Silicon soil amendments for enhancing disease resistance while improving overall crop health for cucurbits in organic farming systems

Silicon soil amendments for enhancing disease resistance while improving overall crop health for cucurbits in organic farming systems

Summary

Greenhouse-grown cucumber seedlings maintained in soilless, peat-based media were subjected to one of two silicon (Si) treatments and evaluated for disease after inoculation with Colletotrichum orbiculare, causal agent of cucumber anthracnose. Si is a known elicitor of chemical defense mechanisms in some plants and has previously been shown by our research group to significantly improve disease resistance in cucumber plants treated with Si (APS 2009 see below). Our current goal is to determine if the improvement in disease resistance is due to the same mechanism as that discovered by Fawe et al. (2002) in cucumber inoculated with powdery mildew and the elicitor Milsana. The Fawe group isolated rhamnetin (3,5,3’,4’-tetrahydroxy-7-O-methoxyflavone) from plants treated with both the pathogen and the elicitor. Rhamnetin was not isolated from plants treated with just the pathogen or just the elicitor. Silicon-treated cucumber plants infected with Colletotrichum orbiculare had significantly less disease symptoms compared to control plants infected with the pathogen. Chemical analysis of harvested, freeze-dried leaf tissue is currently underway. Attempts to determine antifungal activity of plant extracts using bioassays have been inconclusive.

Objectives/Performance Targets

We are planning to use mass spectrometry techniques to determine if rhamnetin is present in control and treated cucumber plants and at what levels in response to varying Si and control treatments (see Fawe et al 2002.

Accomplishments/Milestones

We have optimized methods to sample leaves following treatment with silicon and pathogen. The samples collected were processed to find the best tissue extraction procedure. Currently leaves from plants were collected and stored at -80C freezer for further analyses for the following treatments: (a) no treatment control, (b) plants grown in soil amended with Si, (c) plants treated with the pathogen, (d) plants treated with Si and the pathogen. Extracts from these samples will be analyzed in HPLC-mass spectrometry. In addition to these experiments, we also made an attempt to create a novel, simple qualitative bioassay technique for testing plant extract activity against fungal plant pathogens in vitro. This research served as a tool for training one graduate student in phytochemistry.

Impacts and Contributions/Outcomes

This project has confirmed the ability of silicon amendment to significantly delay the development of symptoms and reduce the severity of cucumber anthracnose under greenhouse conditions (Palenchar et al 2009). This disease reduction could potentially be significant for cucumber producers in the southeastern U.S. since many use hydroponic greenhouse systems and could incorporate Si amendments into the water solutions. This system would, of course, require further experimentation and validation. However, the potential exists to limit disease and reduce fungicide reliance.
The elucidation of an additional plant defense pathway is of further scientific interest and has relevance and application in the fields of plant physiology, pathology, and genetics. Further understanding of host resistance genetics will lead to improved plant breeding.
Finally, many of the compounds known to plant pathologists as phytoalexins function as phytonutrients in the human diet (such as resveratrol in red wine and the isoflavanoids in soy) and may play a role in the maintenance of good health. Future research in this area of plant defense may affect not only agriculture, but public nutrition by increasing our understanding of enhanced dietary value in secondary plant compounds.
In summary, our findings are significant and may have impact on the producer and consumer by 1) reducing fungicide reliance for improved producer, worker, environmental, and consumer safety, and 2) potentially increasing the phytonutrient content of plants if phytoalexin-eliciting growing protocols are widely adopted.
References
Fawe, A.; Abou-Zaid, M.; Menzies, J.G.; Belanger, R.R. 2002. Silicon-mediated accumulation of Flavonoid phytoalexins in cucumber. Phytopathology 88(5):396-401

Palenchar, J.; Taber, S.; Datnoff. L.E.; and Gevens, A.J. 2009. The impact of silicon soil amendments on cucumber anthracnose in the greenhouse. Joint Meeting of the Florida Phytopathological Society and the Caribbean Division American Phytopathological Society Holiday Inn Hotel & Suites Universal Studios, Orlando, FL. May 17, 60 attendees. (Palenchar: 3rd place graduate student award)

Palenchar, J.; Treadwell, D.; Datnoff, L.; and Gevens, A.J. 2009. Cucumber Anthracnose in Florida. Institute of Food and Agricultural Services. University of Florida. EDIS Publication PP266. Online (http://edis.ifas.ufl.edu/pdffiles/PP/PP26600.pdf)

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