Disease Update: Cleaning Up the Messy 2018 Season

There were many diseases affecting tree fruit growers in 2018. Sanitation will be very important while preparing for the 2019 season.
Disease Update: Cleaning Up the Messy 2018 Season - News

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Sanitation will be critical for managing disease for the 2019 season. Photo: K. Peter, Penn State

The end of the 2018 season is finally here. Currently, many folks in PA have several inches of snow on the ground; however, the snow probably won’t stick around too long depending where you are located. If you haven’t winterized your sprayer yet, you have time to get a jump start managing diseases for the 2019 season. Considering how many disease issues folks in the region encountered during 2018, sanitation will be imperative while preparing for the 2019 season. Below is a review of what to have on your radar as you’re putting the 2018 season to bed, especially as the leaves are falling.

Apple Scab and Marssonina Blotch

Leaf removal is key!

If you noticed any scab or Marssonina blotch in your orchard this season, you want to be proactive in mitigating problems for next year. Orchards are self-infecting when it comes to apple scab and Marssonina. Spores do not travel very far, typically around 100 feet for apple scab, and originate from old fallen infected leaves on the orchard floor. Even if your fruit are clean of apple scab this season, there is still a possibility of leaves being infected. Reducing leaf litter and the scab spores they contain is an important defense strategy for any good scab and Marssonina management program.

To reduce the available inoculating spores for next season, growers are encouraged to spray trees with urea as close to leaf drop as possible. Spores need the leaf tissue to survive the winter and urea assists in the microbial breakdown of the tissue: leaves with extra nitrogen stimulate the growth of these beneficial microbes. Using urea will reduce inoculum by 50 to 80% for the next season. Dissolve 40 pounds of feed-grade urea in 100 gallons of water (5% solution), spraying 100 gallons per orchard acre. Feed grade urea is recommended due to the ease of dissolving it in water. If you choose not to use urea, be sure your nitrogen comes from an ammonium source.

Good coverage of the leaves is desired for leaves to absorb the urea. If the leaves have already fallen off the tree, urea can also be sprayed to the fallen leaves on the orchard floor. Additional breakdown of the leaf tissue can be assisted by using a flail mower, which will chop up the leaves. Using urea and a flail mower can reduce spores for the next season by at least 90%. When there are no sources of spores on the orchard floor or within 100 feet, there is a very low risk of early infections from these diseases. Finally, late-season urea application does not compromise cold hardiness and has shown to help with tree health for the next season.

Cherry Leaf Spot

Leaf removal is key!

Cherry leaf spot and apple scab are very similar when it comes to infection: fallen diseased leaves are the culprit for creating spring infections. I saw many tart cherry trees already defoliated in August this year due to cherry leaf spot, which means there is already high inoculum pressure lurking on the orchard floor for next season. Like apple scab, sanitation is critical for effective management. Follow the same sanitation method for managing cherry leaf spot as you would for scab.

Peach Leaf Curl

Control needed when the leaves have all fallen

When the leaves have fallen from the trees the peach leaf curl spores are exposed. This is your only time to manage peach leaf curl. It is important to manage this disease as soon as the leaves have fallen this fall, especially on trees that are early varieties. Do not wait until late dormancy (late February to early March) to make that fungicide application. We have experienced 80°F in late February, which may cause early bud swell of these varieties.

Spray the trees with a fungicide, such as copper, ziram, lime sulfur, or chlorothalonil. If you are unable to apply your spray this fall, fungicides can be applied during late winter before bud swell and before any drastic warm-ups during late winter. As a result, you will reduce the chances of getting peach leaf curl in 2019.

Bacterial Canker

Cooler temperatures favor high bacterial populations

Unlike fire blight bacteria, the bacterial canker bacteria love this very cool, frosty weather and will be reproducing in very high numbers. You will want to avoid large dormant cuts since wounds provide an open door for infection for any bacteria around. The only successful control that has been found to manage the disease is repeated applications of the old Bordeaux mixture in September, October, and November and repeated in the early spring before bud break. Since the bacteria are active right now, they will be most susceptible to the bactericidal property of copper. Bordeaux mixture consists of hydrated lime (builders lime) and copper sulfate.

Since the leaves are falling off the trees right now, phytotoxicity of copper is not so much of an issue. To prepare tank-mix Bordeaux, use only good quality hydrated lime (calcium hydroxide) also called builders lime. The hydrated lime should be fresh, that is, not carbonated by prolonged exposure to air. Hydrated lime is stable and usually is readily available under several trade names. Magnesium lime, a mixture of Ca(OH)2 and Mg(OH)2, may also be used.

Bordeaux formulas are stated as three hyphenated numbers: 8-8-100. The first number refers to the pounds of bluestone (copper sulfate), the second number to the pounds of spray (hydrated) lime, and the last number to the gallons of water to be used. Thus, an 8-8-100 Bordeaux contains 8 lb copper sulfate, 8 lb spray lime, and 100-gal water. Have your tank ½ full of water, and the agitation turned on, add the copper sulfate or copper sulfate solutions, and then finally add the hydrated lime solution. For additional information, please refer to Fruit Notes, Volume 79, Fall 2014.

Fire Blight and Fruit Rot

Dormant pruning will be important

When pruning your fruit trees during late fall and winter, be mindful of cankered wood and mummified fruit lurking in the trees. Both serve as an overwintering source for bacteria and fruit rot spores for next season and should be removed as best as possible to limit future infections during the following season. Cankered wood, which is dead wood, has distinct characteristics that can be easily recognized when pruning. Be on the lookout for:

  • Localized roughened or cracked bark, especially around wounds, branch stubs, old pruning cuts.
  • Bark that is darker than the surrounding bark tissue, which is healthy.
  • Roughened/darkened areas appearing “wrinkled” or “sunken.”
  • Small pimple-like fungal spore-forming structures – may be red, dark brown, or black (depending on the fungus).
  • Wood-decay fungi, which attack dead wood and often appear as white protrusions growing out of the bark.

When pruning this season, it is best to pay extra attention to those orchard blocks with a known history of fire blight. This may require you to visit orchard blocks more than once, especially during different lighting of the day, to be able to spot cankers that could have been missed during the initial round of pruning. If trees were pruned during the season to remove fire blight strikes, you will most likely see a canker at the site where you pruned. Don’t forget to remove this canker.

When you see a canker, prune 6 – 12 inches from the canker’s visible edge into 2-year-old wood or older since older wood is more resistant to the bacteria. This will be easier in larger trees and more challenging in smaller dwarf trees. Judgment calls about whether to remove a tree or not will have to be made for smaller trees depending on the severity of the infection. Since the bacteria are dormant during the winter, disinfecting pruning tools is not necessary. When it comes to cankered wood, it is best to burn all tissue when possible to ensure destroying any overwintering bacteria.

Dormant Copper Applications on Apple Trees

Fall, spring, or both?

Many folks ask about dormant copper applications on apple trees during the fall, the spring or both times. I believe the best time for dormant copper applications on apple trees is before bud break in the spring. Not much is happening in the fall for fire blight and scab for the copper spray to be very useful on apple trees. Even if this was a bad fire blight year, a copper spray will not be very effective because the bacteria are dormant. Right now, focusing your resources on getting rid of the leaves is most important to control for scab; canker removal later in the dormant season will be critical to control fire blight.

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