A Perfect Storm for Crop Performance

(Tomatoes) By Stefan Vogrincic

Recent weather patterns — particularly across the North Island — have delivered another period of warm, humid conditions combined with persistent rainfall. While not unusual for autumn, the intensity and duration of these conditions are creating compounding challenges for greenhouse growers.

Following on from earlier reports around fruit set, I’ve observed a noticeable decline in fruit quality and consistency in trusses currently or recently set. This appears to be driven not by a single issue, but by a combination of factors creating a negative spiral:

• Weather: Unseasonably warm nights and rapidly declining light levels
• Humidity: High moisture levels reducing pollen viability and fruit set
• Whitefly pressure: Continued migration from outside due to mild temperatures
• Disease: Increased incidence of fungal diseases such as botrytis and blight in tomatoes
• Reduced heating: Lower energy use slowing plant activity and delaying fruit development

For tomatoes in particular, high humidity leads to “sticky pollen,” reducing effective pollination and fruit set. At the same time, fruit is remaining on the plant longer due to slower ripening, increasing load pressure on the head and affecting subsequent truss development.

Cucumbers have generally held up better, provided day temperatures have been maintained and a sufficient day/night differential achieved. However, disease pressure remains a key risk in these conditions.

Energy Constraints and the Risk of Reduced Dehumidification:

Ongoing global energy uncertainty — including geopolitical tensions — continues to impact fuel availability and cost. For some growers, recent issues with waste oil supply have compounded this further.

In response, many are understandably reducing heating or conserving fuel for later in the season (e.g. frost protection). While this is a practical decision, it comes with risk.

Heating in autumn is not just about temperature — it is critical for moisture management and plant activity. Reducing or eliminating heating significantly limits dehumidification, increasing the risk of:

• Poor fruit set
• Increased disease pressure
• Reduced calcium uptake
• Slower crop turnover and reduced yield

The impact of these decisions may not be immediate but is likely to become evident in 6–8 weeks’ time through reduced production and fruit quality.

For growers with alternative heating sources, maintaining a minimum pipe temperature remains essential to keep the crop active and balanced.

Practical Strategies to Manage Risk

For growers needing to minimise energy use, the focus should shift to managing risk as effectively as possible:

1. Strengthen Disease Prevention

High humidity creates ideal conditions for fungal diseases such as botrytis. A proactive approach is essential.

• Consider biological protectant products, such as those available through Horticentre  (e.g. TripleX range), to reduce disease pressure
• Maintain regular spray intervals — prevention is far more effective than cure in these conditions

2. Maintain Air Movement

Even without heating, air exchange remains critical.

• Use light venting strategies to promote airflow and remove excess humidity (see image below)
• Ensure minimum vent settings are active, particularly on the windward side

3. Adjust Irrigation Strategy

With reduced transpiration, plants require less water.

• Reduce irrigation frequency and volume
• Avoid excessive runoff, which can further cool and saturate the root zone
• Monitor substrate EC — lower uptake may cause EC to rise if not managed carefully

4. Manage Crop Load

If fruit is not moving due to lower energy input, plants can become overloaded.

• Review truss pruning strategy
• Avoid carrying excessive fruit load that can weaken the head and reduce future set

5. Support Calcium Uptake

Calcium movement in the plant is driven by transpiration and flow.

• Under low transpiration conditions, calcium deficiency symptoms (e.g. leaf mottling, tip burn) may appear
• Introducing small amounts of heat at the start and end of the day can stimulate transpiration and improve calcium distribution.

Looking Ahead

What is ultimately needed is a return to dry, sunny conditions to reset crop balance and reduce disease pressure. However, this may also bring cooler southerly changes — creating a different set of challenges.

As always, the key is balance. Reducing inputs may be necessary, but understanding the consequences and managing the associated risks will be critical to maintaining crop performance through autumn.

See example above. Note, it doesn’t need to be 10% venting — even small amounts of venting on both the windward and leeward sides can create a naturally positive airflow through the house.