Research highlights the importance of the orchard in pre- and postharvest Botrytis infections.
From the grower’s field to the consumer’s fridge: Botrytis is ever-present and ready to invade and degrade plant tissues. Hence, BerriesZA and the Post-Harvest Innovation Programme are co-funding a multifaceted project that aims to attack the problem from several angles.
“We are never going to get rid of Botrytis,” says Prof. Karin Jacobs, “but if we can reduce its numbers, we’ll have better outcomes.”
Jacobs researches microbial ecology and fungal taxonomy in the Department of Microbiology at Stellenbosch University. She is leading the assault on Botrytis and recently shared some preliminary results of field and pack-house investigations.
In the orchard
Botrytis cinerea is the most common species of Botrytis affecting blueberries. The fungus is incredibly adaptable, thriving on a wide range of living and dead plant tissues. It forms various structures that allow it to survive unfavourable conditions, resuming active growth and reproduction as soon as its environment improves.
Most new Botrytis infections are initiated by spores called conidia. Conidia are tiny — about 23 000 would fit on the head of a pin — and easily carried by air. To study Botrytis dynamics in blueberry orchards, Jacobs and her team counted the spores in the air from two Western Cape farms.
They collected three replicates of 50 litres of air at four sites in each orchard every two weeks over the course of a season. The air samples were processed in the laboratory to capture any spores on artificial growth medium in Petri dishes.
The Petri dishes were incubated, and the resulting fungal colonies were identified and counted. Counts are expressed as CFUs (colony-forming units) per m3 of air. The underlying idea is that each colony represents a viable spore.
Spore counts fluctuated substantially between sampling events. On the one farm, the values varied from zero to a peak of about 65 CFUs per m3. Samples collected under two types of netting were compared with samples collected in un-netted berries. None of these consistently had the highest counts.
On the other farm, counts varied from zero to a peak of about 35 CFUs per m3. Samples were collected in orchards of two cultivars, one of which yielded higher counts on almost all the sampling dates.
For context, Chinese researchers conducted controlled trials in greenhouse-grown grapes. They found that 42 CFUs per m3 could cause disease in about a third of exposed plants. This suggests that the spore numbers detected by Jacobs’ team are likely high enough to cause infections under conducive environmental conditions.
Jacobs’ team is currently analysing the Botrytis numbers, together with temperature and rainfall data, to understand the factors associated with elevated spore counts. So far, the findings align with work on other crops, showing that most infections are perpetuated within those crop systems, rather than originating from external sources.
“We wanted to know where the Botrytis comes from,” says Jacobs. “The assumption was that it comes from neighbouring grapevines. But we didn’t see this.”
In the pack house
Another part of the Botrytis project focused on pack houses. Air samples were collected at three Western Cape pack houses every two weeks during the six-month-long packing season. Baseline samples were taken after the pack houses were sanitised but before packing started.
Sampling was conducted in various areas within each pack house, and control samples were collected from the air outdoors. The air samples were processed in the same way as those from orchards. Temperature and humidity data were also recorded.
“When we assess indoor air, we compare the results to outdoor air,” explains Jacobs. “If the counts from outdoors are higher than indoors, then you know the indoor environment is relatively clean. You also look at species composition. If the species indoors and outdoors are more or less the same, then the indoor environment is acceptable.”
In general, the results for the outdoor and indoor air samples suggest that fungal spores were not originating in the pack houses. Spore counts were frequently lower inside than outside, except in the main packing areas, where the counts were often significantly higher than outside. “We suspect that the berries are bringing fungi from outside,” says Jacobs.
Botrytis was not consistently present in the pack-house air samples. When it was detected, spore counts were typically very low. Counts above 40 CFU per m3 of air were only recorded in one area of one pack house at the November sampling dates.
In two of the pack houses, Cladosporium was the most common fungal genus, and in the third, it was Penicillium. Unfortunately, this isn’t cause for celebration, as some Cladosporium and Penicillium species can also cause postharvest decay.
Implications for control
The preliminary analysis of orchard spore counts indicates that the largest numbers of Botrytis spores are present during flowering and harvesting. Increases in Botrytis during flowering are well-established for other crops — the fungus seems to ramp up its activity in response to pollen.
Jacobs believes that more traffic in orchards during the season contributes to higher spore counts. “The Botrytis spores are so light and loosely attached that anything moving past can knock them into the air,” she says.
Regarding the other fungi, the research team is currently inoculating blueberries with Cladosporium and Penicillium isolated from pack houses to investigate their potential to cause decay.
“We want to see whether the symptoms look similar to those of Botrytis,” says Jacobs. “Perhaps people are assuming mouldy berries have Botrytis when it’s actually Cladosporium, given the tremendously high Cladosporium levels in pack houses.”
Whatever fungi are present, sanitation reduces spore numbers. During the sampling phase of the project, Jacobs’ team provided the counts to the pack houses, prompting some of them to clean and sanitise their facilities, which significantly lowered subsequent counts.
“Pack houses should think about testing their air regularly,” says Jacobs. “Then they can clean as soon as high spore counts are detected.”
This season, Jacobs and her team are delving deeper into the relationship between fungal spores in the air and on berries. They will be sampling air and berries simultaneously to see whether more airborne spores are associated with higher spore counts on berries.
Meanwhile, producers already implement many measures to control Botrytis in the field and the pack house. Read the top tips from industry members in our companion article. Or watch a video clip of research in action.
