Disease in tomato plants can just creep up on you so you need to be vigilant and take action at the first sign of a problem.
Here are a few tips to keep your tomatoes disease free:
Prepare your soil properly, making sure that it’s weed free, well dug and with added nutrients suitable for tomatoes.
If you’re growing from seed, choose a disease resistant variety of tomato.
If you’re growing from seedlings, make sure that you only transplant the healthy ones so you don’t introduce disease at the outset.
Keep your tomatoes healthy, using a proprietary brand tomato feed, such as Tomorite, which should be applied in accordance with the manufacturer’s instructions.
Water early in the day if you live in a cool climate so that the foliage is dry before nightfall. In a hot climate, water after the sun has gone down so that the leaves aren’t scorched and the plant has the chance to absorb the moisture over night.
Keep greenfly at bay so that they can’t damage your plants or spread other diseases from plant to plant. You can use either washing up liquid, well diluted or a commercial brand greenfly spray which will probably include a fungicide as well.
Keep your whole garden as weed free as possible as viruses which live on weeks can be transferred to your tomato plants by insects or the wind or water.
Check for signs of disease such as wilted leaves, spots on the leaves, cankers on the stems and odd markings on the fruit or anything that doesn’t look normal.