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

The purpose of this project is to address diseases in organic farming systems by targeting soil health as the fundamental principle towards achieving a healthy cucumber crop. The role of silicon, a naturally occurring but often not available and therefore deficient soil element, will be studied for enhancing the ability of cucumbers to resist or tolerate plant diseases. The element silicon has been demonstrated to reduce many soil-borne and foliar plant diseases in a number of crops, including rice, sugarcane, some vegetables and flowering ornamentals. Research also has shown that host resistance of susceptible and partially resistant cultivars can be augmented with silicon to the same general level as those containing complete genetic disease resistance. In this study, the effect of amending soils naturally low in plant available silicon will be investigated on the cucumber’s ability to tolerate or resist plant diseases, assess the role of plant generated phytolexins (low molecular weight antifungal compounds produced by the plant) as the potential mechanism for the cucumber’s response, determine if specific cover crops can be utilized to sequester silicon during the summer and winter fallow periods, and determine the economic costs and benefits of this soil amendment in the organic cropping system.

Objectives/Performance Targets

Objective 1. Survey natural organic farm ecosystem for diseases infecting cucumber amended without and with silicon.
Objective 2: Evaluate the performance of Si-amended soil on disease control of cucumber anthracnose in the greenhouse.

Accomplishments/Milestones

Objective 1: The effect of silicon on naturally occurring cucumber diseases were tested at one organic farm, Hammock Hollow Herb Farm, and at the University of Florida’s organic research plots in Citra at the University of Florida’s Plant Science Research and Education Unit (UF/PSREU). These sites were chosen because they were found to be very low (< 4 ppm) in silicon. Previous research has demonstrated that soils less than 19 ppm are considered to be extremely low in this element. The cucumber cultivar ‘Straight Eight’ was selected for all experiments because it is very susceptible to a number of plant diseases. The experimental design was three silicon rates x two years with a two level cover crop factor as a split plot treatment. This cover crop factor will occur during the non-cash crop period when the plots will be split so that one half is seeded with either rye grass (winter non-cash crop period) or sorghum (summer non-cash crop period and the other half is left fallow. All treatments were arranged in a randomized complete block design with four replications. Silicon amendments were applied at three rates: 0, 200, 400 and 600 kilograms of elemental silicon/ha. Wollastonite (calcium silicate, CaSiO3), a naturally occurring mined mineral ore was the silicon source used. Cucumbers were grown following the NOPS. Field trials went out at Hammock Hollow Herb Farm and at UF/PSREU in spring of 2008. Based on the discovery of a heavy soil infestation of root knot nematodes at the grower cooperator location, the trial at Hammock Hollow Herb Farm was discontinued for further study. Field trials at UF/PSREU continued through the summer and fall of 2008 and spring of 2009. The primary disease observed and quantified on cucumbers in the field was Downy mildew (Pseudoperonospora cubensis). In the fall 2008 planting, we experienced heavy Downy mildew disease pressure followed by an early frost on November 1. These two factors resulted in plant death and we were unable to continue to collect yield data. In spring and fall of 2008, and spring 2009, plant were monitored biweekly and assessed for the incidence and severity of downy mildew development. Plant tissue was collected for silicon analysis at transplanting, first flowering, first fruit set and two weeks before the last harvest. At first fruit set, fruit was harvested every other day for about 5 weeks. Currently, all data are being either processed or analyzed, and the results of this information will be forthcoming. Between cucumber crops, we planted cover crops: sorghum in the summer and rye in the winter. Cover crop plant tissues were collected and submitted for silicon analysis. Objective 2: We have carried out inoculation experiments in the greenhouse using ‘Straight Eight’ cucumber plants and the anthracnose pathogen, Colletotrichum orbiculare. Two-week-old cucumber plants were inoculated with a conidial suspension of C. orbiculare, incubated under conditions of high relative humidity, and evaluated for foliar disease. We have identified a disease limiting effect on plants grown in soil amended with silicon (600 kg/ha Vansil W50). Significant differences were identified at 14 days post inoculation. We suspect the upregulation of flavonoid compounds in the cucumber tissue in response to anthracnose inoculation. Silicon soil amendments may also be playing a role in this active defense response. Inoculated plant tissues are not being collected for further analysis in the laboratory of Dr. Bala Rathinasabapathi, UF/Horticultural Sciences. Further tests may include thin layer chromatography, mass spectrometry, and high performance liquid chromatography.

Impacts and Contributions/Outcomes

From spring 2008-2009 several activities were carried out to provide outreach/extension of our research results to the academic community and to organic producers.

Activity 1: Oral presentation at the Joint Meeting of the Florida Phytopathological Society & Caribbean Division American Phytopathological Society.

Abstract for oral presentation (presentation received 3rd prize in the graduate student oral competition on May 17, 2009, 70 attendees)

J. Palenchar (Doctor of Plant Medicine, recent graduate) presenting

The impact of silicon soil amendments on cucumber anthracnose in the greenhouse

J. Palenchar, Taber, S., Datnoff, L. E.*., and Gevens, A. J. Department of Plant Pathology, University of Florida, Institute of Food and Agricultural Sciences, Gainesville, FL 32611. (* current address: Dept. of Plant Pathology and Crop Physiology, Louisiana State Univ.)

Cucumber is an economically important crop in Florida and in other parts of the U.S. In Florida, one of the most common diseases on cucumber is anthracnose, caused by the ascomycetous fungus Colletotrichum orbiculare. Anthracnose can cause serious yield and quality losses, and produces symptoms on all aboveground plant parts at any stage of growth. Organic producers have limited options for anthracnose control. The use of silicon (Si) as a tool for disease control has been established in other crop systems. In the greenhouse, the control of anthracnose was demonstrated on 2-week-old ‘Straight Eight’ cucumber seedlings by amending soil (organic Fafard FOF 30) with a high rate of Si (Vansil W50). Treatments included cucumber seeds planted into 1) Si-amended soil (600 kg Si/ha) + no C. orbiculare inoculation 2) Si-amended soil (600 kg Si/ha) with C. orbiculare inoculation, 3) non-amended soil with no C. orbiculare inoculation, and 4) non-amended soil with C. orbiculare inoculation. Each treatment included 5 replications and the experiment was repeated 4 times. Disease evaluations (Horsfall-Barrett scale) were recorded for leaves at 3, 7, and 14 days post inoculation (dpi) and Si levels in plant tissues were determined. Significant differences between treatments were observed at 7 and 14 dpi. Si treatment reduced disease severity on leaves by 20-60% when compared to the inoculated control. This is the first study demonstrating the efficacy of soil-applied Si for the control of cucumber anthracnose.

Activity 2: A project summary was provided to the Florida Organic Growers newsletter to inform and update interested organic producers in the state of Florida. The summary went out in both online and hard copy formats.

Activity 3: An informational report was generated on cucumber anthracnose in Florida based on a literature review for this project. The document is published in the University of Florida’s EDIS information system and will be available online in the near future (PP266, Cucumber Anthracnose in Florida, authored by Palenchar, Datnoff, Treadwell, and Gevens).

Collaborators: