Effects Of K₂HPO₄ On Resistance Induction Of Cucumber Against Diseases

The induced systemic resistance of phosphate (K₂HPO₄),KH₂PO₄ plus KOH,K₂HPO₄ plus fungicide and fungicide was bioassayed to Pseudoperonospora cubensis, Sphaerotheca fuliginea and Colletotrichum lagenarium in cucumber. The effects of different concentrations of K₂HPO₄ on the sporangium germination, the changes of main enzymes activity, the histopathology of the host plant and the field application were investigated in order to reveal their resistance mechanism and make basic study for the application in the field.K₂HPO₄ and KH₂PO₄ plus KOH could induce the cucumber to render resistance against Pseudoperonospora cubensis, Sphaerotheca fuliginea and Colletotrichum lagenarium. In those cases in which the number of fungicide applications was reduced by 50% by alternative phosphate sprays with fungicide applications, better protection against downy mildew than with K₂HPO₄ alone and KH₂PO₄ plus KOH. This treatment offered new possibilities for disease control.The influence of K₂HPO₄ on the sporangium germination of Pseudoperonospora cubensis was measured. The results demonstrated that K₂HPO₄ (1⁵0mmol/L) had no inhibition to the sporangium germination, but K₂HPO₄ (75mmol/L) had some inhibition to the sporangium germination. Inducing times had a big effect to systemic acquired resistance (SAR), and the more the times, the better the prevention. At the same time, the results also showed that K₂HPO₄ had induced systemic resistance in cucumber against downy mildew. Four foliar sprays significantly inhibited development of downy mildew in the field. Treatments of fungicide with K₂HPO₄ (50mmol/L), however, not only improved plant health, but also significantly inhibited downy mildew. This phosphate salt seemed to be more efficient than others, and its controlling effect catch up with the fungicide.The changes of POD,PPO,PAL and SOD activity were determined with treatments of K₂HPO₄ (50mmol/L). The results showed that the activity of POD,PPO and PAL in cucumber leaves inclined continuously on the second day after treatment, among which POD and PPO had their peaks on the fourth day after treatment, while PAL had a peak after 6 days. By contrast, SOD activity had no obviously change at...