No carbon limitation after lower crown loss in Pinus radiata.

BACKGROUND AND AIMS: Biotic and abiotic stressors can cause different defoliation patterns within trees. Foliar pathogens of conifers commonly prefer older needles and infection with defoliation that progresses from the bottom crown to the top. The functional role of the lower crown of trees is a key question to address the impact of defoliation caused by foliar pathogens. METHODS: A 2 year artificial defoliation experiment was performed using two genotypes of grafted Pinus radiata to investigate the effects of lower-crown defoliation on carbon (C) assimilation and allocation. Grafts received one of the following treatments in consecutive years: control-control, control-defoliated, defoliated-control and defoliated-defoliated. RESULTS: No upregulation of photosynthesis either biochemically or through stomatal control was observed in response to defoliation. The root:shoot ratio and leaf mass were not affected by any treatment, suggesting prioritization of crown regrowth following defoliation. In genotype B, defoliation appeared to impose C shortage and caused reduced above-ground growth and sugar storage in roots, while in genotype A, neither growth nor storage was altered. Root C storage in genotype B decreased only transiently and recovered over the second growing season. CONCLUSIONS: In genotype A, the contribution of the lower crown to the whole-tree C uptake appears to be negligible, presumably conferring resilience to foliar pathogens affecting the lower crown. Our results suggest that there is no C limitation after lower-crown defoliation in P. radiata grafts. Further, our findings imply genotype-specific defoliation tolerance in P. radiata.

Phytophthora pluvialis Studies on Douglas-fir Require Swiss Needle Cast Suppression.

Phytophthora pluvialis is associated with early defoliation and shoot dieback in Douglas-fir in Oregon and New Zealand. In 2013, P. pluvialis was described from mixed tanoak-Douglas-fir forests in the Pacific Northwest and concurrently recognized as the main causal agent of red needle cast (RNC) in New Zealand radiata pine plantations. Little is known about its infection cycle and impact on host physiology. P. pluvialis studies in Douglas-fir are challenging due to the ubiquitous presence of the endophyte Phaeocryptopus gaeumannii, which produces similar symptoms and premature defoliation with persistent needle wetness, known as Swiss needle cast (SNC). Nonetheless, our study showed P. pluvialis infection in the presence of SNC. Exclusive expression of P. pluvialis is difficult to achieve as both diseases are promoted by high humidity. Here we established a 'dry leaf' strategy to suppress SNC when inoculating Douglas-fir needles for RNC studies. Sheltering plants along with drip irrigation to avoid needle wetness during the P. gaeumannii sporulation period suppressed its development in the new season flush. The diminished endophyte inoculum enabled bias-reduced studies of P. pluvialis impacts on Douglas-fir without the confounding effects of stomatal blockage and premature defoliation caused by P. gaeumannii.