into Nebraska. Purchasing alfalfa varieties with high leafhopper resistance could be key, particularly if you are not aggressively monitoring for leafhopper presence.

Leafhopper damage can typically be misdiagnosed as dry weather taking a toll on the plant or as a nutrient deficiency. But if you scout, you may find leafhopper presence that warrants spraying.

As noted previously, with the price of alfalfa hay today, we’re at an economic threshold that’s lower than it used to be when hay was cheaper. So it makes sense to scout and spray a little earlier than you might normally.

Bottom line: Stay on top of potato leafhoppers, and if you don’t, then plant a variety with top-tier resistance.

Weevils and aphids
Alfalfa weevils and aphids cover a wider geography than potato leafhoppers, and the majority of alfalfa varieties have resistance to most aphid species.

But again, there will be times when you will need to scout fields, note what the economic threshold is for your region and either make an insecticide application or cutting to remove these destructive pests.

Investing in alfalfa seed that has resistance to the most prevalent diseases or insects in your region is crucial. But don’t forget to proactively scout for pests, too, no matter how resistant your varieties. Diligently practicing both approaches will help ensure that your yield and quality goals are achieved and will position you well for success in years to come.  FG

Aphanomyces goes undiagnosed in many fields. The variety on the left was developed with aphanomyces resistance, while the variety on the right did not have the resistance. Photo courtesy of Jeremy Hayward.

Jeremy Hayward
  • Jeremy Hayward
  • Brand Manager
  • W-L Research

Tips for scouting alfalfa fields

1. Take solid samples. Get representative samples from various sections of the field. The more samples you can get, the better. If you’re taking a sweep sample of insects, you certainly want to cover as much of the field as you can. But if you have a 20-acre field, it doesn’t make sense to walk every acre.

A good rule of thumb is to make at least 10 sweeps per acre. As an example, if I’ve taken 10 sweeps and collected 70 potato leafhoppers in a section of my field, I’m probably going to spray that field. But if I sweep that field and I’ve got four potato leafhoppers, I probably won’t spray.

2. Be proactive. Know when the pest has the potential to affect your alfalfa stands and make sure you’re on the front end of that. Given the choice, most of us don’t want to pursue a rescue treatment – we want to do something before the pest has time to negatively affect the crop.

So make an insecticide application that’s very timely. If you have to cut, make that timely, too.