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"Spate" is a singular collective noun, therefore it is singular.  

A spate of attacks has ...

Treat it similarly to the following, for example:

A gang of children is over there; a clump of bushes is in the middle of the garden; a murder of crows is very loud; etc.

"Spate" is a collective noun, therefore it is singular.  

A spate of attacks has ...

Treat it similarly to the following, for example:

A gang of children is over there; a clump of bushes is in the middle of the garden; a murder of crows is very loud; etc.

"Spate" is a singular collective noun.

A spate of attacks has ...

Treat it similarly to the following, for example:

A gang of children is over there; a clump of bushes is in the middle of the garden; a murder of crows is very loud; etc.

1
source | link

"Spate" is a collective noun, therefore it is singular.

A spate of attacks has ...

Treat it similarly to the following, for example:

A gang of children is over there; a clump of bushes is in the middle of the garden; a murder of crows is very loud; etc.