How to recognise it
Spotting of leaves can take many forms. Generally speaking they are due to the death of cells in a small area of the leaf which leads to discolouration. Mostly these are caused by fungal or bacterial infections, though some viruses can also cause similar symptoms. If there are black, dot like bodies on the discoloured area, then it is likely to be a fungal disease, although the absence of these do not discount a fungus as the cause.
Other causes of leaf spots can include chemical sprays (eg weedkiller chemicals drifting onto plants), which can usually be identified by the spots occuring in a fairly localised part of the plant which is near to, or downwind from, the spraying. Damage from environmental conditions such as hail can also cause spotting, although this is likely to be lighter in colour and appear as flecks rather than spots.
Finally, spots appearing in late summer or autumn could just be due to the natural process of the leaf dying prior to the dormant winter period. Some plants can also naturally have a spotted appearance.
Why it’s a problem
Leaf spots can be the external expression of a far more deep-rooted disease, therefore it’s important to identify exactly what the cause is so that further treatment can be administered, if appropriate. The spot generally marks the entry point for the pathogen.
Where you are likely to find it
On the leaves of the plants, usually (but not always) more visible on the upper surface.
How to prevent it
Appropriate (ie for the plant in question), hygienic growing conditions and careful handling of the plant can help to prevent entry of pathogens causing leaf spot. In addition, careful monitoring of leaves will help identify an infection early on.
How to get rid of it
Genearlly speaking leaf spot diseases don’t require treatment and (with exceptions) there are no chemicals licensed for use. Identification of the disease is key to working out the best treatment.
Is it good for anything?!
No, but often the problem is just cosmetic and won’t seriously threaten the plant.
Other useful information
We have further details on the site of the following diseases causing leaf spots: