What damping off is
Condition
seedling rot underground, at the soil line, or at the crown, not one single pathogen
Fungi
Rhizoctonia and Pythium are the usual names
Blame
it is often mistaken for bad seed because seedlings can die before they emerge
Setup
warm, nitrogen-rich media that stay wet; even hydro can do it if the plug stays saturated
Read the stem, not just the leaves
Start
yellowish-brown stain on the lower stalk, then sunken reddish-brown cankers between nodes
Nodes
the third, fourth, and fifth nodes are hit most often
Roots
unlike root rot, damping off leaves roots intact while the stem fails
Fall
the soft lower stem cannot hold the plant, so it topples
Look-alikes
Burn
excess fertilizer can wilt and yellow a seedling the same way
Salts
high soil salts or leftover hydro solution can mimic the droop
Climate
too much heat or cold, or too much or too little water, can look similar
Tell
brown discoloration near the soil line points to damping off, not feed burn alone
Keep the surface dry
Mix
plenty of perlite or vermiculite so water can leave
Test
water only after the surface is dry; check near the pot edge so you do not wreck roots
Depth
sow no deeper than 1/4 inch (6 mm); deeper soil stays wetter
Nitrogen
rich mixes slow roots and make damping off worse; sterile, soilless blends drain better
Transplant
wait for several true-leaf sets before outdoor planting
Rescue
brand-new sprouts rarely recover; older veg plants may, if you cut out rot, treat wounds, and foliar-feed while the stem is weak