Fungicide efficacy update for cherry leaf spot management

A review of 2015 cherry leaf spot infection and fungicide efficacy for management in 2016.

Cherry leaf spot infection in 2015

In 2015, spring and early summer conditions were cool and wet in northwest Michigan, and these conditions were favorable for early cherry leaf spot development. Cherry leaf spot infections can occur as soon as bract leaves with open stomata are present, which in some areas of northwest Michigan was during the first week of May in 2015.

While bract leaf infection is the first opportunity for cherry leaf spot infection to occur, there were 12 “official” cherry leaf spot infection periods recorded by the Michigan State University Enviro-weather station and cherry leaf spot model for the Northwest Michigan Horticulture Research Center between May 10, 2015, and the end of July (Table 1). Several of these infection periods were long, and in general, conditions were wet between many of the official infection periods which likely prolonged the wet conditions and contributed to the overall very high potential for disease. In particular, periods of frequent rain events from the end of May into mid-June were a difficult time during the season because many growers were not able to apply fungicides due to poor spray conditions.

Furthermore, when ideal spraying weather occurred, the window for spraying was short and growers were faced with prioritizing sprays. Leaf spot took a foothold during this time, and because pressure was so high, control was difficult. Unfortunately, the 2015 season was unforgiving for delayed or missed fungicide applications. MSU Extension also received reports that some growers had higher levels of cherry leaf spot in blocks where alternate row middle applications were made and in lower priority/non-bearing orchards.

Table 1. Cherry leaf spot disease report from MSU Enviro-weather May 10-Aug. 23, 2015.

Start of wetting period

End of wetting period

Duration (hours)

Avg. temp. (F)

Rainfall (in.)

