Thermotherapy: An effective technique for post-harvest treatment of apples

Many operators have equipped themselves to treat harvested fruits, especially apples, with hot water to prevent disease development. The latest trial results from CTIFL are reviewed.

Thermotherapy is a technique that is developing in the organic sector as well as in integrated fruit production, with a possible reduction in fungicide use (-2 to 4 IFT depending on the year).

Necessary conditions for successful treatment

According to trial results on five apple varieties, the effectiveness of thermotherapy on gloeosporioses ranges from 50 to 100%, which is comparable to, or even higher than, pre-harvest treatments. To achieve maximum effectiveness, treatment should be performed as quickly as possible, within 7 days after harvest. Similarly, for Phytophthora, the earlier the treatment, the better the effectiveness, which reaches 90 to 100% within 24 to 48 hours after contamination. For both targeted diseases, the ideal treatment is at 48/49°C for 2/3 minutes on fruits at room temperature, to avoid the risk of lenticels and energy loss. Thermotherapy also performs well on Colletotrichum and sooty molds. Claude Coureau (CTIFL) notes that "if the fruits arrive at the station already affected by sooty molds, the treatment will not remove it, but it will prevent its development during storage.

On the equipment side

There are currently two thermotherapy machines on the market: the Xeda machine, which is a pallet immerseur in a hot water bath (soaking), and the Covara machine, which has two stacked pallets showered with hot water. Their performance was tested in cooperatives in Limousin, as part of a regional project. For both machines, the results are promising with a drastic reduction in rotten apples after applying 50°C for 2 minutes immediately after harvesting. When using these machines, it is necessary to ensure that there is no decrease in water temperature during treatment due to the large volume of fruit. Therefore, this parameter must be constantly monitored with a maximum regulation of + or - 0.5 °C, with a display large enough for the forklift driver, in order to maintain a high level of efficiency. Water management is also an issue to be considered, especially for losses incurred during treatment, reheating after leveling the bins, and draining the soiled baths each evening and refilling them the next day.

Beware of phytotoxicity

Excessive temperatures can cause phytotoxicity. However, since 2019, lenticel blotch pit punctuations have sometimes appeared on apples. Symptoms generally appear within 24 hours after treatment. Factors that contribute to this include the variety and late picking. Other influencing factors are being studied, such as climate change, plot, and amplification of underlying physiological problems.

More tests need to be conducted on plums

To date, there is much less experience and results on the effectiveness of thermotherapy on plums than on apples and peaches. Several trials have already been conducted on the reduction of Monilia according to the types and varieties of plums, and different temperature/shower duration combinations. It is still necessary to test the sensitivity of varieties to burns and to verify the impact on bloom and fruit color evolution. Laboratory results cannot be directly applied to industrial conditions, and adaptations must be made to temperature/shower duration combinations and the machines used. Pascale Westercamp (CTIFL / CEFEL) suggests that "it would be interesting to combine hot water showering with a complementary treatment, using biocontrol products such as yeast or antagonist fungi or essential oils or possibly line disinfection techniques." The thermotherapy process must be adapted from one type of plum to another, which will be different, for example, for American-Japanese plums harvested in bins and stored for a long time or mirabelles harvested on trays and stored very little.

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These results were presented during the CTIFL webinar on February 4, 2021.