Fungus? Virus? Heat? Please help!!

Hey everyone,

I am starting a bunch of seedlings under a hoophouse. I'm in zone 6, it's been getting cool in the 30's at night and reaching the upper 60's, low 70's during the day....making it in the 80's-ish inside my hoophouses.
They have all been doing wonderfully until just this week......
Yesterday I noticed the cucumber and canteloupe seedlings two leaves were starting to wilt and mildly discolor. The stems still looked healthy and green and strong so I figured it was heat stress and that they would recover during the cooler night.
Today, when I went out to check on them, they had NOT recovered. And I noticed the tops of my tomatoes were starting to discolor just a tad bit, as well as some other seedlings!

I've posted pictures below. Do you all think it's a virus? A fungus? Or something to do with the cooler nights and direct sun/heat during the days as of just recently?

Thanks so much for any advice you can offer!

I'm suspicious it could be a Fusarium infection: (1) Fusarium acts to quickly dessicate and kill plants (2) It's not really active until temperatures get up into the 80s (3) it is an equal opportunity fungus that can attack tomatoes and cucumbers (and melons).

Where did you get your soil to start your seedlings? If you tell me you used sterilized potting soil, that will put me off the Fusarium trail, but if you dug it out of the garden that would make me more suspicious.

I had a bad Fusarium problem when I first moved into my place. I got on top of it by growing a cover crop of mustard and tilling it under. The sulfur compounds in the mustard will kill off Fusarium. So will a lot of competition from other fungi, which is another advantage of hugelkultur.

Hmm, John, that is interesting.

The beds have logs at the very bottom, on top of which is top soil, on top of which is about a foot thick mix of composted manure, vermiculite, perlite & peat moss...

From what I've just researched on Fusarium, it will impact the stem of a seedling .... my problem began in the leaves, the stems were left strong and green until more recently. I'm really wondering if it's overwatering since I have an entire bed, using the very same soil, thriving.

The plants look 'sunburned' to me, do you think it might have gotten above 80 degrees? Maybe the damaged ones were closer to the plastic? I grow plants in my living room and this spring I am having a hard time hardening them off because of the extremes of temperature. The pale spots on the leaves of your plants remind me of a time or two when I exposed my indoor grown plants to to much direct sun in the spring and of hot summer time 'sun scalded' tomatoes.


EDIT...I think overwatering the soil would show damage on the edges of the leaves. Is it possible the leaves were wet, either from condensation or overhead watering along with intense sun/ I think that, also might look like your plants. The water on the leaves, will intensify the sun, I think.

OK, maybe I was a little quick to pull the trigger on Fusarium. I've enough bad experience with that one that when I see scalded leaves it immediately springs to mind. You're right, if it is just the leaves and the stems are healthy, it's probably not Fusarium and the plant has a chance to regrow more leaves.

looks a bit like spider mite damage. Check to see if there is any sign of faint webbing over the damaged leaves and tiny, tiny reddish or brownish mites…..might need a magnifying glass to see them if your eyes aren't good. They are a tough problem on indoor plants….high humidity and water on the leaves sometimes (as would happen outside with rain and dew) seems to discourage them (though these are the very things that encourage fungi!)

Thank you all for your input. As I observe further, taking into account all the seedlings that are doing fine, I do think it is a mixture of overwatering + cold nights + hot days.