Quantifying radial growth loss from red needle cast in Pinus radiata D.Don plantations

Background: Red needle cast (RNC), caused by Phytophthora pluvialis Reeser, W.L. Sutton & E.M. Hansen, is a significant foliar disease impacting Pinus radiata D.Don in New Zealand. First detected in 2005, the disease has now been observed in all regions of the country. In the most severe cases, defoliation of entire tree crowns can occur at a landscape scale. While some evidence of growth loss and productivity reduction has been reported, quantitative estimates of the effect of RNC on productivity are needed to inform disease management and mitigation decisions. This study aims to assess both short- and long-term losses in radial growth due to RNC. Methods: We used tree cores to quantify yearly basal area increments at two plantations: a 32-year-old stand in Wharerata Forest, with documented history of outbreaks both severe and cyclic in nature, and a 26-year-old stand in Kinleith Forest, where 8 years of continuous disease severity monitoring has been conducted at the tree level. A Bayesian multilevel modelling framework was used to predict growth losses due to RNC at both sites, accounting for yearly weather and outbreak severity. Results: We predicted a 31% to 51.5% radial growth loss in the year following an RNC outbreak, with reduced growth detectable for 3 to 4 years after disease, amounting to up to 30.6% growth loss over the course of a single event. Recurring disease events every three to four years can lead to a 20% reduction in total radial area growth over the period encompassing the presence of the disease, with no evidence that each additional RNC event aggravates growth loss. Conclusions: