WHY DOES THIS HAPPEN
If you start to notice small black spots on your plant, often particularly noticeable on its leaves, this is likely due to a fungal infection caused by the fungus Diplocarpon rosae.
It's often referred to as Rose Black Spot Disease due to the fact that it very commonly affects garden roses, however, these infections can occur on other types of plants too.
It typically occurs in the springtime or fall when conditions have been particularly moist and humid, and is spread from plant to plant via rain splash as well as by wind. Therefore, if you have an infected plant, it's important to try and isolate it before it can affect your other plants too.
SYMPTOMS OF BLACK SPOT DISEASE
As its name suggests, Black Spot Disease can be identified by the presence of small, circular black spots that can occur on your plant's foliage, as well as on the stems in some cases. These spots sometimes have perforated edges. Your plant may also show:
General overall weakening
Yellowing of the leaf tissue
Premature leaf drop (advanced cases may result in complete defoliation)
Note that these circular spots may be a bit trickier to identify in plants that have been badly affected, as they can eventually combine to form wider areas of darkened leaf tissue. The infected areas may potentially also appear more gray or brownish in color than black.
Black spots can alternatively be caused by Pseudomonas bacteria, which is a type of bacteria that can be found in soil and / or water. However, depending on the exact strain of this bacteria, the symptoms can differ slightly to Black Spot Fungus. For instance, the spots are more likely to be brown in color, surrounded by yellow halos, and can quickly enlarge to darken entire leaves and branch tips. It's also common for Pseudomonas bacteria to cause blackening of the leaf veins.