1Golan Research Institute, University of Haifa, Kazrine 12900, Israel; 2Department of Plant Pathology, ARO, Newe Yaar Northern Research Center, Haifa 31900, Israel

Background and objectives
Powdery mildews caused by Oidium mangifera, Podosphaera leucotricha and Sphaerotheca pannosa are major foliar pathogens of field-grown mango, apple and nectarine trees, respectively. Timely application of effective fungicides, such as sterol biosynthesis inhibitors (DMI), are the main factors in disease control in the field. Reducing pesticide levels on food crops highlights the need for alternative methods for disease control. Foliar sprays of sodium bicarbonate, potassium silicate and mineral or plant oils offer a new method of reducing fungicide usage and severity of powdery mildew. Our previous studies [1, 2] demonstrated the effectiveness of monopotassium phosphate fertilizer (MKP=KH2PO4) in inducing local and systemic protection against powdery mildew, and its control on cucumbers and wine grapes. This study evaluates the efficacy of foliar sprays of MKP in alternation and in tank-mix with DMI fungicides as a new approach in controlling powdery mildew in field-grown fruit trees.

Materials and methods
Field experiments using mango, apple and nectarine trees were conducted in their respective commercial orchards during 1993-97 in the Golan region of Israel. Various numbers of treatments for each crop and year were arranged in a randomized complete block design. Plots consisting of four to six trees were replicated five times. Treatments included a control, foliar applications of commercial DMI, 0.4-1% solutions of MKP, and a treatment using alternating applications of MKP and the DMI fungicide. MKP plus Triton X-100 (0.025%) and fungicides were sprayed to run-off at various intervals. In other trials MKP (1%) was tank-mixed with DMI. These treatments were further evaluated on large-scale demonstration plots. Naturally infected bloom clusters, leaves and fruits were rated based on a 0-4 scale: 0, no symptoms; 4, >50% of the selected organ area infected with powdery mildew. The percentage of infected organs was determined. ANOVA was used for statistical analysis.

Results and conclusions
Foliar sprays of MKP, DMI and an alternating treatment with MKP and DMI inhibited development of powdery mildews on flowers and bloom clusters, leaves, and fruits and leaves of field-grown mango, apple and nectarine trees, respectively. The alternation treatments were similar to DMI fungicides in controlling powdery mildew. However, application of DMI only, without application of MKP on the dates on which it was applied in the alternation treatment, was less effective in controlling the disease than either the DMI or alternation treatment. This indicates that the use of MKP has a significant role in disease control and can reduce the number of fungicide treatments against powdery mildew by at least 50%. These results were confirmed in large-scale demonstration trials conducted in commercial orchards. Our findings also revealed that tank-mix treatments of 1% MKP solution with DMI at a recommended rate were the most effective and provided >93% protection against O. mangifera and P. leucotricha compared with control. MKP solutions were not phytotoxic to plant tissue. Lower yield was obtained on control, untreated trees, probably due to powdery mildew infection on flowers or fruit. The inhibitory effectiveness of MKP makes it a potential major component of an integrated pest management program.

1. Reuveni M, Agapov V, Reuveni R, 1995. Plant Pathology 44, 31-39.
2. Reuveni M, Agapov V, Reuveni R, 1995. Canadian Journal of Plant Pathology 17, 247-251.