The Citrus Exocortis Disease: A Complex of Viroid-RNAs

Citrons inoculated with different field sources, displayed a variety of symptoms ranging from very mild leaf bending and necrosis to the severe reaction normally associated with exocortis disease. Nucleic acid preparations from shoot samples were analyzed by sequential polyacrylamide gel electrophoresis. All source from both California and Spain contained one to four viroids with distinct physical and biological properties. The size range was estimated from 371 nucleotides for the citrus exocortis viroid (CEV) to 275 for the smallest viroid. The recovery of single viroids suggested a relationship between the distinct viroids and the symptom reaction expressed in citron

A method of determining where to target surveillance efforts in heterogeneous epidemiological systems

The spread of pathogens into new environments poses a considerable threat to human, animal, and plant health, and by extension, human and animal wellbeing, ecosystem function, and agricultural productivity, worldwide. Early detection through effective surveillance is a key strategy to reduce the risk of their establishment. Whilst it is well established that statistical and economic considerations are of vital importance when planning surveillance efforts, it is also important to consider epidemiological characteristics of the pathogen in question—including heterogeneities within the epidemiological system itself. One of the most pronounced realisations of this heterogeneity is seen in the case of vector-borne pathogens, which spread between ‘hosts’ and ‘vectors’—with each group possessing distinct epidemiological characteristics. As a result, an important question when planning surveillance for emerging vector-borne pathogens is where to place sampling resources in order to detect the pathogen as early as possible. We answer this question by developing a statistical function which describes the probability distributions of the prevalences of infection at first detection in both hosts and vectors. We also show how this method can be adapted in order to maximise the probability of early detection of an emerging pathogen within imposed sample size and/or cost constraints, and demonstrate its application using two simple models of vector-borne citrus pathogens. Under the assumption of a linear cost function, we find that sampling costs are generally minimised when either hosts or vectors, but not both, are sampled