Most common tomato problems – especially when growing outdoors – are caused by low temperatures, over-watering or wet weather.
Leaves turning mottled, purple veined or reddish under leaves is usually caused by nutrient deficiency owing to low temperatures. Roots are unable to absorb nutrients when the temperature is cold.
Low temperatures and poor nutrient intake (calcium) can also be the cause of blossom end rot.
A poor root system may prevent a plant from fighting off diseases and also reduce yield. Roots need air as well as moisture – too much water and saturated soil on a regular basis means that roots won’t receive enough air.
Tomato Blight is probably the biggest threat when growing tomatoes outdoors.
This is a fungal infection caused by wet or damp leaves. It shows itself with brown/black patches on leaves – it spreads from plant to plant and can destroy all your tomato plants.
Try to keep leaves dry with some kind of shelter when the weather is wet for more than a day or two.
The last few seasons have been very wet in the UK during August – the worst I’ve ever experienced for tomato blight caused by a wet summer.
However, there are many other tomato problems as well as diseases and some of them have several strains so identification can often be difficult.
The two links below fall into the main categories of leaf and fruit diseases.
Please follow link below to leaf disease identification.
Disorders of Tomato Leaves – Vegetable Resources Vegetable Resources (tamu.edu)
Follow link below to fruit disease identification.
Disorders of Ripe Tomato Fruits – Vegetable Resources Vegetable Resources (tamu.edu)
(Many thanks to Texas A&M University System for such a good resource).
Prevention is much easier than cure!
Many tomato growing problems can be prevented before they affect your plants and tomatoes, so knowing how to keep your plants healthy and happy is the best disease prevention.
Sterilizing pots and containers and using new compost will prevent diseases being carried over from a previous season.
Keep outside tomatoes sheltered from the rain if possible – plants hate wet leaves.
Water in the morning rather than late evening because when the temperature drops your plants will be stood in cold water all night.
For more tomato growing tips go here.
Sometimes over feeding can be an issue, so feeding little and often is another way to keep plants healthy.