Question
I am a beginner and I am starting some perennials from seed for the first time. The tips on the leaves of my Dianthus seedlings that sprouted 4 days ago are starting to turn black and crispy! I have a heat mat under the tray, but the tray is elevated with tiles. The thermometer placed on top of the tiles reads 78. I have fluorescent light bulbs 3 inches directly over the Dianthus. My soil was moist when I planted the seeds 8 days ago (I uncovered their saran wrap after they sprouted). I have not watered since then except to spritz them once with a spray bottle two or three days ago. Why is this happening and what should I do???!! I have several different types of seeds in one tray, some of which have not yet sprouted. That is why I dampened the soil from the top rather than bottom soaking because the rest are not up yet and are plenty moist under the saran wrap. Thanks.  

Answer
Julie, the condition your describing is black rot, also known as black spot. Black spot is a fungus disease that is most prevalent during humid conditions. If the problem gets bad enough, it can defoliate the entire plant.

To control the disease, you need to spray the seedlings with a sulfur-based fungicide at 7 - 10 day intervals until the condition is reversed. Also, be sure to remove any of the fallen leaves that have been affected to prevent them from generating soil-borne spores that will reaffect the plant.

What caused this is anyone's guess. It is a soilborne disease, but that would not be a factor assuming you used a soiless starter mix and streilized containers. These soiless mixes are sold in garden centers and are primarily composed of peat moss, vertimiculite, and perlite.

The only thing I can think of based on your description is that the high level of humidity resulting from the seran wrap, in addition to the warm temperatures from the heat mat, may have induced the disease. That is why I stopped using a heat mat years ago. I also use a plastic dome with a ventilator on top so that the growing environment remains humid, but the plants still receive fresh air. If you decide to start over, that would be my recommendation. Otherwise, try the sulfer fungicide and monitor closely.

I hope this helps. Please write again if I can ever be assistance.

Regards,

Mike