Calibrachoas are susceptible to several diseases and nutrient disorders during production. Here is a list of common problems to watch for and prevent this growing season.
Root diseases -Thielaviopsis (black root rot), Pythium (root rot), Phytophthora (stem and root rot) – Early symptoms often show up as stunted growth, yellowing and wilting. Regularly monitor plant roots and stems (at base of the plant), from the time plants are plug stage to finishing. Healthy roots are white and firm; decayed roots are dark colored and rotted. Since symptoms can be mistaken for other causes such as high soluble salts, suspicious plants should be diagnosed by a diagnostic lab. Prevent root rot diseases by avoiding re-using containers, trays and growing media and by managing fungus gnats. Maintain a medium pH at 5.5 to prevent Thielaviopsis. Fungicides provide healthy plants protection from becoming infected.
Botrytis cinerea (leaf blight and stem canker) – Branches may wilt as if the plant has root rot, but in fact, they have Botrytis stem canker. Pick up the canopy and look for fuzzy spores around the stems. To prevent and manage botrytis, water early in the day, so foliage can dry rapidly. Reduce greenhouse humidity by providing adequate ventilation and provide adequate plant spacing for good air circulation. Rotate fungicides with different modes of action groups to delay resistance.
Powdery mildew – Carefully monitor the lower, interior leaves for signs and symptoms of powdery mildew (yellow or brown dying leaves in addition to white, talcum-like fungal growth) to detect an early infection. Manage powdery mildew by reducing greenhouse humidity; providing adequate ventilation; and spacing plants to provide good air circulation. Note that the fungus that causes powdery mildew on cucurbits (pumpkins, squash, melon and cucumber) Podosphaera xanthii is also known to infect calibrachoas, verbena and petunia so avoid growing squash and cucumber transplants in the same greenhouse as susceptible verbena or calibrachoa. Adhere to a strict spray schedule (seven days is recommended), apply fungicides at the full label rate and rotate fungicides with different modes of action.
Calibrachoa Mottle Virus (CbMV) - CbMV was first identified in the US 2003. CbMV causes small dark spots on the leaves which can be mistaken for other causes such as spray injury. It may also cause leaf yellowing and dieback that can be mistaken for nutrient deficiency. Symptoms tend to appear most often when plants are stressed. The virus is spread by handling of infected plants and through propagation of infected mother plants. Petunias are also susceptible to this virus. Chenopodium species have been infected in the laboratory, but natural occurrence has not been reported.
The best way to avoid this virus is for propagators to buy certified virus-free stock. Incoming plants should be inspected for suspicious leaf spots. Infected plants should be carefully rogued and destroyed. A quick test from Agdia is available for diagnosing this disease.
Iron deficiency – Symptoms appear as interveinal chlorosis, normally starting at the shoot tips, but often occur throughout the entire plant. Sometimes the leaves turn almost white. Prevent Fe deficiency by maintaining a medium pH at 5.5 and if needed, use a petunia fertilizer with added iron or use an iron chelate fertilizer. In addition to pH, cold media (cold roots) or poor root development will affect uptake of iron.
This month in diseases: Calibrachoa by A.R. Chase and Margery Daughtrey, June 2011 GPN Magazine.
Squashing Powdery Mildew in Calibrachoa by C. Warfield
Greenhouse Product News 2015 Ornamental Disease Digest (with photos)
Agdia (Test strips)
Tina Smith, UMass Extension
with input from Leanne Pundt, UConn Extension and Angie Madeiras, UMass Extension Plant Diagnostic Lab