Integrated control of powdery mildew on apple trees by foliar sprays of mono-potassium phosphate fertilizer and sterol inhibiting fungicides

Foliar sprays of 0.5-1% solutions of KH₂PO₄ (plus Triton X-100), commercial systemic fungicides, and an alternating treatment of phosphate fertilizer and systemic fungicides inhibited development of the powdery mildew fungus, Podosphaera leucotricha (Ell. and Ev.) E. S. Salmon, on shoots and leaves of apple trees. The effectiveness of alternating an appropriate systemic fungicide with a 1% solution of mono-potassium phosphate (MKP) was similar to that of the commercial treatment with the systemic fungicides. However, application of the systemic fungicides only, omitting the phosphate treatments when they were scheduled, was significantly less effective than either the phosphate or the alternation treatments. These results indicate that the use of phosphate fertilizer has a significant role in disease control, enabling reduced numbers of fungicide treatments against powdery mildew by up to 50%. These results were confirmed in large-scale demonstration trials conducted in commercial orchards in 1996 and 1997. In these trials, the tank-mix of 1% mono-potassium phosphate solution with a half rate of an appropriate fungicide was as effective or superior to that obtained by the standard fungicide treatment. Phosphate solutions were not phytotoxic to plant tissue. The inhibitory effectiveness of mono-potassium phosphate fertilizer makes it a potential major component of an integrated pest management program.