Cherry leaf spot management at second and third cover timings

Recent reports of cherry leaf spot infections in northwest Michigan orchards range from substantial to low infections at this time.

Many orchards are past the first cover timing, and growers are planning their leaf spot management strategy moving forward. The strategy recommended by Michigan State University Extension has been to use a succinate dehydrogenase inhibitor (SDHI) fungicide at the first cover timing to target cherry leaf spot and powdery mildew. The first cover spray is critically important, particularly for powdery mildew management, as 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. Although the SDHIs are among the best materials for cherry leaf spot control, MSU Extension recommends that growers wait to use a second SDHI spray until the pre-harvest timing to prevent brown rot and to provide the longest residual control of cherry leaf spot after harvest. However, preventing the spread of conidia in already infected orchards will be critical for keeping this disease under control through harvest.

Fortunately, there are other materials to consider for leaf spot management at the second and third cover timings. These materials include Syllit, Gem, Captan alone and copper products. Syllit is rated excellent for leaf spot and Gem is rated good/excellent. Both materials are at risk materials for cherry leaf spot resistance development and as a result we remind growers that these materials should be tank mixed with Captan. Copper products are also excellent options for leaf spot, but we caution growers that coppers can be problematic with hot weather, which is predicted for the remainder of the week. The forecast is calling for cooler temperatures after this week’s heat wave. Copper could be a good leaf spot option at that time or around the third cover timing. Additionally, we remind growers that Syllit and copper will not provide powdery mildew control, but Gem is an excellent mildew material. Please read the below sections for additional information regarding these materials.

SDHIs (Group 7, 11) – Excellent first cover options for cherry leaf spot and powdery mildew

The SDHI fungicide class, such as Luna Sensation or Merivon plus Captan, are excellent for cherry leaf spot and powdery mildew control at the first cover timing. The SDHIs are the best fungicides currently available for cherry leaf spot, and we recommend their use at the first cover timing to coincide with high cherry leaf spot spore discharge as well as for powdery mildew protection. 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 (Figure 1). There is high risk for the development of resistance to SDHI fungicides and a protectant such as Captan should be tank mixed with these materials. Using the highest label rate will aid in effectively killing the pathogen and also prevent the development of cherry leaf spot resistance to SDHIs. These materials are also recommended at the pre-harvest timing.

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.

Syllit (Group U12) and 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, copper is also an option for cherry leaf spot when conditions are cooler. If Syllit or copper is used during first cover, an efficacious fungicide for powdery mildew should be included in the disease management program 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.

2012 field trial

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 because is it also rated ‘excellent’ for powdery mildew, it is a decent option for preventing 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 (3.0-3.8 fluid ounces per acre) and including a protectant fungicide is recommended for effective cherry leaf spot control and resistance management. 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. Because Gem has a higher likelihood to developing resistance in the leaf spot pathogen, we recommend tank mixing with Captan. If cherry leaf spot resistance to Gem were to occur, the Captan component of a Gem + Captan mix should provide cherry leaf spot control. Furthermore, data from the 2015 efficacy trial showed that a season-long Captan program effectively managed cherry leaf spot. However, Captan alone will not provide activity against powdery mildew.

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

Table 1. Cherry leaf spot and powdery mildew fungicide efficacy results, 2015

Treatment

Timing

Percent Infection

Percent Defoliation

July 20 2015

Percent Defoliation

Sept. 9 2015

Percent Mildew Infection

July 20 2015

1. Bravo Weather Stik 4 pt Luna Sensation 5 fluid ounces + R56 0.125 percent

 

AB

CDEF

62.1    bc

7.3  b

82.2  bc

0.8  c

2. Bravo Weather Stik 4 pt Sensation 5 fluid ounces + R56 0.125 percent + Captan 80 WDG 2.5 pounds

 

AB

CDEF

42.5     d

5.2  b

66.8  cd

1.0  c

3. Bravo Weather Stik 4 pt Merivon 5.5 fluid ounces + Sylgard (0.03 percent)

 

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 pounds

 

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

Captan – Rated ‘good/excellent’

Recent data suggest that Captan alone at a rate of 2.5 pounds per acre provides good to excellent control of cherry leaf spot disease (Table 1). MSU Extension initially investigated Captan alone for leaf spot control to provide growers with an early season leaf spot material that could be used between sprays of chlorothalonil if needed. Captan is a protectant fungicide that must be applied prior to rain to be efficacious. Captan does not provide back action and will not ‘burn out’ infections that have already occurred; hence, this material is best used in orchards that have no or very little existing leaf spot infection.

Copper – Rated ‘excellent’ for cherry leaf spot

Copper does not provide control of powdery mildew and is best for targeting cherry leaf spot at second or third cover. Growers who are planning to spray copper for cherry leaf spot should use caution as this material can be phytotoxic in hot conditions. 

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