Evaluating tissue localization and pathogenicity determinants of Serratia marcescens in cucurbits, and optimizing mesotunnel systems for organic cucurbit production
Date
2024-05
Authors
Mphande, Kephas
Major Professor
Advisor
Gleason, Mark L
Beattie, Gwyn A
Leandro, Leonor F.S.
Nair, Ajay
O'Neal, Matthew E
Committee Member
Journal Title
Journal ISSN
Volume Title
Publisher
Altmetrics
Abstract
Insect-vectored vascular pathogens are among the most notorious bacterial pathogens to cause diseases on cucurbits across the world, collectively causing crop losses worth millions of U.S. dollars. These diseases include bacterial wilt of cucurbits, which is caused by Erwinia tracheiphila (E. F. Smith) Holland and transmitted by striped cucumber beetles (Acalymma vittatum F.) and spotted cucumber beetles (Diabrotica undecimpunctata howardi Barber), and cucurbit yellow vine disease (CYVD), which is caused by Serratia marcescens and vectored by squash bugs (Anasa tritis DeGeer). Management of these diseases is particularly important in organic production systems due to the limited availability and efficacy of organic insecticides and alternative control methods for these insect-bacterial pathogen complexes. Here we evaluated mesotunnels as an alternative control method for insect-vectored bacterial diseases in organic cucurbit production systems. Our study focused on evaluating pollination strategies under mesotunnels in commercial-scale plots, assessing the effectiveness of living mulches as an alternative weed control strategy in mesotunnels, and comparing the economic efficiency of the strategies. In the pollination experiment, we showed that keeping mesotunnels closed for the entire growing season and utilizing purchased bumble bees (Bombus impatiens) for pollination resulted in higher net revenue than either opening the tunnel ends or uncovering the tunnels during bloom. Through our weed control trial, however, we showed that teff (Eragrostis tef) can suppress weeds effectively in the furrows between muskmelon beds under mesotunnels, but that mowing it 3 weeks after transplanting the crop is essential to avoid a yield drag associated with competition between this vigorous living mulch and muskmelon. Although landscape fabric effectively eliminated weeds in the furrows and resulted in the highest net returns, it required considerable labor to install and remove. Further, in this work we developed and validated a robust bioassay for S. marcescens that yielded consistent and quantifiable CYVD symptoms on squash in the lab. Here we highlight that mimicking the natural process of S. marcescens transmission by squash bug feeding through injection produced robust and quantifiable CYVD symptoms and that this was not achieved using a mechanical wounding method. We investigated the localization of S. marcescens in cucurbit tissue using microscopy and demonstrated that, as a pathogen causing CYVD on cucurbit crops, S. marcescens localizes to the intercellular spaces in the phloem tissue but does not accumulate in the sieve tubes. Our work has shown that fimbriae contribute to virulence in S. marcescens but are not required for pathogenicity, and that fimbriae play a negative role in biofilm formation in S. marcescens CYVD strains in vitro, suggesting they may do the same in planta. Finally, we demonstrated that both striped and spotted cucumber beetles can harbor and transmit S. marcescens in cucurbits. To our knowledge, this is the first report of cucumber beetles vectoring S. marcescens. Our results show the versatility of S. marcescens in adapting to colonizing different hosts, making it one of only a few pathogens with such attributes. These studies have highlighted the distinct etiology of S. marcescens CYVD strains, illustrating how much we do not yet know of this pathogen.
Series Number
Journal Issue
Is Version Of
Versions
Series
Academic or Administrative Unit
Type
dissertation