© 2005 Plant Management Network.
Determining the Efficacy of Disease Management Products in Organically-Produced Tomatoes
Annette L. Wszelaki, Ohio Agricultural Research and Development Center, Department of Horticulture and Crop Science, The Ohio State University, 1680 Madison Ave., Wooster 44691 (currently, Department of Horticulture, University of Puerto Rico, P.O. Box 9030, Mayaguez, PR 00681 9030); and Sally A. Miller, Ohio Agricultural Research and Development Center, Department of Plant Pathology, The Ohio State University, 1680 Madison Ave., Wooster 44691
Wszelaki, A. L., and Miller, S. A. 2005. Determining the efficacy of disease management products in organically-produced tomatoes. Online. Plant Health Progress doi:10.1094/PHP-2005-0713-01-RS.
Sixteen disease control products or product combinations used in organic production systems were tested for efficacy against tomato diseases. Disease pressure was low in 2002, and no treatment significantly reduced disease relative to the control. In 2003, early blight and Septoria leaf spot developed late in the season, and Bordeaux mixture, copper hydroxide, garlic and neem oils, seaweed extract, and Serenade reduced disease development compared to the control. Plots treated with Sonata yielded the most marketable fruit.
While the use of pesticides is greatly restricted in organic agriculture, various natural products are permitted for disease management under the standards of the National Organic Program (NOP). Copper-based fungicides are widely used in organic vegetable production, but pose concerns due to potential accumulation in soil, contamination of run-off water, and subsequent toxicity to non-target organisms (3). Other materials allowed under NOP standards include bicarbonate salts, essential oils, plant and soil extracts, and biological control organisms. Potassium bicarbonate has been shown to reduce postharvest decay development (9) and foliar diseases (5) in citrus. Essential oils, such as tea tree, garlic and neem, have been tested in vitro (8), in soil (2), and on fruit and vegetable crops (7,12,13), but with variable results. Foliar applications of humic acid and extracts of horsetail (Equisetum) and seaweed have been tested for plant growth enhancement (10) and/or inducing plant disease resistance (11). Lastly, biological control agents have been successful in combating disease by inducing plant resistance, producing antibiotics, and out-competing pathogens (6).
The objective of this study was to test the efficacy of 16 products or product combinations approved or likely to be approved for organic production to control fruit and foliar diseases of tomatoes.
Fungicide Efficacy Trials
This experiment was conducted at the Ohio Agricultural Research and Development Center (OARDC) in Wooster, OH on Wooster silt loam soil in a transitional organic (managed according to organic regulations but not yet certified) field. Composted (~12.5 tons/acre) and fresh (~8.5 tons/acre) straw- and sawdust-bedded dairy barn manure (OARDC) were broadcast and incorporated into the test field on 19 April, 2002 and 16 June, 2003. The field was cultivated and beds prepared on 28 May, 2002 and 23 June, 2003, respectively. ‘Peto 696’ (Seminis, Inc., Oxnard, CA) processing tomatoes were used in 2002 and ‘Celebrity’ (F1) (Johnny’s Selected Seeds, Winslow, ME) fresh-market tomatoes were used in 2003. The tomato seeds were hot water-treated (122°F for 25 minutes) and sown on 10 April, 2002 and 15 April, 2003 into 288-cell plug trays containing organic potting mix #423 (35% composted pine bark, 50% Canadian sphagnum peat, 15% perlite v/v/v) (Paygro Co., South Charleston, OH). Transplants were fertilized twice weekly with Bradfield Gold 3-1-5 fertilizer tea (2 cups/55 gal; N at 39 ppm) (Bradfield Industries, Inc., Springfield, MO) until planting. On 29 May, 2002 and 23 June, 2003 seedlings were transplanted 14 inches apart into single row plots 20-ft long on 5-ft centers, and watered in. Wheat straw (0.75 lb/ft2) was applied between rows to suppress weed growth. Treatments (Table 1) were arranged in a randomized complete block design with four replications.
Table 1. Application rates and timing for products evaluated in organic tomato field trials conducted at the OARDC, Wooster, OH.
