- I. Single Application Trials
- II. Multiple Application Trials (1998-2005)
- III. Plant Growth Regulators and Fungicide Standards
I. Single Application Trials (1991-1995)
Early blight trials began in 1991 and were designed to evaluate the longevity of the effectiveness of a single fungicide application against early blight (Table 1). Effectiveness was determined weekly after application as percent of each plant that is infected with early blight at increments of 0, 1, 5, 10, 25, 50, 75, and 100% (dead). Measurements were taken on 20 plants per plot with four plots per treatment. Based on these individual plant measurements of infection, an early blight index was calculated that depicts the percentage of the plot infected with early blight. This may be the most accurate method to compare treatments, varieties and conditions.
Table 1. Trial locations, cultivar and date of first early blight symptons (1991-1995).
| Year | Location | Cultivar | Trial | First Symptons* | ||||||||||
| 1991 | Broadwater, NE | Norgold Russet | Excellent | 18 July | ||||||||||
| 1992 | Bridgeport, NE | Russet Norkotah | Excellent | 16 July | ||||||||||
| 1993 | LaGrange, WY | Goldrush (Russet) | Good | 18 August | ||||||||||
| 1994 | McGrew, NE | Russet Norkotah | Excellent | 12 August | ||||||||||
| 1995 | McGrew, NE | Russet Norkotah | Good | 15 August | ||||||||||
Trial Comments:
*Treatments were applied at first sign of early blight symptoms, |
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At 1 WAT, all treatments delayed early blight onset (Table 2). The 2 and 3 WAT data showed the greatest range of effectiveness between the treatments. At 2 WAT, Maneb and Maneb+Zinc (mancozeb) were not significantly better than check. Bravo (chlorothalonil), Rovral (iprodione), and Microthiol (wettable sulfur) showed some control while the two Penncozeb (mancozeb) formulations kept early blight away. By 3 WAT, only the Penncozebs were still effective (Table 2).
Table 2. Five-year (1991-1995, 5 years) Averages of Early Blight Index.|
Product |
Product Rate lb/a | Early Blight Index * | |||
| 1 WAT ** |
2 WAT |
3 WAT |
4 WAT |
||
| Check (X-77) | 0 | 4.6 A *** | 17.2 A | 57 A | 96 |
| Maneb 75D | 1.5 | 3.1 B | 11.1 ABC | 45 A | 92 |
| Maneb + Zn 4F | 1.5 | 2.6 B | 13.4 AB | 47 A | 94 |
| Bravo 500 | 1.0 | 2.7 B | 7.3 BCD | 42 A | 96 |
| Microthiol 80D | 4.8 | 1.9 B | 8.4 BCD | 42 A | 93 |
| Rovral 4F | 0.75 | 2.8 B | 9.4 BCD | 45 A | 92 |
| Penncozeb 80D | 1.5 | 1.8 B | 4.2 CD | 21 B | 87 |
| Penncozeb 75W | 1.5 | 1.9 B | 2.5 D | 20 B | 87 |
| LSD0.05 | 1.3 |
7.5 |
18 |
NS |
|
Early Blight Index (EBI) was used to compare treatment effectiveness on different reading dates. It indicated the percent of each plot showing early blight symptoms.
The general formula was the sum of the number of plants at each infection level times the infection level as a percent. This number was divided by the number of plants measured to give the % of the plot showing symptoms.
In these trials 20 plants were measured in each plot and the formula used was
EBI= ((# plants without symptoms x 0) + (# plants with 1% infection level x 1) + (# plants with 5% x 5) + (# plants with 10% x 10) + (# plants with 25% x 25) + (# plants with 50% x 50) + (# plants with 75% x 75) + (# plants dead x 100)) / 20 (or total # plants).
II. Multiple Application Trials (1998-2005)
All early blight trials from 1998 to 2005 were conducted on the variety Atlantic at the UNL Panhandle Research and Extension Center, Scottsbluff, NE (Table 3). Multiple treatment applications were used; in 1998 and 1999, three fungicide treatments were applied, and in 2000 to 2005, there were four application dates. All applications were started when the physiological days (p-days) reached between 350 and 450 and were spaced at 5 to 7 day intervals in August. The last treatment was applied when p-days was between 480 and 540. Early blight symptoms appeared within three days of last application. Results are grouped based on comparisons.
