Control of Moldy-Core Decay in Apple Fruits by β-Aminobutyric Acids and Potassium Phosphites

Alternaria alternata is the predominant fungal pathogen responsible for moldy-core decay in the 'Red Delicious' apple fruits. The failure of registered fungicides to control the disease necessitated the search for alternative methods. Here we report that DL-β-aminobutyric acids (BABA) and potassium phosphite are capable of controlling moldy-core in apple fruits in the laboratory and the field. Laboratory tests involving inoculation of wounded mature fruits revealed that decay by A. alternata was 40 to 58% inhibited by 50 μg/ml and completely inhibited by 500 μg/ml of potassium phosphite. DL-BABA, R-BABA, and S-BABA at a concentration of 500 μg/ml inhibited decay formation by 82 to 90%. Potassium phosphite, BABA, and S-BABA inhibited decay formation even when applied 6 to 48 h postinoculation. In vitro tests indicated that BABA compounds did not affect conidial germination or mycelial growth, whereas potassium phosphite partially inhibited both developmental stages of the fungus. Results suggest that BABA inhibits fruit decay development indirectly, probably by inducing resistance in the host tissue, whereas potassium phosphite may act directly against the fungus. A preliminary field trial in a commercial apple orchard showed that three foliar applications of BABA or potassium phosphite, starting from the beginning of bloom until petal fall, reduced the number of fruits infected with moldy-core by 40 and 60%, respectively, relative to fruits from nontreated control trees. Results suggest that potassium phosphite and BABA could provide adequate control of moldy-core disease in apple.