The Horticulture and Food Research Institute of New Zealand Ltd, Ruakura Research Centre, Private Bag 3123, Hamilton, New Zealand

Background and objectives
Plants resist infection via a combination of preformed and inducible defence mechanisms. Inducible defences can be activated not only by infection but also by a variety of biotic and abiotic elicitors. Elicitors have been shown to accelerate and intensify the expression of inducible defences to subsequent pathogenic infections and so their practical application for disease control has been proposed. Salicylic acid (SA) plays an important role in the induction of both local and systemic resistance in plants. Exogenous application of SA and chemically related derivatives have been shown to induce resistance in several plant species.

In conifers, elicitors have been shown to induce defence responses in suspension cultures of Pinus elliottii and Pinus banksiana, and at wound sites in Pinus contorta. Here we investigate elicitor induced responses in Pinus radiata seedlings and suspension cultures and the subsequent resistance of seedlings to infection with Sphaeropsis sapinea (Diplodia pinea). S. sapinea causes shoot blight, crown wilt, canker and sap stain on a wide range of conifer species worldwide. The most common symptom associated with S. sapinea infection in P. radiata is shoot blight. Infection of terminal shoots can result in leader dieback or deadtop, this is considered the most damaging form of shoot blight as it results in stem malformation and a reduction in the useable length of the bole.

Results and conclusions
Elevation of phenylalanine ammonia-lyase (PAL) activity, which regulates the synthesis of antimicrobial phytoalexins and the production of the monomers necessary for lignin biosynthesis, has often been used as an indicator of resistance. There was a transient increase in PAL activity in P. radiata seedlings following application of SA or 5-chloro-SA (5CSA). 5CSA was the more effective and induced a 3-fold increase in PAL activity after 24 h with activity peaking after 48 h at 4 times the level of the untreated control. SA treatment induced a 1.4 fold increase after 24 h with peak activity occurring after 48 h at 2.3 times the untreated control. Stimulation of PAL activity in P. radiata seedlings may be associated with the induction of lignification and other host resistance mechanisms. Indeed, in P. radiata suspension cells SA treatment causes induction of lignin biosynthesis over a similar time course to fungal cell wall elicitor-induced induction of lignin biosynthesis. Similarly fungal elicitor induced an 8-fold increase in PAL activity [1].

A series of experiments were carried out to assess whether SA and 5CSA affected the susceptibility of P. radiata to subsequent infection by S. sapinea. Only 10-20% of seedlings treated with 1mM 5CSA developed deadtop compared to 40-60% of seedlings treated with 1mM SA. In control seedlings 75-95% developed deadtop. SA had no effect on S sapinea growth at concentrations up to 2mM whilst 1mM and 2mM 5CSA reduced growth rate by 12 and 27%, respectively. However, 5CSA concentrations of 0.5 and 0.25 mM were not directly inhibitory to growth of S. sapinea but still afforded significant reductions in deadtop development (P<0.001) to treated seedlings [2].

In order to assess the duration of disease control P. radiata seedlings were treated with 1 mM 5CSA up to 64 days before wound inoculation with S. sapinea. Significant reduction in the incidence of deadtop (P<0.05) was observed on seedlings treated between 2 and 32 days before inoculation.

More detailed biochemical studies are in progress to determine the nature and duration of the 5CSA induced responses in P. radiata and to assess the viability of induced resistance for controlling other diseases in forestry nurseries.

1. Hotter GS, 1997. Australian Journal of Plant Physiology, 24, 797-804.
2. Reglinski T, Stavely FJL, Taylor JT, 1998. European Journal of Forest Pathology (in press).