Spatial and Temporal Analyses of Bacterial Blight of Onion Caused by Xanthomonas axonopodis pv. allii

Abstract

Bacterial blight of onion is a severe disease, which emerged over the past decade in several onion-producing areas. This disease currently is observed in both the Old and New Worlds. Although the causative agent, Xanthomonas axonopodis pv. allii, is potentially seedborne, seed transmission and significance of seedborne initial inoculum for the development of bacterial blight of onion previously has not been assessed. This article describes experimental work designed to evaluate the biological importance of seedborne X. axonopodis pv. allii as an initial inoculum source, and examine the temporal and spatial dynamics of the disease. Over 3 years, outbreaks of bacterial blight of onion always were induced in experimental plots sown with naturally contaminated seed lots, with a contamination rate determined as 0.04%. Analyses of disease patterns indicated a likely seedborne origin for the inoculum associated with the early stages of epidemics. Spatial analyses performed with several statistical methods indicated aggregated patterns of disease incidence data. Primary foci enlarged over time, and a few distinct secondary foci sometimes were established after occurrence of wind-driven rains (with gusts up to 15 m s(-1)). Distances between primary and secondary foci ranged from less than 1 m (satellite foci) to 25 m. It remains possible that longdistance dispersal of inoculum was at least partly involved in the later stages of epidemics.

Modelization of the spatial distribution of corn head smut (Sporisorium reilianum Langdon and Fullerton) in Mexico

“…These results fit with the ones of Roumagnac et al (2004) who obtained maps of Xanthomonas axonopodis pv. allii in onion with a non uniform distribution of the disease, Gavassoni et al (2001) obtained irregular maps of Heterodera glycines distribution in soybean, meanwhile the non-uniform distribution of Colletotrichum kahawae in coffee was obtained by Mouen Bedimo et al (2007).…”

Multiplex Nested PCR for Detection ofXanthomonas axonopodispv. allii from Onion Seeds

“…The sensitivity threshold of the multiplex nested PCR assay was found to be 1/27,340 when it was determined with the same naturally contaminated seed lot mixed with healthy seeds. This level of sensitivity is greater than the contamination rate reported for BBO outbreaks in a tropical environment (4.5/10,000) (43). This threshold is also consistent with the tolerance standards currently recommended in the International Rules of Seed Health Testing for phytopathogenic bacteria and with a lower threshold recently defined for X. campestris pv.…”

“…allii transmission (42). Inoculum associated with seeds contaminated at a rate of 4/10,000 has been used as the primary inoculum in field epidemiological studies (43). Recently, onion seeds were identified as the most probable pathway for the introduction of the pathogen into Réunion Island from the neighboring country Mauritius (33).…”

O father where art thou? Paternity analyses in a natural population of the haploid–diploid seaweed Chondrus crispus

“…However, the assumption of one binomial distribution that can describe the pattern of paternity across all 29 females used in this study may not be valid (Young-Xu and Chan, 2008). Thus, a test for overdispersion was computed following Roumagnac et al (2004) and Sparks et al (2008) and then the fit of both the binomial and β-binomial models were compared using a likelihood ratio test (see Supplementary Figure 2 and Supplementary Note 1). Using the model with the best fit, the two shore levels were then compared using the same likelihood approach as implemented to compare the two distribution models (see Supplementary Note 1).…”