1. Diseases

Fuchsia Rust [Pucciniastrum epilobii] This is the scourge of the fuchsia world. It can be bought or given unwittingly on infected stock and once introduced is extremely difficult, if not impossible to eradicate.

The first signs are a slight discolouration to the top of the leaf. If examined more closely the underside of the leaf will have clusters of red orange pustules like small warts. Once the pustules ripen and change colour to brown they erupt and exhaust their spores to the atmosphere infecting every fuchsia in close proximity. There are several fungicides which will give a modicum of control but again eradication, once infected, is extremely difficult. Here again, the 'Hot Water' treatment is the answer. This particular fungus is rather a nasty one. During the growing season, especially during March, April and May, keep a sharp look out for the tell tale signs more especially on newly acquired stock. If rust is suspected, isolate the plant well away from your main stock. Remove any obviously affected leaves and destroy them. At this time of the year it would be more prudent, if possible, to dip the whole plant in a solution of fungicide then water the root ball with the remaining solution. Take care of course to read the manufacturers instructions, although in the past, I have used this method with varying degrees of success using different fungicides. Collect and destroy any fallen leaves whether infected or not. I have studied this fungus over a period of time and found it to be extremely virulent especially in the confines of a closed heated propagating frame. If your are an exhibitor and have exhibited plants in a flower show, never return them to the greenhouse/conservatory where your main stock is growing. Isolate and dip before returning them to their original growing environment.

Here is another source that is directed toward home gardeners.
https://gardenerspath.com/plants/flowers/fuchsia-not-blooming/

Fuchsias are susceptible to several different diseases, usually fungal in nature. Of the most common diseases, one of these can cause flowers to not form.

Botrytis blight, also known as gray mold, is caused by Botrytis cinerea.

Most often, it causes existing flowers to turn brown and rot, but it can also cause the young, developing buds to turn brown and die.

Good sanitation and being careful not to crowd plants can help prevent this disease, as can being certain to water at the soil level rather than on the foliage.

A product containing the beneficial bacteria Bacillus amyloliquefaciens strain D747 can be used as an effective treatment.
This disease is more common in specimens that experience drought stress.