Table 3. Trial comments from 1998 to 2005.| Year | Trial Comments1 |
| 1998 | Early blight appeared Aug. 20; no late blight. |
| 1999 | Early blight appear on Aug. 20; late blight on Sep. 17 on one of a few remaining plants that had not died from early blight. |
| 2000 | Early blight appeared on Aug. 30. Readings ended on Sept. 22 due to snow storm and killing freeze. |
| 2001 | Early blight appeard Aug. 31 and rapidly spread in late September. August was drier and hotter than usual. Trial was ended by a freeze in early October. |
| 2002 | Early blight appeared on Aug. 28. Indications suggested that the pathogen may be azoxystrobin-resistant. The trial was short due to the rapid spread of early blight and a killing frost on Sept. 22. |
| 2003 | Early blight appeared shortly after first fungicide application August 2 and began spreading rapidly two weeks later. Area remained in severe drought and temperature was below normal As in 2002, due to dryness and highly fluctuating temperatures, plants were under extreme stress and physiological days accumulated quickly. This resulted in a rapid progression of early blight. |
| 2004 | Early blight appeared around Aug. 7 and began spreading rapidly 10 days later. The drought persisted. Planting was delayed due to a a cool early May. Temperature fluctuated greatly through season. |
| 2005 | Early blight appeared Aug. 15 and began spreading a week later. The drought continued. Temperature was unusually high in July and August. |
1 All trials were conducted at Scottsbluff, Nebraska on the cultivar Atlantic. |
|
A. Effectiveness of Selected Standards
Mancozeb and chlorothalonil-Zn are broad-spectrum protectant-type fungicides used in agriculture. These were compared and used as comparative standards throughout these trials. They both delayed the onset of early blight but chlorothalonil-Zn was significantly more effective than mancozeb (Table 4). Averaged over the 7 trials (1998-2005), mancozeb gave 5 days of protection while chlorothalonil-Zn gave 9 days of protection. Chlorothalonil alone (Terranil 6L @ 1.5 pt/a) or with Cu or Zn (Terranil Cu @ 3.34 pt/a and terranil Zn @ 2.2 pt/a) were compared in 1998. Terranil Zn behaved as Bravo Zn (Table 4); chlorothalonil-Cu was less effective than the Zn form by 5 days, and chlorothalonil was less effective than the Cu form by another 5 days (data not shown).
Table 4. Delaying early blight: Comparison of chlorothalonil Zn (CS) and mancozeb (MZ) to untreated (UN), 1998-2005 (7 years).
|
Year
1998 1999 2000 2001 2002 2003 2004 2005 mean days after last treatment to 50% early blight symptoms in plot |
|||||||||
| UN1 | 16 | 16 | 11 | 28 | 2 | 5 | 8 | 15 | 13 C2 |
| MZ | 21 | 21 | 19 | 33 | 5 | 7 | 11 | 24 | 18 B |
| CZ | 31 | 25 | 22 | 39 | 7 | 8 | 11 | 27 | 22 A |
| days after lst treatment to 75% early blight symptoms in plot | |||||||||
| UN1 | 21 | 18 | 15 | .3 | 7 | 11 | 8 | 23 | 15 C4 |
| MZ | 26 | 25 | 22 | . | 10 | 13 | 11 | 28 | 19 B |
| CZ | 41 | 28 | 25 | . | 12 | 15 | 11 | 32 | 24 A |
1UN = untreated check, CZ = chlorothalonil Zn (Bravo Zn 4F @ 2.125 pt/a), MC = mancozeb (Dithane DF @ 2 lb/a in 1998; Manex II 4F @ 3 pt/a in 1999, 2000, 2002, 2003, 2004; Penncozeb 75DF @ 2 lb/a, and Manzate 4F @ 3 pt/a). 2Means with different letters indicate that they are significantly different at the 95% confidence level using least significant difference (LSD0.05 = 3). 3 Plants died from frost before infection level reached 75% in 2001. 4 Means with different letters indicate that they are significantly different at the 95% confidence level using least significant difference (LSD0.05 = 4).
B. Effectiveness of Sequential Applications Chlorothalonil-Zn and Azoxystrobin
Azoxystrobin is a natural fungicide originally discovered in mushrooms in the Czech Republic. It was subsequently identified as being active against early and late blights in potato and tomato. In the 1999 trial, azoxystrobin (Quadris) was included at two rates, 6.2 and 12.4 fl oz/a, in a sequence with chlorothalonil-Zn. Both sequences were more effective than chlorothalonil-Zn alone and the two azoxystrobin rates did not differ from each other. Chlorothalonil-Zn (Bravo Zn 4F @ 2.125 pt/a) gave 10 days of protection, the sequence of azoxystrobin at both rate (Quadris 2.08SC @ 6.2 fl oz/a or @ 12.4 fl oz/a) followed by chlorothalonil-Zn protected plants for 14 days. From 2000 to 2004, azoxystrobin at the lower active ingredient (Quadris 2.08SC @ 6.2 fl oz from 2000 to 2002 and Amistar 80WD @ 2 oz/a) followed by chlorothalonil-Zn was not significantly more effective than chlorothalonil-Zn alone (Table 5). Reversing the sequence did not increase effectiveness (Table 5). Several other sequences were tested as well and their effectiveness are given in Table 6.
