Hi there are different facets to your question - I shall do my best to answer.
I have plants under cover in polytunnels - mainly my parent plants and they are under cover mainly for my benefit as I work on them pollinating, seed collecting etc. Also for visitors to enjoy as they are on raised benches.
I have plants in pots outside and in large display containers outside.
I have plants in the garden outside and plants "wild" in hedgerows outside in deep unmown grass.
The only plants that suffer from any problems are those under the polytunnels where I get botrytis when the pollen drops as it rots and can cause the bract scales of developing flower shoots to get botrytis despite excellent ventilation.
I believe the problem is that the polythene absorbs too much UV light which acts as a sterilising agent on out door plants where I don't get the problem.
I never spray out door plants unless they get aphids when I use a bee friendly insecticide to prevent aphids tranferring viruses. I never use fungicide.
I get no problems with any plants outside despite living in the second wettest area of England. Double and semi-doubles thrive and set seed on their own so I don't find a problem with doubles.
The only problem I find outside is with plants that I bred to meet the consumer demand for up wards facing flowers - as I predicted they can rot off if there is rainfall over a long period when the anthers are shedding pollen. This is exactly as I predicted and if I am asked for upwards facing flowers I say yes but give a warning that some flowers may be lost to rot if it is very wet - the plants are fine just flowers rotting.
This is as one would expect as they like galanthus etc have evolved nodding flowers to shed rain and snow as they flower during winter/early spring - I also believe it keeps the reproductive parts a few degrees warmer and so aids pollination. In their native habitats hellebores can flower when there is deep snow and night time temperatures well below freezing.
With regard to colours I found that some of the early yellows from the 80's and 90's did tend to show less resistance to leaf spots in wet winters and the plants were generally less vigourous. I don't find any other consistent patterns with the plants I breed today. I regularly get people emailing me with photos of plants they've bought and saying how well they're flowering.
I think that there is a problem that many gardening books and magazines perpetuate the myth that hellebores are shade loving moisture loving plants so people put them in deep shade often facing north in waterlogged soils and they will sstruggle and rhizome rot becomes an issue.
The truth is that hellebores grow best in full sunshine during flowering and early leaf growth i.e. winter and early spring. In summer a little dappled shade from deciduous trees can do no harm but isn't essential. Hellebores are drought tolerant - many species in the wild go completely summer dormant their leaves shrivel and the rhizome stores food and water. I have rarely killed a plant by under watering but in the early days killed many by over watering!!! - I believed the so called gardening experts
I hope this answers your concerns. I do not feel that breeders have generally caused plants to become less disease resisitant. Line breeding only causes problems if there are genes that are recessive and harmful to survival. One concern I have is that I believe some breeders use a high nitrate feeding regime in pot plants to get them flowering in 2 years - this causes plants that are overly tall too quickly grown with huge flowers and fleshy stems these in my experience can struggle in the first year or two in the garden and I have seen plants die because they were all flower with too little leaf as they had been "forced" by over feeding and possibly atrificial lighting? After a couple of years they are usually then ok. I don not follow this practice and all of my hybrids are 3-4 years old before sale and not heavily flowering they do best once transplanted outside with extra room for the root development.
This winter which has been the wettest I have ever seen a wild H. Orientalis has shown the worst damage from black spot than any of the x hybridus which surprised me.
Generally leaf black spot fungi are not important as they are only mildly disfiguring and tend to attack old leaves which are dying from August onwards anyway. Botrytis and rhizome rots are the biggest problems and if grown in full sun in well ventilated areas not some dank shady corner against a wall and if the ground is well drained or they are grown on a slope then they are nto a problem.
Follow this advice and grow the Queen of winter flowers and enjoy their long lasting beauty
