Calibrachoa is a stunning plant when in full bloom from May through frost. Because of its performance for the end user, it has become a major crop. As with many crops, it too has production challenges that have given growers headaches. The two most common problems with Calibrachoa are chlorosis of new growth and root disease.

1. Interveinal Chlorosis of New Growth: Most growers have seen calibrachoa displaying symptoms of interveinal chlorosis of the new leaves. This is quite common and is typically caused by iron deficiency induced from high pH in the growing medium. The ideal growing medium pH for calibrachoa is 5.4-5.8. If the pH exceeds 6.2, then the iron becomes unavailable to the plant.

However, we have seen more problems in which the pH of the growing is ideal but, iron deficiency is occurring because of insufficient application of iron. Most fertilizers simply do not provide sufficient quantities of iron for calibrachoa unless they are constant feed at 250 ppm nitrogen or the fertilizer is formulated with extra iron. Unfortunately constant feeding calibrachoa at high rates just to get enough iron, produces soft, stretched growth. It may be better to reduce the constant fertilizer application rate to 150-200 ppm nitrogen and apply an iron chelate once per month to compensate for reduced iron levels or use a complete fertilizer that provides elevated iron levels and better iron chelates.

Calibrachoa exhibiting iron deficiency symptoms.

If calibrachoa are exhibiting iron deficiency, the fastest way to correct the problem is applying an iron chelate. The most effective chelate is Iron-EDDHA. It allows iron to remain soluble and available for plant uptake even if the pH of the growing medium exceeds 9.0. Iron-DTPA is a fairly effective chelate while iron-EDTA is ineffective if the growing medium pH exceeds 6.5.

New Leaf Interveinal Chlorosis = Iron Deficiency? In most cases this is true, but start by checking the plant roots. When root tips are attacked by plant pathogens, they cannot take up fertilizer elements from the growing medium, causing deficiencies. Iron deficiency caused by root diseases is often ‘hit or miss’ through the crop. If the symptoms are expressed uniformly throughout a single cultivar or the entire crop, then it most likely is not caused by root disease.

Interveinal chlorosis can also be a symptom of manganese deficiency. This will be obvious when after applying an iron chelate, the plant temporarily has some improvement in foliar colour but then after a few days shows more severe chlorosis. Before assuming that an iron deficiency is occurring, test the growing medium and tissue to confirm the problem prior to adding iron chelates.

2. Root Disease: Calibrachoa are sensitive to root diseases caused by Pythium and Thielaviopsis, or black root rot. Both pathogens need high moisture conditions to survive, so plants that are likely to be attacked are those that are staying too wet or are overwatered. Use a growing medium such as PRO-MIX® HP BIOFUNGICIDE™ + MYCORRHIZAE™ which has lower water retention, good drainage and the added benefit of BIOFUNGICIDE™ to help suppress root disease. Remember to allow the growing medium surface to turn light brown to tan as an indicator of when to water.

Calibrachoa with Pythium rot.

Calibrachoa are susceptible to these pathogens at any stage, but are particularly vulnerable when they are young. To reduce this threat, it is best to keep the growing medium temperatures above 68°F (20°C) until the crop is well rooted. Then calibrachoa can be grown cooler as they have an established root system that can use the water from the growing medium, reducing incidence of root disease. Try to encourage good airflow to aid in the dry-down of the growing medium and maintain normal fertility levels to keep the plants actively growing. Avoid reusing containers unless they have been cleaned of debris and then properly sanitized.

Cultivar selection is also important. Those with compact growth habits grow slower and thereby use less water. They are easy to overwater compared to vigorous cultivars. It is best to separate compact from vigorous plants in order to watch watering requirements.

Problems can occur with any crop at any time. It is best to be proactive and adjust cultural practices to minimize problems with calibrachoa and other crops.

If you have questions and would like to talk with someone about fertility, water quality or growing medium needs, contact your Grower Services representative.

For more information
Premier Tech Horticulture Office
1, avenue Premier
Rivière-du-Loup (Québec)
Canada G5R 6C1
T: +1 418 867-8883
Toll free: +1 855 867-5407