It’s that time of year again, when unrooted cuttings or rooted plug trays of Million Bells (Calibrachoa) are first arriving in the greenhouse.
When they go right, Calibrachoa are a relatively easy, staple spring crop. However, when million bells go bad, they go bad BIG time.
To help your crop turn out this year, Chevonne and I have compiled some info on how to prevent and deal with common issues in Callies.
One of the biggest issues I get calls about is Callies with Black Root Rot (Thielaviopsis). I’ve seen several farms get hit hard with this disease in the last 2 years, despite growing Callies successfully for years with no issues. By the time the issue was noticed, no treatments were effective, and a good chunk of the crop needed to be thrown out.
Symptoms of Thielaviopsis include:
- Stunting of foliage and roots
- Yellowing of leaves
- Plants in a tray will have uneven heights
- Black areas on roots
- Wilting and plant death (sometimes just 1 plant in a container)
Confirming Thielaviopsis in Calibrachoa:
Once symptoms are seen, confirming diagnosis of black root rot is relatively easy. All you need is some symptomatic root material (i.e. black-looking roots) in a shallow dish of water, and a microscope that goes up to 40X, since the spores are large and distinctive (see photo). (Note that the cost of sending off 1 sample for DNA diagnostics would more than pay for a decent microscope, and you don’t have to wait days for tests to come back.)
Myself and industry consultants are still trying to sort out why some farms are getting hit so badly with Thielaviopsis outbreaks: where are the spores coming from? Are certain cultivars/colors more susceptible? Is there fungicide resistance ? Or was the pathogen load too high due inadequate sanitation and/or spread by fungus gnats?
Until we have those answers, I’m going with what I like to call “an AGGRESSIVELY PREVENTATIVE approach” with this disease. After all, an ounce of prevention is worth a pound of cure with a disease that’s VERY difficult to eradicate once established. Important steps for Thielaviopsis prevention 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). Power washing benches to dislodge any further organic material may not be a bad idea either.
- Use prophylactic applications of fungicides as soon as the rooted trays come in, or as soon as stuck cuttings start to produce roots. The only effective products are Senator (thiophanate-methyl) and Medallion (fludioxonil). More than one application and rotation of these chemicals will likely be needed if there’s a history of Thielaviopsis in your operation. 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. Make sure to monitor your pH regularly throughout the crop cycle.
- Avoid over-watering, as this can encourage root rot. A “little-and-often regime” is encouraged when plants are young.
- Manage fungus gnats and shoreflies. This is such an easy step – and an incredibly important one – since it’s known these insects spread infection of Black Root Rot plant-to-plant. Feeding larvae also damage roots and make plants more susceptible to disease. Treatments include weekly applications of nematodes or Hypoaspsis mites, or single applications of Dimiln (diflubenzuron) or Citation (cyromazine).
Unfortunately, once a Thielaviopsis infection is established, rotated applications of Senator and Medallion may limit infection, but are unlikely to cure it. Thus, it’s best to cull or separate infected plants from uninfected plants as soon as possible, and up your sanitary measures.
Another factor in avoiding disease in Calibrachoa is to keep the plant from becoming stressed. 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), 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.
Although some varieties of Calibrachoa are attractive to western flower thrips, thrips seem to rarely cause visible/economic damage on this plant.
Thus, plug trays of Callies and hanging baskets likely won’t need extra protection from thrips in the greenhouse. 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.
However, Calibrachoa are highly susceptible to aphids. With baskets hung up in the greenhouse, they can be “out of sight, out of mind”, and aphid populations can get out of control in a matter of days. Thus, regular monitoring is needed to prevent 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!), pesticide applications will be necessary. Although I always love to suggest biological control over chemicals when I can, growers and IPM specialists have noticed for years that parasitism of aphids by parasitic wasps (e.g. Aphidius species) has always been poor on Calibrachoa, specifically. Dr. John Sanderson at Cornell University is currently looking into why this is, but until he comes up with an answer, your best bet is an application of Beleaf (flonicamid), Enstar (kinoprene) or Endeavor (pymetrozine) . However, be aware that all of these insecticides take around 4-5 days to start causing aphid death.