Originally, I posted this in the Ferns, Mosses, & Fungi forum, but after several days got no replies. So, I figured I try my problem over here...
I have several Autumn Ferns in the ground in various spots throughout my backyard, and each area has varying degrees of sun, although, none are in full sun. A few receive direct morning sun for a few hours, while others receive late afternoon sun, or filtered/dappled sun throughout the day. The ferns continue to grow fine and even now in December continue to put up new fronds.
Probably back in early September, I noticed on most of the ferns that a healthy green frond would eventually develop one or two white leaflets in the middle or near the top. Sometimes these white leaflets may or may not have brown tips. On some ferns, a white leaflet will develop on one side of a fern, and on the other side a perfectly green frond will have a brown curling tip, or a few slightly browning/or dead tips on some leaflets, but not on all of them. (I recently noticed as of yesterday 12/10, that on a few ferns, there is a dead frond frond or two at the base. They are small as if they were new growth that only grew 3 to 5 inches before they turned white and started dying.)
This didn't happen last year at all, so I am not sure what is going on with them. I tried to research this issue on the internet, and one website said the white leaflets were bleached from receiving too much sun, while another website said it was lack of water and the fern is unable to to send enough water to ends of the fronds resulting in white leaflets. I could understand the latter but on some fronds the white leaflets are in the middle and the ones at the top are green. So I think it would be odd that water reached the whole frond except for two leaflets in the middle. As for the explanation being that it received too much sun...I don't think this is the case since the ferns are in the same spots as they were last year and received the same amount/degree of sun and this did not happen. Besides, even the ferns in dappled shade are having this same issue.
I hope someone here can help me understand what is happening here.
I agree that it does not look like a cultural issue. Sun/shade, and watering problems would much more likely present as more uniformly distributed over the whole frond, not isolated leaflets like are in the pictures. Sunburn for example is most likely to show up as a patch over the mid-leaf area, on the part in the most sun. It would affect several neighboring leaflets, including parts of some, not usually the whole leaflet and not the neighboring ones.
Watering might affect the new growth, especially if you had a hot spell and were not quick enough on the water. The new growth unfurling during that period could die, without killing the whole plant. I do not think it would turn white, though. Just brown and crispy.
I wonder if it is a virus like Mosaic or related to the way some plants are variegated?
I very occasionally find the same problem and my ferns grow in shade, I just snip the white leaf off and the space is soon filled in when the fronds mature, I have honestly never looked into the problem so cant help you out. My fern garden is on the edge of a wooded glade where not much sun gets to the ferns and most have sent seeds out and germinated themselves, I love watching ferns unfurl there new fronds and it really is one of natures amazing mysteries, well for me any way.
I would perhaps get in touch with either a Botanical garden or university that has Botany or gardening courses. maybe I'm clutching at straws for you but someone somewhere must be able to give you a proper answer to the problem, hope so.
Best of luck.
I am including another picture of one of the ferns. (The others are not doing this...yet.) After taking the above pictures, a few days later I went to check on that same fern and found a large frond browning down the center of the leaflets. Not sure what to make of it.
As for the white leaflets, sometimes I snip them off, sometimes I don't. depends on if I have something to cut with in my hands at the time LOL We had a really good rainstorm 2 days ago...if the whitening continues, then I will know it surely isn't lack of water because they got watered well from the rain. I guess I'll wait and see what happens over the next few days.
Sorry to be getting back to you so late on this, but my computer has been down.
The most recent picture looks like a disease or cultural sort of problem: It is more general, all over the frond, As to WHICH problem, I do not know.
Is this one fern exposed to something different?
Soil problem: Maybe a different pH, more rocks, less compost, old concrete thing under there...
Water: standing water? poor drainage? This could lead to disease issues, or cause a problem directly.
Disease: Is it spreading from one to another as if it is an organism that is moving frond-to-frond?
Does the under side of the frond show the same thing? Any pitting, or sunken tissue?