Tips and tricks to mitigate 2018 disease problems this fall. Photo: K. Peter, Penn State.
The 2017 season is almost in our rearview mirror. Although the fruit harvest may be winding down, vigilance is still needed for managing tree fruit diseases, especially in preparation for 2018. No rest for the weary! The cooler weather is setting in and before you know it, the leaves will be on the ground. Below is a review of what to have on your radar as you’re putting the 2017 season to bed, especially as the leaves are falling.
Rot prevention for next season: Leftover fruit in the trees
For whatever reason, you may have some leftover apples hanging on the trees. If some fruit are rotting, they will eventually mummify and ultimately become little spore factories for next season. At FREC, we have been dropping leftover fruit by using Ethephon 3 pt/A + JMS Stylet oil 1 qt/A. If fruit rots have been problematic for you, this may be something to consider if any leftover fruit is hanging around in the trees and not easily able to be dropped by hand.
Apple scab: Leaf removal is key
If you noticed any scab in your orchard this season, you will definitely want to be proactive in mitigating problems for next year. Orchards are self-infecting when it comes to apple scab. Scab spores don’t travel very far, typically around 100 feet, and originate from old fallen leaves infected with scab on the orchard floor. Even if your fruit are clean of 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 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 in order 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 in order for leaves to absorb the urea. If the leaves have already fallen off of 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 scab spores for the next season by at least 90%. When there are no sources of scab on the orchard floor or within 100 feet, there is a very low risk of early scab infections. 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’ve seen many tart cherry trees already defoliated in August 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
Several growers experienced high incidence of peach leaf curl during the 2017 season despite their “dormant” fungicide application. We had an unusually warm February this year and I suspect that pushed some of the early varieties of peach and nectarine trees so the buds swelled enough for the peach leaf curl fungal spores to be protected from dormant sprays applied in March. Fungicide sprays do not control the disease when bud swell occurs or when leaves are present since the leaves (and swollen buds) protect spores in hard-to-reach spaces. Spores overwinter in bark crevices and around the buds. When the leaves have fallen from the trees the spores are exposed.
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 - March) to make that fungicide application. Spray the trees with a fungicide, such as copper, lime sulfur, or chlorothalonil. If you aren’t able 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 2018. For growers with severe cases of leaf curl on their trees during 2017, they may want to consider a fungicide application now and late winter.
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 again in the spring. 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. Additional information can be found in the September 11, 2012 issue of Rutgers Plant and Pest Advisory.