Managing Million Bells: 2025 Updates
It’s that time of year again, when Million Bells (Calibrachoa) are being started in the greenhouse.
This post will discuss common insect and disease pests, such as aphids, thrips and black root rot and viruses.
Stay tuned for an upcoming post about production tips and tricks for this popular spring favourite.
Disease Issues:
Anyone who’s grown million bells knows they are highly susceptible to Black Root Rot (Thielaviopsis) –I’ve seen this take out a good chunk of a crop, especially in crops that are grown mainly on the floor or on landscape cloth, where resting spores of this disease can hide.
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:
- Choose less-susceptible varieties of million bells whenever possible. Ask your breeder or supplier which colours and varieties are the most susceptible to this disease, or ask other growers.
- Proper Sanitation. If you’ve had black root rot in your crop, and you want to avoid an issue with year after year, sanitation is key. During the crop, immediately dispose of diseased plants and limit water splashing. After the crop, make sure you sanitize benches and floors, and NEVER re-use pots or plug trays that may have come into contact with this disease. Zerotol or a 10% solution of chlorine bleach are best for disinfecting areas that have been exposed to black root rot.
- Consider prophylactic applications of fungicides on plug trays. Products that are effective against black root rot 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.
- Carefully monitoring your pH. Some pathologists state this disease is significantly inhibited by a lower pH – between 5 and 5.5. However, others think the best course of action it to make sure you stay within optimal ranges of the plant (5.5-6.0), to avoid stress that can promote disease. Either way, carefully monitoring pH in this crop is key.
- Manage fungus gnats and shoreflies, since these insects can spread black root rot between plants. Treatments include nematodes, Hypoaspsis mites or applications of Cyrus (cyromazine).
If already established, rotated applications of Senator (thiophanate methyl) and Medallion (fludioxonil) may limit black root rot, but are unlikely to cure it. Rotating these chemicals every 2 weeks may be necessary to keep this disease in check – NEVER rely on only 1 chemical for this disease. (*Reminder to always follow label instructions for maximum allowable uses per crop cycle).
Other diseases to watch out for in calibrachoa are viruses, including Chili Pepper Mild Mottle Virus (CPMMoV) . This is a new virus in this crop, first appearing in 2022. An E-Gro alert from 2022 does an excellent job of outlining this new pest.
Tomato Mosaic Virus appeared at a stock farm in 2024, and Tobacco Mosaic Virus has surfaced a handful of times. To reduce the risk of spreading viruses, some breeders are using novel growing methods this year, e.g. promoting branching with light and PGRs instead of pinching.
If viruses are found in your greenhouse, use strict sanitation measures, which are outlined in the E-Gro link, above.
Insect pests:
Some varieties of Calibrachoa are attractive to western flower thrips, resulting visible damage, but this seems to occur later in the spring (likely from thrips fly-ins from outside). Plug trays of Callies and hanging baskets likely won’t need protection from thrips in greenhouses if they are being shipped out in April. The exception to this is if they planted with a highly susceptible plant species in a mixed basket (e.g. verbena). In this case, 1 sachet of Swirskii mites/basket is highly recommended when the basket is hung, and again 4-5 weeks later. Consider using a long-duration sachet product for baskets. This way, only 1 sachet may be needed until sale.
Lastly, Million Bells are highly attractive to aphids. With baskets hung up in the greenhouse, they can be “out of sight, out of mind”, so many growers choose to treat preventatively. Although Beleaf has been the go-to pesticide in the past, some growers are finding it less effective (see this article for more info).
As alternatives, sprays of Rycar or Kontos can be effective on plug trays. Drenches of Altus or Kontos may be needed to provide long-term control once plugs are in baskets. Most growers are aware of the phytotoxicity risks of Kontos on crops, however, Rycar and Altus have also recently shown phytotoxicity in spring crops in the form of marks on open blooms (on Gerbera and Kolanchoe). Care is needed if these products are used later in the crop (note that Kontos is limited to 1 application on petunia, which is closely related to calibrachoa, due to phytotoxicity). Always test on a small batch of plants, first, and wait at least 5 days for phytotoxicity to appear.
For growers who don’t do preventative pesticide applications for aphids, 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. Be aware that most of the insecticides for aphids take around 4-5 days to start causing aphid death.
Unfortunately, biocontrol for aphids is simply not a viable approach on Calibrochoa, specifically. Studies by Dr. John Sanderson (Cornell University) confirmed a few years back what IPM consultants have suspected for years: parasitism of aphids by parasitic wasps (Aphidius species) is poor on this crop due to the number of trichomes. Other natural enemies, like Anystis, Dicyphus, lacewings and Aphidoletes may be better options, but their use on calibrachoa is currently experimental.
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