Susceptibility of commercial Michigan plum cultivars to spotted wing Drosophila

Based on our most recent survey of spotted wing Drosophila (SWD) susceptibility in Michigan peach cultivars, we have new information for growers on cultivar susceptibility and management strategies.

While plums are not a major crop in west central Michigan, many growers still have small plantings of European and Japanese plums. These fruits are sold to processing companies, on the fresh market and to distilleries for brandy production. The wide array of available plum cultivars, extended season, variability in ripening time, typical grower practice of mixing several different cultivars of plums in within a block, and the widespread presence of spotted wing Drosophila (SWD) has led to questions regarding if, when and how to manage this pest.

Most growers are unsure of whether their plums should be managed for SWD or whether certain cultivars can be considered less susceptible than others attacked by this pest. If management is necessary, growers do not know when a program should be initiated or how frequently a new application should be made. Most growers assume earlier plums are less susceptible to SWD and that Japanese plums are more susceptible than European plums.

To begin addressing these concerns, Michigan State University Extension conducted a survey of susceptibility of commercial plums in west central Michigan during the 2017 growing season. Plums sampled included 20 cultivars spanning early, mid-season and late season European and Japanese types. Growers did not have a management program specific to SWD in place at the time of sampling.

For each cultivar, 20-30 fruits were sampled, fruit pressure was recorded and the condition of the fruits was rated at time of sampling. Only fruits with no visible signs of insect feeding or rot were sampled. Fruits were submerged in a simple sugar solution of 7 pounds of sugar per 5 gallons of warm water, gently squeezed and left to stand for at least 15 minutes. Fruits were then strained through fine mesh to remove any larvae that had exited infested fruits. SWD larvae were counted under a dissecting microscope.

Survey results of susceptible commercial plums to spotted wing Drosophila (SWD)

Plum cultivar

Type

Sample date

Firmness (lbs)

SWD larvae

Early Golden

Japanese

7/25/2017

8

0

8/1/2017

0

99

Beauty

Japanese

7/25/2017

0

52

Methely

Japanese

8/1/2017

0

0

Santa Rosa

Japanese

8/2/2017

0

3

Early Magic

Japanese

8/2/2017

0

0

Shiro

Japanese

8/8/2017

0

0

8/2/2017

0

1

8/31/2017

0

0

Black Ice

Japanese

8/8/2017

<3.5

8

Queen Rosa

Japanese

8/8/2017

<3.5

3

AU Rosa

Japanese

8/8/2017

<3.5

8

Castleton

European

8/16/2017

1.79

8

Bluebyrd

European

8/16/2017

10.82

0

8/22/2017

0.3

2

Unknown yellow

Japanese

8/22/2017

0

1

Damson

European

8/31/2017

5.25

0

Ozark Premier

Japanese

8/16/2017

3.57

0

8/22/2017

0.2

10

Starking Delicious

Japanese

8/16/2017

7.17

0

8/30/2017

3.68

0

8/30/2017

2.25

82

Redheart

Japanese

8/30/2017

3.4

0

Unknown red

Japanese

8/30/2017

2.92

0

Italian

European

9/14/2017

3.42

11

Stanley

European

9/14/2017

7.69

0

NY9

European

9/14/2017

3.57

0

Discussion

Both early and late season plums were at risk of being infested if left on the trees until soft (under 3.5 pounds of pressure). The same was true for both Japanese and European plum cultivars. Growers with an early plum cultivar cannot assume that the seasonality of the fruit will protect it from SWD. SWD is present in large numbers well before even the earliest plum cultivars are being harvested, so SWD management plans are needed for early and later ripening cultivars. Fresh market plums that are traditionally left on the tree until flesh is “melting” in consistency need to be managed for SWD regardless of harvest timing, cultivar or type.

The findings of the survey also demonstrate that a visual analysis is a poor way of determining whether or not plums are infested by SWD. This same phenomenon is frequently observed in sweet and tart cherry samples. Intact, nice looking fruit often turn out to be infested by SWD. Growers cannot assume that fruit is not infested and safe for market sales just because there are no visible signs of infestation in the field. All of the fruit tested in this study were of acceptable market quality based on the visual assessment.

None of the plum cultivars were infested by SWD if fruit firmness was greater than 3.5 pounds. This is an important observation because plums do not typically fall below this mark until a few days before harvest. This was true in both Japanese and European plums. If consistent between seasons, this observation could have significant management applications for growers. If fruits are not susceptible until they reach this mark, weekly pressure testing could allow growers to initiate spray management programs only when fruit reach this firmness.

While this broad survey did not validate specific SWD management programs for Michigan plum production, the findings do indicate that growers can likely achieve acceptable management if they initiate a spray program before fruit reach 3.5 pounds of firmness, and maintain a cover of “excellent” products every seven days until harvest has concluded. Growers could also consider avoiding SWD by harvesting plums earlier than normal while fruits are in firmer condition (thus, presumably not susceptible to SWD infestation). Plums are a climacteric fruit, which means they will continue to ripen after they are harvested. As a result, plums picked slightly earlier than traditional harvest dates can still be ripened off the tree and sold at full ripeness.

In summary, a spray program initiated before fruit firmness reaches 3.5 pounds and slightly earlier harvest is likely to achieve acceptable management in most situations, but more research is needed to validate the approach.

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