Numerous plants can develop leaf spots that are caused by several different fungal – or even bacterial – diseases. Severe infestations can kill the leaves and even weaken or kill the plant, depending on its size and overall health.
Leaf spots can be of various colours – grey, brown or black (roses in particular). The spots are in fact dead leaf tissue caused by the fungus that spreads the disease. Spots can sometimes join together to form larger areas of dead tissue. Some roses also develop smaller black spots on stems.
There are few other symptoms than the spots themselves. However, diseased plants frequently have other problems that cause weaknesses, which then allow the fungal disease to develop. It is worthwhile carefully checking any plant with leaf spot.
Immediately remove and destroy all infected leaves and plant parts. Pick up any fallen foliage and destroy them too. To reduce the chances of re-infection for the following season, prune shrubs back hard.
For roses and other ornamental plants, products with one of the following active ingredients will enable control, although directions should be checked to ensure suitability for specific plants: triticonazole, myclobutanil. Continue spraying at fortnightly intervals throughout the season.