Leaf spot comes in many forms and are caused by a variety of fungi. In most cases leaf spot is not very harmful to plants but due to their unsightly nature it is certainly undesirable.
Some of the more commonly found forms of leaf spot include the following...
- Black spot on roses which will develop each year on roses if not treated. Caused by a fungus called diplocarpon rosae
- Tar Spot on maple trees caused by a fungus called rhytisma acerinum
- Leaf spots on hydrangea, Can be either Cercospora or anthracnose
Leaf spots can take many forms. Generally they start as small brown dots, usually forming on older leaves. As the disease develops, the spots will take on a more distinctive look. Tar spots form into large rounded black spots approx. 15mm in diameter. Black spot on roses develop into small rounded purple / black spots with yellowing around the edges.
While there are many types & forms of leaf spot, the treatment & management is often the same. Fungi thrive in warm, damp conditions where airflow is poor. To reduce the build-up of leaf spot you should consider the following...
- Rake up & dispose of all fallen & infected leaves in Autumn. This will prevent the disease being carried over into the following season
- Control watering - It is best to water your plants in the morning and try to avoid watering & wetting the plant's foliage as this creates the ideal breeding conditions for the fungus. Fungal spores require water to spread
- Space plants out to allow for good airflow
- Increase airflow in greenhouse
Chemical control options
While plant management is the same regardless of which type of leaf spot your plant has, chemical controls are often specific to a particular fungus. Therefore, before choosing a fungicide it is important to carefully identify which type of leaf spot your plant has.
Rose clear is the most well known garden fungicide & this can be used to control leaf spot & rusts on roses
Other common fungicides include
- Myclobutanil - Doff Rose Shield Bug & Fungus Killer 1L
- Trifloxystrobin - Interface Fungicide 5L
- Tebuconazole - Bayer Garden - Systhane Fungus Fighter
- Chlorothaloni - BRAVO 500
- Thiophanate - Topsin M WSB Fungicide
- Baking soda can be used to control a number of fungi that cause leaf spot & powdery mildew
- To make your own home brew simply mix 4 teaspoons of baking soda with 3.5 litres of water