Thorns

I'm not feeling 100% so sorry if this comes out garbled but I am rather worried. Do any of you know what would cause several different species of plants to just die off in sections, in some cases dying completely and others being fairly healthy but with the occasional new shrivelled bit?

They look like they're suffering from underwatering, no signs of disease or bugs or whatever (well, none I recognise). But they're getting a reasonable amount of water, and the golden diosma(*) are dying in neat discrete sections rather than shrivelling in bits like the others (and the golden diosma I killed fom underwatering at the house without automatic reticulation). Also the not-sick plants all look perfectly happy, and the sick plants only wilt as a prelude to shrivelling and do not perk up after water.

So far I have a small tree, a golden diosma and a grevillia dead with two trees (of the same type-I-don't-recognise as the first) and two shrubs looking a bit peaky, and was ok with that while it was stable (which it has been for like 2 months) but now another golden diosma is dying off in sections! :(

And no, they're not all near each other, but the four plants which have been sickest longest (one of which died) are all together. Hmmmmm.

On the plus side, the icon for "worried" is cute.

(*)I think that's what they are, from this page I found on google. Man I suck at gardening!