Current weather forecasts predict an unstable period for this week. These conditions present risks for Apple Scab, Fire Blight and Bacterial Spot. There are predictions for showers mid-week and heavy rains predicted for Friday.
Apple Scab and Fire Blight
Growers have had plenty of time to cover up since the last infection period. If you have made a solid application in the last few days you can probably ignore the mid-week rains. However if your fungicide applications are more than a few days old it may be a good idea to put on a protectant fungicide since there has been a great deal of growth over the weekend.
A greater concern are the heavy rains predicted for Friday. If these predictions are accurate, Friday may be a significant scab and blight infection. We are nearing the end of the primary scab infection period. According to the model at NEWA, we are at about 93% ascospore maturity statewide. Light rains occurring mid-week would be a good thing since it would release most of those spores without completely removing protectants. The worst case scenario is rains mid-week fail to materialize and heavy rains on Friday remove all protection. If there is any more than an inch of rain mid-week, reapply protectants before the predicted rain Friday. If there is no rain mid-week and more than 2” rain Friday, come back in with Rally at the full rate plus a protectant. If you suspect the DMI’s are no longer working, any of the QoI’s (Sovran, Flint, Pristine, Luna Sensation) may reach back 48 hours, unless there is resistance to these chemistries also. Mancozeb products and captan may have 18-24 hours back action. The AP’s Vangard and Scala have 48 hours back action but these materials will do nothing for rust or mildew.
Regarding Fire Blight, in southern counties bloom is still present but mostly off. Northern counties may have more bloom present. The wetting mid-week may be significant if much bloom is present, but if there are heavy rains on Friday there could be trauma that results in shoot blight regardless of how much bloom is present. The NEWA Cougar Blight model is predicting extreme risk throughout the week. Open blossoms should be protected with streptomycin products, or oxytetracycline products. Where the bloom is off avoid these antibiotics unless there is a traumatic event. More information on scab and fire blight can be found at Dr. Dave Rosenberger’s blog.
Windy, wet, and warm. We’ve had a windy period today that could have injured some leaves. Temperatures have been hot and fruit is out of the shuck and at its most susceptible stage to bacterial spot. Therefore we are at risk for bacterial spot infection especially on highly susceptible varieties. Any wetting in the next few days may be an infection event. Even if the winds subside, heavy rains on Friday may still cause infection. Bottom Line: Now is the time for oxytetracycline products (Mycoshield or Fireline). These products have very limited residual but may be more rainfast than copper. Apply just before any wetting or immediately after.