The lettuce was the only plant (seeds) I tried. I was trying to see if I could get it going when it was till pretty cool outside: Ie., marginal risk of frost, but I could cover if necessary.
I might try a small patch where I scrape off the top couple inches of soil, add new (purchased from Menards?) topsoil, and plant in that.
You’d think some sort of fungicide applied to the soil would work, but I’m not finding much except those intended for lawns. Might not be safe for veggies? Daconil, maybe?
Read the lettuce packet for when lettuces can be safely planted outside for YOUR zone.
The fresh soil on top should do the trick. You may not even have had a fungal issue - you may have had an ill-timed seeding issue?
Once lettuce is established, it prefers cool to heat. Germination is a whole ‘nother matter. :)
I’m in Zone 5a. Early spring, until mid-June for me, and then late September through freeze (October 15th) is good for me to direct seed lettuces.
Of course - we’ve had Spring that goes STRAIGHT TO HOT and Spring that rains and rains and rains. It’s a balancing act every, d@mn. year. ;)