Agriculture and Agri-Food Canada, Southern Crop Protection and Food Research Center, Vineland Station, Ontario L0R 2E0, Canada

Background and objectives
Brown rot of peach caused by Monilinia fructicola is the most important disease of peach in North America. Pre-harvest applications of fungicides are currently used to control brown rot, but there is a public interest in the use of safe biological alternatives. Isolates of Pseudomonas syringae from apple leaves significantly suppressed apple scab under field conditions [1]. Some of these isolates showed promising results in controlling post-harvest brown rot on peach (Northover and Zhou, unpublished, 1996). The objective of this research was to determine the potential use of isolate MA-4 as a pre-harvest application to reduce brown rot on peach fruits.

Materials and methods
The experiment was conducted in a 6-year-old research orchard (cv. Loring), spaced 6x4.5 m, with a Latin square (5x5) design. The biocontrol agent (BCA), isolate MA-4 of P. syringae, was cultured in potato dextrose broth in a bioractor (12 l) with continuous stirring at 200 r.p.m. for 72 h at 22C. Cells of the BCA were harvested and adjusted to 5x107 cells/ml for the field applications. There were five treatments: (i) BCA 1f-MA-4 applied once at Aug 15, 1997; (ii) BCA 1s-MA-4 applied once at Aug 29; (iii) BCA 2x-MA-4 applied twice; (iv) water check 2x; (v) calcium check 2x (1% CaB'y-10% Ca; 0.5% boron). Treatments 3-5 were applied on Aug 15 and Aug 29, 1997. The percentage of infected peaches was assessed both on and under each tree every 2-3 days. On Sep 8, 1997, 100 apparently healthy peaches were randomly harvested and placed without touching each other in wooden trays. After incubation at 22C for 2 days, the percentage of peaches with brown rot lesion(s) was determined.

Results and conclusions
At harvest, the incidence of brown rot-infected fruits in the water check was 14.7%. Single applications of MA-4 (BCA 1s and BCA 1f) reduced the disease by 30-34%, similar to the treatment of CaB'y. The treatment of two MA-4 applications (BCA 2x) gave a 65% disease reduction. After 2 days' incubation of visually uninfected fruits, the incidence of brown rot was 48% in the water check. Single applications of MA-4 (BCA 1s and BCA 1f) and treatment of CaB'y gave numerical reductions of 18-19%, whereas two applications of MA-4 gave a significant reduction of 40%. The results indicate that two pre-harvest applications of the isolate MA-4 can significantly delay the development of brown rot of peach fruits, resulting in lower disease incidence and longer shelf life.

1. Zhou T, DeYoung R, 1996.In Tang W et al., eds, Advances in Biocontrol of Plant Diseases. China Agricultural University Press, Beijing, pp. 396-399.