The Alberta Greenhouse Growers Association (AGGA) has been organizing and holding a Greenhouse Chat since 2021. During the pandemic, growers started organizing virtual sessions to keep in touch and check in with how growers were handling isolation and managing all the pandemic protocols.

This ‘Greenhouse Chat’ was an instant success as a means of communication and an opportunity to share and learn. My specific role was to share and discuss problems and solutions. In 2023, the AGGA decided to create a YouTube channel where all the programs were uploaded and thus all the knowledge was preserved. Follow the link at the end of this article to explore some of the topics we discussed.

In this month’s article for Greenhouse Canada, I decided to share our most recent chat, held in early May, when it is the peak season for all sorts of problems. We covered issues from diseases and insects to heat-related stresses on tomato, pepper and lettuce crops. 

This article will focus on tomatoes. It is important to note that a correct diagnosis is the key to finding solutions. This article will also point out where a wrong diagnosis resulted in economic losses. 

Tomato Blossom End Rot (BER)

Blossom End Rot (BER) is a well-known physiological disorder of tomato fruit where rot starts from the end of the fruit. In the case of Image 1, the crop is being grown in soil but this can occur in any soilless production systems. The grower in this case lost three first clusters to this rot which is a significant economic loss. 

I have seen situations where grower thought that it was a rot due to a fungus and already sprayed different chemicals but to no avail. 

Here are key points to understand and manage BER:

• It is a temporary shortage of calcium during critical periods when fruit is rapidly sizing. Most of the time, a proper amount of calcium is present in the fertilizer solution but it is not taken up by the roots or not being transported to the site where it is needed most due to a competition between fruit and leaves for calcium.
• Calcium is considered to be an ‘immobile’ element, meaning that once it is absorbed by the roots, it is transported to the sites where it’s needed and gets fixed in cell walls and thus not available to be moved unlike mobile elements like nitrogen and potassium. Its uptake and transport depends on active transpiration by the leaves. 
• Active transpiration means that the greenhouse relative humidity is not very high and temperature is not in stress range. Growers may be familiar with the term VPD or Moisture Deficit, which is a combination of temperature and relative humidity. The ideal is to maintain a VPD of between 3 to 7 grams/m³ of air. With a VPD of over 11-12 grams/m³ of air plants are under very big stress and this is happening due to a relative humidity of around 25 per cent and a temperature of over 28ºC. Under such a stress, plants selectively absorb water for transpiration and cooling and calcium deficiency may occur causing BER. Growers should try to use a misting system and a shade screen to bring the VPD down. 
• Try to bring the Electrical Conductivity (EC) of the leach solution below 3.0 mS/cm² by reducing feed EC and by introducing pH-adjusted water when light intensity goes above 1200 joules. 

Scorching

In the case of Image 2, the entire crop is showing brown leaves, which can be described as scorching. This happens when the air temperature goes above 30ºC, and VPD values are close to 15 grams/m³ of air. Leaf temperature in such cases could go as high as 40ºC and a plant just cannot cope with such a high stress. The fruit sizing is seriously affected and it matures before reaching adequate size. Tomato plants set three leaves and one set of flower cluster per week and flower abortion occurs. The following strategies are recommended to avoid leaf scorching:

• Focus on climate control by increasing relative humidity through misting and fogging.
• Since light and temperature are related, shade the greenhouse with internal screens or external shade.
• Bring feed and leach EC down so that plants can take up water easily. High EC restricts water uptake.
• Avoid chloride-based fertilizers because they increase EC. In this case, the grower was using calcium chloride as part of the fertilizer application. Some growers use potassium chloride which also contribute towards high EC.

Generative plants 

Thin heads, like those seen in Image 3, indicate that plants are more generative. There is reduction in leaf size, flowers are small and yellowish instead of being orange colour. There is reduced pollen production and viability, and bumble bees generally avoid visiting these flowers. Here are some recommendations to help:

• Reduce fruit load on earlier clusters. Many growers twin head their crops early in March without adding a dripper to provide extra feed solution. It is recommended to twin head every fourth plant but in this case every plant was twin headed, which increased the density to 4.2 plants/m². 
• Bring 24-hour average temperature to around 18ºC by reducing night temperature from 12:00 to 3:00 pm. There is no fixed rule for this window to bring the temperature down. You can adjust the window to a different period of time frame based on how cool the nights are.
• During high day temperatures CO₂ enrichment becomes difficult due to early ventilation. CO₂ can be supplied at sunrise for two to three hours before vents are fully open. This will help to reduce thin heads. 

Wilt

In Image 4, plant wilt is scattered throughout the crop and symptoms described as wilting during high light periods and recovery at night. The plants are fully loaded with fruit. 

Generally, wilt is associated with fungus Pythium, which attacks roots under water-logged conditions. In this case, the grower observed that the rockwool slab drain holes were not cut properly and water was collecting under the slab and roots stayed in that area. Some suggestions to prevent wilt:

• Recut the slab plastic to drain properly
• Adjust the drain volume based on sunlight intensity
• Increase the relative humidity to bring VPD to proper range. 
• Apply fungicides if needed.

Dr. Mohyuddin Mirza, Ph.D., P.Ag. is an industry consultant in Alberta and can be reached at drmirzaconsultants@gmail.com

Explore more on AGGA’s Youtube channel here.

Read more on managing greenhouse crops through summer from Greenhouse Canada here.