Penn State: Watch for apple scab, fire blight April 21-22
Open blooms need protection from fire blight infection.
We’re in various stages of bloom right now, which makes controlling for fire blight very important at this time. We’ve had great pollinating weather and pollinators have been flying around carrying not only pollen, but possibly fire blight bacteria, as well. Insects unknowingly deposit bacteria onto the stigma, where the bacteria will replicate like crazy during warm weather. When any kind of wetting event comes along (rain, high humidity, sprays), the bacteria get washed into the base of the flower from the stigmas, into the nectar pores, which is how the bacteria enter the plant. The temperatures have been perfect this week so far for bacteria to reach high numbers on the stigma, so it’s important to be mindful of any upcoming wetting event.
According to the fire blight models, areas that are experiencing bloom will be in an infection period April 21 – 22. Now is the time to apply streptomycin for protection on apples and pears in bloom. The addition of an activator-type surfactant (Regulaid, LI-700, NIS-80, Activator 90, etc.) will improve coverage and penetration of the flower structure, especially the nectaries where most infections occur. When applying streptomycin, it is effective for 24 hours before and 24 hours after a rain event. Blossoms have to be open for the bacteria to do damage; if you’re at pink and no blossoms are open yet, this is not the time to spray streptomycin.
Since the temperatures are forecasted to be mild over the next 10 days, bloom time may not be as fast as last year. As a result of new flowers opening, you will have to consider reapplying streptomycin in order to protect newly open flowers that were not protected the first go-around. For resistance management, do not exceed four antibiotic sprays a year.
In addition, consider applying the plant growth regulator, prohexadione calcium (Apogee or Kudos), to reduce the threat of shoot blight on vigorous trees of susceptible varieties. Prohexadione calcium causes shoots to start hardening off approximately 10-14 days after application, resulting in reduced susceptibility to shoot blight. Research has shown that prohexadione calcium can be safely tank-mixed with streptomycin plus a surfactant, allowing prohexadione calcium to take effect while there is residual protection from streptomycin. Prohexadione calcium is not to be considered a replacement for streptomycin sprays for blossom blight control.
For those seeking alternatives for fire blight management, options are limited, unfortunately. Right now, the best option is a low rate of lime sulfur.
Apple scab
This has been a very weird scab year so far. The few infection periods that have been predicted didn’t come to fruition; however, the Pittsburgh area has seen a few more infection events than other parts of the state. Depending on your location, the scab spore maturity within the overwintering leaves is from 30 – 94%. The dry weather has been working to our advantage so far and, if it continues, this could possibly be a light scab year again.
However, we’re not out of the woods yet. With the forecasted rain, we’re under the gun for another potential apple scab event April 21 – 22. Our average temperatures right now are perfect for scab conditions and only 6 hours of leaf wetness are necessary to cause an infection. Many options right now are available for control: fungicides in FRAC group 3; 7; and 9 will work well, along with the premixes (7+11; 3 + 9). Remember to tank mix with a broad spectrum (manzate or captan) and rotate the chemicals you use by FRAC group code to practice fungicide resistance management. IF you chose to use a product that is FRAC group 11, which is the strobilurin class, I highly recommend it is used as a premix considering fungicide resistance for this chemistry does exist in the region.
Resources:
Follow Penn State TreeFruit Doctor on Twitter (@drtreefruit) for the latest up-to-the-second alerts!
For more resources, visit the Penn State Tree Fruit Production website.
For specific recommendations for disease management, visit The Disease Control Toolbox via the Tree Fruit Production website.
When controlling for disease, weather and tree growth conditions need to be monitored at a local level within one’s own orchard. Before chemical products are applied, be sure to be in compliance by obtaining the current usage regulations and examining the product label. Product information can be easily obtained from CDMS.
Source: Penn State Extension