Blueberries leaves problem - sudden colour change and defoliation

Hi,

I am looking for help regarding this issue on my 1st year blueberries field:

- Fertigation: 100Kg/Ha N, 35 kg/Ha P, 25kg/Ha k finished 6 weeks ago no other nutrient was fertigated

Suddenly in the last 10 days, leaves in 30% of the field (and increasing) have started changing colour and falling.

The field has extreme temperatures now (high 90s F during the day and and low at 35-40 F) and it rained last week (it hadn't rained for 3-4 months before). However as I said the field is drip irrigated and no signs of over irrigation or drought.

Could this be nutrition (I sent a 2 weeks ago a sample of leaves but they were green and healthy then)
or a fungus?

I would really appreciate the opinion fo an expert.
Thank You

Joao