Diseases can be detected by colour - if you see a new colour appear on the leaves or stems - black, orange, white or brown are the key ones you’ll see - this is your warning sign. If it’s not green (and it isn’t some specially cultivated variegated colour) then it’s not right. Yellow is the most common colour of leaf trouble, but it doesn’t relate to a specific disease, it’s a general illness colour and its a plant feeling weak and sickly for any number of reasons.
However, the most common reason for yellow leaves is watering problem and this is also the most common cause of death in cultivated plants. Click here for more information on watering.
Top diseases to watch out for
Fungal Leaf Spot