Apr 27, 2015
Penn State: Tree fruit diseases to manage during May

May is the battleground month for disease management: growers need to be on alert for apple scab, fire blight, powdery mildew, rust, cherry leaf spot, brown rot and bacterial spot infection conditions. Here is what growers need to keep in mind as they approach the month of May:

Apple scab

The release of the overwintering scab spores started the beginning of April and the number of available spores is increasing weekly, with the maximum number peaking (~20,000 +) soon around bloom through petal fall. Anticipate high numbers of spores being released throughout the month, so control must be vigilant during infection periods. During the peak time, consider using the following fungicides and be sure to practice resistance management:

FRAC Group 3 (Indar, Topguard, Inspire Super, etc – use at the highest labeled rate)

FRAC Group 7 (Fontelis, Luna Sensation, Luna Tranquility, Merivon)

FRAC Group 9 (Scala, Vangard, Luna Tranquility)

Tank mix with a protectant is a must: EBDC (dithane, mancozeb, ziram, or something similar), sulfur, or captan (Including an EBDC will also help control rust).

If you choose to use FRAC Group 11 (strobilurins), I recommend using one of the premix fungicides, such as Luna Sensation or Merivon. Due to the high incidence of strobilurin tolerance in apple scab fungal populations across the region, using strobilurins during peak apple scab conditions is cautioned.

“Spray By The Numbers”: Rotate at-risk fungicide.

If it rains after spraying, assume all fungicide protection, regardless of product, will be removed after 2 inches of rain. If we have a rainy period with few completely dry days, it is important to apply fungicides in the rain, particularly if it is a light rain (not a down pour) or misting. Consider applying an EBDC, sulfur, or captan, in the rain. Other information to remember:

Do not use Captan with oil or within 7 to 10 days of an oil application.

Do not apply Merivon with EC formulations or crop oils.

Do not tank mix Fontelis with Captan.

Fire blight

To review what was said in the April Fruit Times about managing fire blight:

Remember: blossom sprays protect only flowers that are open and only protect blossoms prior the infection event. Since blossoms do not open all at once, it is necessary to apply several sprays when infection conditions are frequent during bloom. It is important to be vigilant in monitoring weather conditions: average temperatures >60°F and wetting events (rain, heavy dew). Unfortunately, applying fungicides or plant growth regulators during bloom using high volumes of water can provide a wetting event necessary for infection when all other conditions for blossom blight are present.

Options available to protect blossoms and considerations to keep in mind:

Apply antibiotics as complete sprays and add an adjuvant or surfactant. Antibiotic sprays are most effective when they are applied the day before or the day after an infection event (within 24 hrs!).

Streptomycin is still the best option since it kills the bacteria and has partial systemic activity. Note: the systemic activity does not persist like fungicides and you have about a 48 hour window. Streptomycin still works in the Mid-Atlantic.

Kasugamycin is new to the market this year. It is different from streptomycin in that it reduces bacterial growth and reproduction, rather than killing it directly. Research in Michigan has shown this product has helped regions where streptomycin resistance is a big problem.

Oxytetracycline is an antibiotic that functions similarly to kasugamycin in reducing bacterial growth.

There is a 4 spray maximum when applying antibiotics and do not apply antibiotics after bloom. This is necessary for resistance management. Please do not think that just because 3 antibiotics are available you are able to apply 12 antibiotic sprays. Not only is it expensive, it is unnecessary and generally not a good idea.

Blossom Protect is a live yeast product that colonizes the flower and prevents the bad fire blight bacteria from entering the nectaries. Research on the West Coast indicates this is a very successful product for controlling fire blight. HOWEVER, this product is not as effective for our conditions on the East Coast at the present time. This is most likely due to the natural flower microbial community, which seems to prevent good colonization of the Blossom Protect. I tested this product last year during very high pressure conditions and only achieved ~40% control, whereas I had 76% control using streptomycin. In addition, the strep treated trees had significantly few instances of shoot strikes compared to the trees treated with Blossom Protect. Research is currently underway in Michigan to see what measures can be taken to make Blossom Protect work better in our conditions.

Although applying copper at bloom will kill bacteria, copper can cause fruit russetting and should be used with caution.

Be mindful of rattail bloom. All blossoms are susceptible to infection if the bacteria and conditions are present.

Shoot blight will be limited by applying the plant growth regulator, Apogee. The effect of Apogee occurs 10 -14 days after application and can be tank mixed with streptomycin. It is not a streptomycin replacement. Consider including Apogee in an antibiotic spray. Apply during late bloom when active shoot growth is 1 – 3 inches. Apogee will harden off shoots, which will make the shoots not susceptible to shoot blight.

DO NOT spray antibiotics post petal fall. A hail event is the exception.

Powdery mildew

Powdery mildew is considered a “dry weather” disease. The fungus does not like prolonged leaf wetness; high humidity is enough for the spores to germinate. Be mindful of dry weather and protect your trees. Fungicides for controlling apple scab, such as the FRAC Groups 3 and 7, are useful for controlling powdery mildew and be sure to tank mix with a broad spectrum protectant.