Cherry leaf spot

Progress towards infection

5/10 9-10PM

5/12 3-4PM

Wet: 33; Span: 43

49.7

0.22

Moderate

178%

5/15 2-3AM

5/16 9-10AM

Wet: 29; Span: 32

48.9

0.35

Low

121%

5/17 6-7PM

5/17 8-9PM

Wet: 3; Span: 3

72.9

0.32

None

40%

5/24 8-9PM

5/25 2-3PM

Wet: 17; Span: 19

59.6

1.6

Moderate

214%

5/26 Midnight-1AM

5/26 3-4AM

Wet: 4; Span: 4

63.2

0.06

None

77%

5/26 5-6PM

5/27 11PM-Midnight

Wet: 7; Span: 7

65.5

0.31

Low

137%

5/27 Noon-1PM

5/27 3-4PM

Wet: 4; Span: 4

57.5

0.03

None

45%

5/29 Noon-1PM

5/30 3-4PM

Wet: 21; Span: 28

54.8

0.36

Moderate

180%

6/7 8-9AM

6/9 11AM-Noon

Wet: 39; Span: 52

57.4

0.6

High

439%

6/11 4-5PM

6/12 11AM-Noon

Wet: 20; Span: 20

55.4

0.55

Moderate

182%

6/13 5-6PM

6/15 10-11AM

Wet: 32; Span: 42

63.9

0.27

High

590%

6/18 3-4PM

6/18 4-5PM

Wet: 2; Span: 2

60.6

0.03

None

31%

6/22 2-3PM

6/22 4-5PM

Wet: 3; Span: 3

71

0.07

None

48%

6/30 8-9AM

7/1 11AM-Noon

Wet: 24; Span: 28

52

0.01

Moderate

169%

7/6 10-11PM

7/7 8-9AM

Wet: 11; Span: 11

59.3

0.41

Low

109%

7/13 10-11PM

7/14 1-2PM

Wet: 16; Span: 16

64.7

0.22

Moderate

308%

7/17 4-5AM

7/17 2-3PM

Wet: 11; Span: 11

64.6

0.03

Low

207%

7/18 10-11AM

7/18 11AM-Noon

Wet: 2; Span: 2

67.6

0.03

None

40%

8/2 11AM-Noon

8/3 9-10AM

Wet: 22; Span: 23

63.1

1.19

High

348%

8/7 6-7PM

8/8 8-9AM

Wet: 15; Span: 15

63.6

0.11

Moderate

280%

8/13 6-7PM

8/14 8-9AM

Wet: 9; Span: 15

69.8

0.3

Low

158%

8/18 7-8PM

8/19 Noon-1PM

Wet: 14; Span: 18

69.5

0.33

Moderate

259%

8/20 5-6PM

8/20 6-7PM

Wet: 2; Span: 2

61

0.02

None

31%

8/23 Noon-1PM

8/23 3-4PM

Wet: 4; Span: 4

67

0.31

None

80%

Fungicide efficacy 2015 results

If there is an upside to such a challenging cherry leaf spot year, it is possibly that last season’s conditions were ideal for assessing cherry leaf spot fungicide efficacy on Montmorency tart cherries at the Northwest Michigan Horticulture Research Center. In this trial, Bravo Weather Stik (4 pints per acre) was applied to all treatment trees for the first and second applications of the season. The following applications of four treatments were as follows:

  • Treatment 1: Luna Sensation 5 fluid ounces plus R56 0.125 percent
  • Treatment 2: Luna Sensation  5 fluid ounces plus R56 0.125 percent plus Captan 80 WDG 2.5 pounds
  • Treatment 3: Merivon 5.5 fluid ounces plus Sylgard 0.03 percent
  • Treatment 4: Captan 80 WDG 2.5 pounds, and an untreated control

Applications were made every nine to 10 days with a handgun (300 gallons per acre), and data on the percentage of leaves on terminals with infection and percentage of terminal defoliation was collected July 20 and Sept. 9. While growers should not be applying four consecutive applications of fungicides such as Luna Sensation or Merivon due to the risks of fungicide resistance, we need to study fungicide efficacy in this manner on a small scale to most effectively compare compounds.

We found that all of the treatments were significantly more effective for preventing infection and defoliation compared with the untreated control (Table 2). Treatments 2 and 4 that contained Captan were the most efficacious against cherry leaf spot. Furthermore, the efficacy of Luna Sensation was improved by adding Captan. Although the percent defoliation numbers are high on Sept. 9 in this experiment, it should be noted there are unsprayed control trees in the block and so the disease pressure was consistently high all season. We also found mildew incidence was significantly lower on trees treated with either Luna Sensation or Merivon compared with the untreated control. These results indicate that even during seasons with extremely high cherry leaf spot pressure, these materials are still providing good cherry leaf spot control.

Table 2. Cherry leaf spot fungicide efficacy results, 2015

Treatment

Timing

% Infection

% Defoliation July 20

% Defoliation Sept. 9

% Mildew infection July 20

1. Bravo Weather Stik 4 pt

Luna Sensation 5 fl. oz. + R56  0.125%

AB

CDEF

62.1 bc

7.3 b

82.2 bc

0.8 c

2. Bravo Weather Stik 4 pt

Luna Sensation 5 fl. oz. + R56 0.125% + Captan 80 WDG 2.5 lb

AB

CDEF

42.5 d

5.2 b

66.8 cd

1.0 c

3. Bravo Weather Stik 4 pt

Merivon 5.5 fl oz + Sylgard (0.03%)

AB

CDEF

53.6 bcd

11.3 b

63.4 d

0.0 c

4. Bravo Weather Stik 4 pt

Captan 80 WDG  2.5 lb

AB

CDEF

45.2 cd

3.5 b

53.0 d

9.7 ab

Untreated control

 —

95.5 a

31.2 a

99.7 a

23.9 a

Fungicide spray dates and respective growth stages were as follows: A = May 18 (early petal fall); B = May 28 (shuck split); C = June 8 (first cover); D = June 17 (second cover); E = June 26 (third cover); and F = July 6 (fourth cover).

Preventing early cherry leaf spot infections will help to minimize the risk of a cherry leaf spot epidemic in the 2016 season

This strategy is particularly important for trees stressed by early leaf loss last season; potentially two years of early defoliation will contribute to poor tree health and mortality, especially if we have another hard winter. Furthermore, many orchards have a high level of inoculum as we head into the 2016 season due to severe infections last year, and there is the potential for a very high discharge of cherry leaf spot spores during this week’s predicted rain and temperatures above 60 degrees Fahrenheit.

Therefore, we would like to encourage growers to protect orchards that have open bract leaves or true leaves from infection prior to rain. If retreatment is needed before the 10-day retreatment interval for chlorothalonil is met, Captan 80 WDG (2.5 pounds per acre) is a good alternative for cherry leaf spot.

Drs. Rothwell and Sundin’s work is funded in part by MSU’s AgBioResearch.

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