Botrytis is the most intractable disease we have in strawberries, and we have very limited options when it comes to new fungicides in the pipeline.

In order to direct the next work in that area, we have done a review of practices and technologies that can shift the needle when it comes to botrytis management, and produced a review document for SGNZ. It’s 20 pages and a relatively dense read, but the executive summary is only 2.5 pages, and reproduced below.

If you are a NZ strawberry grower with a somewhat scientific mindset and an interest in contributing to the industry’s direction in regards to botrytis strategies for the future, read on.

If, when you have read the summary below, you’re still keen to engage and willing to do so a handful of times as part of a “botrytis working group” via online video meetings, email me (Molly Shaw). I’ll send you the full review document to digest, and we’ll work to set up some group conversations over the winter months.

Executive Summary

An exploration of current and emerging practices and tools to combat botrytis in strawberries in New Zealand and reflection on their efficacy.

Botrytis in strawberries

Botrytis is ubiquitous, willing to rot almost any plant material that is dead or dying, and commonly resides quiescently in leaves of strawberry plants received from the nursery.  Heat treatment has been used successfully to eliminate botrytis on nursery transplants, but starting out clean at transplant does not eliminate the subsequent need for controls during flowering/fruiting.

Fruit rot most commonly starts through senescing anthers in the flower peduncle, and can lie dormant in the fruit until ripening and/or warm/wet conditions.

Botrytis grows fastest at 15–25°C, but can still grow even at very low temperatures near freezing.  Leaf wetness for 13 continuous hours at optimal temperatures can result in infection in strawberries.

Current and emerging practices

A review of current and emerging practices in botrytis in strawberries is reported.

Prediction systems – testing of a system using weather conditions to identify infection periods has been successful in Mid-Atlantic US states (a climate similar to NZ), and its use has reduced fungicide sprays while maintaining disease control.  Using the model to identify infection periods specifically in tunnels would help tunnel managers identify where they should improve humidity management.

Plant resistance – there are variations in susceptibility of different strawberry cultivars to botrytis. However to date the commercial fruiting characteristics (including yield) have been more of a driver of variety choice than botrytis susceptibility.

Fungicides – Applying fungicides extensively to the open flowers to protect senescing anthers has been the most successful strategy, for botrytis fruit rot prevention, even in dry climates. Fungicide resistance is a major problem.  Also, where old broad-spectrum materials are used, market tolerances are becoming problematic.  As a small industry in a small nation, NZ strawberry growers do not have access to the newest products, and residue tolerances of new botrytis materials are not established in NZ, so we are limited to use patterns that give <0.1ppm residue.  Consequently, the NZ growers’ repertoire is very limited and it is not growing significantly.

Biological fungicide performance has been uninspiring in many studies.  Botrytis is uniquely challenging to biocontrols as it is ubiquitous and active at much lower temperatures than the biocontrols are active.  However, there could be a unique application for biological fungicides delivered to flowers by bumblebees, for covered-crop growers.

Two nil-residue sprays seem worth trialing in NZ.  Surprisingly good results were achieved with peroxyacetic acid applied 3 days preharvest.  Adding potassium silicate to the fungicide spray tank or liquid feed regime also gave enhanced botrytis control.

Post harvest management – At harvest, holding picked fruit under 5°C is key to delay the development of infections, but the NZ produce cold chain is inadequate to achieve this.  

Cultural practices – Growing strawberries under cover creates a rain-free environment, and as such it is easier to control botrytis. However, the disease it is still a major issue in the autumn (cooler, with more humidity and higher plant density).

• Plant spacing – Planting at wider plant spacings only shows a slight improvement in botrytis control and only later in the season when plant canopy is dense.  Less dense planting isn’t profitable compared to the denser spacings, even when botrytis losses are considered.
• Leaf removal – Leaf removal has a yield penalty associated with it, and did not consistently improve botrytis control either.  Removing rotten fruit from the field gave no benefit, but removing it from within the canopy is still recommended.
• Humidity management in greenhouse tomatoes is crucial for avoiding botrytis.  Goals for tomatoes are to keep RH < 88%.  To achieve this, plants are “put to bed dry,” preventing leaf gutation.  Tunnel strawberries have room to improve humidity management.
• Plant shaking – This new cultural practice seems worth investigating in strawberries.  To date, the experience is only in NZ grapes, but grapes have seen significant botrytis reductions such that canopy shaking has been widely adopted in Sauv Blanc grown in Marlborough.

Plant nutrition – Avoiding excessive nitrogen, makes plants less susceptible to botrytis in a soilless planting system.  In soil, the connection of excessive N to botrytis levels is variable.  In general, plants with low or moderate vegetative growth tend to have firmer fruits, which is often correlated with less botrytis. Getting firmer fruit through foliar calcium applications gives variable results, seemingly dependent upon overall plant calcium status and use of a penetrant with foliar Ca applications.  The Grapes Futures program in NZ had success increasing fruit calcium levels when Ca was applied with a penetrant.

“Generative” plant growth parameters –  “Generative” plants have a strong ‘stocky’ structure, with shorter internodes, and thicker leaves, and stronger cell walls, while “Vegetative” plant structure could be categorized as ‘weak’ or ‘floppy,’ with thinner, larger leaves and weaker cell walls.  A consequence of stronger cell walls is a higher plant resistance to botrytis attack; conversely, plants grown in a “vegetative” environment are more susceptible to botrytis.

It takes far more skill to grow a strong generative plant that is resistant to pathogen attack than a vegetative plant.  Learning to use and optimize all the ‘levers’ available to grow a plant whose style of growth resists infection by botrytis is not easy, but is massively impactful and necessary in this era where fungicides will no longer rescue us.

Emerging Technologies

UV-C:  Up until now, overseas efficacy data has been exciting, but application at a practical farm level in NZ has been lacking.  Recently, a 2023/24 season trial in grapevines has shown good potential for use in berry crops. This technology seems to be at a mature enough stage in NZ to warrant strawberry grower trials.

RNAi:  This technology, which can be liked to a “plant vaccine,” is not yet near commercialization.  While exciting, there are variable efficacy results and the reasons for the variability are not well defined.

Recommendations

Establish a focus group of up to 5 engaged strawberry growers to choose the next demonstration/trial steps.

Technologies with low barriers/risks to trial and adopt:

• peroxyacetic acid applied pre-harvest
• potassium silicate added to fungicide applications

Techniques requiring more skill to adopt:

• Applying botrytis disease model to identify infection periods, especially in covered crops
• Applying horticultural techniques to grow plants that have stronger cell walls and are more resistant to infection by optimizing irrigation management, reducing nitrogen excess (potentially), reducing humidity, and increasing air flow.

Emerging technologies:

• UV-C, while not a silver bullet, has good potential to bring significant improvement to both botrytis control and mite control, and the technology is just mature enough to trial at a practical farm level in NZ.
• Plant shaking requires more research to determine its efficacy in strawberries, but the prospects are enticing.  It is new in grapes, and the Plant and Food research team who discovered it are keen to test applicability to other crops, especially trellised (or tabled) crops.  We should investigate what it would take to partner with PFR to test efficacy in strawberries.

‍

‍