Effect of Alternative Fungicides on Alternaria solani Control and Productivity of Organic Tomatoes

Final Report for GNC02-007

Project Type: Graduate Student
Funds awarded in 2002: $7,500.00
Projected End Date: 12/31/2005
Grant Recipient: Iowa State University
Region: North Central
State: Iowa
Graduate Student:
Faculty Advisor:
Faculty Advisor:
Kathleen Delate
Iowa State University
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Project Information

Summary:

The efficacy of organic fungicides and compost teas to control Septoria leaf spot (Septoria lycopersici Speg.) on tomatoes was evaluated. Tomato plants (‘Mountain Spring’) were transplanted into black plastic mulch in May of 2003 and 2004, inoculated with 7.5 x 108 conidia×ha-1 and treated with: (i) control (no foliar spray), (ii) alternated applications of chlorothalonil plus copper hydroxide and mancozeb plus copper hydroxide ( iii) Copper hydroxide only, (iv) Bacillus subtilis only, (v) WCCM tea, (vi) VCCM tea, and (vii) SporanTM. The only treatments effective in reducing disease severity were copper hydroxide and copper hydroxide plus chlorothalonil or mancozeb. Treatments that received B. subtilis, SporanTM , or compost extracts were not effective at controlling S. lycopersici.

Introduction:

Managing diseases of tomatoes organically poses challenges for producers in humid climates. Septoria lycopersici is a common fungal disease of tomatoes. Resistant or tolerant cultivars are not available; therefore growers must rely on cultural and chemical control practices, including the use of copper (Cu) fungicides for disease control (Jones et al., 1991). There are currently five Cu based fungicides allowed in certified organic production. Concerns regarding the use of Cu fungicides have been expressed due to their potential to reduce crop yield (Rhoads et al., 1989) and to cause toxicity to earthworms, nematodes (Burrows and Edwards, 2002), and entomopathogenic fungi (Ropek and Para, 2002). Furthermore, the potential for Cu accumulation in the soil surface (Kabata-Pendias and Pendias, 1992) and concerns about pathogen resistance also have led producers to seek alternatives.

An alternative disease-management technique available to organic producers is the use of biological control agents that are commercially available or producer created. Bacillus subtilisâ (Bacillus subtilis Cohn; AgraQuest, Davis, CA) is a commercially available, broad-spectrum biofungicide registered for use on tomatoes to control foliar diseases caused by bacterial and fungal pathogens. However, independent research regarding its efficacy in controlling S. lycopersici has not been repeated.

Compost tea, a liquid extract with microorganisms (Diver, 1998), is a producer-created biofungicide. There are two main ways in which teas can be made; one includes aeration whereas the other does not. Research on non-aerated compost teas has found that control of plant pathogens in the phyllosphere from beneficial organisms is acquired through induced resistance, inhibition of spore germination, antagonism, or competition. Such studies have not been performed on aerated compost tea (Scheuerell and Mahaffee, 2002). Bacteria and fungi are the agents in compost tea that control foliar diseases, specifically those belonging to the genera Bacillus Cohn, Pseudomonas van Hall, Serratia Sm., Penicillium Link, and Trichoderma Pers. (Brinton et al., 1996). There are many factors that affect the efficacy of compost tea including compost source, dilution ratio, and organism extraction and application methods (Scheuerell and Mahaffee, 2002).
The efficacy of these new commercial organic products and techniques available to farmers has not been evaluated. Scientific evaluation of the efficacy of pesticide products registered for use in organic agriculture is vital for growers’ success.

Literature Cited

Brinton, W.F., A. Tranker, and M. Droffner.
1996. Investigations into liquid compost
extracts (“teas”) for the control of plant
pathogenic fungi. Biocycle 37:68-70.

Burrows, L.A. and C.A. Edwards. 2002. The use
of integrated soil microorganisms to effects
of pesticides on soil ecosystems.
Eur. J. Soil Biol. 38:245-249.

Diver, S. 1998. Compost teas for plant disease
control; a pest management technical note.
National Sustainable Agriculture Information
Service, Fayetteville, AR.

