DURHAM, NORTH CAROLINA

Duke University

BROOKINGS, SOUTH DAKOTA

USDA ARS

1999 field trial to test natural resource for rootworm resistance --Eubanks, E, Riedell, W 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 MNL73:30, 1999; Amer. J. Bot. (suppl.):84:116, 1997; MNL 70:22-23, 1996; MNL68:40-41, 1994). Under the auspices of NSF grant no. 9801386, a field trial testing efficacy of the rootworm resistance trait in crosses between Tripsacum-Z. diploperennis and corn under field conditions was conducted at the USDA Northern Grain Insects Research Lab in Brookings, SD, during the summer of 1998 (M. Eubanks and W. Riedell, MNL 73:29-30, 1999). Results of a second field test conducted during the summer of 1999 are reported here.

Thirty seeds of each of two corn inbreds, B73 and W64A, and seven Tripsacum-Z. diploperennis X corn breeding line crosses (9015, 9094 X7009, [8089 X (4021 X A188)], 97-5 X 97-1, 97-3 X 97-5, JY X 3029, and 5004X JW2) were planted at the USDA Northern Grain Insects Research Lab in May,1999. The 65 ft X 70 ft plot was flagged out and staked in six rows spaced 40 inches apart on May 6. The plot consisted of eight test rows surrounded with a corn buffer row on both sides of the test rows. On May 13, test rows were infested with 1400 viable Western corn rootworm eggs per linear foot of row. To accomplish infestation, eggs were mixed with room temperature agar and were applied to the soil through an anhydrous ammonia knife at a depth of 3 to 4 inches. Kernels were then hand planted into the furrow to a depth of 2 inches and a spacing of 9 inches. Two biophenometers were placed in the soil to a depth of four inches in two of the buffer rows of the experimental plot to monitor GDD accumulation.

All plants were dug and the roots washed and rated according to the Iowa (a.k.a. Hills and Peters) scale July 12, 1999. Root ratings are: 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. Results are summarized in Table 1.

Table 1. Results of Phase II 1999 Field Test, Brookings, SD
 
       
Root Ratings
Line Treated Res.  Susc. 1 2 3 4 5 6
97-3-2 X 97-5-5  26 1 25 0 1 2 4 6 13
97-3-1 X 97-5-2 18 3 15 0 3 6 4 3 2
97-5 X 97-1 28 3 25 0 3 7 11 7 0
97-3-1 X 97-5-3 20 2 18 0 2 3 5 5 5
(8009 X 4021) X A188 26 1 25 0 1 8 1 3 13
9015 24 0 24 0 0 4 3 9 8
9094 X 7009 21 14 7 0 14 2 2 2 1
JY X 3029 7 1 6 0 1 3 4 0 0
B73 25 0 25 0 0 3 7 5 10
W64A 27 0 27 0 0 0 5 8 12

Since the corn controls in this field test were severely damaged (Fig. 1), it is clear we had a heavy rootworm infestation in the field in 1999. This indicates that strong natural resistance is present among the segregating progeny of most of the above F6BC4 families (Fig. 1). Two-thirds of the (9094 X 7009) plants were resistant when tested in the field. Seed from this family is being grown and self or sib pollinated to increase this line for further development in the recurrent selection breeding program. Field test results in 1998 and 1999 have confirmed that the rootworm resistance trait selected under controlled experimental conditions in growth chambers is also present under field conditions.

Figure 1. Top row: roots of resistant plants from three F6BC4 Tripsacum-Z. diploperennis X corn hybrid families. Bottom row: roots of corn inbreds B73 and W64A. Infestation of Western corn rootworm eggs was at a rate of 1400 eggs per ft of row.
 
 


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