Book Description

Bacterial wilt of cucurbits is caused by the bacterium Erwinia tracheiphila (Et) and is considered one of the most destructive diseases of cucurbits in the Midwestern and Northeastern US, causing losses of up to 80%. Recent and early studies demonstrated a preference among Et strains to colonize hosts in the genus from which they were isolated. Non-productive (asymptomatic) colonization of non-preferred squash plants, and root colonization of both preferred and non-preferred hosts was demonstrated for the first time using a bioluminescent Et strain. Asymptomatic but colonized plants could serve as sources of inoculum in the field. Additionally, cucumber beetle larvae could acquire and transmit Et when feeding on roots, a phenomenon not previously considered in the epidemiology of this disease. Et is a pathogen undergoing relatively recent genomic changes, including a variable number of type VI secretion system (T6SS) gene clusters among Et strains. In single inoculations, BHKY, but not TedCu10 and MDCuke, was pathogenic to squash. Plants inoculated with either TedCu10 or MDCuke at 108 colony forming units (CFU)/ml followed immediately afterward by BHKY at 108 CFU/ml in the same sites remained asymptomatic. Single deletion mutants of the three T6SS loci in rifampicin-resistant MDCuke were generated via bi-parental mating. In vitro growth of MDCuke-Rif¿T6SS-2 was slightly higher than that of the wild type strain. The motility of MDCuke-Rif¿T6SS-1 on soft agar was significantly higher than that of the wild type strain. Single deletions did not affect the pathogenicity of MDCuke on melon plants or mediate the apparent competition between the preferred BHKY and non-preferred MDCuke strains in squash. Additional studies, including the generation and testing of double and triple deletion mutants of MDCuke T6SS loci and assessment of population size effects in intra-specific interactions are needed to unravel the mechanisms leading to the disease suppression phenotype observed. Currently, insecticides applied to the seeds or transplants to manage cucumber beetle vectors are the main approach to manage bacterial wilt. Another management approach is perimeter trap cropping. Beetle density was significantly higher in the squash trap crop than in the muskmelon main crop in nearly half of the sampling periods in 2015. Although no significant differences were observed in bacterial wilt incidence, trends indicated reduced disease in muskmelon grown within a squash perimeter trap crop than in those grown without a trap crop. Conventional bactericides are insufficient and ineffective to combat bacterial infections of plants. Novel compounds with potency to inhibit bacterial growth are needed for management of bacterial diseases in various crops. In a high-throughput screening assay, 20 small molecules (SMs) were cidal to multiple strains of this pathogen. These SMs showed minimal toxicity to honey bees, human cells, and beneficial bacteria. Nineteen of the 20 SMs were cidal to Xanthomonas cucurbitae and inhibited more than 50% growth of Pseudomonas syringae pv. lachrymans. In addition, 19 SMs were cidal or static against E. amylovora in vitro. Some of these SMs showed promising results for controlling Et in melon plants when applied to cotyledons or delivered to the roots.