![]() It is unlikely that you will overly stress trees during the few days that make up peak periods of highest fire blight risk. A little soil drying is beneficial, assuming trees are well watered when the really stressful time of year starts. However, studies have shown that trees are not nearly as stressed for water as we think they are in the Spring. Of course, your orchard may need some irrigation during May and June. 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. Keep early season irrigation, cover crop and weed growth to a minimum. The higher the RH, the higher the dew point, and the more likely your orchard will reach the minimum conditions for infection. 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. 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. Data gathered from leaf wetness sensors shows a wide variation in the presence and duration of 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. You cannot stop the rain from wetting blossoms, but you may influence the potential for dew. You can do little to affect the daily temperature in a way that will reduce the potential for blight infection. Heat drives the infection process, and moisture on the blossoms triggers it. With that in mind, I looked up what Tim Smith from Washington state, who developed CougarBlight, had to say about dew and fire blight. The dry weather in New England recently reminds me of Yakima, Wenatchee and Utah, where fire blight is often driven by dew. Avoid spraying during bloom if at all possible when fire blight risk is as high as it is now. Moisture can come from rain, heavy dew or even spray applications. Apple flower showing the stigma where fire blight bacteria reproduce and the hypanthium and ovary, where they infect. Once in the flower, bacteria can move internally to other parts of the plant, destroying tissue and causing blossom blight and shoot blight.įig. If the flower gets wet, bacteria are washed down to the base, the hypanthium, where they can infect the ovary. Our present weather is perfect for fire blight bacteria. On the flower’s stigma, with warm temperatures, they reproduce rapidly. As flowers open, fire blight bacteria are transported to open flowers, usually by insects. 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.īloom is critical. This probably means that multiple streptomycin applications will be needed over bloom. 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. ![]() To counter high bacteria populations, we recommend spraying streptomycin in early bloom, even without rain in the forecast. In plain terms, the risk of major fire blight infection is much higher this year than last, and last year was very bad. 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. ![]() I’ve never seen those levels at Cold Spring Orchard. The MaryBlyt infection potential (EIP) in Massachusetts and surrounding states is extremely high, exceeding 300. The only thing standing in the way of a major infection over the next week is dry weather. Conditions will be ideal for explosive bacterial development in open apple and pear flowers for at least a week. He does a much better job than I can do!Īll predictions indicate extreme to exceptional fire blight risk when apple and pear trees bloom over the next week. For $50 you get excellent advice all season long. Jon Clements gave me permission to copy the fire blight alert he sent out to his Healthy Fruit Newsletter email list.
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