Colletotrichum anthracnose is one of the most common diseases on many ornamentals. It can occur at anytime during the year and is often confused for other leaf spot and dieback diseases. Ann Chase of Chase Horticultural Research answers questions about this disease.
1. What are the symptoms of Colletotrichum?
Anthracnose diseases are usually characterized by leaf spots and blight. Diagnosing anthracnose can be challenging since many people do not recognize that the leaf spots and cutting rot found in propagation are due to the same pathogen as shoot dieback later in the production cycle.
If you do not achieve control this disease during the propagation phase, you will be fighting a generally losing battle a year or more later when the dieback phase becomes obvious. Sometimes there are black specks in the leaf spots. They can form in concentric rings and look like a pin cushion under magnification.
2. How do anthracnose diseases spread?
These diseases spread by spores that are easy to splash with irrigation water or rainfall. However, since they are somewhat sticky they do not easily spread by simple air movement by the wind or fans. Wounding of the plants can increase disease severity, but it is not necessary for infection.
Usually the pathogen starts on infected cuttings and the disease may be apparent in propagation where the mist can easily spread spores and provide ideal conditions for infection. If you start with infected cuttings, you will be applying fungicides the entire life of the crop.
3. What plants are most commonly affected by Colletotrichum?
Many plants can be attacked by anthracnose fungi especially those grown outside of greenhouses like woody ornamentals and tropical foliage plants. There are far fewer examples of disease infection in greenhouse potted crops and the bedding plants. Some of the most commonly affected ornamental plants are camellia, cyclamen, euonymus, ficus (many species), hydrangea, hosta, Vinca minor, lupine, azalea, aglaonema, cordyline, dieffenbachia, palms, yucca, cacti and succulents.
Research published in the 1960s showed that the Glomerella cingulata (Colletotrichum spp.) present in the greenhouse and nursery trade at the time were not host specific. That is, an isolate from a plant like ficus was capable of infecting the other plants tested and visa versa. This is important in deciding which plants might need protecting.