Apple scab and fire blight management

By Terence Bradshaw

Apple scab- this one is simple, we likely have an infection period coming on Saturday that will likely release another quarter or so of the season’s entire ascospore load. Every orchard needs to be covered for this event. Overnight tonight through Saturday morning looks like good spray weather. This is a good time to apply a protectant plus a second material that has activity against powdery mildew rusts, like a DMI, QoI, or SDHI. Organic growers are mostly limited to sulfur (which is excellent on powdery mildew), although I have heard anecdotal reports that Regalia may have some effect on rust.

Fire blight- Many growers are concerned about this, but I still suggest cautious optimism should rule. Remember the four factors I referenced already that are needed for blossom blight infection to occur: bloom, wetting, heat before wetting (simplified as Epiphytic Infection Potential, EIP), and heat during the infection event. We have the first two in virtually every orchard in the state. However, we’re lacking the last two in most orchards in the state. EIP generally need to reach 100 before we consider the population infective, and we’re below that value in every orchard that I’m monitoring.

For orchards that have susceptible cultivars or a history of the dis, I get it- a streptomycin spray is a cheap insurance against this potentially devastating disease. But take this note- I have strep in the shed at the UVM orchard, I have aa block with Gala, Cortland, and Mutsu that gets fire blight fairly commonly, I’m spraying a scab fungicide tomorrow anyway, and I’m not applying strep. The heat just isn’t there for a Saturday infection, and every model is in agreement.

For those that wish to go ahead and treat because the cost of application is much lower than that of an infection occurring because I or the weather forecast is wrong, take heart in knowing that a) use of prophylactic streptomycin at labeled rates against blossom blight has never been implicated in the development of resistance to the antibiotic by the pathogen, and b) based on pretty extensive literature, there is extremely low likelihood of the use of streptomycin as a foliar-applied spray in orchards affecting resistance in human pathogens. If using strep, it should be applied at full rate with a nonionic surfactant, and applied on a full-row basis. Attention may be paid to the most susceptible cultivars and areas with a history of the disease. Again, I don’t see this event as serious enough to warrant attention to the whole orchard, so if you want to focus on spot-treating, that should be fine.

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