Ongoing research aims to optimise old and discover new methods to maintain fruit quality. By Anna Mouton.
Delivering fresh fruit to distant markets partly depends on chemical control of decay-causing organisms. “The importance of orchard practices can never be discounted,” says Dr Pieter Louw, portfolio manager of crop protection at ExperiCo. “Quality and control start there, and you have to get those things right. But you also have to get your chemical applications right.”
Most post-harvest decay of pome and stone fruit is caused by fungi. Preharvest spray applications definitely help to reduce infections in the orchard, but Louw explains that growers should be mindful of the risk of fungicide resistance. “When there is a good product for post-harvest use, in the pack house, the concern is that using it in the orchard can speed up the development of fungicide resistance.”
Any population of fungi will contain individuals that are more or less sensitive to a given chemical due to natural variation. When the population is exposed to that chemical, the more sensitive individuals die. Over time, only the less sensitive individuals survive and reproduce, so the entire population becomes more resistant. The development of resistance is an ongoing challenge wherever humans use chemicals to control other organisms.
According to Louw, fungicide resistance is more likely to occur in an orchard than a pack house, because the orchard is an open environment containing an enormous diversity of fungi, any of which may have genes for resistance. He is leading a Hortgro-funded research project to identify fludioxonil and pyrimethanil resistance in Monilinia laxa and Botrytis cinerea populations from stone-fruit orchards.
“So far, we are happy that we haven’t seen problems with fludioxonil, which is the core of this project,” says Louw. However, research is ongoing, and some isolates are still to be tested.
Surface versus systemic
Chemicals used for decay control in the pack house can be broadly categorised as sanitisers or fungicides. Sanitisers are generally effective against a broad spectrum of organisms and used to reduce microbes in water or on surfaces. “The important thing to understand about sanitisers is that they are contact agents,” stresses Louw. “Dipping an infected fruit in a sanitiser is not going to make a difference.”
Chlorine is probably the most common sanitiser used at pack houses. Although chlorine is inexpensive and effective, safety concerns may limit its use in future. Louw recently concluded a Hortgro-funded project that assessed alternative sanitisers for pome-fruit pack houses. He found that a commercial product containing a combination of peracetic acid and hydrogen peroxide was just as effective as chlorine when tested against Botrytis cinerea and Penicillium expansum spores.
Louw has also obtained good results when applying ozone gas to kill Botrytis cinerea and Monilinia laxa spores on plums. Not as good, however, as fludioxonil treatment. Further research is necessary.
“Even though some fungicides are exclusively contact-acting, similar to sanitisers, many are locally systemic to some degree ,” says Louw. Depending on the fungicide, it will be able to move into or through the skin of the fruit and penetrate wounds. Fungicide residues also remain on fruit after treatment, thereby giving extended protection against infection.
Once infection has occurred, however, treatment is not going to be much help. “If the fungus has already begun developing – and that can happen very quickly – then you will not obtain 100% control,” says post-harvest researcher Arrie de Kock. “This is why you should treat the fruit as soon as possible after harvest. Because during harvest the fruit can suffer small wounds that allow the fungi to enter and start developing.”
While at ExperiCo, De Kock and fellow researcher Ida Wilson conducted laboratory trials of fludioxonil efficiency in stone fruit. He cautions that higher residues do not necessarily provide better decay control. “We know that with fludioxonil we can get good control with a relatively low residue,” says De Kock. “If your residue is 1-2 parts per million, don’t apply more product. It’s not necessary and you are just wasting fungicide.”
Consistency is key
Stone fruit are very susceptible to post-harvest decay caused by wound infections. To prevent decay, most stone fruit are treated with a fungicide on the pack line using an atomiser. Atomisers allow the application of fungicides without excessive wetting.
De Kock and Wilson surveyed post-harvest fungicide application practices at stone-fruit pack houses in different regions. They placed water-sensitive cards on the pack line to assess deposition, and they also measured exposure time and fungicide residues.
“Our main finding was that fludioxonil gives very good decay control, provided people use the atomisers correctly,” says De Kock. “The problem we highlighted was that in some cases coverage was not up to standard.”
The main reasons for poor deposition were too few or incorrectly spaced nozzles, variable delivery by different nozzles on the spray boom, and poor calibration. Spray drift occurred in some cases because the nozzles were not entirely covered by the cabinet.
Besides ensuring that fungicides are uniformly applied to fruit, pack houses should also take note of their residue levels. De Kock recommends that fludioxonil-residue levels should not be less than 1 part per million. His research showed that this can be achieved with exposure times of as little as ten seconds.
In addition to consistency when atomising, De Kock emphasises the importance of consistent temperatures in controlling decay. “You want the relative humidity around the fruit to be as high as possible, but you don’t want condensation on the fruit, so there should be as few breaks in the cold chain as possible,” he explains. “Once your fruit are cold, you must keep them cold.”
Read more about the work De Kock and Wilson did on effective fungicide application on stone fruit in the June 2019 issue of Fresh Quarterly. That issue also contains other articles on post-harvest topics such as minimising moisture loss in pears during storage, and dynamic control systems for fruit storage.