More on Fire Blight

24 June 2014: As noted in my last post 10 days ago, fire blight is apparently pretty easy to find in the Hudson Valley this year.  Several people have told me that younger trees that were protected with streptomycin sprays are generally in pretty good shape, but many folks figured that older blocks of Empire, McIntosh, Stayman, Jonamac, etc. wouldn’t get  blight because they were considered less susceptible than younger trees and/or cultivars such as Honeycrisp and Gala that had blight problems in previous years (when all those Gala and Honeycrisp blocks were still young trees).

This year in the Hudson Valley, the number of degree-hours during bloom was more than double the minimum required to initiate blossom blight, and weather during bloom was very humid even on days when we had no rain. We have known for many years that when  fire blight models show heat unit accumulations at 2, 3, or 4 times the minimum required for infection, then blight tends to show up in more and more places, including in older trees and on cultivars that rarely show blight in years when degree-hour accumulations stay at less than 1.5 times the minimum threshold. Thus, I fully expected this to be a very bad fire blight year.  Nevertheless, I am surprised to hear that blight is showing up on mature trees on some relatively isolated farms that have not seen ANY fire blight for more than 20 years. I had suspected that fire blight inoculum was totally absent on many farms in the Hudson Valley because many farms never got blight even when they applied no streptomycin.  However, the wide spread occurrence of blight this year suggests that either I was wrong or else fire blight bacteria have an uncanny ability to migrate very quickly into areas where weather favors a fire blight party!

At this point, the only solution is to prune out the blight infections unless one wishes to try the combination of Double Nickel plus Cueva that I mentioned in my last post.  Incidentally, after my post of 14 June, several people told me that Double Nickel can work with copper because the active ingredients in Double Nickel are metabolites generated by bacterial growth in the fermentation facilities where this product is produced rather than the live bacteria in the formulated product. Apparently these bacterial metabolites are not degraded by copper.  In fact, the combination of Double Nickel and Cueva have apparently worked better together than they do independently, not only in the fire blight trial mentioned previously, but also in trials with several bacterial diseases of vegetable crops as well.

As I indicated in my last post, I don’t think that we will ever find a completely “safe” copper for apples, meaning that I suspect all copper formulations are capable of causing at least minimal damage to fruit of some cultivars if the copper is applied during summer under slow-drying conditions and/or with the wrong surfactants.  However, damaged fruit may be better than dead trees.  The latter is especially true if you have older trees with considerable blight immediately adjacent to a young block of a valuable cultivar (e.g., Honeycrisp, SweeTango, Snap Dragon, etc.) that was kept blight free via strep sprays during bloom.  Where old trees with active fire blight are next to younger trees that will continue growing into August, it may be worth risking fruit injury in the older trees by spraying them with Double Nickel plus Cueva so as to reduce the inoculum that will otherwise be blown into the adjacent young orchard. Young trees will remain susceptible to shoot blight infection so long as they are producing new terminal leaves. Spraying the young trees with copper may help, but the growing shoot tips will quickly outgrow the copper protection.  Applying copper to the adjacent older trees will help to reduce the inoculum that could otherwise result in heavy infection in the nearby younger trees.

How far will blight bacteria travel from infected trees?  That question cannot be answered because the distance is totally dependent on the kinds of storms that will move through the region over the next four to 6 weeks.  Gentle rains can generate a lot of airborne bacteria, but that bacterial aerosol probably will not move very far in the absence of high winds. However, a violent thunderstorm with gusty winds can easily carry inoculum a mile or more in sufficient quantity to destroy blocks of young trees that have the misfortune of being directly downwind. (I’ve seen this happen in the past!!) When blight is widespread in a region, it is almost impossible to totally prevent continued spread of fire blight that may occur during violent storms.  However, if weather over the next few weeks brings us only gentle rains, then using inoculum reduction (i.e., Double Nickel plus Cueva) in older trees immediately adjacent to younger blocks might prevent losses in the younger trees.  Incidentally, inoculum production will stop (or at least be drastically reduced) when those older trees set terminal buds. Even though young trees may remain susceptible to blight into mid-August because they will continue growing through summer, the risk of blight infection will drop off after adjacent older trees set terminal buds (assuming that those older trees are the primary source of inoculum).

Finally, a few more warnings about using copper during summer:

1.  NEVER combine copper with phosphite fungicides or acidifiers (e.g., LI-700) in the  spray tank. Phytotoxicity of copper is increased as pH is lowered.

2.  Yellow apple cultivars, especially Golden Delicious, seem to be more prone to injury from copper than most red-skinned cultivars.

3.  Phytotoxocity potential of copper sprays can be reduced by applying the copper only to dry leaves under fast-drying conditions and with low volumes of water (50-80 gal/A for medium sized trees) so that the spray droplets dry quickly and do not coalesce on fruit surfaces.

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