Botrytis control in practice

A six-pack of strategies to help prevent pre- and postharvest losses.

Botrytis grows on dead and damaged plant tissues, producing airborne spores. In contrast to some other fungi like Cladosporium and Penicillium, Botrytis doesn’t grow on indoor surfaces such as ceilings. Therefore, Botrytis control must focus on the orchard.

Flowering is associated with increased Botrytis infections in many plants. In blueberries, the long flowering period that overlaps with ripening fruit complicates chemical control. Flowers are most susceptible from opening to senescence, with infections often leading to fruitlets dropping, resulting in yield loss. Severe flower infections may progress to twig blight.

Infections of mature fruit cause postharvest decay. Spores can infect micro-cracks and other openings. As Botrytis will grow even at temperatures just above freezing, decay may advance despite good cold-chain maintenance.

Effective Botrytis control requires a shotgun rather than a silver bullet. Combine the strategies in the following six areas for the best results.

1. A strong start

Countering Botrytis begins with orchard establishment. High relative humidity favours spore germination and increases infection risk. Therefore, Botrytis will be more challenging to manage in damp microclimates with poor air circulation.

The influence of climate is also reflected in regional differences. Botrytis is a bigger problem in the Western Cape than in the more northern parts of South Africa, as winter rainfall often accompanies flowering in the Western Cape.

Cultivars differ in their susceptibility to postharvest decay generally and Botrytis specifically. Thin-skinned berries and tight bunches are more prone to infection.

Dense plantings and canopies limit air circulation, promoting humid conditions that favour Botrytis infections. Wider spacings and open canopies not only create a drier environment but also facilitate better spray coverage and orchard sanitation.

Sparser canopies enable labourers to identify and remove all mature fruit and diseased vegetation that can act as breeding grounds for fungi. Ample light penetration is good for the plant and bad for Botrytis. Research has shown that direct sunlight significantly reduces spore germination.

2. More on management

Besides keeping the canopy of plants open throughout the season, removing any branches that touch the ground will improve air circulation, especially in existing high-density orchards.

Pruning debris is an excellent substrate for Botrytis. Prunings and fallen leaves have been shown to be the primary source of Botrytis spores in some crops. Don’t leave prunings in the orchard.

Botrytis will also grow on weeds, including those treated with herbicides. Weed control and removal are best done before blueberries come into flower. If weeds are problematic during flowering and fruiting, hand-weeding is the safest option. Avoid using strimmers during flowering and fruiting as they scatter weed fragments onto the blueberry plants.

Be mindful of irrigation scheduling in cultivars prone to cracking, especially when rain is expected. High soil moisture and relative humidity combine to increase the water content and pressure inside berries, leading to cracks that open the door to Botrytis and other postharvest decay organisms.

3. Monitor to mitigate risk

Effective Botrytis control relies on combining information from three sources: orchard history, scouting, and weather data.

Everyone knows that history repeats itself, which is why it’s worth keeping a close eye on orchards with a history of Botrytis. These blocks may warrant more aggressive chemical control earlier in the season than blocks without a record of Botrytis problems.

Scouting during the season allows timely detection and control of infections on flowers and fruit. Spray applications may not be necessary if signs are minor and conditions are dry. All risk factors relevant to the block need to be considered when selecting a control strategy.

Weather is a critically important driver of Botrytis risk. Moisture and moderate temperatures boost spore formation and germination. High relative humidity and rainfall also induce berry cracking.

Models exist for estimating the Botrytis risk based on weather data or the number of hours of leaf wetness. The apps associated with certain weather stations incorporate Botrytis risk models to help inform decisions about control. These are not replacements for scouting.

In the pack house, regular testing of air and surface samples can alert producers to rising spore counts, indicating the need to intensify pack-house sanitation.

4. Spray applications

Growers should consult their technical advisers on what to spray when. There are only a few active ingredients registered for Botrytis control on blueberries, and some can only be applied a limited number of times per season. Control options become even more restricted once harvesting starts and residues come into play.

Rotating FRAC codes is essential, since Botrytis has a well-known tendency to develop fungicide resistance if sprays aren’t alternated.

Softer products, such as biologicals and potassium bicarbonate, extend the spray arsenal and are valuable for lower-risk periods.

People have different approaches to spray applications. One strategy is to use the harder chemicals early in the season to suppress Botrytis, preventing populations from building up and releasing masses of spores later. The downside is that growers may exhaust their chemical options before the season ends.

Another strategy is always to spray based on a risk assessment and use softer products whenever possible. The harder chemicals are kept in reserve for high-risk periods, such as extended wet weather.

The choice of products and timing of applications also depend on factors such as the persistence of the active ingredient and its safety to beneficial organisms, including bees. This is particularly important during pollination, when bees are active, but Botrytis risk can be high.

Regardless of what product is applied, its efficacy is only as good as the spray coverage. Growers should calibrate their spray equipment carefully and consider using apps such as Dropsight to check spray deposition.

5. Your first loss is your best loss

Decay organisms such as Botrytis have co-evolved with fruit. Senescence and decomposition are necessary for fruit to release their seeds. The success of postharvest practices in delaying these processes hinges on optimal harvest maturity.

When planning the week, prioritise decay-sensitive cultivars and high-risk blocks. When rain is forecast, pick as much as possible, especially of high-risk cultivars or in high-risk blocks. Don’t resume harvesting until the plants have dried.

If harvesting falls behind due to weather or other reasons, growers should strip-pick the orchard by removing all fruit that has developed any colour. Likewise, it’s advisable to strip-pick mature fruit when it has been exposed to more than 10–15 mm of rain.

Strip picks can be painful, but in fruit production, your first loss is your best loss. There is nothing to be gained by packing post-optimal or high-risk fruit.

Also, keep in mind that fruit quality and shelf life often decline towards the end of the picking season, typically after most of the crop has been removed. These later berries may be more suitable for local sales or processing.

6. Postharvest practices

Botrytis enters the pack house with fruit, so be vigilant about removing damaged or decayed berries. Keep the pack house clean and sanitise regularly. This will reduce Botrytis spores, as well as discourage decay-causing fungi that can establish themselves inside the pack house.

Sanitation should extend to anything that comes into contact with berries, including picking buckets and harvest trays.

Rapid postharvest cooling is critical as it slows berry senescence and suppresses fungal growth. Cold-chain maintenance does the same, as well as preventing condensation inside packaging and on fruit.

SO₂ fumigation and sheets are effective in limiting decay, and controlled atmosphere containers, modified atmosphere bags, and ethylene absorbers can be helpful.

Lastly, packaging profoundly impacts postharvest fruit quality, as it affects temperature, humidity, and SO₂ levels. Packaging was the topic of a recent SATI newsletter. Although aimed at table-grape growers, the principles apply to other fruit types.