Zinnias

This is the 1st time I tried Zinnias and I failed completely.
I purchased 12 small plants in the spring and lost 2 before I planted them to a sudden collapse, black on stems.
Of the other 10, 8 were planted in pots and 2 directly in the herb garden.
I have 1 left, all the rest slowly had the stems blacken after being fine for weeks.
Help, what did I do wrong?
The potting mix was organic with added compost.
The 2 in the earth were put in the herb garden that has sandy quickly draining soil.
I used a liquid organic fertilizer, a mix of seaweed and fish emulsion once.

If you could post pictures of the black on the stems, it might help.

Watery Soft Rot is a plant disease caused by a fungus (Sclerotinia sclerotiorum) that can attack a wide range of plants, including zinnias. A water soaked area develops on the stem near the soil line. As it extends upward, leaves die, wither, and droop toward the ground. It could be that your compost harbors that disease. No chemical controls Watery Soft Rot. You should remove and destroy all plants when symptoms are first observed.

There might be time for you to start over again with zinnias from seed. Seeds cost less than buying plants, and commercially available zinnia seeds are unlikely to carry any plant diseases. It is possible that the plants you bought already had the disease when you bought them.

ZM

Thanks Zen Man, I took photos of the few I didn't pull.
These look more like leaf damage then the stems are dried out.

I don't think the one on the left has Watery Soft Rot. The one on the right could be succumbing to Watery Soft Rot, but its stem near the ground seems OK, so probably not. That one on the left appears to have holes eaten in its foliage by some critter -- check the pot for snails, slugs, pillbugs, or earwigs. It is interesting that the main growing point, the flower bud part, is gone on both zinnias. That could be caused by a nutrient deficiency, possibly Boron and/or Calcium.

The death of the lower leaves could also be caused by a nutrient deficiency, possibly Potassium or Phosphorus. I like to use Miracle-Gro Tomato Food as a water soluble plant food, complete with both macro-nutrients and micro-nutrients, for my outdoor zinnias. Also, zinnias need full sun, and the pictured environment looks rather shady.

ZM

Zen Man, the stem ends deteriorated when the flowers were pinched off . The flowers had also dried out and turned brown.
It could be the potting soil and compost but I have 4 pots of huge, bushy, flowering sedum in the same mix but did lose 1 dill plant in the same potting mix.
The pots are in full sun near lavender.

Thanks for your suggestions. I think I'll try seeds next year.

There is probably still time to try seeds this year. Zinnias normally bloom in about six weeks from when you planted them.

Well I got a very nice surprise when in an area I weeded and cleared out at about the time the zinnias bloomed, before their decline,
I recently found a very happy self sown zinnia. It has budded but has not bloomed.
It is growing in a small patch of soil a few inches deep right next to the asphalt driveway.
It seems they prefer hot, dry soil with low fertility and no supplemental watering.
Aren't plants wonderful?