https://www.researchgate.net/publication/339865185_The_genus_Ravenelia_Pucciniales_in_South_Africa

https://www.inaturalist.org/projects/rust-covered-vachellia-pod-galls-of-serowe-botswana

I have found a group of 12 small vachellia trees (3-4 m high) growing on deep black cotton soil on the bank of a water course that only flows after very heavy rain.
The ID of the trees is so far unconfirmed. Some have iD them as
vachellia grandicornuta
Vachellia sieberiana var. woodii
vachellia hebeclada var tristis
vachellia lueduritzii
so there is no consensus yet about the ID of the vachellia tree host.

THE RUST
IS IT RAVENELIA ?
The trees had varying numbers of galls. Some more than 50 while others had one or two.
In March 2022 many of the galls were covered wih tiny, macaroni-like tubes which may be a ravenelia sp rust.
https://www.inaturalist.org/observations/108509610

A similar rust sp appears in a picture in this paper on page 15 ( photo at the bottom)
https://www.researchgate.net/publication/258050120_Survey_and_prioritisation_of_potential_biological_control_agents_for_prickly_acacia_Acacia_nilotica_subsp_indica_in_southern_India

The rust in the picture is identified as ravenelia. acaciae-arabicae which has the host vachellia nilotica.
Vachellia nilotica trees are very abundant and fruiting in the area around the galled vachellia , but I have not yet seen any galls or rusts on v.nilotica in the vicinity.

On one occasion I have found tubes that may be ravenelia inside an old gall, but normally they are on the outside.
https://www.inaturalist.org/observations/121539483

OR IS IT CEPHALOTELIUM( Vachellia Rust ) ?
https://www.inaturalist.org/taxa/1151370-Cephalotelium

OLD GALLS
By June many of these galls has presumably fallen to the ground and those remaining on trees had lost their rust'tubes'. In late april to June galls had changed from red-brown to black and many of them were pitted with small holes, perhaps where the rust tubes had once been.
see https://www.inaturalist.org/observations/113235212

On 12 June 2022 some of the black galls were found to be covered with an orange/white fungus and were white and powdery inside.
https://www.inaturalist.org/observations/121537969
More pics of old galls with this fungus can be seen here
https://www.inaturalist.org/observations/121537639
This was the tree that had most galls on it on June 12 22
I the article the picture on P15 shows ravenelia. acaciae-arabicae which has he host vachellia nilotica.

I have found a variety of insects in and emerging from the galls
Larvae
https://www.inaturalist.org/observations/108509106.
https://www.inaturalist.org/observations/108508709
https://www.inaturalist.org/observations/108508829

Pupae
https://www.inaturalist.org/observations/121122991
https://www.inaturalist.org/observations/109729345
https://www.inaturalist.org/observations/108808959

Adult moths
https://www.inaturalist.org/observations/121206820
https://www.inaturalist.org/observations/108662018

OTHER GALLS ON THESE TREES
A
I have see several galls like this
https://www.inaturalist.org/observations/108365449

Is it possible it is a galled flower and an ealier stage of the longer galls with rust ?