Cherry leaf spot and powdery mildew control materials

First cover is the optimal time to target powdery mildew in tart cherries.

Many cherry orchards are at first cover timing for disease management, and cherry leaf spot and powdery mildew are the main diseases of concern at this time. With the dry conditions earlier this spring, most orchards are free of disease symptoms so far this season. However, there are some isolated orchards where we have detected cherry leaf spot lesions on the first true leaves. Preventing the spread of secondary phase of this disease, the conidial stage, is critical for keeping this disease under control for the duration of this season.

Powdery mildew can be a problem in tart cherry orchards, particularly in young orchards and infection tends to occur on the most actively growing tissue. Cherry leaf spot and powdery mildew can cause early defoliation if these diseases are not managed effectively, and with the potential for a large cherry crop, adequate leaf area will be necessary to ripen the crop and to have healthy trees as we head into next winter.

The biggest issue for powdery mildew control is the prevention of initial fungal infection. There are no fungicides that will eradicate powdery mildew, so Michigan State University Extension advises controlling this disease before it appears on the leaves. Powdery mildew control is achieved by using protectant fungicides ahead of the infection. By the time white mycelium is visible on leaves, it is too late for control.

The most important spray timing for powdery mildew control is the first cover timing, i.e., the first spray application after shuck split. This spray is critically important. Previous research has shown that if this timing is missed, the amount of powdery mildew-infected leaves can increase by at least threefold at harvest (data not shown). Northwest Michigan Horticulture Research Station research has also found that if fungicides targeting powdery mildew are only applied closer to harvest, powdery mildew infection can reach epidemic levels by mid-August (about 70 percent incidence of leaf infection) (Table 1). There are several materials that provide activity against cherry leaf spot and powdery mildew; however, not all fungicides are active against cherry leaf spot and powdery mildew. Therefore, we remind growers to include materials that are effective for both diseases, particularly at the first cover timing.

Table 1. Fungicide timing trail/cherry at the Northwest Michigan Horticulture Research Station

Treatment

Timing

% Leaves infected (June 29)

Pristine 12 ounces

Late bloom/petal fall, shuck split, first cover, second cover, third cover, fourth cover

1.1d

Pristine 12 ounces

Shuck split, first cover, second cover, third cover, fourth cover

2.6d

Pristine 12 ounces

First cover, second cover, third cover, fourth cover

3.7d

Pristine 12 ounces

Second cover, third cover, fourth cover

7.1c

Pristine 12 ounces

Third cover, fourth cover

19.5a

Pristine 12 ounces

Fourth cover

15.2b

Gem 3.8 ounces

Late bloom/petal fall, shuck split, first cover, second cover, third cover, fourth cover

2.2d

Untreated

22.3a

SDHIs (Group 7, 11) – Best options for cherry leaf spot and powdery mildew at the first cover timing

The SDHI fungicide class, Luna Sensation or Merivon, are excellent for cherry leaf spot and powdery mildew control at the first cover timing. We recommend this class of fungicide be tank-mixed with Captan. The SDHIs are the best fungicides currently available for cherry leaf spot, and they are recommended at the first cover timing to coincide with high cherry leaf spot spore discharge; SDHIs are also rated excellent for powdery mildew control.

Growers have been concerned that the SDHIs are expensive, but a well-timed first cover application of these newer materials will provide ideal control of cherry leaf spot and powdery mildew (see figure). Additionally, because these materials are systemic, they are less likely to wash off during a rain event. However, there is high risk for the development of resistance to SDHI fungicides, which is why we recommend a protectant (e.g., Captan) be tank-mixed with these materials. Using the highest label rate will aid in effectively killing the pathogen; killing the pathogen will delay the development of cherry leaf spot resistance to SDHIs by reducing selection of fungi that are less sensitive to the SDHI class of fungicides.

Note: According to the Luna Sensation and Merivon labels, it is not permitted to apply more than two sequential applications of a Group 7 or 11 fungicide before rotating with a fungicide from a non-Group 7 or 11.

 Bar graph with the control group showing the highest percentage of defoliation on August 1.

2012 Field Trial B at the Northwest Michigan Horticulture Research Station. The first two applications are Bravo Weather Stik, 4 pints.

