The Effect of Foliar-Applied Manganese in Mineral and Complex Forms with Amino Acids on Certain Defense Mechanisms of Cucumber (Cucumis sativus L.) Against Powdery Mildew

Manganese (Mn) is an essential micronutrient with positive effects on enhancing plant tolerance against fungal diseases. On the other hand, it has been suggested that metal–amino acid complexes are more effective than mineral sources in supplying Mn and improving the translocation and distribution of these metal nutrients in plants. Meanwhile, certain amino acids, that is, lysine and methionine, play an important role in crop resistance to biotic stresses. Little information is, however, available regarding the effect of foliar-applied Mn in mineral and complex forms with amino acids on crop tolerance to powdery mildew caused by Podosphaera fuliginea. In this nutrient solution experiment, the effect of foliar-applied Mn, in the form of MnSO₄, Mn-methionine (Mn-Met) and Mn-lysine (Mn-Lys), on certain defense mechanisms of cucumber (Cucumis sativus L. cv. Espadana RZ) against powdery mildew was investigated. Different Mn sources were applied as foliar spray 4 days before and 6 days after inoculation with P. fuliginea. The results revealed that Mn application prior to pathogen inoculation decreased the disease in the treated and newly growing leaves; however, the efficiency of Mn–amino acid complexes in disease suppression was greater than that of MnSO₄. Post-inoculation spray of Mn had no effect on the disease reduction of the newly growing leaves, but all Mn sources diminished disease severity equally in the treated leaves. In such leaves, regardless of the application time, the higher content of cell wall lignin was found in the plants treated with MnSO₄, in comparison with those treated with Mn–amino acid complexes. In the newly growing leaves, the foliar application of Mn–amino acid complexes before inoculation resulted in a significant increase of leaf cell wall lignin content, whereas MnSO₄ was ineffective. Pre-inoculation spray of MnSO₄ led to increasing PO and PPO activities in the treated leaves, whereas Mn–amino acid complexes had no significant effect on the activity of these enzymes in the same leaves. Application of Mn-Met and Mn-Lys after pathogen inoculation led to a significant decrease of PPO activity in the treated leaves. In general, the results of the present study revealed that the time of Mn foliar application could have a considerable effect on controlling cucumber powdery mildew. Foliar application of Mn before infection with P. fuliginea could strengthen the defensive structures of the plant and help to prevent the disease incidence.