T. F. Branson (Ann. Entomol. Soc. Amer. 64:861-863, 1971) reported that Tripsacum dactyloides is resistant to corn rootworm. A bridging mechanism for moving Tripsacum genes into corn has been achieved through wide cross hybrids between Tripsacum dactyloides and Zea diploperennis (M. W. Eubanks, Econ. Bot. 49:172-182, 1995). Efficacy of this genetic bridge for conferring natural rootworm resistance to corn has been demonstrated through a series of insect bioassays (M. W. Eubanks Amer. J. Bot. (suppl.):84:116, 1997; MNL 70:22-23, 1996; MNL 68:40-41, 1994). Under the auspices of NSF grant no. 9801386, a trial testing efficacy of the rootworm resistance under field conditions trait in crosses between Tripsacum-Z. diploperennis and corn was conducted at the USDA Northern Grain Insects Research Laboratory in Brookings, SD, during the summer of 1998.
Twenty to thirty-six seeds of each of the corn inbreds B73 and W64A, and four hybrid lines [B73 X Tripsacorn, B73 X (W64A X Sun Star), W64A X Sun Star, and W64A X Tripsacorn] were planted in a 65 ft X 45 ft plot in rows spaced 40 inches apart in May, 1998. The plot consisted of three test rows surrounded by buffer rows planted in NK4242. The first test row contained B73 and W64A; the middle row was planted in [B73 X (W64A X Sun Star)], and the third row was planted in (W64A X Sun Star) and (W64A X Tripsacorn). The plot was fertilized with 50-35-35 NPK at a rate of 215 lbs/acre on May 8, then planted on May 14. The test rows were hand planted one seed per hill to a depth of 2-2.5 inches at 9-inch plant spacing and infested with an approximate total of 400,000 Western corn rootworm eggs at a rate of 1400 eggs per foot in the three test rows. A biophenometer was placed in the soil to a depth of four inches. All rows were cultivated with a single row blue on May 28 and June 22. On July 16, all plants were dug from the test plots, and the roots were washed with a pressure washer. Then the plants were refrigerated until the roots were rated on July 27.
The roots were scored using the 1-6 Iowa rating scale (Hills and Peters ): 1 = no damage or only a few minor feeding scars; 2 = feeding scars evident, but no roots eaten off to within 1.5 inches of the plant; 3 = several roots eaten off to within 1.5 inches of the plant, but never the equivalent of an entire node of roots destroyed; 4 = one node of roots completely destroyed; 5 = two nodes of roots completely destroyed; 6 = three nodes of roots completely destroyed. Plants that have a root rating of 1 or 2 are resistant. The results are presented in Table 1.
Table 1. Root ratings of 1998 rootworm resistance field trials
|Line Tested||No. of plants||1||2||3||4||5||6|
|B73 X Tripsacorn||19||0||1||1||2||4||11|
|B73 X (W64A X Sun Star)||16||0||4||6||4||1||1|
|W64A X Sun Star||12||3||3||5||0||0||1|
|W64A X Tripsacorn||30||1||11||6||8||4||0|
Compared to the maize inbreds which had root ratings ranging from 3
to 6, at least one resistant plant with a root rating of 1 or 2 was recovered
in all of the hybrid lines tested. Lines that demonstrated greatest resistance
were (W64A X Sun Star) and (W64A X Tripsacorn). The field test confirmed
that the natural corn rootworm resistance trait observed in growth chamber
bioassays is also expressed under field conditions.
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