The vegetative propagation and quality specifications of Pinus Patula hybrid cuttings

Abstract

Pinus patula has for many years been the most important softwood species along the eastern regions of southern Africa but as a result of the pine pathogen, Fusarium circinatum, there have been serious establishment issues and difficulties in propagating P. patula from either seed or cuttings. In response to this threat, research has been conducted on the development of F. circinatum-tolerant P. patula and hybridising it with more tolerant Pinus species such as P. tecunumanii. Not enough hybrid seed can, however, be produced to replace P. patula seedling production. It has therefore become necessary to investigate the production of tolerant P. patula families and various P. patula hybrid alternatives through vegetative propagation. The suitability of P. patula hybrids to the current and future vegetative propagation systems, that is hedges in polythene bags with composted pine bark growing media and hydroponic sand beds. It is also necessary to determine what the ideal plant specifications for those hybrid cuttings being produced are in order to ensure good survival and stocking. The potential gains from high quality planting stock include optimum stocking and volume growth. While some plant quality work has been undertaken on P. patula seedlings there has not been any research into the plant quality specifications required for the successful deployment of P. patula hybrid cuttings. The objective of this study was thus to: investigate the propagation potential of these taxa in the two vegetative propagation systems, with natural infection by Fusarium circinatum; and to test the morphological plant quality specifications for Pinus patula x Pinus tecunumanii (low elevation, LE) rooted cuttings required for optimal survival and growth after planting. The taxa propagation investigation comprised two experiments; a taxa production experiment and a propagation system experiment. Experiments were carried out at the Sappi Shaw Research Centre located near Howick, South Africa (S29°28.53’ E30°10.75’). The taxa sele cted represented a range of predicted F. circinatum tolerance. Each experiment comprised 23 family treatments (across eight hybrid and pure taxa). A total of 2300 hedges were included in the taxa production experiment, planted in the current commercial standard which is composted pine bark growing media in black polythene bags. The propagation system experiment, compared two hedge system types, the hydroponic sand bed and current commercial standard. A total of 1200 hedges were included in this experiment. A pine mini-hedge system was employed to produce juvenile shoots for vegetative propagation in both experiments. Rooted cuttings were produced between October 2008 and June 2012. Over the 45 month period a total of 23 shoot harvests were set. All dying hedge plants were collected and sent for laboratory confirmation of infection by F. circinatum. Needle samples from 493 hedges across selected hybrid crosses and P. patula as well as all 714 hedges of P. patula x P. tecunumanii (LE) were submitted for DNA fingerprinting to confirm their hybrid status. Significant differences (p < 0.001) in mortality associated with F. circinatum were observed between the P. patula x P. tecunumanii (LE) hybrid (6%) and P. patula (19-23%). No significant differences in mortality associated with F. circinatum were observed within P. patula x P. tecunumanii (LE) families which ranged from zero to 15 percent. Significant mortality differences (p < 0.001) were observed between P. patula families which ranged from eight to 44 percent. The number of rooted cuttings produced, per hedge established, over the four year period was significantly better (p < 0.001) in the P. patula x P. tecunumanii (LE) hybrid (52) than in P. patula (29-33). Significant differences (p < 0.001) were also observed in the number of rooted cuttings produced per family, with P. patula x P. tecunumanii (LE) families ranging from 35 to 70 cuttings per hedge plant established and P. patula families between 20 and 42 cuttings. Over the four year duration of the trial all taxa showed increased productivity in hedges grown in a hydroponic sand bed system, which received more consistent fertilisation and yielded an average of 55 rooted cuttings per hedge, over those grown in polythene bags with composed pine bark medium which yielded 41 cuttings on average. To investigate the morphological plant quality requirements for Pinus patula x Pinus tecunumanii rooted cuttings, rooted cuttings aged between 2 and 23 months, from time of setting, were selected from multiple families to establish a plant quality field trial. Cuttings were grouped into five age treatments and planted in a randomised complete block design in 7 by 7 tree plots and with 6 replications. A total of 20 hybrid families were included. Cuttings were raised in 90ml inserts in a containerised system with composted pine bark growing medium. Plant quality measures were assessed across 1470 individual cuttings, with age being used as a grouping factor at field planting. The cutting quality parameters included in this study were; plant age, height, RCD, needle colour, root plug colonisation, visual presence of ectomycorrhizae and number of visible white root tips. Survival and growth for each individual cutting was recorded at one year after field planting. The ideal raising period for P. patula x P. tecunumanii (LE) cuttings, grown in a 90ml cavity was 10 months from setting. The ideal height for cuttings was 28-32cm and the ideal root collar diameter range was 3.5mm - 4.5mm. The root plug was optimal when no growing medium fell off the plug when extracted from the insert; the root plug was firm but not hard and the plug was well colonised with a high proportion of thin brown roots. It was optimal to have at least three or more actively growing white root tips present and visible evidence of ectomycorrhizae. Needles in the dark mid-green to dark green range were shown to be optimal. These plant quality recommendations were based on findings from a single trial site that experienced good planting conditions and good rainfall. As a result, the effects of significant water stress on the survival and growth of these cuttings was not adequately assessed and would require further testing. This study showed that the P. patula x P. tecunumanii (LE) hybrid is a feasible substitute for P. patula in both vegetative propagation systems, as it not only shows improved survival, through increased F. circinatum tolerance, but also improved productivity. It also showed that even under ideal planting conditions cutting age, height, root collar diameter, needle colour, root plug integrity and the number of white roots all had a significant effect on survival and growth of cuttings a year after field establishment.