Fire Blight information from Jon Clements and Dan Cooly

Hi fruit growers,

Jon Clements gave me permission to copy the fire blight alert he sent out to his Healthy Fruit Newsletter email list. You should all receive his messages. For $50 you get excellent advice all season long. He does a much better job than I can do!

Heather

FIRE BLIGHT ALERT!

Dan Cooley and Jon Clements

All predictions indicate extreme to exceptional fire blight risk when apple and pear trees bloom over the next week. Conditions will be ideal for explosive bacterial development in open apple and pear flowers for at least a week. The only thing standing in the way of a major infection over the next week is dry weather. The MaryBlyt infection potential (EIP) in Massachusetts and surrounding states is extremely high, exceeding 300. I’ve never seen those levels at Cold Spring Orchard. Heat units in CougarBlight are well over what we saw last year, at CSO a high of 613 in 2014, and a predicted high of 1088 this year. In plain terms, the risk of major fire blight infection is much higher this year than last, and last year was very bad.

To counter high bacteria populations, we recommend spraying streptomycin in early bloom, even without rain in the forecast. Dew is enough to trigger infection (see below), and though at worst only light dews are forecast over the next few days, it is probably better to be conservative and spray. This probably means that multiple streptomycin applications will be needed over bloom. I tried different streptomycin scenarios at CSO using MaryBlyt, and two applications, one on May 10 and a second May 12 kept EIP below the critical 100 level for the next 8 days, with the exception of May 11, when it was 109.

Bloom is critical. As flowers open, fire blight bacteria are transported to open flowers, usually by insects. On the flower’s stigma, with warm temperatures, they reproduce rapidly. Our present weather is perfect for fire blight bacteria. If the flower gets wet, bacteria are washed down to the base, the hypanthium, where they can infect the ovary. Once in the flower, bacteria can move internally to other parts of the plant, destroying tissue and causing blossom blight and shoot blight.

Fig. 1. Apple flower showing the stigma where fire blight bacteria reproduce and the hypanthium and ovary, where they infect. From Quan Zeng, Conn. Agric. Expt. Sta.

Moisture can come from rain, heavy dew or even spray applications. Avoid spraying during bloom if at all possible when fire blight risk is as high as it is now.

The dry weather in New England recently reminds me of Yakima, Wenatchee and Utah, where fire blight is often driven by dew. With that in mind, I looked up what Tim Smith from Washington state, who developed CougarBlight, had to say about dew and fire blight.

Heat drives the infection process, and moisture on the blossoms triggers it. You can do little to affect the daily temperature in a way that will reduce the potential for blight infection. You cannot stop the rain from wetting blossoms, but you may influence the potential for dew. When a period of abnormally high temperature comes and goes, without rain, blight outbreaks may occur in low, flat “frost pockets” or valleys in the orchard, where dew forms on flowers earlier and stays longer. Data gathered from leaf wetness sensors shows a wide variation in the presence and duration of dew. It appears that as few as two or three hours of wetting is sufficient to trigger infection if the four-day degree hour total is over the high risk threshold.

What may you do to reduce dew? The orchard microclimate may have a higher than ambient RH, due to irrigation, frost control, and the transpiration of trees and cover crop. The higher the RH, the higher the dew point, and the more likely your orchard will reach the minimum conditions for infection. Keep early season irrigation, cover crop and weed growth to a minimum.

If blossoms are present and the weather has been warm, the light wetting that may occur from sprinkler mist reaching higher in the tree or along the edges of irrigation zones can also trigger blight.

Of course, your orchard may need some irrigation during May and June. However, studies have shown that trees are not nearly as stressed for water as we think they are in the Spring. A little soil drying is beneficial, assuming trees are well watered when the really stressful time of year starts. It is unlikely that you will overly stress trees during the few days that make up peak periods of highest fire blight risk. Keep the intervals between irrigations as long as possible, and let the soil surface dry.

http://county.wsu.edu/chelan-douglas/agriculture/treefruit/Pages/Fire_Blight.aspx

Action items include:

Spray streptomycin (Agri-Mycin 17 or similar) at no less than 16 oz. per acre on small trees, at least 24 oz. per acre on larger trees (label rate is 24 to 48 oz. per acre) beginning as early as Saturday and likely need to be repeated 2-3 times over the next 5-6 days. Consider the fact you only get about 24 hours of streptomycin working backwards and forwards from the time of your spray. Once any open blossom is protected, it might be good for that blossom, but you have blossoms opening up over the course of several days (at a minimum). Include Regulaid (1 pt. per 100 gallons, do not concentrate) or other NIS in the first spray, but you might want to omit in subsequent sprays.

Give spraying susceptible varieties (Gala, Golden Delicious, Paulared, Cortland, etc.), blocks in heavy bloom, and young trees first priority when spraying streptomycin, however, given the current conditions and history of fire blight last year, all apple and pear blocks with open bloom should be treated.