The 2011 ASHS Annual Conference

Citrus diseases are a serious threat to profitable fruit yields and quality. Disease control with continuous use of agrochemicals only is not an acceptable, sustainable option due to environmental contamination, pesticide residues in the food chain, and pesticide 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 (Mycosphaerella citri) infection of 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 ten replications. The treatments included full strength Hoagland’s nutrient solution (T1), one tenth concentration Hoagland’s nutrient solution (T2),  full strength Hoaglands minus Mg (T3), minus Ca (T4), minus B (T5), minus Mn (T6), and minus Zn, Cu, Mo, Fe (T7). Nutrient solution specific to each treatment was applied weekly. A high humidity was maintained in the greenhouse to encourage greasy spot spore germination and leaf infection. The foliar symptom expression and defoliation of citrus plants naturally infected with greasy spot fungus was highest in T2, T4, and T7 treatments. The full strength balanced nutrient solution greatly reduced the greasy spot infection. Electrolyte leakage, which is an indicator of compromised cell membrane integrity, was highest in T2, T4 and T7 treatments. Leaf sap pH was lower in the different nutrient deficient treatments (pH 6.0-6.2) compared to the balanced nutrition (pH 6.3). Stem diameter and SPAD chlorophyll index values were lowest for T2 and T4 treatments. 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.