Foliar applications of mono-potassium phosphate fertilizer inhibit powderymildew development in nectarine trees

Foliar applications of 1% solution of mono-potassium phosphae (MKP = KH2PO4 ) fertilizer plus Triton X-100 (0.025%), sterol inhibiting (SI) fungicides, and lan alternating treatment of phosphate fertilizer and SI fungicides in hibited development of the powdery mildew fungus Sphaerotheca pannosa on fr uits and leaves of nectarine trees in 3 consecutive years. The effectivenes s of the alternating treatments with an appropriate systemic fungicide and 1% solution of MKP was similar to that of the commercial treatment with the systemic fungicides alone. However, application of the systemic fungicides only, omitting the phosphate treatment when they were scheduled, was signi ficantly less effective than either the phosphate or the alternation treatm ents. These results indicate that the use of phosphate fertilizer has a sig nificant role in disease control, enabling up to 50% reduction with number of fungicide treatments required for powdery mildew control. These results were confirmed in a large-scale demonstration trial conducted in a commerci al orchard in 1995. This trial also revealed that a tank-mix treatment of 1 % MKP solution with an SI fungicide provided the best protection against po wdery mildew. Phosphate solutions were not phytotoxic to plants. Fruit harv ested from MKP-treated trees by either alternation or tank-mix were similar or larger in their size distribution to those harvested from the commercia l fungicide-based treatment. Fruit harvested from control, nontreated, tree s were much smaller and produced ca. 23% nonmarketable fruits. The inhibito ry effectiveness of MKP against powdery mildew in nectarine makes it a pote ntial major component of all integrated pest management program.