Jones, J.B., J.P. Jones, R.E. Stall, and T.A.
Zitter, eds. 1991. Compendium of tomato
diseases. American Phytopathological
Society, St. Paul, MN.

Kabata-Pendias, A. and H. Pendias. 1992. Trace
elements in soils and plants. 2nd Edition.
CRC Press, Boca Raton, FL.

Rhoads, F.M., S.M. Olson, and A. Manning. 1989.
Copper toxicity in tomato plants. J. Environ.
Qual. 18:195-197.

Ropek, D. and A. Para. 2002. The effect of
heavy metal ions and their complexions
upon the growth, sporulation and
pathogenicity of the entomopathogenic fungus
Verticillium lecanii. J. Invertabr. Pathol.
79:123-125.

Scheuerell, S. and W. Mahaffee. 2002. Compost
tea: principles and prospects for plant
disease control. Compost Sci. Util. 10:313-
338.

Project Objectives:

Our objectives were to evaluate the efficacy of organic fungicides and compost tea from either windrow-composted cattle manure (WCCM) or vermicomposted cattle manure (VCCM). We also compared the efficacy of organic and conventional fungicides.

Cooperators

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  • Hank Tabor

Research

Materials and methods:

Research was conducted for two seasons at the Iowa State University Horticulture Station near Gilbert, IA. The field site was a Clarion loam soil defined as a fine loamy, mixed, superactive, mesic Typic Hapludoll. The 2003 field site was planted in muskmelons (Cucumis melo L.) in 2001 and soybeans [Glycine max (L.) Merr.] in 2002. The 2004 field site was planted in muskmelons in 2003 and tomatoes, squash, and green beans in 2002.

For 2003 the average temperature close to normal at the beginning of the season and rainfall was below normal. The deviation from normal temperature in May, June, July, and August was + 0.1, -0.5, -0.7, and +2.0 °C, respectively (Figure1). Rainfall deviation from normal for May, June, July, and August was -14, -45, +10, and -73 mm, respectively (Figure 2).

For the 2004 growing season fruit ripening was delayed because of lower than normal temperatures. The deviation from normal temperature in May, June, July, and August was +1.6, -1.0, -2.0, and -2.4 C respectively, below average for June, July, and August. Temperatures reached 32 C only twice during the growing season and did not exceed 32 C. This resulted in delayed fruit set. Average rainfall in 2004 was greater than normal at the beginning of the season. Rainfall deviation from normal for May, June, July, and August was +89, -71.0, -12.5, and +6.0 mm, respectively.

