Oct 30, 2015

Get a jump on disease management this fall

While the fruit harvest may be winding down, vigilance is still needed for managing tree fruit diseases, especially in preparation for 2016. Below is a review of what to have on your radar as you’re putting the 2015 season to bed.

Apple scab: Leaf removal is key

Thanks to the dry May, we dodged a bullet this year as far as a major scab headache is concerned. Our untreated Rome Beauty apple blocks at FREC had only 40% incidence of fruit scab—in contrast to last year where we saw 99% incidence. 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 important for a 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 to not 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. Like apple scab, sanitation is critical for effective management. Follow the same sanitation method for managing cherry leaf spot as you would for scab.

Fire blight: Time to start canker removal

This was another doozy of a season for fire blight and there is a lot of inoculum in the orchards thanks to cankers that formed as a result of numerous infections. Now is the time to get ahead of managing fire blight for next season. While leaves still remain on the trees, you have a better chance seeing the fire blight infections; consequently, growers are encouraged to prune out fire blight strikes now. Once the leaves fall off the trees, it will be difficult to easily prune out those strikes. If you don’t have time, another option would be to go through the orchard and spray paint or flag those diseased areas with a bright color so it’s not overlooked when it comes time for winter pruning. When pruning, be sure to remove all dead branches and cankers.

In addition, dead wood is a haven for infectious fungi for the next season. The trees have stopped growing, which means the bacteria have stopped growing. There isn’t a threat of spreading any bacteria since the disease isn’t active. Consequently, disinfecting pruning equipment is not necessary. For visible cankers, prune at least 12 inches beyond the canker or to the next limb.

Bacterial canker: Bordeaux mixture now

Cherry blocks that have had a history of bacterial canker are most likely oozing something fierce right now. 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 keep the disease in check 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.

Peach leaf curl: Control needed when the leaves have all fallen

The only time peach leaf curl can be controlled is when all of the leaves are off the trees, which will be pretty soon. Spores overwinter in bark crevices and around the buds. When the leaves have fallen from the trees, spray the trees with a fungicide, such as copper, lime sulfur, or chlorothalonil. This can be done late fall or late winter before bud break. As a result, you will reduce the chances of getting peach leaf curl next year.

Fruit rots: Check storages this winter

This was an interesting year for fruit rots. Weather conditions and large amounts of rain caused issue with the cherry season, and the rain and warm temps in August and September encouraged bitter rot, in particular, a little too much. We also had several rain events in the last 2 months that produced more than 2 inches of rain, washing off any residual fungicides. Bitter rot spores can lay in wait on the fruit without causing infection. Although your fruit may have looked clean when you harvested, it might be a different story after a period of time in storage. It will be important to scout your bins in cold storage and remove any infected fruit (see picture).

For infection prevention, remove mummified fruit from trees, both for stone fruit (brown rot mummies) and apples (bitter, black, and white rot mummies). Mummified fruit are loaded with spores, which will overwinter and be infectious for next season. Also be mindful of the mummified fruitlets that didn’t fall off when thinning occurred in the spring. We have noticed this in several apple blocks where there was a higher incidence of rot when mummified fruitlets were nearby.

– Kari Peter, Penn State Extension