August 1, 2003
From: Dick Hamilton, ProMED-mail Plant Disease Moderator <hamilton@promedmail.org>

A ProMED-mail piece dated 31 Jul 2003 regarding the bacterium that causes cucurbit yellow vine disease (CYVD) in several cucurbit species requires clarification.

While it could be construed that _Serattia marcescens_ [Sm] was indicted as a human pathogen in the posting, that is only one aspect of the ecology of this cosmopolitan bacterium. Bruton and colleagues in Oklahoma have provided substantial evidence that CYVD strains are phloem-inhabiting bacteria that
are physiologically very different from human strains. Moreover, there are many other Sm strains, including those that are being evaluated as biocontrol agents for plant disease control, others that cause disease in insects, and a endosymbiont that fixes nitrogen in rice plants. Sm is ubiquitous, but it is
very unlikely that strains from most habitats are human pathogens.

Nevertheless, the fact remains that some strains/isolates of Sm are opportunistic human pathogens. As a general rule, Sm is a hospital-acquired pathogen affecting debilitated individuals, especially those with catheters and/or endotracheal tubes. It is uncommonly acquired outside the hospital.

There are similar instances of phytopathogenic bacteria causing infection in humans. For example, _Burkholderia cepacia_ causes nosocomial infections that may be a significant problem in patients with cystic fibrosis -- and causes sour-skin disease in onion.

I am indebted to Benny D. Bruton (USDA, ARS, Lane, OK) and Larry Lutwick (ProMED-mail Bacterial Disease Moderator) for their assistance in preparing this comment on CYVD etiology.

Dick Hamilton
ProMED-mail Plant Disease Moderator
Senior Research Scientist (Retired)
Research Branch, Agriculture and Agri-Food Canada
Richmond, BC CANADA
hamilton@promedmail.org

July 31, 2003
Source: American Phytopathological Society, PLANT DISEASE [edited]

Serratia marcescens, a Phloem-Colonizing, Squash Bug-Transmitted Bacterium: Causal Agent of Cucurbit Yellow Vine Disease.

B. D. Bruton, United States Department of Agriculture-Agricultural Research Service (USDA-ARS), Lane, OK 74555; F. Mitchell, Texas Agricultural Experiment Station, Stephenville 76401; J. Fletcher, Department of Entomology & Plant Pathology, Oklahoma State University, Stillwater 74078; S. D. Pair, USDA-ARS, Lane, OK; A. Wayadande, Department of Entomology & Plant Pathology, Oklahoma State University; U. Melcher, Department of Biochemistry & Molecular Biology, Oklahoma State University; J. Brady, Texas Agricultural Experiment Station; B. Bextine, Department of Entomology & Plant Pathology, Oklahoma State University; and T. W. Popham, USDA-ARS, Stillwater, OK 74075. Plant  Dis. 87:937-944.

Accepted for publication 26 Mar 2003.

Cucurbit yellow vine disease (CYVD), which can inflict heavy losses to watermelon, pumpkin, cantaloupe, and squash in U.S. production areas from the midwest to northeastern states, causes phloem discoloration, foliar yellowing, wilting, and plant decline.

Bacteria were cultured from the phloem of crown sections of symptomatic plants of _Citrullus lanatas_ and _Cucurbita pepo_. Those bacteria testing positive in CYVD-specific polymerase chain reaction (PCR) were all gram-negative and appeared morphologically identical, producing creamy white, smooth, entire, convex colonies on Luria-Bertani or nutrient agar.

Characterized cucurbit-derived strains of _Serratia marcescens_ were introduced into greenhouse-grown squash plants by puncture inoculation and into field-grown squash plants by enclosure with _S.
marcescens_-fed squash bugs, _Anasa tristis_. Up to 60 percent of the bacteria-inoculated plants in the greenhouse and up to 17 percent of field plants caged with inoculative squash developed phloem discoloration and tested positive for _S. marcescens_ by CYVD-specific PCR. None of the controls developed phloem discoloration or tested positive by PCR.

Of the diseased field plants, 12 percent (2 of 35) also yellowed, wilted, and collapsed, exhibiting full symptom development of CYVD. However, neither plant collapse nor decline was observed in the greenhouse-grown, puncture-inoculated plants. The morphology, growth habit, and PCR reaction of bacteria cultured from crown tissue of a subset of plants in each experimental group were indistinguishable from those of the inoculum bacteria.

Evidence presented from our studies confirms that the squash bug can transmit _S. marcescens_, the CYVD causal bacterium. The _S. marcescens_-_A. tristis_ relationship described here is the first instance
in which the squash bug has been identified as a vector of a plant pathogen. Our experiments represent a completion of the steps of Koch's postulates, demonstrating that _S. marcescens_ is the causal agent of CYVD and that the squash bug, _A. tristis_, is a vector of the pathogen.

[This report confirms that a strain of the ubiquitous bacterium, _S. marcescens_ [Sm], is the causal organism of CYVD in cucurbits and that the squash bug, _Anasa tristis_, is a vector that transmits the pathogen to its cucurbit hosts. Whereas the results of earlier attempts to identify and characterize the pathogen were equivocal, this report shows that Sm is a walled bacterium that can be grown on nutrient agar, thus allowing for completion of Koch's postulates. CYVD occurs on cucurbit crops in Oklahoma, Tennessee, Massachusetts, and Arkansas.

>From a public health perspective, Sm is now considered a harmful human pathogen known to cause urinary tract infections, wound infections, and pneumonia. A pertinent references re: human infections:
<www.projectlinks.org/serratia/> - Mod.DH]

[Elkhorn coral is being killed by _S. marcensens_ (see ref. below). Another example of a microbe pathogenic to both plants and humans is _Burkholderia cepacia_ (formerly _Pseudomonas_), the cause of pneumonia, especially in cystic fibrosis patients, as well as the cause of sour skin disease of onions. - Mod.LL/JW]