Diseased (?) tomato seedlings

Asked March 30, 2017, 12:41 AM EDT

Q: My tomato seedlings came up fine and grew well to the two true-leaf stage. First they started developing dark (silver-gray) spots on their cotyledons, which then turned yellow and dropped off or simply shriveled. Now the true leaves seem to be affected as well. The stems seem OK. The plants have also more or less stopped growing. The seedlings are in a well-lit south facing window (well lit by Portland, Oregon standards, anyway...), were sparsely planted and not particularly heavily watered. Essentially all plants from a number of seed sources and types of tomato are affected. Chili plants and tomatillos sprouted at the same time appear fine to date. Early blight, or something else? Any tips on how to avoid contamination of subsequent attempts? Given all seedlings were affected at pretty much the same time is the seed starting mix a likely source? Thanks!

Multnomah County Oregon

3 Responses

Thank you for including images with your inquiry. Unfortunately, it’s difficult to determine the characteristics of the spots. My best guess about the spots on the leaves in the first 2 pictures is that the spots are not infections and are likely to be unimportant. The 3rd image reveals what appears to be an expanding lesion at the edge of the cotyledon (seed leaf) as well as a shriveled seed leaf (perhaps dried out?) on the adjacent seedling. The slight brown coloration on the white particles (perlite?) at the edge of the pots suggests the seedlings are short of water. (Water at the base of the seedlings, adding sufficient to drain out of the bottom; discard the drain water.)

It would be helpful if you could bring several of the affected seedlings to the office of the Multnomah County Master Gardeners to be inspected in person. The office is in Montgomery Park, 2701 NW Vaughn St. Suite 450, Portland, OR 97210. Hours are 10 AM to 2 PM, Monday through Friday, closed holidays.

The seedlings need more light. A 4-foot long shoplight, with 2 fluorescent tubes (one cool white, the other warm white) suspended an inch or 2 above the seedlings.

The leaf purpling indicates the seedlings need fertilizer. Use a liquid fertilizer, prepared at quarter- to half-strength, applied as needed.

See “Seed Starting” at https://extension.illinois.edu/hortihints/0402a.html

Thanks so much, Jean, I think your diagnosis of too little light and fertilizer was spot on. Ten days of regular fertilizer and some artificial lighting and they have come back from the (near) dead and tripled in size. Not sure why they did so much better last year in the same spot with the same potting mix - maybe just a reflection of it being so much more overcast this year. Thanks again, Ben

Pleased to hear you've been able to turn them around.

Enjoy your garden!