Mar 13, 2017

Apply dormant sprays to manage fungal, bacterial diseases

I have to be brutally honest: I am nowhere near ready for the commencement of the 2017 season. Typically, I enjoy the breather between the marathon winter meeting schedule, which ended March 2, and the start of the season. Like last year, there is no rest for the weary: We are here. Whether I like or not. In the spirit of facing the music, here are some nuggets of wisdom to help folks get ahead for disease management in 2017:

Managing fungal and bacterial diseases

Growers are encouraged to apply dormant copper sprays on apples and pears for fire blight and scab; and on peaches (and other stone fruit) for bacterial spot and peach leaf curl. Ziram and chlorothalonil (e.g. Bravo) are alternatives to copper and will also control peach leaf curl. Since peach leaf curl can only be managed when leaves are off the trees, applications should be made prior to bud swell If using copper, growers will want to aim for 2 lb/A of metallic copper: pay attention to the % metallic copper equivalent (and amount of metallic copper per unit) listed on the label of the copper you use. Also, during dormant sprays, it is okay to mix oil and copper. Since minimal green tissue is present, the risk of phytotoxicity from the copper-oil mix is very low. Consequently, emergence of green tissue will want to be monitored when this combination spray is used.

Get rid of overwintering scabby leaves

I know it’s the burning question on everyone’s minds: have the apple scab spores started to fly yet? As of March 6, we started to detect the first mature ascospores, albeit in very low numbers (single digits). Depending on your location, very early green tip is being reported on some varieties. Considering the Polar Vortex will be gripping the region over the coming week, panic need not ensue, but the season is officially here whether we are ready or not. However, it is not too late to get your orchards in shape to prevent apple scab in 2017. Apple scab can be kept in check by reducing the number of available overwintering spores in last year’s leaves present in the orchard through sanitation.

Remember: Orchards are self-infecting when it comes to scab since spores can travel about 100 feet.

Options for reducing spores

• Apply urea sprays (40 pounds per 100 gallons of water per acre) to the orchard floor including the sod row middles. Spores need leaf tissue to survive and urea helps the breakdown of the tissue thereby eliminating the food source for the spores. If urea is applied, your spring nitrogen applications need to be reduced based on the amount of urea applied to the tree rows.
• Shred leaf litter using a flail mower or remove leaf litter by raking, sweeping, or vacuuming. Shredding leaf litter assists the decay of the plant material, as well as aids in the reorienting the leaves, thereby disrupting ascospore discharge.

Bacterial Canker

One disease to think about during the swings of extreme temperatures this time of year is bacterial canker. Bacterial canker, which is often referred to as a “cold weather disease,” is caused by Psuedomonas syringae bacteria that have proteins that allow water in plant cells to freeze at higher temperatures. Consequently, this causes damage to plant tissue sooner during the freeze process. When plant nutrients leak out of damaged cells, the bacteria will feed on those nutrients, causing the bacterial population to explode. Unfortunately, few management options exist, but reports around the region indicate the disease is kept in check using the Bordeaux mixture (bluestone plus hydrated lime). If acquiring bluestone or blue vitriol (soluble non-fixed copper) is a challenge, combining a fixed copper with hydrated lime should also be able to decrease the bacterial populations. Although there may be little to no green tissue yet on the trees, cankers may be present and the bacteria overwinter in buds. As previously mentioned, the goal is to reduce the bacteria available in the orchard.

Kocide for frost protection: the misconception

Last year during our early April freeze, there was some discussion through an email listserv about the use of Kocide for frost protection. There were a lot of unanswered questions that had to be taken into consideration about the anecdotal evidence with regard to the “success” of using Kocide to prevent frost damage. After rereading the Kocide label, it came to my attention folks were misinterpreting the information on the product’s specimen label.

In short, Kocide does not give frost protection the way people hope it could do.

Here is the low down:

If you read the Kocide 3000 label, applying Kocide to prevent frost injury is in relation to the protection of plants from damage caused by ice nucleating bacteria, such Pseudomonas syringae, which is the main culprit for bacterial canker as described above. If you are experiencing bacterial canker in your stone fruit, I’d follow the recommendation for applying Kocide for the purpose of disease control and subsequent damage prior to a frost event.

The Kocide 3000 label does not describe protection from frost injury when bacteria are absent. If Psuedomonas syringae bacteria are not present, the water in plant cells are not at risk for freezing at a higher temperature and plant damage will not occur.

I do not believe Kocide will offer any other kind of frost protection and would caution using it (or any copper) considering it may actually do more harm than good since the copper could cause further damage to frozen tissue.

For more information, visit extension.psu.edu/plants-tree-fruit.

— Kari A. Peter, Penn State University

Source: Penn State University Extension