Risks for infection of strawberry plants with an aerosolized inoculum of Xanthomonas fragariae

Whole-genome sequencing and comparative genome analysis of Xanthomonas fragariae YM2 causing angular leaf spot disease in strawberry

Background

Angular leaf spot disease caused by plant pathogenic bacterium Xanthomonas fragariae seriously threatens strawberry crop production globally.

Methods

In this study, we sequenced the whole genome of X. fragariae YM2, isolated from Yunnan Province, China. In addition, we performed a comparative genome analysis of X. fragariae YM2 with two existing strains of X. fragariae YL19 and SHQP01 isolated from Liaoning and Shanghai, respectively.

Results

The results of Nanopore sequencing showed that X. fragariae YM2 comprises one single chromosome with a contig size of 4,263,697 bp, one plasmid contig size of 0.39 Mb, a GC content ratio of 62.27%, and 3,958 predicted coding genes. The genome of YM2 comprises gum, hrp, rpf, and xps gene clusters and lipopolysaccharide (LPS), which are typical virulence factors in Xanthomonas species. By performing a comparative genomic analysis between X. fragariae strains YM2, YL19, and SHQP01, we found that strain YM2 is similar to YL19 and SHQP01 regarding genome size and GC contents. However, there are minor differences in the composition of major virulence factors and homologous gene clusters. Furthermore, the results of collinearity analysis demonstrated that YM2 has lower similarity and longer evolutionary distance with YL19 and SHQP01, but YL19 is more closely related to SHQP01.

Conclusions

The availability of this high-quality genetic resource will serve as a basic tool for investigating the biology, molecular pathogenesis, and virulence of X. fragariae YM2. In addition, unraveling the potential vulnerabilities in its genetic makeup will aid in developing more effective disease suppression control measures.

Dynamics and Spread of Bacterial Spot Epidemics in Tomato Transplants Grown for Field Production

Tomato transplants are the primary means of establishing commercial tomato production fields in the eastern United States. Transplants are often suspected as the source of inoculum for major outbreaks in production fields of bacterial spot of tomato (BST) caused by Xanthomonas perforans (Xp). A combination of high plant densities with overhead irrigation, high humidity, and high temperatures are conducive to BST outbreaks during transplant production. In addition to chemical control, transplant growers use roguing to remove diseased transplants, as a primary way to manage BST during transplant production. The value of roguing is often questioned, because information about the rate of pathogen spread and the incubation period between infection and symptom development is limited. In this study, we evaluated the extent of X. perforans spread on tomato transplants relative to symptom development by using a rifampicin-resistant X. perforans strain and conducting experiments in an environmentally controlled greenhouse simulating grower practices and also at a commercial transplant facility in Florida. BST symptom development typically lagged behind X. perforans dispersal by at least 5 to 7 days depending on environmental conditions. Furthermore, X. perforans was capable of aerosolization, which resulted in long-distance dispersal of ≤2 m under highly favorable conditions. Growers should rogue diseased plants and surrounding nonsymptomatic plants by >1 and ≤3 m, depending on outbreak severity, to limit disease spread. As a result, proper disease management should reduce introduction of nonsymptomatic transplants into the field and subsequently reduce pesticide applications.