Treatment rows were alternated with border rows. Treatments were applied according to manufacturers’ instructions using Hudson Heavy Duty Perfection Plus Pumpless Sprayers (2 gal capacity) (H. D. Hudson Manufacturing Company, Chicago, IL). Foliar disease severity (% disease) was evaluated using a modified Horsfall-Barratt rating scale (% disease: 1 = 0%, 2 = 1-3%, 3 = 4-6%, 4 = 7-12%, 5 = 13-25%, 6 = 26-50%, 7 = 51-75%, 8 = 76-87%, 9 = 88-94%, 10 = 95-97%, 11 = 98-99%, 12 = 100%) on 22 July, 1 and 16 August, and 4 September in 2002; and 22 and 29 July, 6, 19, and 31 August, and 8, 15, and 26 September in 2003. Disease ratings were converted to midpoints (% disease) and the area under the disease progress curve (AUDPC) was calculated. In 2002, abiotic leaf curling was observed and rated on a 1 to 3 scale, where 1 = slight, 2 = moderate, and 3 = severe curling; no leaf curling was observed in 2003. Insect damage (% foliar damage) was evaluated on 6, 19, and 31 August, and 8, 15, and 26 September 2003 using the modified Horsfall-Barratt rating scale. Fruit were harvested from five plants in the center of each treatment row on 5 September, 2002 and 19 and 30 September, 2003 and weights of marketable fruit, green fruit, fruit with physiological disorders (blossom end rot, cracks, and zippers), fruit with pest or predator damage (insect, bird, and mouse), and diseased fruit (anthracnose, early blight, and ‘other’) were recorded. Average maximum temperatures for 29 to 31 May, June, July, August, and 1 to 5 September 2002 were 82.1, 83.4, 87.9, 86.0, and 85.8°F; minimum averages were 60.2, 58.2, 62.4, 60.4, and 56.9°F; and rainfall was 0.65, 3.25, 0.86, 1.97, and 0.17 inches, respectively. Average maximum temperatures for 23 to 30 June, July, August, and September 2003 were 84.8, 82.3, 82.9, and 73.4°F; minimum averages were 58.2, 60.2, 60.9, and 50.8°F; and rainfall was 0.68, 7.17, 3.74, and 5.45 inches, respectively. Data were analyzed, within each year, by ANOVA using SAS statistical software (SAS Institute, Cary, NC). Means were separated using Fisher’s Protected Least Significant Difference test.
Effects of Fungicides on Early Blight in the 2002 Growing Season
The 2002 growing season was relatively dry and disease pressure was low. Early blight (Alternaria solani) (Fig. 1) was the only foliar disease observed in the plots, reaching a maximum of 24% severity at the final assessment on 4 September (Table 2). Tomato plants treated with Sonata alone or tank-mixed with Kocide 2000, Serenade tank-mixed with Kocide 2000, Garlic Barrier, or Oxidate had more early blight than the control at the final assessment, although the AUDPC for plants treated with Oxidate did not differ significantly from the water-treated control. Marketable yield of tomato plants treated with Sonata was higher than that of the control. Bordeaux mixture caused significantly more leaf curling than the other treatments and the non-treated control. There were no differences in incidence of fruit diseases among the treatments, with all having greater than 84% marketable red fruit. There were no differences among treatments in physiological disorders of the fruit or pest or predator damage.
Table 2. Percent early blight (Alternaria solani) at the end of the growing season, area under the disease progress curve, and marketable yield of processing tomato ‘Peto 696’ treated with fungicidal products for organic production, 2002.
x See Table 1 for rates and application schedules.
y Means followed by different letters within each column are significantly different at P = 0.1 (+) or 0.05 (**).
z AUDPC (area under the disease progress curve) calculated using midpoints of a modified Horsfall-Barratt rating scale.
Effects of Fungicides on Early Blight and Septoria in the 2003 Growing Season
Rainfall was higher in the 2003 growing season than in the 2002 growing season, and early blight and Septoria leaf spot (Septoria lycopersici) (Fig. 2) were the principle diseases. Tomatoes treated with copper fungicides, except for Sonata + Champion, had significantly less foliar disease during (AUDPC) and at the end of the growing season (26 September) than the control (Table 3). Bordeaux mixture, Champion alone, Champion alternated with StorOx, and Champion in combination with Serenade significantly reduced Septoria leaf spot and early blight. While Serenade, SW-3, Garlic Barrier and Trilogy applications resulted in foliar disease at the end of the season similar to the water-treated control, multiple evaluations of disease severity expressed as the AUDPC showed significant reductions in disease. Neither StorOx alone, Sonata alone or in combination with Champion, Timorex, Timor, Biodynamic 508, Kaligreen, nor Humega reduced foliar disease severity during or at the end of the season. The two foliar diseases were not assessed separately, but field notes indicated that Humega, SW-3, and Biodynamic 508 were ineffective against Septoria leaf spot. Yields were variable and no treatments increased yield significantly compared to the water-treated control. However, average yield was highest for plots treated with Biodynamic 508, Timorex, and Sonata. There were no differences in incidence of fruit diseases, which occurred at low frequency, in physiological disorders of the fruit or in pest or predator damage. No leaf curling was observed in 2003.
