STEM-END ROTS: THE INFECTION PORTAL

To design and implement an effective control strategy for stem-end rots, the pathway for infection needs to be determined. Three possible pathways for infection (1. aerial borne spores infect the picking wound, 2. fungi growing inside the stem tissue are activated by picking, 3. fungi growing just under the cuticle are smeared over the wound by secateurs) were tested using a variety of harvesting techniques. These included plucking, clipping pedicels to a button as in commercial practice, clipping pedicels at 2, 4 and 6 cm long, removing the extra-cambial layers before cutting the stem, and treating the picking wounds with various fungicides. The fruits were then stored and ripened at 20 o C in a controlled temperature room. Avocados were cut when ripe, assessed for rots and fungi from stem-end rots isolated and identified. Results showed that entry via the picking wound is a major portal for pathogens initiating post-harvest rots because there was an inverse relationship between the incidence of post-harvest rots and the length of the pedicel remaining after harvest. Disease incidence and severity was highest in fruits harvested by plucking. This was mainly due to an increase in the incidence of rots associated with Colletotrichum spp. and to a lesser extent with Phomopsis sp. Applications of fungicides to the picking wounds had variable effects. Dipping the wounds of plucked fruit in a Sportak/Benlate mixture significantly reduced rot incidence and severity. The mixture had no effect when applied to the cut end of pedicels clipped to a button. Dipping secateurs in the mixture before clipping each pedicel was also ineffective in reducing rots. However, dipping the button in Benlate alone was more effective. Slicing off the extra-cambial layers before clipping the pedicel through the xylem elements was ineffective, probably because it proved impossible to remove the extra-cambial layers without damaging the xylem elements. This study shows that control of stem-end rots can be achieved by applying a systemic fungicide to the cut end of clipped fruit at harvest.