Root-knot nematodes problem in corn

The promise of potential profits is spurring many Southeastern growers to plant corn in 2007. But this could be a mistake for farmers who have fields with a history of Southern root-knot nematode problems.

Prior to 1970, corn was considered a non-host of the Southern root-knot nematode. And even today, reference to corn as an effective rotation alternative for Southern or cotton root-knot nematode can be found in Extension literature nationwide.

Unfortunately, all corn hybrids on the market today apparently are good hosts for Southern root-knot nematodes and will maintain, if not increase, their populations rather than suppress them, according to researchers. It is believed that resistance to this particular species of nematode was lost over the years because plant breeders failed to screen corn breeding lines against it.

A Southern root-knot nematode-resistant corn hybrid is badly needed to help avoid aggravating already serious nematode problems in cotton, soybean and many vegetable crops, according to Auburn University researchers who currently are working on the problem.

For example, cotton production throughout the South is threatened by both the reniform nematode and the Southern root-knot nematode. Corn is a non-host for the reniform nematode and can be used successfully as a rotation partner to effectively manage this nematode in a cotton production system. However, cropping corn before cotton can greatly increase the risk of sizable yield losses in cotton due to Southern root-knot nematodes, particularly in fields with lighter soils.

The only alternative rotation partner for cotton growers with Southern root-knot nematode problems is peanuts, which is a non-host to both the reniform and the Southern root-knot. However, peanut production is not only limited to certain soils, but also to the availability of costly harvesting equipment and marketing options. Therefore, peanuts is not a viable option for many Alabama cotton producers.

The Alabama study, conducted by Kathy Lawrence, Austin Hagan and Bill Gazaway, focused on identifying corn hybrids that are resistant to the reniform and Southern root-knot nematode. Corn hybrids selected were based on availability of those hybrids suitable for cultivation in the South.

Greenhouse evaluation of corn hybrids for resistance to the Southern root-knot nematode and reniform nematode were conducted at the Plant Science Research Center of Auburn University. Ninety-two transgenic and non-transgenic corn hybrids were evaluated for host suitability to both nematodes species and compared to a nematode susceptible cotton standard — DPL 555 BG/RR.

Reniform nematodes were collected from various cotton field locations across Alabama and the Southern root-knot nematode originated from the Auburn University, E. V. Smith Research Center. Reniform and Southern root-knot nematodes were propagated and increased on cotton (ST 5599BG/RR and DPL 555BG/RR, respectively) in the greenhouse. After 60 days, cotton plants were removed and each nematode species was extracted separately from the soil by combined gravity screening and sucrose centrifugal floatation.

Nematode eggs were collected from the root systems by shaking the excised root systems for four minutes in 0.6 percent NaOCl and sieving. Both nematode species were standardized to 1,000 juveniles or vermiform life stage and eggs per 2 milliliters of water for inoculation.

Separate evaluations were conducted for each nematode species. Corn hybrids were grown in 150cm3 Conetainer filled with a loamy sand soil (72.5-percent sand, 25-percent silt, 2.5-percent clay, ph 6.4). The soil was autoclaved twice at 121 degrees C. and 103.4 kPa for two hours on two consecutive days. Seeds were planted and allowed to germinate and grow for five to seven days after which the hybrids were inoculated with the standardized solutions of each nematode species.

Greenhouse experiments were arranged in a randomized complete block design with five replications and each test was conducted twice. Sixty days after inoculation, nematodes were extracted from the soil and eggs were removed from the roots as previously described. After enumeration of reniform vermiform and egg populations, reproductive factor values (Rf = final population / initial population) were determined. Any RF values more than one would indicate the nematode species is increasing in population numbers. Nematodes are not reproducing when the RF values are less than one.

All data was analyzed using general linear model procedures (GLM) of SAS. Means were separated with Fisher's protected least significant difference test (P < 0.05).

Researchers currently are in the process of screening current and discontinued corn hybrids for signs of resistance to Southern root-knot nematodes in the greenhouse. So far, none of the corn hybrids screened exhibit any resistance to Southern root-knot nematodes. Increases in Southern root-knot populations as well as considerable root galling were noted for all corn hybrids tested. All of the corn hybrids screened for resistance to the reniform nematode are resistant to the nematode and do not allow for reproduction of this nematode species.

More corn hybrids are being screened, but until a corn hybrid with good resistance to Southern root-knot is identified, corn should not be grown in fields with established populations of this nematode species, the researchers recommend.

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