A sudden and widespread change in symptoms and incidence of vascular streak dieback of cocoa (Theobroma cacao) linked to environmental change in Sulawesi, Indonesia

Unstable new-encounter relationships between plants and pathogens form when organisms are transported be-tween once-isolated biogeographic regions. Ceratobasidium theobromae is a new-encounter pathogen that has plagued the cocoa industry causing vascular streak dieback in Southeast Asia and western Melanesia. The disease underwent a sudden change in symptoms associated with an increase in disease incidence in Indonesia and Malaysia from about 2004. These changes have not been linked to changes in the pathogen or host and this shift could signal an effect of region-wide environmental changes in the responses of cocoa trees to infection. To study the effect of ambient temperature on symptom expression, this study assessed disease symptoms in cocoa trees along altitudinal gradients in Sulawesi, Indonesia. It recorded types of VSD symptoms (necrotic or chlorotic) in clones and local hybrids on managed farms, and in a reciprocal transplant experiment. The results found that consistently across landscapes, in trees of varying inherent susceptibilities to VSD, the incidence of new, necrotic symptoms decreased with altitude and was restricted to lowlands, while the chlorotic symptoms that originally characterised the disease were found throughout the altitudinal range. The new symptom pattern in cocoa ap-pears to be an environmentally driven change in tree responses to the pathogen, possibly related to recent climate warming or elevated atmospheric concentration of CO2 due to widespread burning of forest and peat beds in Indonesia. The increased occurrence of a necrotic response of leaves to invasion by C. theobromae is typical of a hypersensitive response often associated with resistance to invasion by pathogens and could indicate enhanced resistance, although prolonged attachment of diseased leaves allows increased sporulation of the fungus and increased disease incidence. These are the first results to show an altered plant host-fungus pathogen response likely due to climate change, indicating that global warming may cause subtle but ecologically important changes in plant-parasite in-teractions. New-encounter relationships that are less stable than long-term co-evolved relationships might pro-vide early-warning of ecological changes due to climate change. Determining whether the changed symptoms are a new threat is vital knowledge for farmers' livelihoods in this region.