Whanganui foresters are worried by watching needles turn red and drop off radiata pines, Middle Districts Farm Forestry Association member Denis Hocking says.

He's had lots of inquiries about pine diseases, and the association is organising a workshop and field visit to hear some possible answers and get some discussion going.

The gist seems to be that the diseases are the result of long periods of warm, wet weather- a humidity that doesn't suit Pinus radiata.

Read more: Dieback disease affecting pines

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The free workshop is on September 19 in Ashhurst and is open to anyone. It starts at 10.30 at the Valley Village Centre and moves to a site visit on the Pahiatua Track, finishing mid afternoon. Participants are asked to wear outdoor clothing and bring their lunch.

The disease problem is across the southern North Island, Mr Hocking said.

The most obvious disease is red needle cast, Phytophthora pluvialis. It can cause needles to fall off the tree, leaving its canopy very thin. It's at its worst from July to September and can affect trees of any age.

It is common in the wider Whanganui region, but more prevalent further north.

It's seldom fatal, Mr Hocking said, with new growth coming on in spring.

Red needle cast isn't the only disease that attacks New Zealand's commonest forestry tree. Dothistroma and Cycloneusma are two other fungal diseases also favoured by humid conditions.

One of the speakers at the association's workshop on September 19 will be Dr Nari Williams, from Scion, the leader in its foliar diseases group. The other will be Brent Rogan, the Ministry for Primary Industries' regional forestry health contractor.

They and other foresters will talk about the various diseases, possible treatments and ways to avoid them.

For more information contact Mr Hocking, jdhocking@xtra.co.nz or 06 322 1254; or Patrick Murray, murraysnurseries@xtra.co.nz or 06 376 5212.