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American brown rot has developed strong resistance to Indar in parts of Michigan

Fungicide screens in northwest and west central Michigan show widespread resistance to all labelled rates of Indar in populations of the American brown rot fungus.

American brown rot infection on ripening sweet cherries
American brown rot infection on ripening sweet cherries. Photo by George Sundin, MSU.

Introduction

Indar (fenbuconazole) has been a key tool for managing American brown rot (Monilinia fructicola) in Michigan stone fruit orchards for more than 30 years. While resistance to this product at lower labeled rates (6 fluid ounces per acre, 8 fluid ounces per acre) has been previously documented, growers have been able to continue using this material in the past several years by increasing the rate applied per acre to the maximum labelled rate (12 fluid ounces per acre). This practice has prolonged the life of Indar in Michigan stone fruit orchards and kept a valuable tool in growers’ pockets.

Unfortunately, resistance tests conducted on 296 American brown rot isolates from west central and northwest Michigan in the 2019 growing season demonstrate that functional resistance to even the 12 fluid ounce rate is now widespread in American brown rot throughout these regions, and growers are going to need to look elsewhere for American brown rot management in 2020 and beyond.

Methods

Sweet cherries infected with American brown rot were collected from 15 orchards in west central and northwest Michigan in July of 2019. One hundred American brown rot isolates were collected from sampled northwest sites and 196 American brown rot isolates collected from sampled west central sites. All American brown rot isolates were grown on PDA medium amended with 0.3 µg/ml of fenbuconazole, and relative growth (RG) of all isolates was measured. A relative growth value greater than 30 for any American brown rot isolate indicates functional resistance to Indar in the tested orchard at all labelled rates.

For the duration of this article, we will call any American brown rot isolate with a relative growth value greater than 30 “resistant.” This indicates that the isolate is not sensitive to fenbuconazole at any labelled rate, including the 12 fluid ounce rate.

Results

The majority of American brown rot isolates from both west central and northwest Michigan displayed relative growth values of greater than 30, indicating functional resistance to Indar in the orchard (Table 1). While a wide range of relative growth values was detected in both regions, nearly all orchards had at least one resistant isolate (Tables 2 and 3). At most orchards, the majority of isolates displayed resistance (Tables 2 and 3).

Table 1. Relative growth (RG) values and ranges from northwest and west central Michigan. An RG value greater than 30 indicates than an orchard has functional resistance to even the 12 fluid ounce rate of indar.

Region

Number of orchards

Number of isolates

Range of RG values

Average RG

Northwest

6

100

17.3-100

41.1

West central

15

196

2.3-94.1

36.1

Total

22

304

--

--

The relative growth values for most isolates collected in 2019 fell in the 30-89.9 range (Fig. 1). While a range of relative growth values is observable, this means the majority of isolates fall in the range of resistance to Indar.

Figure 1 graph
Figure 1. Relative growth values of American brown rot isolates on media amended with 0.3 µg/ml propiconazole or fenbuconazole. X axis = RG Values, Y axis = % of isolates, n=304 for 2019, DD = 0.3 µg/ml active ingredient Propiconazole and Fenbuconazole.

In northwest Michigan, all but two sites had at least one isolate that was resistant to fenbuconazole (Table 2). In all orchards where resistant isolates were found, the majority of the isolates from the orchard were resistant (Table 2).

Table 2. Individual orchard results in northwest Michigan.

Orchard code

Number of isolates

Range of RG values

Number of resistant isolatesª

Average RG for orchard

19A

18

32.2- 100

18

57.8

19B

17

26.7- 65.7

15

37.7

19C

19

17.3- 44.7

15

32.7

19D

11

22.9- 61.6

8

39.8

19E

18

21.2- 48.5

7

31.0

19F

17

23.5-80.0

16

47.7

19G

8

0.0- 21.5

0

10.2

RG greater than 30.

All orchards sampled in west central Michigan had at least one American brown rot isolate that was resistant to fenbuconazole (Table 3). Unlike northwest Michigan, resistant isolates were universally found in orchards throughout the region as opposed to a majority of orchards as was seen in the northwest region. Average relative growth values for some individual orchards was below 30, but the majority of orchards were above 30. It is important to note that orchards that had an average relative growth of below 30, there were still individual isolates within these orchard populations that tested above a relative growth of 30.

Table 3. Individual orchard results in west central Michigan.

Orchard

Number of isolates

Range of RG values

Number of resistant isolatesª

Average RG for Orchard

19H

22

8.8- 94.1

12

35.5

19I

18

19.0- 60.1

7

31.4

19J

17

16.4- 75.0

9

33.4

19K

18

18.3- 72.5

17

55.5

19L

20

10.6- 98.0

13

50.9

19M

18

0.0- 70.9

6

26.4

19N

12

17.9- 37.1

11

32.8

19O

17

18.2- 88.1

10

55.0

19P

16

11.0- 45.0

4

23.0

19Q

7

16.6- 39.4

5

29.5

19R

4

27.9- 37.9

2

32.9

19S

9

2.3- 41.0

1

20.2

19T

8

12.8- 34.3

2

21.7

19U

6

25.3- 39.6

3

31.0

19V

4

15.8- 32.6

1

23.4

RG greater than 30

Discussion

The efficacy of Indar against American brown rot has been declining for several years as regional populations of the fungus gained resistance to increasingly higher rates of the fungicide. While growers were able to prolong this product’s life, its usefulness as an American brown rot material is at an end in most orchards in west central and northwest Michigan. The year 2019 was an extremely challenging year for brown rot infection in sweet cherries across the state. While the warm, wet start to the growing season contributed to the issue, the widespread discovery of resistance to the highest labelled rates of Indar now has to factor in to our explanation of the problems that we experienced this year.

While some readers may point out that not all American brown rot isolates collected in this work displayed resistance to fenbuconazole, it is important to remember that resistance in any percentage of an orchard’s American brown rot population is still a major issue because this fungus has an extremely high reproductive capacity. The average relative growth for an orchard is valuable for us to observe general trends, but it is important to note that the overwhelming majority of orchards had several isolates that were well above the resistance threshold relative growth of 30, even orchards whose average relative growth fell below this mark.

Resistant isolates (relative growth greater than 30) are capable of growing and producing sufficient inoculum to cause problems in any orchard even if there are still susceptible isolates mixed in to the population. A pathogen as prolific as American brown rot does not need 100% survival of individual members of the population within an orchard following a spray of Indar to cause economic damage.

Growers do still have effective materials available to fight American brown rot and should be planning on leaning on these options in 2020 and beyond to get control. Flint Extra (strobilurin) and Merivon/Luna Sensation (SDHI plus strobilurin premixes) are systemic materials that still provide excellent control of American brown rot. All of these materials should always be mixed with 2.5 pounds of Captan WDG to manage against resistance, and none can be relied upon for any back-action.

Also, consider horticultural management practices such as improving canopy air circulation, decreasing canopy depth/density and planting new fresh market sweet cherry acreage on dwarfing Gisela rootstocks to limit tree/canopy size. American brown rot growth is highly favored by warm, humid conditions and reducing canopy density and tree size means a reduction in humidity within the canopy microclimate. It will also help improve coverage of fungicide, which is particularly critical given that this pathogen is both highly prolific and extremely fast-growing under conducive conditions.

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