Gosh Lesuko, I feel for you, as I had this exact same problem this spring already. I'm not sure if we have the same reasons, but we sure have/had the same results...unfortunately I lost every plant that showed the signs you have. You may have to start over from scratch like I did. For me - problem number 1 was too much heat. I learned on these forums that after the seedling sprouts, they like to grow in a much cooler environment. Also, add air. A little desk fan running 24/7 in the corner made a big difference for me too. The soil was also a problem for me but I can only guess as to the cause based on what happened AFTER I quit using it. I thought I had primo stuff...(I made it myself afterall!)...but as I was taught here, it was just not porous enough, held water, and sort of compacted inside the cell-pots. Also, it had no nutrients in it, which some say is okay for the first few weeks of plant life, but in my mix the combo spelled death. (But it's soooo black, and it has the perfect ingredients, and it looks so ideal, I lamented.) Well, maybe so, but the proof was in the pudding, and I had a high mortality rate. Same as you, I also had a fungus growing on the soil. So, I switched to a pre-mix formulation that my local feed store carries. 2nd batch faired much better. Now the water....I too, felt that DOD didn't seem right since I have hardly any humidity, and I'm still not convinced that it was a factor or that I was overwatering. Until, I dumped the dirt out in my hand after the plant finally croaked. It was moist, but not wet. Crumbly, soft, fluffy, [smelled moldy] which maybe the one indicator of a obvious moisture problem. Indeed, I had to listen to folks on these forums that know better than I, and I made a crucial adjustment. I stopped watering just because the top looked dry or felt dryish to the finger test. Instead, to satisfy my need to see nice black, moist dirt on top, I bought a pump sprayer and misted the tops, leaving the watering alone until it really WAS needed. To that mist I added foliar feeding fertilizer, and man, what a difference. Within two weeks, yellowing stopped and green started to take over. I really can't claim any credit, because before these forums helped me, my poor plantlets looked just like yours!

There is a shock from relocating out of warm humid greenhouse by the way of semi trucks to the store and to your much drier house. Can cause blossom drop. Or this bud stem was someway damaged on the way. None of these are a problem. AV will adjust and give you more blossoms in time. Congratulations with your very pretty purchase. i.

Pictures please. One gallon pot, It might be to small and young.

This is much more common in fruits like apples than roses. Basically the plant decides it has more flower buds/fruits than it can support, and spontaneously drops some. It isn't usually considered a bad thing since fruit growers will often thin young fruit to improve the size and quality of what is left. It isn't something discussed much with roses since few roses do it without an enormous amount of stress. Rugosas are one of those roses. What you can or should do about it is more than a bit of a mystery. I will say that rugosas are much finickier than most people give them credit for. They like acidic sandy soil, and struggle in heavy soil, or neutral or alkaline pH. Since some of the leaves in the pictures are beginning to show signs of chlorosis, I'd start with a standard soil test. It is possible they were originally planted in soil that had been pH adjusted in some manner, and that treatment is wearing off. I have this issue with roses with multiflora heritage. However, you really need to establish a base line reading before messing around with things.