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Compost tea for suppression of xanthomonas in carrot production
By Joel Reiten & Cindy Salter
April 2002 |
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This article summarizes an ongoing collaborative effort between Bejo Seeds, Inc., Growing Solutions, Inc., and BBC Laboratories, Inc. to evaluate the ability of compost tea to inhibit infection of carrot by Xanthomonas campestris pv. carotae.
Project Participants
Bejo Seeds (www.bejo.com) specializes in the development, production and sales of both conventional and organic vegetable seeds. Bejo leads the European seed market in cabbage, carrots and onions, and for many years has offered its products to organic farmers. The organic seed market is small but is growing worldwide, especially in European countries. Bejo’s Organic Seed Program focuses not only on organic seed production, but also on organic breeding. Bejo Seeds has created an infrastructure of organic seed producers in the Netherlands, Italy, France, and the United States in particular. Bejo Seeds also performs research on organic methods to improve the quality of the seed. Joel Reiten, Bejo’s Seed Production Manager, is optimistic about the use of compost tea in organic production. Because of his previous successes, Reiten initiated a field trial in 2002 to evaluate the use of compost tea on carrots for seed production in the Willamette Valley, specifically to achieve inhibition of Xanthomonas campestris pv. carotae.
Growing Solutions, Inc. (GSI) (www.growingsolutions.com) is a Eugene-based manufacturer and distributor of compost tea systems and other products for sustainable agriculture. GSI collaborated with Bejo Seeds on this project by providing compost tea made in one of their systems using a known source of compost, GSI’s Compost Tea Catalyst, and assistance with field application. Growing Solutions, Inc. is dedicated to research and information exchange that will demonstrate the economic and environmental benefits of compost tea.
BBC Laboratories (www.bbclabs.com), based in Tempe, Arizona, provides microbiological testing and research for the agricultural community. Among other microbiological assays, BBC offers a Pathogen Inhibition Assay as a screening technique to determine if compost, compost tea, biological products or soil contains organisms that have the potential to actively inhibit the growth of specific plant pathogens. For this project, samples of the compost tea used in the field were submitted to BBC Laboratories for the Pathogen Inhibition Assay to assess its ability to suppress Xanthomonas campestris pv. carotae.
Description of the disease
Xanthomonas campestris pv. carotae, or bacterial leaf blight, is a common disease in the carrot crop. Severe outbreaks can cause significant yield reductions and loss of storability. The bacteria that causes this disease can be carried on the seed or can over-winter in carrot debris left in the field. In the case of seed crops, the bacteria can be spread from mature fields to a newly emerged field by the wind blowing contaminated chaff and dust between fields or in established fields by water splashing and via insects, animal or machinery transmission.
Bacterial leaf blight of carrots is caused by Xanthomonas campestris pv. carotae, an aerobic, gram-negative, rod shaped bacterium that is mobile in the presence of water on the leaf surface. On the leaf surface the bacteria grows epiphytically. When the bacterial populations reach levels greater than 106 CFU/g of leaf tissue, disease symptoms develop. In seed production fields, the bacteria colonize the umbels and contaminate the developing seeds.
The Hypothesis
Bacterial diseases are difficult to control in both organic and conventional production. Copper and antibiotics have been used in some crops but both are undependable and may have harmful consequences to the environment.
Compost teas have been reported to have an inhibitory effect on plant pathogenic organisms. Compost teas are an extraction of beneficial organisms from compost in an aerobic, liquid brewer. The teas are then applied to the plant, and beneficial organisms attach themselves and begin to colonize the leaf. Reasons for this inhibitory effect range from competition from the beneficial organisms to that of an inhibitory compound excreted by the beneficial organisms. Systemic acquired resistance (SAR) induced in the host plant by the application of the compost tea has also been shown to occur.
We hypothesize that compost teas can be used to control the growth of X. campestris pv. carotae on the leaf surface and keep the population from reaching infectious levels.
In an effort to more fully evaluate the feasibility of using compost tea to help prevent X. campestris pv. carotae in carrots, we designed an experiment to demonstrate this hypothesis.