Table 3. Percent foliar disease (early blight (Alternaria solani) and Septoria leaf spot (Septoria lycopersici) at the end of the growing season, area under the disease progress curve, and marketable yield of fresh-market tomato ‘Celebrity’ treated with fungicidal products for organic production, 2003.
w See Table 1 for rates and application schedules.
x Means followed by different letters within each column are significantly different at P = 0.05.
y AUDPC (area under the disease progress curve) calculated using midpoints of a modified Horsfall-Barratt rating scale.
z Septoria leaf spot and early blight.
Weather and Product Performance in 2002 and 2003
Weather conditions during the two years of this study were very different, resulting in differences in disease pressure. In 2002, the average maximum temperatures tended to be 5 to 10°F warmer than in 2003. Also, 2002 was a dry year with only 6.9 inches of rain during the growing season, while 17.0 inches of rain fell during the 2003 growing season. These conditions likely contributed to the difference in disease progress between the two years. Nonetheless, the results show that copper compounds were most effective for controlling Septoria leaf spot and early blight. Bordeaux mixture caused phytotoxicity in the form of leaf curling in year 1 on ‘Peto 696’, a processing tomato variety, although not in year 2 on ‘Celebrity’, a fresh market variety. This difference may be variety related or may be the result of hotter, drier conditions in 2002 than in 2003. Because copper-containing products are classified as “restricted” on the OMRI (Organic Materials Review Institute, Eugene, OR) list, alternative disease control measures must be in place if the established limit is reached in any given crop cycle to prevent toxic buildup of copper in the soil. Therefore, it is essential for organic growers to adopt an integrated approach to disease management that does not rely on copper alone. Integrated control methods might include choosing disease-resistant varieties, disinfecting seeds with heat treatments, rotating crops, applying compost, managing weeds, and, when necessary, using other effective, approved products for disease management. Garlic and neem oils, B. subtilis, and seaweed extract significantly reduced foliar disease on tomato compared to the control in this study. Two of the principle compounds in garlic oil are diallyl-disulphide and diallyl-trisulphide (8), which are likely to contribute to its antimicrobial properties. In neem oil, the biologically active compound is a terpenoid, also a known contributor to host plant defense (1). Recent findings also suggest that seaweed has targeted antimicrobial defense strategies, including production of secondary metabolites (4). Serenade (B. subtilis QST 713) produces a series of secondary metabolites with multiple modes of action, including suppression of spore germination, disruption of germ tube and mycelial growth, and inhibition of pathogen attachment to the plant surface. Sonata (B. pumilus QST 2808) produces degradative enzymes that prevent spore germination (B. Highland, personal communication). While the Sonata treatment did not significantly reduce disease severity, tomato plants treated with Sonata tended to yield better than control plants. This was clearly observed in year 1 in processing tomatoes. In year 2, yields were numerically higher in Sonata-treated plants than in the control, but the difference was not significant, probably as a result of variability in fruit yield per plant in fresh-market tomatoes. Alternating Storox and Champion in year 2 reduced foliar disease severity, but Storox applied alone did not. Therefore, it is likely that the reduction in disease observed can be attributed to the copper fungicide.
Many products are advertised for disease control in organic production systems, most of which have not been adequately evaluated in independent, replicated trials. Bordeaux mixture, copper hydroxide (Champion, Kocide), garlic (Garlic Barrier) and neem (Trilogy) oils, seaweed extract (SW-3) and Serenade reduced disease development compared to the control. Plots treated with Sonata yielded the most marketable fruit. Additional work is needed to provide information to organic growers on the relative efficacy of these materials. While none of the products should be relied upon as the sole means of managing diseases, those with demonstrated efficacy may be used in integrated management programs for organically-produced crops, such as tomato, that are at high risk of economic loss due to disease.
We gratefully acknowledge the contributions of Christopher Steiner, Sonia Walker, Elizabeth Burnison, Tobias Butler, Melanie Ivey, and Jhony Mera. This work was supported by state and federal funds appropriated to The Ohio State University and by the Ohio Vegetable and Small Fruit Research and Development Program. Agraquest, Inc., Biomor Israel, Inc., BioSafe Systems, Certis USA, L.L.C., Garlic Research Labs, Global Organics, L.L.C., and Nufarm, Ltd. donated products. We thank Michael Ellis and Brian McSpadden Gardener for pre-submission review of the manuscript.
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