09/15/1993

Look Out Nematodes, The Geraniums Are Coming

AUBURN, Ala. - Field crop growers know there's just no getting around the problem of nematodes in the soil. In Alabama, as well as in other sandy soils of the Southeast, the microscopic organisms, which reduce yields and kill plant roots, are a problem in virtually all major field crops.

Fortunately for growers, Auburn University Plant Pathologist Rodrigo Rodriguez-Kabana is leading the battle against the pest. An unlikely knight, the Alabama Agricultural Experiment Station researcher is fighting the nematodes with unlikely weapons--geraniums, velvetbeans and pasture grass. And, of course he's not using ordinary methods.

Typically, managing for nematodes consists of developing resistant varieties, using nematicides (chemicals) or using selected cultural practices. Researchers have done well with resistant varieties of crops such as soybeans and cotton, says Rodriguez-Kabana, but they have not fared so well in developing resistant peanuts. "What you can do through genetics is limited," he says.

Nematicides are declining in favor as an option. They are generally expensive, somewhat ineffective and becoming less available. This leaves cultural practices at the head of the list of nematode-fighting measures. Rodriguez-Kabana's studies have already led to successful rotational and cropping sequences that keep damage low.

"Crop rotation is a must. There are dozens of systems we can use in soybeans, peanuts and cotton," he says. "These systems suppress the nematode population, increase yield, and give responses beyond what we expected. This also tells us we are solving other problems through the use of rotations."

Not only do rotations pay, but Rodriguez-Kabana says the research can prove it offers huge differences in yield, while monocultures (replanting the same type crop), on the other hand, may lose money.

The best rotations are those that include livestock; the combination not only helps control nematodes, but also can be profitable. Bahiagrass pastures combined in rotations gave great nematode control, although it's not a grass particularly relished by cattle. But the overall results are a plant pathologist's victory. "It's rare when we don't at least double the yields of peanuts, soybeans, or cotton with this rotation," he says. "All the crops we've tried respond this way."

But, in order to keep the cattle happy, work has switched to switchgrass pastures. So far, it has the same effect on nematodes, while providing a high quality palatable pasture for the animals. Since the rotations prove beneficial in nematode control, and offer the producer more flexibility, Rodriguez-Kabana, in cooperation with Agronomist David Bransby, has since taken to looking at winter pasture grasses; so far, he says several look good for both nematode control and cattle production.

In the meantime, he realized that growing cattle as a crop rotation wasn't an option for everyone. He then decided to see how it was done elsewhere. History again provided a clue: until the 1940s, velvetbean was the "premiere rotation legume for the South." Velvetbean is still a popular rotational choice for some countries in Latin America and the tropics.

Once again, despite common sense, he won in the ongoing battle with nematodes. The AU researchers more than doubled crop yields. Not only did velvetbean aid in soil improvement (by fixing nitrogen), it helped prevent erosion. As if that wasn't enough, velvetbean can serve as a nutritious, quality livestock feed. The only negative seems to be that velvetbeans grow as creepers, making the harvest somewhat difficult.

It was at this point that fellow researchers had begun to think the battle went to Rodriguez-Kabana's head. His next defense seemed to be to rather quaint; give the nematodes some flowers. But his reasoning was clear. Some plants--herbs and scented geraniums among them--never seemed to have the nematode troubles that plagued their field crop cousins. The pathologist decided to find out why. By knowing where the resistance originates, the properties may be able to be used later.

Strange sights began showing up in his Alabama greenhouses and field plots; sesame, hairy indigo and joint vetches. Native partridge pea. Toxic castor beans. And a number of herbs and aromatic plants. Rosemary, thyme and a number of other scented herbs were even planted.

He explains, "We didn't get into all these plants, especially not the toxic ones, because we think they have a future here. But we need to know what is in those plants that kills or repels nematodes."

For example, oregano has no nematode problems. Researchers discovered that it produces terpenes, and terpene-like compounds. While these compounds are common enough to be used in everyday cooking, they can have either positive or negative effects on nematodes. Thymol and furfural are both natural compounds in plants that manage to discourage nematodes with their toxicity. While some of the compounds kill the pests outright, others just make the roots so unattractive that the nematodes go elsewhere for dinner.

While the work is feasible at the nursery horticultural level, much of it is not yet ready to be incorporated into row crops. Cropping systems, however, can be developed for almost any farmer interested in breaking the nematode cycle in their field. Future work will focus more on row crops.

In the war on nematodes, the scientist has obviously stumbled on to more than just a control measure. "It all argues for profitable cropping systems, combined with resistant varieties, as a way to control soil-borne diseases and nematodes," he says. "If people would just come out and look at one of the study fields, I wouldn't have to explain anything. The differences are that obvious."

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News from:

Office of Ag Communications & Marketing

Auburn University College of Agriculture
Alabama Agricultural Experiment Station
3 Comer Hall, Auburn University
Auburn, AL 36849
334-844-4877 (PHONE) 334-844-5892 (FAX)

Contact Jamie Creamer, 334-844-2783 or jcreamer@auburn.edu
by Reginia Broadway

September 15, 1993