Scary Weather for Apple Diseases

Friday, May 16, 2014: The current weather pattern is generating high risk for a number of apple diseases:

Fire blight risks have been very high through much of the last week.  I have not gone back through our historical records, but I suspect that it has been a very long time since the Hudson Valley has seen this duration of continuous fire blight infection conditions.  The long infection window means that there may well be gaps in protection provided by strep sprays. (Hopefully everyone has protected at-risk orchards with at least two strep sprays by now!)  No one knows how many orchards in the Hudson Valley actually contain fire blight inoculum and/or have been exposed to inoculum by wind-blown rain or pollinating insects, but this may be the year that we find out!

Given the high potential risks for fire blight, this is a year when Apogee (6 oz/100 gal of dilute spray) may prove valuable as a second line of defense against fire blight. Unfortunately, you can’t wait to see if you get fire blight before making a decision on Apogee.  To be effective for fire blight suppression, Apogee must be applied now (applications earlier this week were even better) because it takes about 10 days after application before an Apogee application will result in increased resistance to fire blight. If Apogee is applied during bloom or at petal fall, then the increased resistance to fire blight will kick in at about the same time that the first symptoms of fire blight appear on blossom clusters that were not adequately protected by strep sprays. Apogee-induced resistance to blight can be a critical component in reducing secondary spread of fire blight after bloom and will provide time to remove blossom blight infections before the disease becomes epidemic.

Remember that you should NOT apply Apogee to Empire or Staymen and that a water conditioner must be added to the spray tank before Apogee is added.  Read the product label! Also, applications of Apogee may interfere with thinning, but I suspect that most folks would rather have a thinning problem than a fire blight problem.

Quince rust is favored by long wetting periods with intermittent or light rains (like yesterday!). Heavy rainfall tends to wash the spores out of the air. I recall visiting a farm in Columbia County back in the 1980’s where a fungicide gap allowed about 40% of fruit to become infected with quince rust. Thus, this “minor disease” can cause major losses if trees are left unprotected. Captan does not control rust diseases, and 3 lb/A rate of mancozeb is too low to provide complete control if there is an abundance of inoculum (i.e., infected cedar trees) in the vicinity of the orchard.

The good news is that quince rust is fairly easy to control in most orchards so long as fungicides are applied ahead of infection periods. Fungicides that control quince rust include mancozeb, Polyram, Flint, DMIs, and SDHIs (Fontelis, Merivon, Luna Tranquility, and outside of NY, Luna Sensation). These fungicides protect against quince rust, but with the exception of the DMI fungicides, they lack any significant post-infection activity.

Rally has been shown to have really incredible post-infection activity against quince rust (perhaps as much as 7-days).  Other DMIs may also have post-infection activity, although I am uncertain about the post-infection capability of Indar and Inspire Super against quince rust.  The latter two fungicides are more surface-active than Rally. The quince rust pathogen grows toward the center of the fruit after infection and therefore may outgrow the reach of Indar and Inspire Super within a few days after infection. Thus, while Indar and Inspire Super will control quince rust when applied as protectants, the duration of their post-infection activity is unknown (at least to me).

Where there is any concern about whether fungicide protection was adequate during the past week, including Rally in the petal fall applications should eliminate quince rust.

Apple scab is obviously a significant threat at this point, but hopefully everyone knows how to control scab and has fungicide protection in place.

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