‘Mountain Spring’ tomato plants were transplanted on 16 May 2003 and 12 May 2004 into black plastic mulch. All treatments except for the conventional treatment received 8 Mg×ha –1 of compost that contained 1.4% nitrogen and 27% moisture in 2003 and 22 Mg×ha –1 of compost that contained 0.52% nitrogen and 27% moisture in 2004. Compost used in 2003 was windrow composted cornstalk bedded cattle manure from Terra Fact, Corp. near Lake Park, IA. Compost used in 2004 was windrow sand bedded cattle manure from the Iowa State University dairy farm. The nitrogen (N) applied by compost in each year was 83 kg×ha –1. The conventional treatment received 67 kg×ha –1 of N as urea in both 2003 and 2004. Compost and urea were broadcast uniformly and incorporated to a 20- cm depth before laying plastic mulch. Plants were pruned once at the first flower cluster on 12 June 2003 and 17 June 2004 and staked and tied according to the Florida method. Plants were irrigated with a trickle system to maintain the 20-cm-depth tensiometer readings at < -3 MPa. Each treatment row was 7 m long with ten plants plus two guard plants at each end. Guard rows of ‘Mountain Spring’ were planted between each treatment row. Treatments were (i) control (no foliar spray), (ii) alternated applications of chlorothalonil (Bravo®; tetrachloroisophthalonitrile; Syngenta Crop Protection, Greensboro, NC) plus copper hydroxide (Champion®; Nufarm Americas, Sugar Land, TX) with mancozeb (Dithane DL®; methyl (E) –2-{2-[6-(2-cyanophenoxy) pyrimidin – 4-Y10XY] phenyl} – 3- methoxyacrylate; Syngenta Crop Protection, Greensboro, NC) plus copper hydroxide, (iii) copper hydroxide only, (iv) B. subtilis only, (v) windrow composted cattle manure compost tea (WCCM), and (vi) vermicomposted cattle manure compost tea (VCCM). A rosemary oil based certidifed organic fungicide (SporanTM; EcoSmart Technologies Inc, Franklin, TN) was added in 2004. Commercial fungicides were applied according to label recommendations. Compost teas were made at a 10:1 water/compost ratio, by weight, in SoilSoupä compost brewers, (SoilSoup Inc., Seattle, WA), aerated for 24 h and applied at 2000 L×ha –1. The adjuvant Nu-Film 17TM (Miller Chemical & Fertilizer Corp, Hanover, PA) was added to all treatments prior to application. All treatments were applied once weekly from 19 June to 13 August 2003 and 17 June to 10 August 2004. All plants except those in the data rows were inoculated with 7.5×108 conidia×ha-1 of S. lycopersici on 28 July 2003 and 6 July 2004. The inoculum source (infected foliage) was collected from field-infected plants in Ames, IA, in 2002 and stored in a freezer. To increase the amount of inoculum, leaves were macerated in water that was filtered and then sprayed onto tomato plants in a greenhouse. The field was overhead- irrigated immediately after inoculation to disperse the conidia. Treatments and inoculum were applied with a backpack sprayer. Weekly disease severity ratings began on 28 July 2003 and 27 July 2004 and were taken by using a modified version of the scale of Horsfall and Barratt (1945). Disease rating began one week after the first disease symptoms appeared from infection caused by naturally occurring pathogens, and the same day that inoculum was applied in 2003. In 2004, disease ratings began three weeks after inoculation when disease symptoms began to show. Weather conditions in 2003 were particularly favorable for Xanthomonas campestris pv. vesicatoria Doidge, which resulted in an epidemic of bacterial spot before we inoculated with S. lycopersici (Jones et al, 1991). It was impossible to distinguish between symptoms caused by the two pathogens on leaves in the field. In fact, symptoms were primarily bacteria spot. Consequently, disease severity ratings reflect both X. campestris and S. lycopersici for 2003. In 2004, however, disease symptoms were caused primarily by S. lycopersici. Weekly harvests of ripe fruit were from 29 July to 2 Sept 2003 and 11 August to 8 Sept 2004. All green fruit was harvested at the end of the season. Fruits were sorted as marketable or unmarketable (diseased, cracked, or zippered) and then counted and weighed. The experimental design was a randomized complete block with four replications and seven treatments. Data were analyzed by using the general linear model procedure and Tukey’s multiple comparisons and orthogonal contrasts option of SAS.

Research results and discussion:

Results
Differences in disease severity occurred early and were consistent throughout the 2003 growing season. Plants in the two treatments that received copper hydroxide remained disease-free and there was no additional effect on disease severity of adding conventional fungicides to copper hydroxide. Treatments that received copper hydroxide had 50% lower disease severity than the untreated control by the end of the season. There was no difference in disease severity between plants that received B. subtilis or compost extracts and the control treatment as 50% of leaves on plants in all four treatments showed disease symptoms by the end of the season. The composting method did not affect disease severity between the two types of compost teas, VCCM and WCCM. Again, 50% of leaves on plants that received compost tea applications showed disease symptoms.

