Balanced Mineral Nutrition Decreases Greasy Spot Incidence in Citrus
Diseases are a serious threat to profitable yields and quality of citrus. Disease control solely by continuous use of agrochemicals is not an acceptable, sustainable option due to environmental contamination, residues in the food chain, and potential development of pathogen resistance. Therefore, alternative integrated pest management (IPM) techniques need to be developed and evaluated. Mineral nutrition could increase or decrease disease resistance by affecting both plant and pathogen growth. To evaluate the effect of mineral nutrition on greasy spot disease incidence in citrus, a greenhouse hydroponics experiment was established for precise control of nutrient elements. ‘Valencia’ orange plants [Citrus sinensis(L.) Osb.] were grown in plastic pots filled with graded quartz sand automatically drip-irrigated with deionized water. The experiment was conducted in a completely randomized design (CRD) with seven nutritional treatments and 10 replications. The treatments included full-strength Hoagland’s nutrient solution (T1), one-tenth concentration Hoagland’s nutrient solution (T2), full-strength Hoagland’s solution minus Mg (T3), minus Ca (T4), minus B (T5), minus Mn (T6), or minus Zn, Cu, Mo, and Fe (T7). The nutrient solution specific to each treatment was applied weekly. A high humidity was maintained in the greenhouse to encourage greasy spot spore germination and natural leaf infection. Greasy spot incidence was up to 90% lower in the full-strength balanced nutrient treatment (T1) than in the nutrient-deficient treatments. Electrolyte leakage, which is an indicator of compromised cell membrane integrity, was higher in T2 and T4 than in the full-strength nutrient treatment. Leaf sap pH was lower in different nutrient-deficient treatments (pH 6.0–6.2) than in the balanced nutrition treatment (pH 6.3). Leaf nutrient concentrations, principal component analysis (PCA), and canonical discriminant function analysis (DFA) revealed that omitting any one of the nutrients can increase susceptibility to greasy spot. The results confirm Liebig’s law of minimum, and suggest that a complete balanced nutrition supply for citrus may significantly reduce the occurrence of greasy spot, reduce pesticide spray requirements, promote overall tree health, and enhance production efficiency.