Biological and Agronomical Strategies for Controlling Skin-pitting of Kiwifruit

Skin-pitting of kiwifruit is a post-harvest disease caused by Cadophora luteo-olivacea. Inoculation occurs in the field but symptoms appear after about three months of cold storage. Incidence of the pathology is very variable over time but it can be quite high causing considerable damage. The aim of this study was to search for biological and agronomical control measures, which can reduce skin-pitting. For biological control, isolates of potential antagonists were made from the surface of stored kiwifruit sourced from fruit produced in two different orchards: one with high and the other with low incidence of disease. Five and nine different microorganisms were obtained from fruit of the first and second orchard, respectively. Isolated microorganisms were inoculated on PDA plates together with C. luteo-olivacea. The majority of the isolates did not influence pathogen growth, except for two strains very closely related to Pseudomonas libanensis (P. fluorescens subgroup), isolated from fruit from the orchard with low incidence of symptoms. The bacteria were tested in vitro against five isolates of each of the two most frequent strains of C. luteo-olivacea (A and B); all were significantly inhibited in growth, presenting a compact, mucous and orange-brown mycelium. The strong antagonistic activity of the two P. libanensis strains represented a promising result for further studies. For agronomical control, it is known that light has a positive effect on calcium accumulation in fruits and that a low calcium content contributes towards creating favourable conditions for pathogen development. In 2009, heavy summer pruning and calcium chloride treatments were practiced in three different orchards, to verify whether a higher light availability and an increase of the calcium content would have a positive effect on reducing disease incidence. Skin-pitting incidence was monitored during storage.