Furrow irrigation results in large gradients in water and residual nitrate nitrogen from upper to lower field positions. Water is much less and nitrogen much higher on the lower end of furrow irrigated fields. Reduced water lowers crop yield on the lower end and increased nitrogen can adversely or positively affect different crops. In sugarbeet, less water lowers root yield while increased nitrogen lowers sucrose and increases loss to molasses. Root aphids were much worse on the lower field position resulting in a substantial loss in sucrose but no affect on yield or loss to molasses. Wet soil can limit root aphid but resistant cultivars are more effective in an IPM approach to root aphid control.

Sugarbeet variety trials were conducted on Pullman clay loam soil with furrow irrigation at Bushland, Texas in 1993, 1994, and 1995. The fields were 1200 ft long and were irrigated with 18 to 36 hr sets and 2 to 4 hr of tail water. Nitrate nitrogen was measured to 4 ft depth at 200 ft intervals down the field prior to planting sugarbeet. Water intake time was estimated at the same points. Sugarbeet yield, quality, and root aphid infestations of resistant and susceptible cultivars were also estimated for 200 ft intervals.
Root aphids were rated visually on a scale of 1 to 5 with 1 = no aphids and 5 = totally infested plot areas. Ratings were taken on each plot of 36 cultivars replicated six times each year. The cultivars were divided into three categories. The 12 most susceptible and 12 most resistant were grouped into susceptible and resistant categories, respectively.

As expected water intake time was much longer near the water input end of these fields (Table 1). Average intake time ranged from 24 hr at 100ft to 5 hr at 1100 ft. The actual amount of water intake would not vary nearly as much since intake is much greater early in a wetting event than later. However, actual intake probably varied by a factor of nearly 2-fold over the distances measured. Water intake time was not measured in the crops preceding sugarbeet, which was mostly wheat. Intake times in wheat would be similar since they were irrigated in a similar manner. Wheat yield was noticeably lower on the lower field positions with less water intake time.
The differences in water intake time in wheat resulted in a substantial gradient in residual nitrate nitrogen at sugarbeet planting (Table 1). This occurred even though the preceding wheat crop was fertilized more on the upper end of the field than the lower end. Differential fertilization of wheat was done whenever a substantial gradient in nitrogen was measured prior to planting wheat. Thus, to a considerable extent the gradient in nitrogen occurred despite our attempts to avoid it by differential fertilization. Higher residual nitrogen adversely affects sugarbeet sucrose content and loss of sucrose to molasses. This explains our interest in controlling residual nitrate nitrogen at uniform and low levels.

Sugarbeet root aphid were nearly absent on the resistant cultivars at all field positions (Table 1). In contrast, root aphid were higher and increased much more with distance down the field for susceptible cultivars. Root aphid management by IPM includes a component of soil water. Frequent watering or rainfall and high soil water limits the reproduction and spread of the root aphid. Some of this effect is probably physical since this soil cracks badly when dry allowing easy spread of the insect. Other factors such as temperature, disease, or parasites could impact the aphid adversely in wet soil.

Root aphid infestation on susceptible cultivars varied considerably depending on field position. On the upper, wetter end of the field aphids were moderate with aphid scores of 1.3 to 2.21 at 100 and 500 ft, respectively. Aphid score increased to greater than 3 at 900 ft. This is a fairly severe infestation. By 1100 ft mean aphid score was 3.6. Damage was limited and similar all three years because infestations did not occur until near harvest in October. In severe drought years, such as 1980, there was severe plant wilting due to root aphid by July. Damage in 1980 was 50% loss in root yield and nearly 50% loss in sucrose. This rendered susceptible cultivars a total loss due to root aphid. Damage on resistant cultivars in 1980 was very slight. No gradients in damage with either resistant or susceptible cultivars were noted in 1980. The damage seemed uniform across furrow irrigated fields.

Since resistant cultivars in 1993-95 were nearly uninfested at all field positions, field gradients in yield or quality can be assumed to be a response to water and nitrogen. The root yield decline of resistant cultivars with distance down a furrow irrigated field can be assumed to be a response to less water (Table 2). The yield loss with resistant and susceptible cultivars was nearly identical. Thus, one can conclude that the late infestation of root aphids noted here did not further decrease root yield.

Sucrose response to distance indicates losses to both agronomic factors and root aphids (Table 2). Sucrose of resistant and susceptible cultivars was identical at 100 ft however by 1100 ft sucrose was 1.0% lower for susceptible cultivars. This indicates a 0.7% loss to increasing nitrate nitrogen and an additional 1.0% to root aphid with susceptible cultivars. This is a substantial and significant economic loss.

Loss to molasses increased with distance for both cultivar groups (Table 2). Root aphid do not substantially worsen loss to molasses because they remove some impurities from the root while increasing others.

This research has substantial implications for water, nitrogen, and root aphid management with graded furrows. Nitrogen gradients in furrow irrigation are substantial and have significant implications for management of all crops. Water gradients affect yield and can affect pest problems such as the root aphid. Wet soil can substantially limit damage from the root aphid however resistant cultivars are more effective and have been stable for resistance for 25 years or longer in Texas.