I've had this happen time to time. It just seems that that leaf didn't develop properly. Eventually it'll either grow to normal size or turn brown and die. The latter is more common. Don't be to alarmed your plant has healthy new leaves so that's good.

Thanks for your thoughts. We just had 4-5 days of super heavy rain, and the plant oddly looks perked up! It’s so weird. The yellowing I understand... it was just those weird blotches and the die-back that made me think of a fungus. It’s looking a bit better now, so I may never know what happened. Lol!

Forgive me for not taking "and it is not being over-watered" at face value. While it could certainly be a reduction in photo load causing leaf abscission, odds suggest that root health/function issues should not be overlooked w/o something to corroborate the contention. Too, if you're using a digital device to check moisture levels in the soil (your finger), odds of it being related to over-watering (or just an ordinarily poor medium) increase. Try using a wooden tell to keep you informed about what moisture levels are deep in the pot. Something I wrote about using a 'tell': Using a 'tell' Over-watering saps vitality and is one of the most common plant assassins, so learning to avoid it is worth the small effort. Plants make and store their own energy source – photosynthate - (sugar/glucose). Functioning roots need energy to drive their metabolic processes, and in order to get it, they use oxygen to burn (oxidize) their food. From this, we can see that terrestrial plants need air (oxygen) in the soil to drive root function. Many off-the-shelf soils hold too much water and not enough air to support good root health, which is a prerequisite to a healthy plant. Watering in small sips leads to a build-up of dissolved solids (salts) in the soil, which limits a plant's ability to absorb water – so watering in sips simply moves us to the other horn of a dilemma. It creates another problem that requires resolution. Better, would be to simply adopt a soil that drains well enough to allow watering to beyond the saturation point, so we're flushing the soil of accumulating dissolved solids whenever we water; this, w/o the plant being forced to pay a tax in the form of reduced vitality, due to prolong periods of soil saturation. Sometimes, though, that's not a course we can immediately steer, which makes controlling how often we water a very important factor. In many cases, we can judge whether or not a planting needs watering by hefting the pot. This is especially true if the pot is made from light material, like plastic, but doesn't work (as) well when the pot is made from heavier material, like clay, or when the size/weight of the pot precludes grabbing it with one hand to judge its weight and gauge the need for water. Fingers stuck an inch or two into the soil work ok for shallow pots, but not for deep pots. Deep pots might have 3 or more inches of soil that feels totally dry, while the lower several inches of the soil is 100% saturated. Obviously, the lack of oxygen in the root zone situation can wreak havoc with root health and cause the loss of a very notable measure of your plant's potential. Inexpensive watering meters don't even measure moisture levels, they measure electrical conductivity. Clean the tip and insert it into a cup of distilled water and witness the fact it reads 'DRY'. One of the most reliable methods of checking a planting's need for water is using a 'tell'. You can use a bamboo skewer in a pinch, but a wooden dowel rod of about 5/16” (75-85mm) would work better. They usually come 48” (120cm) long and can usually be cut in half and serve as a pair. Sharpen all 4 ends in a pencil sharpener and slightly blunt the tip so it's about the diameter of the head on a straight pin. Push the wooden tell deep into the soil. Don't worry, it won't harm the root system. If the plant is quite root-bound, you might need to try several places until you find one where you can push it all the way to the pot's bottom. Leave it a few seconds, then withdraw it and inspect the tip for moisture. For most plantings, withhold water until the tell comes out dry or nearly so. If you see signs of wilting, adjust the interval between waterings so drought stress isn't a recurring issue. AL

Thanks both of you. I’m not exactly sure what I’m looking for, but I haven’t seen any of the signs of physical damage I read about in some googling — either entry holes from cane borers, or tunnels inside the canes from borers/girdlers (I clipped a couple canes off to look inside). My understanding is that there should be some telltale signs of damage with either of these pests. Is that right? Another thing I notice looking closely at the rugosa is that there are a few new green shoots coming out of canes where everything else had previously wilted. I’m not sure whether that’s to be expected with girdlers or borers? I’m pretty new to rose pests, and haven’t had any trouble with the rugosa before, so please excuse my ignorance. I appreciate all the info.