Statistical analysis of the spatio-temporal dynamics of rubber tree (Hevea brasiliensis) trunk phloem necrosis: no evidence of pathogen transmission By N. Peyrard 1 , F. Pellegrin 2 , J. Chadœuf 1 and D. Nandris 2 1 Applied Mathematics and Computer Science Department, National Institute for Research in Agronomy, INRA, 84 914 Avignon cedex 9, France. E-mail: nathalie.peyrard@avignon.inra.fr; 2 Plant Pathology Laboratory, Institute for Research and Development (IRD/CBGP), 34988 Montferrier, France Summary Trunk phloem necrosis (TPN) of Hevea brasiliensis is an irreversible syndrome of the phloem that spreads from the collar towards the tapping cut. It is responsible for the cessation of latex production, the main constraint in rubber plantations worldwide. Numerous investigations have been undertaken to understand the mechanisms of this disease. The apparent linear spread of TPN supported the initial hypothesis of a biotic causal agent for TPN. However, previous and recent aetiological analyses remained inconclusive and the pathogen hypothesis is tending to be abandoned. In this paper we present a complementary statistical analysis of spatio-temporal epidemiological data collected in a rubber plantation in Co ˆ te d’Ivoire. There, four study plots were surveyed each year from 2000 to 2003, with tree-by-tree disease assessment. In two plots, the tapping knife was systematically disinfected with sodium hypochlorite to stop any virus or viroid transmission. Based on permutation tests, our analysis confirmed the aetiological results: there was no evidence for spread by pathogen transmission. However, the spatial structure of the disease was clear. These results strengthen the current alternative scenario of a multi-factor physiological disease caused by an accumulation of exogenous and endogenous stresses. Spatial heterogeneity of the risk factors probably results in the presence of areas of stress that can explain the spatial patterns observed among the TPN cases. The final contribution of this study was confirmation of the curative effect of sodium hypochlorite in the earliest stages of the disease, thus opening the way for control of this disease. 1 Introduction The main constraint in rubber plantations worldwide is the cessation of latex production caused by two different syndromes: (i) tapping panel dryness (TPD), which is considered to be a reversible physiological response to overexploitation; and (ii) trunk phloem necrosis (TPN), an irreversible syndrome of the phloem that spreads from the collar towards the tapping cut. TPD was already considered to be a serious problem at the beginning of the 20th century during the rise of Hevea brasiliensis cultivation in Asia (Bobilioff 1919; Keuchenius 1924). In the 1970s, it became a major problem, which affected latex yield in ÔmodernÕ industrial plantations (Paranjothy and Yeang 1977). The TPN disease was first detected in a single estate plantation in southwest Co ˆ te d’Ivoire in the 1980s (Nandris et al. 1984). Beyond the obvious dryness of the cut, TPN was shown to quickly cause a severe disturbance in the trunk phloem, starting from the collar and moving upwards towards the tapping cut (Nandris et al. 1991a). Nowadays, TPN is prevalent in most modern rubber plantations worldwide, with a wide range of severity across sites. Evidence of immature trees with TPN enabled confirmation of the For. Path. 36 (2006) 360–371 Ó 2006 The Authors Journal compilation Ó 2006 Blackwell Verlag, Berlin Received: 17.03.2005; accepted: 05.04.2006; editor: S. Woodward www.blackwell-synergy.com