Three experts on post-harvest disease share advice for growers. By Anna Mouton.
Post-harvest diseases start in the orchard. “What you do before harvest will largely affect what happens after harvest,” says Dr Pieter Louw, portfolio manager of crop protection at ExperiCo. Implementing the steps below will set fruit up for successful storage.
1. Keep a clean house
If there’s one thing that entomologists and pathologists agree on, it’s the importance of orchard sanitation. “Especially the removal of rotten fruit in the orchard,” says Dr Julia Meitz-Hopkins, post-harvest pathologist in the Department of Plant Pathology at Stellenbosch University.
Meitz-Hopkins explains that decaying fruit release masses of spores that perpetuate the disease cycle. She has even observed fruit with symptoms of bull’s-eye rot still hanging on trees. “I could clearly see the spores of the fungus on the fruit,” she recalls.
Fungi thrive on the concentrated nutrients in fruit. Don’t feed the fungi in your orchard – remove all fruit that remain after harvest.
2. Weed out fungal hosts
“Weed management is really important, especially the closer you get to harvest,” says Dr Cheryl Lennox, leader of the Fruit and Post-harvest Pathology Programme in the Department of Plant Pathology at Stellenbosch University.
Fungi such as Botrytis and Penicillium survive on weeds, and love growing on flowers. Lennox cautions against allowing weeds to flower at the same time as trees because more flowers mean more fungal spores that can land on blossoms and lead to calyx infections. She also advises against planting cover crops that flower simultaneously with trees.
When it comes to harvest, Lennox recommends mowing and removing weeds. Alternatively, in warm weather, mow early so that the organic material has dried completely by the time picking starts.
3. A breath of fresh air
“All of these fungi want moisture to be able to establish in the orchard on the fruit surfaces,” says Lennox, “so it’s important to open up the canopy of your trees to let the proper airflow through.” Open canopies also discourage fungal infections by increasing exposure to ultraviolet light.
“If you’re going to be putting on fungicides, then the penetration of the fungicides into the tree is so much better if you’ve opened the canopy,” adds Lennox.
In general, cultural practices that improve tree health and fruit quality will also reduce losses from post-harvest diseases. “Fruit quality at harvest affects susceptibility of fruit to agents of decay,” comments Louw. “A fruit must be grown correctly, in a clean orchard, and harvested at the right time, and handled in the right way, to have legs.”
4. Optimal harvest maturity
Growers know that optimal maturity at harvest is the key to maintaining fruit quality and minimising disorders during storage. Increasing maturity also increases susceptibility of fruit to post-harvest diseases.
More mature fruit have higher sugar levels that promote the growth of decay-causing organisms. Softer fruit is more prone to damage and easier for microbes to infect, and increased maturity is accompanied by a drop in defences against decay. All these changes are simply more reasons to focus on getting the timing of harvest right.
5. An eye on the weather
“Harvesting wet fruit is an absolute no-no,” asserts Lennox. Moisture encourages germination of spores and promotes growth of fungi. Meanwhile, wet conditions make the skin of especially stone fruit more sensitive to infection. “If you put wet fruit into a bin, the chances of it drying off are zero,” says Lennox.
In the case of red and bicoloured apples, says Meitz-Hopkins, one of the problems is that the growers tend to keep the apples on the trees as long as possible to get good colour, putting them at risk of early rains.
Harvesting in extreme heat or cold can also increase the risk of post-harvest diseases. “If it’s warmer, then the fruit is probably going to be dryer, which is better,” says Lennox, “but then your fruit is probably going to have a very high field temperature.”
Bins should be put in a cool area, out of the sunshine, and taken to the pack house as soon as possible. Lennox recommends step-down cooling before fruit goes into cold storage.
6. Damage control
“Harvest time is always so stressful and pressured,” says Lennox. “But your pickers need to know how to pick properly.” She points out that pickers need to be monitored, and attention paid to items such as jewellery that can wound fruit. Pickers also need to be trained to put fruit in the bin gently.
“Every little bang, every little wound, is a potential portal for ingress of pathogens,” warns Lennox. She adds that this applies equally to transport. “Be careful – especially when driving over dirt roads – of the fruit bouncing around.”
Post-harvest diseases usually only show up after harvest – hence the name. But sometimes decay already occurs in the orchard. Meitz-Hopkins recommends that pickers shouldn’t be motivated to maximise volumes, as this can lead to rotten or damaged fruit in the bins.
“It’s really important to not bring the rotten fruit to the pack house,” says Meitz-Hopkins. “When the fruit are washed it just makes a soup of spores and that then infects the other fruit.”
7. Prevention is better than cure
“We are very spray-orientated,” says Louw. “I think the focus should shift toward cultural, physical or biological practices. But rainfall during harvest carries a high risk and requires fungicide sprays.”
Lennox agrees that fungicide applications before harvest can help prevent decay. “What you want is a fungicide with a bit of residual activity, to provide a little insurance.” Both Lennox and Louw stress that spray programmes must not promote the development of resistance, and that applications before harvest must meet regulatory and market requirements.
Meitz-Hopkins believes that growers could be making more use of full-bloom sprays. “Some of the pathogens, like Alternaria and Botrytis, are always around, so you will already get infections at fruit-set.” Early-season fungicide applications could prevent these infections without the residue concerns attached to spraying before harvest.
Ultimately, says Louw, the best control is integrated control. “We’re talking about basic practices from the orchard to the pack house. Don’t think that chemicals will cover up mistakes – you have to include and follow the right steps.”