my plants look lush and green but get dark brown spots on the leaves. plants bloom well, but the leaves turn black/brown. any suggestion of how to treat this would be helpful
This is a common, especially in humid weather.
The link below will help you.
https://www.gardeningknowhow.com/ornamental/flowers/black-eyed-susan/rudbeckia-leaf-spot.htm
When is the best time to transplant Black eyed susans?
Early spring or fall would be best.
https://www.gardeningknowhow.com/ornamental/flowers/black-eyed-susan/black-eyed-susan-flower.htm
They are very tall, but have not yet bloomed. If I cut them back a few inches would they come in thicker with more blooms? Mine have always looks so tall and leggy with small blooms? Thank you, I appreciate any help...Christine
If you cut them now, they will come back thicker and will likely bloom again later in the summer, but you will lose their first flush of flowers.
You may want to wait until after they bloom to cut them back. They will still grow back thicker and you will likely enjoy a second flush of flowers later in the year as well. A little patience will get you two bloom periods in the same season.
Why did my black eyed susan bloom this year with white flowers
We have not heard of this or have any research that would explain this.
Black Eyed Susan's can be effected by Powdery Mildew, which would cause a white powdery film on the leaves.
You can send us an image if you like.
https://www.gardeningknowhow.com/plant-problems/disease/powdery-mildew-homemade-and-organic-remedies.htm
Do I need to dead head a Tundergia and how do I? The seed thingys (?) all look the same .. the new ones and old ones.
Not necessary to deadhead this plant. You can save some of the seeds for next year.
My black eyed Susan vine has and continues to grow larger and larger however there are very few blossoms is there anything I can do to produce/promote them
Try a feeding of Phosphorous, with Bone Meal to help promote flowering.
My black eyed Susan's were eaten down by rabbits last year, before the stems grew. This year, I only have a couple inches of leaf growth and no stem. Since it is the end of July and they have usually stemmed by now, I wonder why their growth has been stunted.
Since your plants were eaten down before they grew out their life cycle, they may be stunted a bit this year.
Give them another season to recover.
https://www.gardeningknowhow.com/ornamental/flowers/black-eyed-susan/black-eyed-susan-flower.htm