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Effect of Nitrogen and Vegetative Growth on Plant Resistance to Bollworm Helicoverpa Zea, in Selected Bt Cotton Varieties

J. Breen-Pierce, R. Flynn, C. Ellers-Kirk and C. French


 
ABSTRACT

The effects of plant nitrate levels and vegetative growth on plant resistance to cotton bollworm, Helicoverpa zea were evaluated in 3 varieties of Bt cotton. Deltapine 90B, Deltapine 33B, Paymaster 1215 (DP5415, DP90B, PM1215) and two recurrent parents DP5415 and DP90, the recurrent parents of DP33B and DP90B respectively.

In August, under low bollworm pressure, the number of damaged squares was significantly lower in the low nitrogen/ low vegetation treatment in DP90 compared to high vegetation treatments and a high nitrogen / low vegetation treatment. In September, under higher pressure, the high nitrogen/high vegetation treatment had significantly more damage in both DP90B and its' recurrent parent. Plant only 7" taller produced significantly less lint. DP33B and DP90B high vegetation plots produced 7 and 13% less lint than the low vegetation plots. Bt varieties produced twice as much cotton as their recurrent parent varieties. Bt varieties had 5 times less minor bollworm damage and 20 times less severe damage to squares. There were no differences in damage among Bt varieties.

High vegetative growth may increase bollworm damage but is most likely due to higher egg deposition rather than a direct influence on expression of resistance. Damage may be higher at very high nitrogen levels, but this effect is not exclusive to Bt varieties, and was, in fact, more evident in the recurrent parent varieties. The effect of nitrogen is unlikely to affect very susceptible insects but can affect the level of resistance against less susceptible pests such as bollworm and beet armyworm.



Reprinted from Proceedings of the 1999 Beltwide Cotton Conferences pp. 1234 - 1236
©National Cotton Council, Memphis TN

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Document last modified Monday, Jun 21 1999