Differences in disease severity in 2004 showed a similar trend to that of 2003; disease severity was less in treatments that received copper hydroxide than those that did not. However, unlike the 2003 growing season there was a significant difference in disease severity between plants that received copper hydroxide only, and those that received copper hydroxide plus conventional fungicides. Plants in treatments that received copper hydroxide only, were < 25% diseased, whereas plants in treatments that received copper hydroxide plus conventional fungicides were 25 to 50% diseased. However, plants that received copper hydroxide fungicides were 75% less diseased than plants that did not. There was no difference in disease severity between plants that received B. subtilis, rosemary oil or the compost teas and the unsprayed control, as all plants in these treatments were 100% diseased by the end of the season. In 2003, plants that received copper hydroxide fungicides yielded 64% more marketable fruit than plants that did not. There was no difference between plants that received copper hydroxide only, and plants that received copper hydroxide plus conventional fungicides. Plants in treatments that received copper hydroxide only, had the least amount of culled fruit; however, this was not different than culled fruit weight from plants in treatments that received copper hydroxide plus conventional fungicides, B. subtilis, or WCCM compost tea. But plants in treatments that received copper hydroxide only, had 75% less culled fruit than plants that were treated with VCCM compost tea or the control. Plants in treatments that received copper hydroxide fungicides produced the largest fruit; however, this fruit size was similar to plants that received B. subtilis or plants in the control. Plants that received copper hydroxide fungicides plus conventional fungicides produced the most fruit (P<.05). The number of green fruit harvested was highest from plants in the conventional treatment. Plants that received copper hydroxide plus conventional fungicides had 326% more green fruit at the last harvest compared with other treatments. Plants in treatments that received copper hydroxide only, had 200% more green fruit than those that did not. However, there was 54% more green fruit on plants that received copper hydroxide plus conventional fungicides then plants that received only copper hydroxide. In 2004, plants that received copper hydroxide fungicides yielded 150% more marketable fruit weight than plants that did not. There was no difference between plants that received copper hydroxide only, and plants that received copper hydroxide plus conventional fungicides. There was no difference in cull weight among the treatments. There was no difference in marketable fruit size among all the treatments. On average, fruits weighed 290 g each; fruit from plants that received copper hydroxide only, produced the largest fruit, 332 g each, while plants in the control yielded the smallest fruit at 276 g each. Plants in treatments that received copper hydroxide fungicides produced 122% more fruit per hectare than plants in treatments that did not. When green fruit was harvested during the last harvest in 2004, plants in treatments that received copper hydroxide fungicides had an average of 56 more green fruit per plot than plants in all the other treatments. Discussion
Neither B. subtilis nor rosemary oil was effective at controlling S. lycopersici, and B. subtilis was not effective at controlling X. campestris. The B. subtilis product label recommends its use for control of X. campestris in conjunction with Cu-based fungicides. Results from this research indicate that B. subtilis alone cannot control X. campestris and S. lycopersici when disease pressure is high. There was phytotoxic damage on plants that received rosemary oil, however, it is possible that during the first treatment application plants may have received a concentrated amount of fungicide because of inadequate sprayer mixing.

Compost tea was not effective at controlling S. lycopersici or X. c. pv. vesticatoria. Furthermore, there was no difference in disease severity between compost tea made from window-composted cattle manure or VCCM, indicating that composting method did not affect efficacy of the compost tea. These results are consistent with those of Plotkin (2002), who showed compost tea was ineffective at controlling Alternaria solani (Elli. & Mart.) L.R. Jones & Grout and Septoria spp. on field tomatoes. However, compost teas have been found to control some important diseases of fruits and vegetables. Tsror (1998) found that a compost tea of commercial cattle manure incubated for 7 or 14 d controlled A. solani on field-inoculated tomato plants. The degree of control was similar to that obtained with Kocideâ (copper hydroxide; Griffin L.L.C., Valdosta, GA) or Funguranâ (copperferroxychlorid; Spiess Urania, Hamburg, Germany), both Cu-based fungicides. Compost tea made from dairy cattle manure reduced disease caused by Botrytis cinerea Pers., on grapes (Vitus L. spp.) by 48% (Trankner, 1992). In growth chambers, compost teas made from either composted cattle or chicken manure controlled B. cinerea on tomato and pepper plants and grape berries (Elad and Shtienberg, 1994). Further research is necessary to determine what preparation and application factors affect the efficacy of compost teas on S. lycopersici. Investigating other composting sources, adjusting the dilution ratio, or changing application rate and timing may increase efficacy.

Copper hydroxide controlled both X. campestris and S. lycopersici. To our knowledge, the efficacy of this product for both pathogens on tomatoes has not been demonstrated previously. This information is useful for organic producers because copper hydroxide is one of six Cu fungicides allowed in certified organic production.

