Stem and calyx mould in pome fruit

A recently concluded Hortgro-funded study investigated the most common culprits and how to control them. By Anna Mouton.

Saprophytic fungi help keep the global carbon cycle spinning by digesting dead organic matter, but, unfortunately, their food sources include the stems and calyx leaves of stored pome fruit. The resulting furry fungal growth is harmless yet understandably off-putting to consumers. Mouldy stems and calyces can lead to significant financial losses when exporters reject affected fruit.

A Hortgro-funded study conducted by master’s student Inge Block looked into the causes and control of stem and calyx mould. She was supervised by Dr Cheryl Lennox of the Fruit and Postharvest Research Programme in the Department of Plant Pathology at Stellenbosch University and co-supervised by Hortgro market-access specialist Dr Julia Meitz-Hopkins.

The main players

Block sampled three apple pack houses in the Western Cape in 2020–2023. “We knew there was a problem,” she says, “but it was important to identify the specific fungi present. If we know that, we can adjust our management strategies for those fungi.”

She examined 2 073 apples representing eight widely-grown cultivars. The fruit had been stored in controlled atmosphere for six months. Most had been treated with either fludioxonil or pyrimethanil after harvest. All the cultivars grew mould — Block found mould in more than 85% of the fruit in half of her sample batches.

Mould was transferred to an artificial growth medium in Petri dishes for further assessment. Block obtained 93 fungal isolates from her apple samples and 19 fungal isolates from a previous survey of pears. She grouped them according to their visual and microscopic appearance. The groups were then further identified based on genetic testing.

Approximately 78% of the fungi turned out to be Alternaria species. Cladosporium and Epicoccum species represented another 10% each. “What I found very interesting was that these fungi would always occur together,” observes Block. “When I put the mould on selective media, two or all three would grow.”

Block induced mould by inoculating these fungi onto fruit stems, but the fungi did not cause decay when inoculated into wounds. Stem and calyx mould were more severe when fungi from different groups were combined than when they were inoculated individually.

“It’s called a complex,” notes Lennox. “The fungi support each other.”

Performance in Petri dishes

Block tested the sensitivity of her isolates to the fungicides fludioxonil, pyrimethanil, thiabendazole, difenoconazole, and fluopyram in the laboratory. She inoculated 22 fungal isolates from apples and six fungal isolates from pears on artificial growth media containing different fungicide concentrations.

She incubated the fungi for five days before measuring their growth and calculating the fungicide concentrations that would provide either 50% or 95% control.

“Fludioxonil and pyrimethanil are the two go-to fungicides that we can use in the postharvest setting for apples and pears,” says Lennox. “We assume we’re bringing fungi in from the orchard, so we need to know if our treatment is appropriate.”

The results showed that Alternaria and Epicoccum species were sensitive to fludioxonil, whereas Cladosporium species were less sensitive.

Pyrimethanil was less effective than fludioxonil for all three groups. “That’s generally known, but pack houses use pyrimethanil as a resistance-management strategy,” explains Lennox. “They’ll use fludioxonil for one year and pyrimethanil the next.”

Thiabendazole failed to inhibit the growth of most of the fungi in the laboratory trials. Difenoconazole and fluopyram were evaluated against only Alternaria arborescens isolates, but both were generally effective.

Fungicides in practice

Block also examined the efficacy of fludioxonil and pyrimethanil drenches in three apple cultivars. Fludioxonil was applied at 299 mg per litre for 30 seconds, and pyrimethanil was applied at 500 mg per litre for one minute. The fruit were then dried before being stored either unpackaged or in bags at -0.5 °C in a regular atmosphere for six months.

Two bags were assessed: unperforated bags made of low-density polyethylene of 50-micron thickness and micro-perforated bags sold for packaging grapes.

Control fruit that was neither drenched nor stored in bags developed no stem and calyx mould. Other results were not consistent across cultivars.

Cripps Pink and Cripps Red apples had the most stem and calyx mould when untreated and stored in unperforated bags. Drenches and perforated bags, either separately or in combination, significantly reduced mould in these cultivars. However, drenching was less effective at preventing calyx mould when apples were packed in unperforated bags.

Fuji had less mould overall than the other cultivars, but fludioxonil drenching did not protect Fuji — it always has to be different — from developing mould in unperforated bags. On the contrary, in unperforated bags, there was more calyx mould in fludioxonil-treated than in untreated Fuji apples.

Sanitiser dips containing either stabilised biocide or a combination of hydrogen peroxide and peracetic acid were compared with a standard calcium hypochlorite solution in Early Red One and Golden Delicious. These apples had been in regular atmosphere storage for three months before treatment.

Results were similar to those for fungicides: treated fruit developed no mould when placed in perforated bags.

“Perforated bags make a huge difference,” says Lennox. “Fungicide drenches are protective on the surface of the fruit, but when fruit is packaged in unperforated bags, mould can re-emerge on calyx sepals and fruit stems.”

Back to basics

“Our research has shown that fludioxonil at the registered dose is effective against the fungi isolated from stem and calyx mould when applied as a postharvest treatment,” says Lennox. “It is important that the fungicide dose and fruit exposure time are carefully monitored to ensure adequate efficacy.”

For drenches to be effective, all the fruit in the bins must receive a fungicide coating sufficient to leave a protective residue. Many variables affect residue deposition — for example, the bins may not be drenched long enough, or the drench solution may have deteriorated over time. Strict quality control of this process is therefore indispensable.

Pack-house sanitation is equally essential. Saprophytic fungi are ubiquitous, and their spores can readily build up in storage facilities throughout the season. More spores mean more opportunities for stem and calyx infections, so pack houses would do well to sanitise aggressively during the off-season and to sample their facilities to ensure that their protocols are working.