56 The Journal of Cotton Science 13:56–66 (2009) http://journal.cotton.org, © The Cotton Foundation 2009 AGRONOMY & SOILS Effect of Irrigation Rates on Three Cotton (Gossypium hirsutum L.) Cultivars in a Root-knot Nematode (Meloidogyne incognita) Infested Field Terry A. Wheeler*, J. Wayne Keeling, James P. Bordovsky, John Everitt, Kevin F. Bronson, Randal K. Boman, and Benjamin G. Mullinix, Jr. T.A. Wheeler, J.W. Keeling, J.P. Bordovsky, J. Everitt, K.F. Bronson, R.K. Boman, and B.G. Mullinix, Jr., Texas AgriLife Research, 1102 East FM 1294, Lubbock, TX 79403 *Corresponding author: ta-wheeler@tamu.edu ABSTRACT A large-plot study was conducted for ive years to determine the effect of irrigation rate and fer- tigated nitrogen on the performance of Fibermax (FM) 989BR, Stoneville (ST) 5599BR, and Pay- master (PM) 2280 BG/RR in a ield infested with the root-knot nematode (Meloidogyne incognita). The objective was to determine if irrigation rate would effect the relative ability of susceptible and root-knot nematode tolerant cultivars to perform in a root-knot nematode infested ield. The split- plot design included irrigation and nitrogen rates as the whole plots (base irrigation rate = B, 75% of base rate [75%B], and 125% of base rate [125%B], with nitrogen applied through the center pivot system and being proportional to irrigation rate) and cultivars as the sub-plot with three replications. The B and 125%B rates had higher lint yield and gross loan value ha -1 (P < 0.05) than the 75%B rate when averaged across the three cultivars. The root-knot nematode tolerant cultivar ST 5599BR, averaged equal or higher lint yields than FM 989BR at all irrigation rates when averaged over ive years. ST 5599BR and FM 989BR had similar gross loan values ha -1 at all three irrigation rates. ST 5599BR had higher lint yields and gross loan values ha -1 than PM 2280 BG/RR (2003 – 2006) at the B and 125%B rates. The Paymaster cultivar is susceptible to root-knot nematode. Increasing irrigation rate did not improve the proitability of susceptible cultivars in a root-knot infested ield, compared to the response of a tolerant cultivar. C otton (Gossypium hirsutum L.) production in the semi-arid Texas Southern High Plains presents numerous challenges. Water availability is of concern since pumping capacities from the Ogallala aquifer are generally limited and cotton is mostly produced under various levels of deicit irrigation. The low energy precision application (LEPA) irrigation concept was developed to maximize the use of seasonal rainfall and increase irrigation eficiencies in arid and semi-arid areas (Bordovsky, et al., 1992). Cultivar choice is also an important factor that affects cotton yield and quality. Irrigation recommendations with LEPA are based on the needs of the crop for water at any given time. Reference evapotranspiration (ET r ) is a calculated estimate of potential water loss from a well-watered turf area and is based on environmental conditions. Crop evapotranspiration (ET) which is equal to ET r x a crop-speciic coeficient is based on both envi- ronmental conditions and the model used to predict the amount of water that a crop needs as it grows and develops. Cotton is particularly sensitive to the cumu- lative water availability during lowering (Guinn and Mauney, 1984). In the Southern High Plains of Texas, deicit irrigation is most likely to occur during the lowering period, when crop coeficients are near 1.0. Nitrogen is the second most limiting factor for semiarid cotton production, following water when considering the three factors of water, nitrogen, and heat units (Morrow and Krieg, 1990). Nitrogen fertil- izer response is positively related to irrigation well capacity and irrigation rate (Bronson et al., 2001). Much of the N fertilizer in center-pivot-irrigated cot- ton produced in the Southern High Plains is injected through the irrigation water (i.e. fertigation). The performance of different cultivars may be affected by water availability. However, diseases can also affect cultivar performance. Cotton is a host for the southern root-knot nematode (Meloidogyne incognita (Kofoid & White) Chitwood). Approxi- mately 40% of the area planted in irrigated cotton in the Southern High Plains, is infested with M. in- cognita (Robinson et al., 1987; Wheeler et al., 2000). There are no commercial upland cultivars available with high levels of resistance to M. incognita. Re- sistance is deined as a reduction in the ability of the