In both 2003 and 2004 plants in treatments that received copper hydroxide fungicides were the only ones in which expected yields the region were obtained, between 29 and 33 Mg·ha –1 (Foster et al., 2003). While there were differences in culled fruit weight, marketable fruit size, and the number of marketable fruit in 2003, a lack of differences in culled fruit weight and marketable fruit size in 2004 indicates that marketable fruit number is likely the main factor affecting yield. Fruit number is a result of flower production and/or percent of flowers that set fruit. On determinate tomato plants, such as ‘Mountain Spring’, flower clusters are often found at the end of plant branches. A healthier plant, i.e. one with less disease, would result in a bushier plant with more branches, and thus more flowers. In 2003 plants in treatments that did not receive copper hydroxide fungicides were 5-25% diseased by the second harvest and 25-50% diseased by the third and fourth harvest. In 2004 plants were 25% diseased by 27 July, two weeks before the first harvest and 85% diseased by the first harvest. Plants in these treatments had less green leaf area for photosynthesis, thus they were smaller and less branched, which could have resulted in less flower production, leading to decreased yields.

The use of compost tea to control S. lycopersici should not be promoted in tomato production at this time. However, further investigations are warranted because there are many factors that can affect the tea efficacy. Future research should focus on compost source, using compost with known populations of beneficial species. Populations of beneficial species should be tracked throughout the entire brewing/ application process to determine methods that favor beneficial species. Finally, composting methods should select for beneficial organisms specific to a particular pathogen if the information is available.

The use of chlorothalonil and mancozeb is only necessary when resistance to Cu-fungicides by X. campestris and S. lycopersici is a concern. This is beneficial because consumers are becoming increasingly aware of the impacts agricultural chemicals have on the environment. Producers will benefit from marketing their tomatoes as either organic, sustainable produced, or “low input”, providing them with a competitive advantage over conventional producers.

Literature Cited
Elad, Y. and D. Shtienberg. 1994. Effect of
compost water extract on grey mould

Foster, R., D. Egal, E. Maynard, R. Weinzierl, M.
Babadoost, H. Taber, L. Jett, and B.
Hutchinson, eds. 2003. Midwest vegetable
production guide for commercial growers. Iowa
State Univ. Publishing Services, Ames, IA.

Plotkin, J.B. 2002. Response of Alternaria spp.
blight and Septoria spp. leaf spot to
biological disease control agents in
tomatoes. USDA-Sustainable Research and
Education Grant no. FNE00-292 final progress
report. Washington, DC. www.sare.org.

Trankner, A. 1992. Use of agricultural and
municipal organic wastes to develop
suppressiveness to plant pathogens Pp. 35-42.
In: E.C. Tjamos, G.C. Papavizas and R.J. Cook
(eds.). Biological control of plant
diseases. Plenum Press, New York.

Participation Summary

Project Outcomes

Recommendations:

Areas needing additional study

Since there are currently no other viable alternatives to copper fungicides in certified organic production future research should focus on ways producers can reduce Cu fungicide applications to address concerns regarding soil accumulation, yield loss, microorganism toxicity and disease resistance associated with Cu.

The use of compost tea to control S. lycopersici should not be promoted in tomato production at this time. Even so, further investigations are warranted because there are many factors that can affect the tea efficacy, including the age of the compost source. The composts used in this experiment were cured, meaning that most of the easily degradable organic substrates were already decomposed. Thus, it is likely that the diversity of organisms present in the compost was low. At this stage, the bacterial population within the compost is low, particularly when compared to the fungi population. There is some skepticism as to how well fungi can reproduce and survive in an aqueous environment. Furthermore, thermophilic bacteria, those that can survive temperatures between 60 and 65 C, such as Bacillus sp. may not present in cured compost (CIWMB, 2001). Compost tea efficacy may have been greater if non-cured compost was used. Future research into compost teas should focus on identifying the beneficial organisms present in the compost source used to make the tea and monitoring their populations throughout the brewing and application process. Tea components, such as salt, pH and other elements should be evaluated for their impact on microorganism survival. Also, making adjustments to how the tea is prepared, such as dilution ratio or fermentation time may increase efficacy. Application rate and timing may also increase efficacy.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.