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

Florida's water resources have been burdened by competition between a steadily increasing population and 2 million acres of irrigated agriculture. During chronic dry weather, regulatory agencies may impose mandatory cutbacks in water drawn for agricultural use to protect groundwater levels, regardless of irrigation methods used by growers. We investigated the impact of water application deficits on tomato (Lycopersicon esculentum Mill. ‘Sunny’) under micro‐irrigation (drip irrigation), where normally little surplus water is applied above crop requirements. Full‐bed plastic‐mulched tomato was grown for three springs on a sandy Spodosol in south Florida. Following 3 wk for crop establishment, three irrigation treatments were applied through the rest of the growing season: (i) full irrigation based on daily evapotranspiration (ET) estimates and soil water tension, (ii) 15% deficit from full irrigation, and (iii) 30% deficit. Treatments were applied daily by adjusting the irrigation time. Estimated seasonal tomato plant ET ranged between 5.5 and 7.4 in., and full irrigation ranged between 3.9 and 7.1 in. Soil water tension at the 6‐in. soil depth under the micro‐irrigation tubing fluctuated between 2 and 20 cbars, and was consistently greater with decreased irrigation. Tomato plant height decreased about 1.5 in., and crop water stress index (CWS1) increased with decreased irrigation. In 1992, early blight disease (caused by the fungus Allernaria solani) incidence was 50% more severe, and in 1994, blossom end rot incidence was five times more severe in the 30% deficit treatment compared with full irrigation. Three‐year mean total marketable tomato yield decreased from 11.09 to 8.83 tons/acre as irrigation decreased. The yield decrease was chiefly due to decreased production of extra‐large fruit, which receive the highest price on the fresh market. An economic analysis showed that 15 and 30% irrigation reductions would reduce gross revenue by 15 and 22%, respectively. Therefore, deficit‐irrigating micro‐irrigated tomato could cause substantial economic loss through decreased crop marketability. Research Question Florida's water‐regulating agencies may impose mandatory cutbacks 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 cutback, 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 microirrigation. Study Description An irrigation deficit study was conducted at the University of Florida Southwest Research and Education Center in Immokalee. Full‐bed, plasticmulched, micro‐irrigated tomato was grown for three springs on a sandy Spodosol. 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. Soil Pomello fine sand (>98% sand, <1% organic matter) Experimental design Randomized complete block, four replications Irrigation treatments (i) fill irrigation based on daily ET estimates and soil water tension, (ii) 15% deficit fiom full irrigation, and (iii) 30% deficit Irrigation method One plastic drip‐irrigation tube per row, 12 in. emitter spacing, 0.5 gal/min per 100 ft flow rate Fertilization N, P2O5, and K2O were applied at 200, 100, and 216 lb/acre, respectively. Most of the N and K2O were applied through the imgation system Tomato cultivar ‘Sunny’ 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 in. shorter, and crop water stress index increased as irrigation deficit increased. Early blight severity was increased by 50%, and blossom end rot incidence was five times higher in the 30% deficit treatment, compared with full imgation. 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 ). 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 ), so an irrigation cutback 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. Irrigation deficit effect on marketable tomato yield, and economic impact. 1992 1993 1994 3‐yr Estimated Irrigation mean gross deficit X Lg. Total X Lg. Total X Lg. Total total revenue % ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐tons/acre‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ $/acre 0 2.20 6.74 8.65 15.74 3.65 10.78 11.09 7490 15 1.48 6.29 6.88 13.99 2.38 8.36 9.55 6360 30 1.39 5.99 5.04 12.61 2.25 7.86 8.83 5820 ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐Probability level‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ Contrasts Linear 0.01 0.30 0.01 0.01 0.03 0.01 0.01 0.01 Quadratic 0.15 0.91 0.97 0.82 0.17 0.12 0.28 0.22 Probability at which the orthogonal contrast is significant.