Syllit (Group U12) + Captan (Group M) and Copper – Rated “excellent” for cherry leaf spot, no powdery mildew activity

Although Syllit is typically suggested as a second or third cover cherry leaf spot material, some growers may consider using Syllit while the weather is cool; copper is also an option for cherry leaf spot in cooler conditions. If Syllit or copper is used during first cover, an efficacious fungicide for powdery mildew should be included as these fungicides will not provide powdery mildew control. Syllit is an at-risk fungicide, and this material should be mixed with Captan for resistance management. Copper has provided excellent control of cherry leaf spot, and many growers are tank-mixing copper with lime to minimize phytotoxicity. We used to recommend 6 pounds of hydrated lime, but many growers are using a 1:1 ratio of copper to lime with good results. The cooler the temperatures, the less potential we have for copper-phytotoxicity.

Pristine (Group 7,11) – Not recommended for cherry leaf spot, rated “excellent” for powdery mildew

Pristine was first registered in 2004 and is a pre-mix fungicide containing boscalid (SDHI) and pyraclostorbin (stobilurin). Previous research at the Northwest Michigan Horticulture Research Station has shown the cherry leaf spot pathogen has reduced sensitivity and resistance to Pristine, specifically the boscalid component of this material, which had been the workhorse of Pristine for cherry leaf spot prior to the development of reduced sensitivity to the fungicide. In Northwest Michigan Horticulture Research Station research trials, there was a significantly higher percentage of defoliation in trees treated with Pristine compared with an SDHI fungicide (see figure). Therefore, MSU Extension is not recommending Pristine for cherry leaf spot management. In the MSU Extension “Michigan Fruit Management Guide,” the fungicide Pristine is rated “fair/good” for cherry leaf spot control and “excellent” for powdery mildew.

Gem (Group 11) – Rated “good/excellent” for cherry leaf spot and “excellent” for powdery mildew

Although not as effective as the SDHIs, Gem is rated “good” to “excellent” for cherry leaf spot and “excellent” for powdery mildew, and is not the recommended material to prevent these diseases at the first cover timing. The label rate for Gem is 1.9-3.8 fluid ounces per acre; however, a higher rate of 3.0-3.8 fluid ounces per acre and including a protectant fungicide is recommended for effective cherry leaf spot control and resistance management.

We recommend Gem be tank-mixed with Captan as data from the 2015 efficacy trial showed that a season-long Captan program effectively managed cherry leaf spot (Table 2). Gem is a strobilurin fungicide, which is a site-specific or single-site fungicide, meaning that only one mutation of the pathogen’s target site is needed for development of resistant strains of the cherry leaf spot fungus. In apples, field resistance occurred quickly, and increasing the rate of Gem will not provide improved control. Therefore, if cherry leaf spot isolates are resistance to Gem, the Captan component of a Gem + Captan mix should provide cherry leaf spot control. Hence, Gem must be tank-mixed with Captan. Captan alone will not provide activity against powdery mildew.

Note: Gem is a Group 11 fungicide, so use caution if using Gem and SDHI products in an orchard’s spray program.

Table 2. Cherry leaf spot and powdery mildew fungicide efficacy results, 2015

Treatment

Timing

% Infection

% Defoliation

July 20

% Defoliation

Sept. 9

% Mildew infection

July 20

Bravo Weather Stik 4 pints

 

AB

62.1 bc

7.3 b

82.2 bc

0.8 c

Luna Sensation 5 fluid ounces + R56 0.125%

CDEF

Bravo Weather Stik 4 pints

AB

42.5 d

5.2 b

66.8 cd

1.0 c

Luna Sensation 5 fluid ounces + R56 0.125% + Captan 80 WDG 2.5 pounds

CDEF

Bravo Weather Stik 4 pints

AB

53.6 bcd

11.3 b

63.4 d

0.0 c

Merivon 5.5 fluid ounces + Sylgard (0.03%)

CDEF

Bravo Weather Stik 4 pints

AB

45.2 cd

3.5 b

53.0 d

9.7 ab

Captan 80 WDG 2.5 pounds

CDEF

Untreated Control

 

95.5 a

31.2 a

99.7 a

23.9 a

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