Cherry leaf spot

Cherry leaf spot is similar to apple scab when it comes to infection conditions: warm and wet. The cherry leaf spot fungus prefers moderately wet conditions (hours of leaf wetness), with temperatures above 60°F. Optimal temperature range for the spread of this fungus is between 60 to 68°F. Serious infection of a tree occurs in years with many rainy periods and cooler summers.

Research at Michigan State University showed that tart cherry trees are susceptible to cherry leaf spot earlier than anticipated. They determined that the bract leaves, which are the small leaves out prior to bloom and before vegetative leaves, are very susceptible to cherry leaf spot and can trigger a fungal epidemic. Bract leaves have natural openings called stomates, which provide the entry point for the fungus to enter the plant. Stomates in vegetative leaves on trees are not open and functional until petal fall, which has been the typical time to apply a fungicide to protect for cherry leaf spot. However, since the bract leaves have openings functional and available for cherry leaf spot spores, it is important to apply a protective fungicide prior to petal fall.

In Michigan, where tart cherry production is huge, the following is recommended to manage cherry leaf spot early:

Prior to shuck split, the recommended fungicide for cherry leaf spot management has been chlorothalonil (Bravo and generics). This fungicide is a multi-site protectant, is excellent for cherry leaf spot control and is not at risk for fungicide resistance development. Chlorothalonil has been the traditional workhorse to control cherry leaf spot at this time during the season. However, a recent and preliminary initial study has shown that chlorothalonil may have some negative impacts on honey bees, particularly when hives have been treated with insecticides for mite infestations.

Until further studies can confirm chlorothalonil’s impacts on honey bee colonies, the recommendation is if growers need to spray at or during bloom to protect open bract leaves, use one of the new SDHI fungicides (Luna Sensation or Merivon). This recommendation is intended to minimize early season cherry leaf spot infection while protecting our valued pollinators.

Brown rot

Rutgers has been posting frequent alerts about brown rot, so it is best to heed the following:

Blossom infections from the brown rot fungus can occur whenever pistils are exposed and a favorable climate exists. Infections can occur during any wetting period when temperatures are between 41 and 86°F. However, optimum conditions for infection occur with wetting and temperatures in the mid 70s. During long wetting periods (several days or more) blossoms can be infected regardless of temperature. Generally infections that occur when conditions are sub optimal are less severe. Blossoms and fruitlets will remain susceptible until the pistil desiccates (sometime between petal fall and shuck split). Keep blossoms protected with fungicides for blossom blight. Be sure to tank mix fungicides with a broad spectrum protectant for fungicide resistance management.

Bacterial spot

Norm Lalancette (Rutgers University) wrote a great article last year about bactericides for peach bacterial spot management. Here are some useful excerpts from that article:

Bactericide applications typically begin at late petal fall or early shuck-split stage and continue on a 7-to-14-day interval throughout the summer. Shorter application intervals should be used when rainy periods are frequent and temperatures range from 75°F to 85°F. A longer 14-day interval is acceptable during extended periods of dry weather.

The table below lists these products along with the recommended rate/A for cover sprays which provides 0.5 oz copper/A (considered to be 1X rate). Note that most of the product labels have much higher rates listed for either dormant application or for just two applications at first and second cover. These rates are too high for a multiple cover spray program and will result in excessive leaf injury and defoliation. Note Cuprofix Ultra and C-O-C-S are not included in the table because they are labeled for use only through shuck split and petal fall, respectively.

Some applications at the 2X level may be worth trying, particularly before major infection events. To determine the 2X rate, simply double the recommended 1X rates listed in the table above. For example, the 1X rate for Cueva is 25 fl oz/A, so the 2X rate would be 50 fl oz/A, which provides 1 oz of copper per acre. When applying at this higher rate, the shoots should be closely monitored for any increase in defoliation.

After applying two-to-three applications of one of these copper products, circular reddish-brown spots should be visible on the oldest leaves. This injury is normal, to be expected, and indicates that the proper amount of copper is present to provide control of the bacterial spot pathogen. Eventually, the leaves will drop, most likely from a combination of bacterial spot lesions and copper injury.

Defoliation will occur from the base of the shoot outward toward the tip. As long as the shoot continues to grow and produce new leaves, an adequate amount of photosynthetically (green) active tissue should be present to produce a crop of fruit. The goal is to apply enough copper to protect the fruit, but not so much that leaf drop exceeds leaf production. For this reason, copper bactericides should only be applied to trees with moderate to vigorous growth; older and/or slower growing trees may not produce new leaves at a high enough rate to compensate for leaf loss.

The application of the antibiotic oxytetracycline is also recommended for bacterial spot control of peach and nectarine. FireLine and Mycoshield are the two oxytetracycline products available for stone fruit. The recommended rate for both products is 1.5 lb/A at 100 gallons/A volume. As with copper bactericides, higher volumes are critical for good coverage. Given the short residual activity, ideal application timing is within 24 hours prior to an infection event. The incorporation of oxytetracycline into a copper program helps reduce defoliation since the antibiotic is not phytotoxic to foliage.

Click here for more information.

Kari Peter, Penn State University

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