Managing Million Bells

By Sarah Jandricic and Chevonne Carlow

It’s that time of year again, when baskets of Million Bells (Calibrachoa) are going up in the greenhouse.  Here’s how to deal with and prevent some of their most common issues.

Fe def calibrachoa

Iron deficiency in Calibrachoa.  The resulting yellowing can look similar to symptoms caused by black root rot or nitrogen deficiency.

From a nutritional standpoint, the best thing you can is keep the pH of your calibrachoa in its ideal range; between 5.5 and 6.0.  A pH higher than this can inhibit nutrient uptake, especially micronutrients such as iron. 


Iron deficiency can be difficult to distinguish from other issues (like Black Root Rot – see below), but typically leads to yellowing of new growth.  Leaves may only show chlorosis between the veins, or it may be spread throughout the leaf.  This is different from nitrogen deficiency where yellowing occurs in the oldest leaves. If iron deficiency occurs, adding a chelated form of iron is best for uptake.

Yellowed plant growth (yellow circle) and dead plugs (orange circle) on a plug tray of Callibrachoa.
Yellowed plant growth (yellow circle) and dead plugs (orange circle) on a plug tray of Callibrachoa from black root rot.

Million bells are also highly susceptible to Black Root Rot (Thielaviopsis) – I’ve seen this take out a good chunk of a crop.  Symptoms include:

  • Stunting of foliage and roots
  • Plants in a tray will have uneven heights
  • black areas on roots
  • yellowing of leaves

Prevention is worth a pound of cure with this disease, as it is difficult to eradicate once established.  Important steps to take include:

  • Proper Sanitation. To avoid an issue with Black Root Rot year after year, immediately dispose of  diseased plants, limit water splashing, and sanitize benches, floors and used pots/plug trays.  Always physically wash surfaces  with a cleaner to remove organic matter, then follow up with a  disinfectant such as KleenGrow (ammonium chloride compound).
  • Consider prophylactic applications of fungicides on plug trays.  Products include Senator (thiophanate-methyl) or Medallion (fludioxonil). Preventative applications are an especially good idea if you’ve issues in the past. Adding bio-fungicides containing Trichoderma harzianum (e.g. Rootshield, Trianum) may also help
  • Lowering your pH. This disease is significantly inhibited by a lower pH – between 5.0 and 5.5.
  • Manage fungus gnats and shoreflies, since these insects can spread Black Root Rot between plants. Treatments include nematodes, Hypoaspsis mites , or applications of Dimiln (diflubenzuron) or Citation (cyromazine).

If already established, rotated applications of Senator and Medallion may limit Black Root Rot, but are unlikely to cure it.

Aphids tend to be found on flowers and new growth of Calibrachoa.

Lastly, Million Bells are highly attractive to aphids.  With baskets hung up in the greenhouse, they can be “out of sight, out of mind”, but  regular monitoring is needed to prevent large aphid outbreaks.  Place sticky cards directly in baskets, and routinely check plant material for aphid cast skins and honeydew.

Once aphids are detected (and they will be!), applications of  Beleaf (flonicamid), Enstar (kinoprene) or Endeavor (pymetrozine) will usually take care of them.  However, be aware that all of these insecticides take around 4-5 days to start causing aphid death.




A Crime Against Callies: Floriculture IPM of Black Root Rot

Diagnosing a plant with vague symptoms like wilting, yellow and stunting is much like being a “plant detective”. First, you need to profile the “victim” — here,  Callibrachoa plugs.  Then, collect DNA evidence.  Finally, use knowledge and instinct to narrow down your “suspects”.  Only then can you come up with a plan to stop the assailant.

Yellowed plant growth (yellow circle) and dead plugs (orange circle) on a plug tray of Callibrachoa.
Yellowed plant growth (yellow circle) and dead plugs (orange circle) on a plug tray of Callibrachoa.
An unhealthy plug; few white roots are visible.
An unhealthy plug; few white roots are visible.

In this case, our DNA evidence (c/o UofG Lab Services) gave me 3 possible suspects: Pythium disotocum, Fusarium oxysporum and Thielaviopsis basicola.   But which of these was the real culprit?

P. disotocum is rarely documented as an aggressive pathogen in flowers, so we can eliminate that.  Similarly, Fusarium is often “around” at low levels without causing a problem.  But, T. basicola, better known as Black Root Rot, is a common problem in Callibrachoa, specifically.  Rotting roots are not always directly evident (even under a microscope), but severe discoloration of the foliage is a good clue, since this pathogen produces toxins that result in yellowing.  Left unchecked,  Black Root Rot causes severe stunting and plant death.

Luckily, there are treatments for Black Root Rot, and preventative measures that can be taken.

A fungus gnat (left) and shorefly (right) caught on a yellow sticky card.  Both of these pests can transmit Black Root Rot and other pathogens.
A fungus gnat (left) and shorefly (right) caught on a yellow sticky card. Both of these pests can transmit Black Root Rot and other pathogens.
  • Drenches of Senator 70 WP (thiophanate-methyl) are the best option for dealing with an existing outbreak of Black Root Rot
  • Black Root Rot is often transmitted by fungus gnats and shoreflies.  Management of these insects is key to prevention.  They can be controlled with soil applications of nematodes, predatory mites (e.g. Hypoaspis), or insect growth regulars like Dimilin (diflubenzuron).  These products are compatible with biocontrol programs for other pests, like thrips and aphids.
  • A high soil pH encourages the growth of Black Root Rot.  pH should be kept below 5.6.
  • Bio-fungicides containing the beneficial organism Trichoderma harzianum (e.g Rootshield) can help protect plants from this Black Root Rot if applied at planting.

For additional information on Black Root Rot, check out


Spray technique affects nematode rates

Spraying nematodes in your greenhouse for fungus gnats, shoreflies, or thrips?  Then check out this article published in the January 2015 issue of Hort Matters (OMAFRA’s Horticultural newsletter):

Researchers realized that nematodes can “stick” to the inside of spray tanks, reducing the number of nematodes coming out of the nozzle as you spray.  This is unlikely to be a problem using backpack sprayers (since the volume is so small), but could be an issue if you use a towed sprayer on wheels.

What’s the solution? Agitation, agitation, agitation.

To keep your nematode numbers consistent throughout the application, do your best to keep nematodes in suspension.  And, if you have any concerns about your application technique, it’s easy enough to assess if you have a microscope (or have a friendly neighbourhood OMAFRA agent with one).

Nematodes (S. feltiae) preparing to enter an adult western flower thrips.
Nematodes (S. feltiae) preparing to enter an adult western flower thrips. Photo by R. Buitenhuis (Vineland).

Nematodes are around 0.5 mm long, and can be easily seen using the low magnification setting on a scope.  A black background makes them easier to see. Live nematodes are usually serpentine or “J” shaped, and often wiggle slightly.  Dead nematodes are stick straight.

By counting the number of live nematodes in a small volume of your original spray solution (1 mL should do it), you can compare this to the number of nematodes in the same volume (1mL) from the nozzle dribble at the end of your application.  Seeing a huge reduction here?  Then you may have a problem.

To ensure nematode health, also follow these other tips:

  • Air temperature should be less than 30 C at application time
  • Apply during low light levels since nematodes are UV sensitive
  • Nematodes can be stored in a refrigerator (4C) but should be used within 4 weeks of receipt
  • Do not apply nematodes though sprayers that exceed 300 psi
  • Use a nozzle aperture of more than 0.5 mm