Hot water treatment, dual action against rot
The primary aim of hot water treatment is to control fungal diseases that affect the storage of apples. The hot water applied directly to the fruit disrupts the development of pathogenic fungi: it inhibits the germination of spores, blocks the elongation of the germ tube and deteriorates the spores. Biochemical studies of fruit compounds have also revealed a defence reaction in the fruit, which is initiated in the epidermis. Several teams have demonstrated the activation of the plant's defence mechanisms via the synthesis of certain enzymes, including heat shock proteins. HWT therefore has a dual mode of action.
The main rots observed on fruit after leaving storage and packaging are lenticel rot, known as Gloeosporium rot, and rots caused by the genus Phytophthora. These are followed by Nectria canker, depending on the geographical area, and wound rots caused by several species such as Penicillium, Botrytis and Monilia. In conditions that are adapted to apples, the technique is not very effective against these wound rots. Scientific articles cite the effectiveness of the technique against Penicillium at a temperature of 55°C for 2 minutes. But at these temperatures, the majority of apple varieties show burns on the epidermis. On the other hand, HWT has proved effective against Gloeosporium rot and Phytophthora spp1 . Most of the trials carried out within the network concerned these diseases, and the application of hot water was always carried out by dipping.