A large tract of Waipū Forest in Northland has suffered an outbreak of red needle cast brought on by Phytophthora pluvialis and the very wet winter.
The disease outbreak is very noticeable in the 10- to 12-year blocks between Taipuha and Waipū, and on the Brynderwyn hills around SH 1, which also have parts of the same forest.
Forest management company Manulife said wetter than usual conditions in areas of Northland together with susceptible age classes of Pinus radiata has seen red needle cast (RNC) appearing.
“Phytophthora pluvialis is carried in airborne water droplets, and wet late summer conditions – mist, cloud and rain – are ideal conditions for the disease to develop,” Northland silviculture manager Peter Houston said.
Needle loss can set back tree growth with defoliation but as this damage is sporadic and unpredictable, normal tree growth resumes.
Already the affected trees are showing some green growing tips.
A beef farmer with land alongside the Waipū Forest on Millbrook Rd, Taipuha, said he noticed discoloured needles followed by needle loss on a large scale over the past few months.
A younger, replanted forest some 5km to the north of Millbrook Rd has shown RNC spread in recent weeks.
RNC is predominately found on the East Coast and Central North Island but more reports are being received from Northland and the top of the South Island, Scion team lead for pathogen diagnostics and collections Darryl Herron said.
The disease tends to follow a seasonal pattern where mild temperatures of 10degC to 20degC and high rainfall favour progression.
“This often results in peaks in disease over the winter months, and very little disease over summer,” Herron said.
“A reliance on moisture means that RNC hot spots can occur in areas with high mist or fog, for example in valleys or in forests exposed to sea mist.
“The 2022 disease season has been very severe, with expression of RNC as early as March, and complete defoliation of radiata pine in some locations.”
Significant needle loss in winter and spring leads to substantial tree growth losses in the following season.
Low-volume aerial applications of copper oxide have been shown to reduce the severity of RNC in trials conducted by Manulife in the Kinleith district in collaboration with Scion.
Houston said Manulife does not plan to preventively spray copper in Northland at this stage.
