This post was originally written by Sarah Jandricic and Chevonne Carlow in 2017. Updates to the post are in blue.
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.
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 (see picture above from UMass). 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.
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:
- Choose less-susceptible varieties of Million bells whenever possible; ask your breeder.
- 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. Immediately after the crop, always physically wash surfaces with a cleaner to remove organic matter, then follow up with a disinfectant. This article states that Zerotol or a 10% solution of chlorine bleach are best for disinfecting trays and pots.
- 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.
- Carefully monitoring your pH. Some plant pathologists state that this disease is significantly inhibited by a lower pH – between 5.0 and 5.5, while others think the better plan is just to make sure you stay within optimal ranges (5.5-6). Either way, carefully monitoring pH in this crop to keep it around 5.5 may solve several issues at once.
- 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. Applications of fungicides every 2 weeks may be needed to keep this disease in check (*always follow the label instructions for maximum uses per crop cycle)..
Some varieties of Calibrachoa are attractive to western flower thrips, with resulting visible damage, but this seems to occur later in the spring. Plug trays of Callies and hanging baskets likely won’t need extra protection from thrips in the greenhouse 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. (Or, consider used a long-duration product for baskets; this way, only 1 sachet may be needed).
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. With Beleaf now registered as a drench, this option can provide up to 5-6 weeks of protection from aphids, and may be a good preventative measure for baskets, starting in late Feb or March. Be aware that all of these insecticides take around 4-5 days to start causing aphid death.
Unfortunately, biocontrol for aphids is not a viable approach on Calibrochoa , specifically. Dr. John Sanderson at Cornell University has recently confirmed what many IPM consultants have suspected for years: parasitism of aphids by parasitic wasps (e.g. Aphidius species) is poor on this crop.
Do you struggle with certain pests or nutrient issues on Million bells? What are your biggest concerns? Feel free to share any comments, tips or tricks for growing this crop in the comments section!