N TEIXIDO¹, M NOGUERAS², J MUNOZ², J USALL¹ and I VINAS ¹

¹Postharvest Unit, CERTA, Centre UDL-IRTA, 177 Rovira Roure Ave., 25198 Lleida, Catalunya, Spain: ²Dept of Food Technology. UdL, 177 Rovira Roure Ave., 25198-Lieida, Cataiunya, Spain

Bakground and objectives
Alternaria spp. are known to be the major pathogens responsible for core-rot of apples [1], a post-harvest disease which is difficult to detect because of the lack of external symptoms and the fact that apparently healthy fruit can be infected and internally rotted [2]. It is difficult to control core-rot, because once inside the fruit, fungi are protected from fungicides, and conditions for their growth are very conducive.

Biological control using microbial antagonists has been considered a desirable alternative to the use of chemicals to control diseases. The aim of the present study was to look for an effective antagonistic agent to control core-rot caused by Alternaria alternata on Starking Delicious apples.

Material and methods
A total of 358 different strains were isolated from fruits and leaves of apples and tested for efficacy to select those which could successfully control Alternaria rot on Starking Delicious apples. Laboratory and field assays were carried out by injecting the best antagonist, the pathogen and both together directly into the apple core to evaluate biocontrol agent. Finally, the population dynamics of the biocontrol agent were studied in NYDB medium and in Starking Delicious apple cores.

Results and conclusions
Following a large screening programme, a strain of the bacterium Pseudomonas syringae (CPA-10) isolated from apple leaves was found to be particularly effective in controlling Alternaria rots and was chosen as the best antagonist to continue this research.

In laboratory assays, control of core lesion diameter by P. syringae was 96 and 73% at 4.10³ cfu/ml and 10⁴ cfu/ml Alternaria concentrations, respectively, in artificially inoculated apples.

Field experiments demonstrated that P. syringae reduced the incidence of Alternaria rot when artificially inoculated in field conditions and after 4 months in cold storage. A study of the population dynamics showed that the biocontrol agent could grow effectively in the apple core.

This strain was demonstrated to have good features to control the core-rot caused by A. alternata in the field, and during cold storage.

References
1. Ellis MA, Barrat JG, 1983. Plant Disease 67, 150-152.
2. Spotts RA, 1988. Compendium of apple and pear diseases, pp. 29-30.