Systemic Immunity against Soil Borne Phytophthora and Control of Ink Disease of Chestnut by Foliar Spray of Potassium Phosphonate

Chestnut ink disease represents a great risk for chestnut (Castanea sativa Mill.) in all regions of chestnut production of Europe and North America. Induced by soil borne oomycetes, Phytophthora cinnamomi and P. cambivora, both parasites infect the root system and cause root and collar rot resulting in dieback, decline and finally, the death of infected trees. There are no single control measures for root rot pathogens and control with systemic and selective oomycetes biochemical substances can be important as part of the integrated disease management of these soil borne parasites. In this work we studied the effect of potassium phosphonate to protect roots from P. cinnamomi. Chestnut seedlings were planted in potting mix previously inoculated with P. cinnamomi (Pr 120). One group of five pots, with three seedlings each were submitted to potassium phosphonate foliar spraying (3 ml LAtlante) and another group of five pots were water foliar sprayed. Seedlings had been grown in a nursery for 120 days. At the end of the experiment, visual symptoms of the crown and root system were assessed. Parameters related with roots: root length, root rot length, root number, root rot number, crown diameter, height and biomass were evaluated. Statistic significant differences between treatments were obtained in all root studied variables. Potassium phosphonate by foliar spraying prevented P. cinnamomi infections of roots of chestnut and can be a key factor in the management of ink disease of chestnut. INTRODUCTION Chestnut ink disease represents a great and continuing threat to chestnut (Castanea sativa Mill.) in all regions of chestnut production. Induced by soil borne oomycetes, Phytophthora cinnamomi and P. cambivora (Tucker, 1931; Pimentel, 1947; Urquijo, 1947; Crandall, 1950; Grente, 1961), both parasites infect the root system and cause root and crown rot. Symptoms can be apoplectic and the tree rapidly dies or results in dieback, long-lasting decline and in the end, death of the infected trees occurs. Dead roots reduce water and mineral uptake which stresses the tree over several years before symptoms of dieback and rearing of the crown are visible above ground. Symptoms of the disease may be non-specific and diverse which hamper diagnoses and management. Despite the presence of the pathogens and their devastating epidemics in Europe since the second half of the 19 century (Zentmyer, 1980) and even though many and varied approaches to control the disease were studied, there are no simple solutions for these root rot pathogens. Chemical control with systemic and selective oomycetes products can be an important factor and part of the integrated management of soil borne Phytophthora. Phosphonates are salts or esters of phosphonic acid (Guest and Grant, 1991) but in recent literature, when these products were applied for protection of plant diseases the term phosphite is also frequent. Daniel et al. (2005) and Daniel and Guest (2006) stated that following phosphonate treatment challenged cells in normally susceptible plant species undergo rapid cytological changes that include nuclear migration and hypersensitive cell death, activate defence-related biosynthetic pathways and accumulate higher levels of phytoalexins and deposit of physical barriers around challenged cells. Activities of Proc. 1 European Congress on Chestnut Castanea 2009 Eds.: G. Bounous and G.L. Beccaro Acta Hort. 866, ISHS 2010 450 enzymes involved in defence mechanisms are also elevated in normally susceptible Eucalyptus marginata seedlings treated with phosphonate that includes phenylalanine ammonia lyase (PAL), 4coumarate coenzyme-A ligase (4-CL), cinnamyl alcohol dehydrogenase (CAD), and the synthesis of phenolic compounds (Jakson et al., 2000). Phosphonates are officially approved to control Sudden Oak Death associated with P. ramorum (Garbelotto and Douglas, 2009) in California and “Jarrah” dieback associated to P. cinnamomi (Jakson et al., 2000) in Australia and some reported experiments reveal phosphonate efficacy in many others Phytophthora host interactions (Wilkinson et al., 2001). A range of responses to phosphonate application are also reported depending on the plant species and time of application (Wilkinson et al., 2001; Navarro et al., 2006). The present work provides data on biological activity of potassium phosphanate (H2PO3), the anionic form of phosphonic acid, against P. cinnamomi in chestnut seedlings and its efficacy in controlling ink disease by preventive foliar application. To take into account some potentially negative effects of phosphonate, direct toxicity against beneficial soil micro-organisms were also evaluated in vitro to provide useful information to aid in the implementation of a viable management treatment for ink disease of chestnut.