Table 5. Delaying early blight: Comparison of sequential applications of chlorothalonil Zn (CZ) and azoxystrobin (AS) to untreated (UN), 2004-204 (5 years).| Year | |||||||||
| Sequence | 2000 2001 2002 2003 2004 mean | ||||||||
| 1st & 3rd 2nd & 4th |
days after last treatment to 50% early blight symptoms | ||||||||
| UN1 | 11 | 28 | 2 | 5 | 8 | 11 B2 | |||
| CZ | 22 | 39 | 7 | 8 | 11 | 16 A | |||
| CZ | AS | 25 | 42 | 7 | 8 | 13 | 19 A | ||
| AS | CZ | 23 | 12 | ||||||
| |
|||||||||
| 1st & 3rd 2nd & 4th |
days after last treatment to 75% early blight symptoms | ||||||||
| UN | 15 | .3 | 7 | 11 | 8 | 10 B2 | |||
| CZ | 25 | . | 12 | 15 | 11 | 16 A | |||
| CZ | AS | 29 | . | 13 | 15 | 13 | 18 A | ||
| AS | CZ | 26 | 11 | ||||||
1UN = untreated check, CZ = chlorothalonil Zn (Bravo Zn 4F @ 2.125 pt/a), AS = azoxystrobin (Quadris 2.08SC @ 6.2 fl ox/a in 2000, 2001 and 2002, and Amistar 80WD @ 2 ox/a in 2003 and 2004). 2Means with different letters indicate that they are significantly different at the 95% confidence level using least significant difference (LSD0.05 = 5). 3Plants died from frost before infection level reached 75% in 2001.
C. New Products Compared to Fungnicide Standards
During the course of the eight years of evaluating fungicides for early blight suppression, many potential products, experimental compounds and new formulations of existing ingredients were tested for one to three years. None of these showed an advantage over existing treatments (data not shown). Table 6 summarizes the relative effectiveness of these products. Table 6. Delaying early blight: Relative effectiveness of new products.| Product | Rate/a | Years | Relative Effectiveness |
| Acrobat MZ | 2.25 lb | 1998 | > Dithane, |
| Bravo Ultrex | 1.4 lb | 1998 | |
| Fluazinam | 1 pt | 1998 | = Bravo; 0.5 pt ~ Dithane |
| Quadris | 18 oz | 1998 | = Bravo Zn |
| SuperTin 80WP | 2.5 oz | 1998 | > Dithane, ~ Bravo Zn |
| Tatoo C | 2.3 pt | 1998 | |
| mixtures | |||
| Curzate 60DF+ Bravo Zn 4F | 3.3 oz 2.1 lb | 2000-01 | = Bravo Zn |
| Curzate 60DF+ SuperTin 80WP | 3.3 oz 2.5 oz | 1998-99 | > Bravo Zn |
| Flint 50WG+ Dithane RS NT | 4 oz 1 lb | 1999 | > Bravo Zn and > Dithane; Flint @ w oz/a ~ Bravo Zn |
| Gavel 75 DF | 2.25 lb | 1998 | > Dithane, ~ Bravo Zn |
| Gavel 75D+ Quadris 2.08SC | 2.3 lb 6.2 foz | 1999-00 | slightly > Bravo Zn |
| Manex C-8 | 24 oz | 1998 | = Dithane |
| SuperTin 80WP+ Manzate 200DF | 2.5 oz 2 lb | 1998 | > SuperTin alone, |
| sequences | |||
| Flint 50 WG Bravo Zn 4F | 4 oz 2.1 pt | 2000 | ~ Bravo Zn ~ Quadris followed by Bravo Zn |
| Gavel 75D Quadris 2.08SC | 2.3 lb 6.2 foz | 2001 | > Bravo Zn, = Bravo Zn followed by Quadris |
| Bravo Zn 4F - Headline 2.09EC | 2.1 pt 6.1 foz | 2001 | > Penncozeb, |
| Polyram 80DF+ SuperTin 80WP- Headline 2.09EC | 2 lb 2.5 oz 6.1 foz | 2001-02 | |
| Polyram 80DF+ SuperTin 80WP- Quadris 2.08SC | 2 lb 2.5 oz 6.2 foz | 2001 | = Bravo Zn, |
less than, = same as, ~ similar to. Refer to Tables 4 and 5 for values of Bravo Zn, Dithane and Bravo Zn/Quadris sequence data.
III. Plant Growth Regulators and Fungicide Standards
Due to interest in the anti-senescence activity of cytokinin plant hormones, trials from 2001 to 2005 included testing a synthetic cytokinin, thidiazuron (e.g. Dropp, FreeFall). This compound is commercially used to defoliate cotton. The question was whether such a compound would delay aging of leaves and thereby assist fungicides in delaying the onset of early blight. The results of this study were published; see reference below. The general conclusion was that cytokinins will delay the progress of early blight and that when added to mancozeb or chlorothalonil-Zn will prolong their effectiveness against early blight.
Pavlista, A.D. and C. Gall. 2011. Delaying Early Blight Onset in Potato with Thidiazuron. American Journal of Potato Research 88:114-120.
DOI 10.1007/s12230-010-9168-x
Reprints can be obtained from author.
