stem thin in middle seedling/early veg days

Hi there hoping someone can shed light on a problem i have encountered.

growing Nirvana Northern lights, 45W 6700K CLF inch1/2 away, small 12v fan blowing on it from some distance.
Growing soil and coco, previously had no troubles with this combo, and as Northern lights is apparently a hardy strain wouldn't expect these problems.

Planted 2 seeds, lost one other was going good but now growing concerns about this one too.

I lost first one due to there being a really really thin section on the stem, turned brown and shrivelled. Now, that one stretched a lot as i didn't have a light at the time, I planted the second when i saw this one wasn't going to make it and this second one didn't stretch at all, was growing really nicely and rooting like mad.
But now it seems to be developing a similar problem to the other one but no where near as bad.

So wondering what the cause of this thinning could be?

I have considered stem rot, and i now keep the current baby very dry around the stem and turned the fan up a notch, but still really quite wobbly and got purple tint to the area just below soil line.
Temps fall lower than they should do at night for moderate periods, could this be the cause?

Help would be greatly received don't wanna be losing another one.

I should've taken a picture of the first one but i put it down to stretching didn't expect a similar problem again, and this time the camera i got really wouldn't help much.

Many thanks RIU people

These pictures dont show any of the stem that i worry about, but shows healthiness of plant which thought may help.

Based on what you described it sounds like damping off disease. That is only an assumption based on what you said and not a certainty based on clear visuals, but it is likely the case.



[h=1]Damping-off[/h] The single term used to describe underground, soil line, or crown rots of seedlings due to unknown causes is damping-off . The term actually covers several soil borne diseases of plants and seed borne fungi.


Rhizoctonia root rot (Rhizoctonia solani) is a fungal disease which causes damping-off of seedlings and foot rot of cuttings. Infection occurs in warm to hot temperatures and moderate moisture levels. The fungi is found in all natural soils and can survive indefinitely. Infected plants often have slightly sunken lesions on the stem at or below the soil line. Transfer of the fungi to the germination room or greenhouse is easily accomplished by using outdoor gardening tools inside or vice versa. The germination room should not be used for mixing potting soils or transplanting seedlings as a general rule.


Pythium Root Rot (Pythium spp.) is similar to Rhizoctonia in that it causes damping-off of seedlings and foot rot of cuttings. However, infection occurs in cool, wet, poorly-drained soils, and by overwatering. Infection results in wet odorless rots. When severe, the lower portion of the stem can become slimy and black. Usually, the soft to slimy rotted outer portion of the root can be easily separated from the inner core. Species of Pythium can survive for several years in soil and plant refuse.


Phytophthora root rot (Phytophthora spp.) are usually associated with root rots of established plants but are also involved in damping-off. These species enter the root tips and cause a water-soaked brown to black rot similar to Pythium. These fungi survive indefinitely in soil and plant debris.


Black root rot (Thielaviopsis basicola) is a problem of established plants. It does not occur in strongly acid soils with a pH of 4.5 to 5.5. It usually infects the lateral roots where they just emerge from the taproot. The diseased area turns dark brown, and is quite dry. The fungi survive for 10 years or more in soil.


Miscellaneous fungi causing similar symptoms include Sclerotinia (white mold), Sclerotium rolfsii, Macrophomina phaseoli, some species of Botrytis (gray mold), Aphonomyces, Fusarium, Cylindrocladium, and others. Hence the need for the collective term known as damping-off.
[h=2]Symptoms of Damping-off:[/h] Seeds may be infected as soon as moisture penetrates the seed coat or a bit later as the radicle begins to extend, all of which rot immediately under the soil surface (pre-emergence damping-off). This condition results in a poor, uneven stand of seedlings, often confused with low seed viability. Cotyledons may break the soil surface only to whither and die or healthy looking seedlings may suddenly fall over (post-emergence damping-off). Infection results in lesions at or below the soil line. The seedling will discolor or wilt suddenly, or simply collapse and die. Weak seedlings are especially susceptible to attack by one or more fungi when growing conditions are only slightly unfavorable. Damping-off is easily confused with plant injury caused by insect feeding, excessive fertilization, high levels of soluble salts, excessive heat or cold, excessive or insufficient soil moisture, or chemical toxicity in air or soil.


Above ground symptoms of root rot include stunting, low vigor, or wilting on a warm day. Foliage of such plants may yellow and fall prematurely starting with the oldest leaves. The roots of a diseased plant will have some shade of brown or black and evidence of water-soaking. Healthy roots are fibrous appearing and are usually white or tan in color. These symptoms are easily confused with severe mite, aphid, scale infestations, or root-feeding by nematodes or insect larvae. Environmental factors such as accumulated salts in the soil, insufficient light or nitrogen, potbound roots, cold drafts, etc. can be eliminated only by examination of the roots.

Thanks for all the info mayyn, interesting read.

It seems to be pulling through now pheww

Had never heard of this damping off! fair chance it was that.
cheers

i have problems in cloning! my stems are turning brown and slimy. only using water no nutrients