Tomato Seedling Emergency!


Hi. Four weeks ago I planted a number of cherry tomato seeds into small pots an placed them under grow lights. They sprouted one week later and they have been growing nicely ever since. I started with three per pot, and one week ago I thinned down to one per pot. However, a few days ago I noticed that some of the leaves were beginning to curl on themselves, and now some leaves on some of the plants are turning yellow on one side and/or reddish on the other side, and even falling off. These plants continue to have other healthy leaves, and the stems have lots of thin hairs and are thickening nicely and look healthy. But still, the discoloured/curling leaves are worrying me! I have attached a picture of some of the leaves that have been afflicted. Can you suggest what the problem might be? (Overwatering, underwatering, not enough airflow, too much light, not enough light, fungu, etc. — the internet is full of possibilities, but nothing is concrete!)

Thank you for your help.


Thank you for contacting the Toronto Master Gardeners concerning your tomato seedlings.

As you found out, there could be a number of reasons why your tomato leaves are turning yellow. Would you happen to know if the yellow leaves are the first formed leaves after the cotyledons? As the plants get older it is not unusually  plants to discard their juvenile leaves as adult leaves grow larger.

Your seedlings could be suffering transplant shock after thinning. Were the roots disturbed during the thinning process?

Are your seedlings receiving at least 12 hours of light/ day followed by a period of darkness of at least 8 hours. .

Too much or too little water could also be a factor. Make sure you allow your soil to dry out in between waterings.

The fact that your plants are  putting out healthy leaves, stems are thickening and covered with trichomes sounds as if it is not a pest or disease. I would monitor them for now and if problems persists or becomes worse please send us another email along with additional pictures.

You may be interested in the following factsheet on Growing Your Own Tomatoes.

May 12, 2021