I grow many begonias. This one is a Rhizomatous and is a fairly new purchase. I have had this problem on other begonias (even discarded one recently) but this new plant has not been near the other affected ones. I have also been careful to sanitize my snips so as not to spread the disease.
It looks more like bacterial leaf spot. These can be confused with fungal leaf spot so it's best to get a diagnosis from a plant disease clinic. Your area Cooperative Extension Service should have one. Rhizomatous begonias are sensitive to water on the leaves and should be watered from below. Here is more information on management:
https://extension.psu.edu/begonia-diseases
https://www.gardeningknowhow.com/ornamental/flowers/begonia/begonia-leaves.htm
https://www.gardeningknowhow.com/ornamental/flowers/begonia/begonias.htm
How to properly cut back begonias?
This article will help you.
https://www.gardeningknowhow.com/ornamental/flowers/begonia/pruning-begonias.htm
How far back should a Begonia be pruned back to store for winter?
This article will help you: https://www.gardeningknowhow.com/ornamental/flowers/begonia/pruning-begonias.htm
Hi, my tuberous begonias have all been destroyed by powdery mildew this summer, despite spraying with insecticide. I have a number of varieties in both baskets and planters, but none in the garden beds. The tuberous begonias are all completely covered in mildew, both tubers from previous years, and begonia plugs bought this spring. Could you let me know if it's best to destroy the tubers, or, can they be started off as normal next spring? I live near Lanark, Scotland thank you. Douglas Fairbairn
Powdery mildew is very easily cured. This is mold, so an insecticide will not help at all. You will have to use fungicides. There are many of which you probably already have in your kitchen. I would toss the plants at the end of the year. No need to overwinter any spores.
These articles will help: https://www.gardeningknowhow.com/plant-problems/disease/get-the-cure-for-powdery-mildew.htm
I live in Redding, CA where the temperature can stay over 110 for days. I plant my tubers in pots with potting soil that has fertilizer. They are in a shaded, screened-in patio. They receive little to no sunlight. I water with a drip mist system every three days-every other day when the temp. gets above 100. The leaves are always large, but the flowers are disappointing. Are the flower size determined by species, or is there something I can add to the fertilizer that increases the size and amount of the flowers. Thank you for any suggestions.
This is, mostly, determined by genetics but environment can have an impact. Usually you can help the bloom process by adding phosphorus and potassium, or fertilizers high in these and low to no nitrogen. This will usually help with the blooming. Also, try moving them where they will get a little bit of sunshine, dappled shade or morning sun.
This collection of articles will give you tips on the care of begonia: https://www.gardeningknowhow.com/ornamental/flowers/begonia
https://www.gardeningknowhow.com/ornamental/flowers/begonia/tuberous-begonia-care.htm
just by putting the leaves in water to root. The blooms are very small, white, on a tall spike and not very impressive. I thought my Mother always called it a wax begonia but I am not sure about the name. What is the name?
It sounds like a wax begonia (Begonia x semperflorens-cultorum), except I wouldn't say the flower was on a tall spike. Hardy begonias (Begonia grandis) have more of a spike, but the leaves are more heart shaped.
https://www.gardeningknowhow.com/ornamental/flowers/begonia/begonia-leaves.htm
Do I plant in Peat Moss and when do I replant for a good show next year.
No. You can dry the tubers then store them over winter. Here's how: