Did you transplant it recently? If you did, it might be going through shock. It probably is also running out of energy for new healthier grow. I went through similar problem with my other berry. I kept them in the platic container from Wal-mart, they had been growing slow and the new leaves were not vibrant. I recently put them in ground, and mix the soil with some "Peat Moss", the new leaves are now looks better, I guess they get more new energy/nutrient now. Raspberry needs acidic soil, get some Peat Moss into your planting area, and get some soil acidifier powder from your local nursery, mix it with water then spray the plant and ground around the plant. It should improve within 2 weeks.

I wouldn't fertilize now either. I agree rooting in spring has its benefits, but since you're trying to save the leaf at this time, you can't hold off till spring.. Like Rjm stated, diving the leaf is one way to root. Actually, I've seen pics of how rooting Sans is done..One leaf is cut into 2-3" pieces, set upright/angle in well-draining soil. The problem is, since your leaf is yellow, which normally happens when Sans are over or underwatered, this leaf may not root..When rooting, heatlhy foliage is recommeded..Neither the oldest nor youngest leaves are advised. I'm not saying your leaf is unhealthy, since it's still firm (no rot, right?) So, it might have a chance of survival with some work. It's also a good experiment. I wouldn't use the entire leaf to root. You've a better chance at success cutting into pieces. If you decide to go this route, there's no need for plastic (humidity) In fact, you want the soil to dry out to a degree, adding plstic will keep soil too wet. The best container is a small, shallow dish/tray. Set cuttings a few inches apart. Place tray in medium light (no direct sun) If you use Superthrive Hormone and Vitamins, add a small amount once a month..If you don't have/use Superthrive, perhaps a light dose of Fish Emulsion will help..a very light dose. Good luck, Toni

I sincerely apologize for the delayed response! I never got any notification of replies to my question so I assumed there were none. I came back on here today because I have a new problem with my marshmallow. I have got them hardened off and they are all outside near my patio. They are definitely growing better but in the last few days, have all gotten purple leaves (still only a very minor issues with the yellowing)! It is only effecting the oldest of the leaves so far but the stems have turned purple too so I'm worried that whatever this is will make it to the beautiful newer leaves as well. I haven't put them in the ground yet because I am terrified of pests destroying them. Something, I think we've come to the conclusion it's birds, has taken every one of the 30 clary sage seedlings and 2 of my comfrey seedlings that I transplanted out. But why on earth would the marshmallow leaves now be turning PURPLE?! Gardening is supposed to relaxing

It is possible you have Early Blight. Mine do the same thing. They will be fine until the first full day of high heat and humidity then it just explodes. The first thing to do is get all the bottom leaves trimmed off - I've read that it is fine to take everything off at least the first foot of your plants and even up to 18". Get them in the trash then wash your shears and gloves with bleach. This will help stop the spread. Copper fungicide or Daconil sprays, or Epsom Salt drenches are usual recommendations. I usually start spraying mine about 2 weeks after planting. Good luck with yours.