Watch alfalfa fields for nematode damage; use resistant varieties

Producers are familiar with varieties of alfalfa, potatoes or corn, but there’s also varieties of nematodes – some beneficial to the soil and some that can stunt the growth of plants, thus reducing yields and reinfecting the next crop.
So which is which?
The “good” nematodes help producers by being biological controls of soil-dwelling insect pests, according to Saad Hafez and other researchers at the University of Idaho.
And the “bad” nematodes?
The “bad” nematodes either attack plants from the outside or they live inside the host plant for part of their lives. A few nematodes, such as the root-knot nematode and the root-lesion nematode, attack many kinds of crops, Haafez said.
Damage from nematodes in alfalfa includes stunting, root damage, and loss of yields of forage and seed, along with stand loss, said Don Miller, plant breeder at Producer’s Choice Seed.
Potato growers that have nematodes in their potatoes may find the processors do not want the potato because nematodes leave structures that damage the appearance, Miller said.
“Nematodes are microscopic worms that live in the soil, most predominantly in sandy, lighter soils,” Miller said, adding that soils that are heavy clay tend to get more root diseases than nematode damage.
The nematodes can cycle into the plant and cause stem and crown bud damage and root system damage, depending on the type, he added.
The most economically damaging types of nematodes that infect alfalfa are: alfalfa stem nematode, root knot nematodes, and root lesion nematodes, according to Hafez.
Miller has been working with the “bad” nematodes and finding alfalfa resistant varieties to the pesky worms for 25 years. He does not work with pototoes or other crops at present.
“It is a huge problem in Idaho, where alfalfa is used in rotation with potatoes,” he said. “Alfalfa is a huge crop in Idaho.”
There are “bad” nematodes that can damage both alfalfa and potatoes, among other crops, but they are different types of nematodes.
“Just because you have resistance to one kind of nematode doesn’t mean you have resistance to another kind,” he added.
Miller said warm, moist conditions can help a population of nematodes explode. But there usually has to be an agent that brings them in to a clean field because the nematode worms move very slowly, he said.
If an infected alfalfa field is cut with a swather and not washed, that equipment moves on to another field, starts cutting, and it can infect the clean alfalfa field.
Other ways nematodes can move to an uninfected field are through flood irrigation when the water is pumped back and reused, and debris in seed and hay, he said.
Nematodes live in the soil and planting into the soil with a non-resistant variety that is susceptible to nematodes is asking for problems, Miller said.
“The most important thing producers can do to manage nematodes is to plant a resistant variety,” he said.
He has developed several types of alfalfa resistant to nematodes and that are specific for a certain region of the country.
Alfalfas are classified into climate types using a  fall dormancy rating 1-10. The lower the number, the better adapted to cold climates, Miller said. The rating 4 is best adapted to cold climates like, the Dakotas and Montana. Lower elevations or slightly warmer areas can still use 4s but also use more 5s, such as areas like Idaho, Utah, and Oregon.
Alfalfa varieties with nematode resistance include:
• Bullseye is the only one with Columbia root knot nematode resistance.
• PGI 424 is highly resistant to stem nematode and resistant to northern root knot nematode.
• PGI 459 is resistant to stem nematode and highly resistant to northern root knot nematode.
• With a fall dormancy of 5, Ruccus  is resistant  to stem nematode.
• PGI 557 is highly resistant (HR) to  both stem nematode and northern root knot nematode.
Miller said it is often easy to spot when nematodes are in an alfalfa field.
“Sometimes I can just drive by and see ‘white flags’ or a circular area in a field where nematodes have stunted the growth or there is uneven plants that have been damaged,” he said.
White flags are what happens when alfalfa stem nematodes get into the leaf tissue and kill chloroplasts. The infected leaf tissue then turns completely white and is called a white flag.
When there are white plants in the field, Miller suspects nematodes. The same goes for circular areas with little or no growth, he said.
Since they live in the soil, cutting the alfalfa plant to the ground does not kill off the nematodes, he added.
When breeding for resistant varieties, Miller travels all over the country making plant selections that he can send off to universities and field trials that will grow the more resistant varieties.
He keeps selecting out the most resistant for three or four generations (about five years) until he comes up with a good, resistant variety.
In addition, he has been working with the University of Idaho to grow the nematodes he needs with resistance work.
“I’ve been working pretty closely with Dr. Hafez at the University of Idaho,” Miller said. Hafez grows the nematodes for Miller so Miller can use them in alfalfa trials.
Miller said the plant parasitic nematodes feed only on living plant tissues, and have a hollow needle-like structure known as a stylet that can puncture the plant and cause a wound. Other diseases can then move in to infect the plant.
Stem nematodes can  infect the crown buds of the plant and cause them to be swollen.
Miller said he often shows the buds to a group of producers, cuts them open and shows them the worms inside. So swollen buds are a sign of nematodes.
But if producers want to know for sure, Miller recommends sending in a sample to a testing lab.
Next week, Miller is going to Utah.
“They have severe stem nematodes, so I’m going down to the field and get some plants that are not stunted and inspect for nematode damage,” he said.
Miller will select 100-200 plants out of 10,000 that have the best resistance and send them off to begin to look for a good, resistant variety.
“We have quite a number of alfalfa resistant varieties,” he added.


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