I'm sorry to say this looks like box blight, a fungal disease which attacks the roots. There is no satisfactory treatment and you may have to dig them out.
Thank you both for your help. I will see what happens in the next few weeks and hope that the damage is due to the caterpillars. If it is blight then I will have to replace them.
Google both blight and the caterpillar and see which pictures most closely resemble yours.
Even if it is blight there is a fungicide available to help control that and a good regime of feeding and plant husbandry should keep on top of things. No need to get rid of the plants unless you want to.
The RHS website is a good starting point for info.
Heaven is ... sitting in the garden with a G&T and a cat while watching the sun go down
It looks like the leaves have definitely been munched by something. The only other possibility is that the plants have been watered from overhead in sunshine , but my money is on the caterpillars that you mentioned. As Topbird says try the RHS, a link here https://www.rhs.org.uk/advice/profile?PID=760
Not sure if this will help at all (new to these forums), but while we've been spared caterpillar issues this year, we do have fungal problems on box caused by excessive heat combined with moisture and high temperature variations. We've a treatment, but are currently (hope to have that answer today) looking for something a little less extreme.
Best indication of box caterpillar I know is droppings and webbing. That photo looks like (not too recent) caterpillar damage to me (and until this year I've spent, on average, 2 to 3 days per year treating infested box, for a good six years or so).
There were definitely caterpillars. I found them earlier in the year and treated the plants I thought successfully but they have reappeared. I have treated them again and will repeat that in about 10 days in case I missed any.
Yep that's definitely caterpillar damage...mine got munched a week ago. May have to admit defeat and replace them as don't want to be spraying all the time when alternatives are available. Sometimes it's worth admitting putting in the wrong plant in the wrong place and I'd rather have healthy strong plants that don't need chemicals to simply survive.
Agree with amancalledgeorge. Maybe in the long run box will be phased out. Here, individuals are looking after their gardens, but local authorities (with bigger volumes) are struggling to keep up.
kmhowarthw7: If the weather's in their favour, you'll get around three infestations a year. Buy a big sprayer (is my advice...)
Posts
Box blight is a fungal disease initially characterised by black / brown spots on the leaves.
Google ‘box caterpillar’ and see if the pictures match your plants.
Even if it is blight there is a fungicide available to help control that and a good regime of feeding and plant husbandry should keep on top of things. No need to get rid of the plants unless you want to.
The RHS website is a good starting point for info.
As Topbird says try the RHS, a link here
https://www.rhs.org.uk/advice/profile?PID=760