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 (CWSI) increased with decreased irrigation. In 1992, early blight disease (caused by the fungus Alternaria solari) 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.