Deficit Irrigation of Micro-Irrigated Tomato Affects Yield, Fruit Quality, and Disease Severity

T. A. Obreza*, D. J. Pitts, R. J. McGovern, and T. H. Spreen

T. A. Obreza, D. J. Pitts, and R. J. McGovern, Univ. of Florida, S. W. Florida Res. and Education Cent., P. O. Drawer 5127, Immokalee, FL 33935

T. H. Spreen, Univ. of Florida, Food and Resource Economics Dept., P. O. Box 110240, Gainesville, FL 32611.

Florida Agric. Exp. Stn. Journal Series No. R-04626. This research was funded by a grant from the South Florida Water Management District.
*Corresponding author (taob@ifas.ufl.edu).

RESEARCH APPLICATION SUMMARY

Research Question

Florida's water-regulating agencies may impose mandatory cut-backs in agricultural pumping to protect the state's limited groundwater resources during droughts, regardless of the producer's irrigation method. Deficit irrigation (deliberate under-irrigation) is likely to reduce yield or quality of horticultural crops, with severe economic consequences. Under a mandatory water-use cut-back, producers who have installed highly-efficient micro-irrigation systems may be severely affected. We investigated the impact of water application reductions to tomato under micro-irrigation, where little surplus water is applied above crop requirements.

Literature Summary

The correlation between evapotranspiration (ET) and crop yield is high. If ET is reduced due to insufficient soil moisture, yield reductions can be expected. Tomato is highly-sensitive to water stress, especially during the fruiting stage. Approximately 10% of Florida's vegetable production land has been converted to micro-irrigation (drip irrigation) from subirrigation (water table management) due to its substantially higher application efficiency, as well as its economic and production advantages. Micro-irrigation systems provide water efficiently by precisely meeting the actual crop ET demand, and apply little surplus water if managed properly. The volume of water pumped to grow a Florida tomato crop can be decreased by about 90% after conversion from subirrigation to micro-irrigation.

Study Description

An irrigation deficit study was conducted at the Univ. of Florida, Southwest Florida Research and Education Center in Immokalee. Full-bed plastic-mulched, micro-irrigated tomato was grown for three spring seasons on a sandy Spodosol.

• Soil: Pomello fine sand (>98% sand, <1% organic matter)

• Experimental design: Randomized complete block, four replications

• Irrigation treatments: 1) full irrigation based on daily ET estimates and soil water tension, 2) 15% deficit from full irrigation, and 3) 30% deficit

• Irrigation method: One plastic drip-irrigation tube per row, 12 in. emitter spacing, 0.5 gal/min/100 ft flow rate

• Fertilization: N, P2O5, and K2O were applied at 200, 100, and 216 lbs/acre, respectively. Most of the N and K were applied through the irrigation system

• Tomato cultivar: 'Sunny'

Irrigation treatments were applied daily by adjusting the irrigation time. Soil water tension, crop water stress, plant height, and plant disease severity were monitored. Yield and size distribution of tomatoes were measured with respect to irrigation treatment at harvest, and results were analyzed from an economic standpoint.

Applied Questions

What are the effects of deficit-irrigating by 15 or 30% on micro-irrigated tomato?

The full irrigation treatment applied between 3.9 and 7.1 in. of water per growing season, depending on tomato plant ET and rainfall volumes and distribution. Soil water tension at the 6 in. soil depth under the micro-irrigation tubing fluctuated between 2 and 20 cbars, and was consistently higher with decreased irrigation. Tomato plant height was about 1.5 inches shorter, and crop water stress index (CWSI) increased as irrigation deficit increased. Early blight severity was increased by 50%, and blossom end rot incidence was 5 times higher in the 30% deficit treatment, compared to full irrigation. Total marketable tomato yield decreased as irrigation deficit increased in two of three seasons. Reduced irrigation consistently reduced production of the most valuable extra-large sized fruit (Table 1).

What are the economic consequences?

An economic analysis showed that 15 and 30% irrigation reductions would have reduced gross revenue by 15 and 22%, respectively (Table 1), so an irrigation cut-back on micro-irrigated tomato could cause substantial economic loss through decreased crop marketability. Because fresh tomato marketability is also affected by visual appearance, increased disease severity due to plant water stress could result in a total crop failure

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