Orchid growers often talk about “Anthracnose” in a way, as if it was a pathogen. Anthracnose is not a pathogen.
The word “Anthracnose”, correctly spelled is ANTHRAKNOSIS, is referring to black spots or blemishes caused by a variety of fungal, bacterial or viral pathogens and observed predominantly on leaves.
It is a Greek word that combines two words:
Anthrax = Black = Carbon = Black as Carbon; AND Nosos = Disease.
TWO names exist for the black blemishes: Anthracnose and Necrosis.
Anthracnose is caused by Pathogens;
Necrosis is caused by other Natural causes. Necrosis (Nekros = Dead) = Natural Death.
In plain English: Black blemishes on leaves or stems of plants, caused by pathogens are referred to as “Anthracnose”, until the name of the pathogen is known. Once the Pathogen has been identified the name of the pathogen is or should be used.
Example: Fungi of the Genus Colletotrichum cause black blemishes on orchids. The blemishes are called “Anthracnose”, until the pathogen, Colletotrichum is positively identified. Subsequently: The plant is suffering from Colletotrichum fungi.
It is important to know the pathogen before you start spraying otherwise you may waste time, money and also damage the environment.
Yellow blemishes are known as Chlorosis.
Chlorosis = yellowing of the leaves due to insufficient production of Chlorophyll. Chlorosis is a sign of: Ageing, nutrient deficiency, translocation of Nutrients, sap-sucking insect attack, excessive sunlight, fungal or viral infection (i.e. Chlorotic Ring-spot Virus), etc.
Bacteria are predominantly involved in the decomposition of leaves, stems or roots and do not cause Chlorosis.
Decomposition creates oily, almost “translucent” patches (Pict.1), soft with smelly fluids and stem or root rot. Some “patches” may turn black (anthracnose). Root rot prevents nutrients getting through and that may cause indirectly chlorosis.
Pic.1: “Oily” blemishes cause by Bacterial infection.
Anthracnose caused by Bacteria. The "Shrinkage" caused by "dehydrtation" of Bacterial damaged areas, after tretment, is clearly visible