Pathogen Inhibition Assays
Samples of X. campestris pv. carotae were supplied to us by Dr. Lindsey DuToit with the Washington State Plant Pathology Dept. at Mt. Vernon, WA. This culture was from isolates collected in infected carrot fields in the Northwest this past year. The cultures were sent to BBC Labs Inc in Tempe, AZ to be used for a pathogen inhibition assay. In this assay, the pathogen, X. campestris pv. carotae was cultured overnight in a broth medium. The pathogen from the culture was then evenly swabbed onto a growth medium in a Petri dish. An undiluted sample of compost tea, provided by Growing Solutions, Eugene, OR, was applied to a small disk of filter paper and placed on the Petri dish and the reaction of the growth of the pathogen was observed. Inhibition of pathogen growth was scored after a suitable time and determined to be strongly inhibitory, partially inhibitory or not inhibitory, according to the size of the zone of inhibition around the compost tea-soaked disk.
In this instance, after two days, large zones of clearing were observed in the test plates when compared to the control plates, indicating strong inhibition.
Further investigation of the plates that showed large zones of clearing yielded 12 isolates of bacteria that appeared to have inhibitory effects on the pathogen. These 12 isolates were then individually identified through fatty acid ester identification, cultured, and subjected to the same pathogen inhibition assay. From these tests, three isolates were identified that showed the greatest potential for disease suppression based on the following criteria:
1. The isolate inhibition against the selected pathogen must be strong.
2. The isolate must be documented as a normal and regular organism found in compost.
3. The isolate must be documented as being able to live for a reasonable amount of time on the leaf surface in field conditions.
The three isolates that were selected were Alcaligenes xylosoxydans xylosoxydans, Enterobacter agglomerans and Alcaligenes faecalis.
Field Application of Compost Tea
The field trial was conducted in the Willamette Valley in July of 2002. Compost tea was supplied by Growing Solutions, Inc., using the same compost source as the previous tests, applied at a 1:1 dilution using a drench nozzle.
Treatments were applied twice at a six day interval to carrot seedlings with 6-10 true leaves in a random design within five, 300- foot rows. In all, there were 10 treatment sections that were each 50 feet long. The treatments were:
Control: Water only
T1: Water + pathogen
T2: Compost tea + pathogen
Three days after the second application of water or compost tea, all but the control plants were sprayed with a culture solution containing the pathogen X. campestris pv. carotae, obtained from a field showing a severe infection of carrot leaf blight in the Northwest.
Results
In August 2002, leaf samples from each of the treatments were submitted to STA Laboratories in Longmont, CO for disease diagnosis. The results of the laboratory analysis showed:
Control plants (water only): tested negative for disease
T1 treatment (water + pathogen): tested positive for disease
T2 treatment (compost tea + pathogen): tested negative for disease
These results indicate that the T2 plants that were given 2 applications of compost tea before the pathogen was introduced were able to resist infection, while the T1 treatment plants that were given water instead of compost tea became infected.
Conclusion
Although done on a relatively small scale and not rigorously replicated, we believe that these trials show that compost teas have an important role to play in the biological control of plant pathogens in organic and conventional growing conditions. It is our intent to explore this potential further during the next growing season.
References
Umesh, K.C., Davis, R.M., and Gilbertson, R.L. 1998. Seed contamination thresholds for development of carrot bacterial blight caused by Xanthomonas campestris pv. carotae. Plant Dis. 82: 1271-1275.
Hoitink, H.A.J., Zhang, W., Han, D.Y., Dick, W.A. 1997. Suppression of rot and foliar diseases induced by composts. Pg 95-100. In: Organic Recovery and Biological Treatment into the Next Millennium. E.I. Stentiford (ed.) Zebra Publishing, Manchester, U.K.
R. James Cook and Kenneth F. Baker, 1983. The Nature and Practice of Biological Control of Plant Pathogens. The American Phytopathological Society, St. Paul, MN.
Lindow, Steven E., Hecht-Poinar, Eva I., and Elliott, Vern J., 2002. Phyllosphere Microbiology. The American Phytopathological Society, St. Paul, MN.
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