A March 7, 2016 blog post by Dr Dave Rosenberger at the Cornell University Hudson Valley Lab has reported that the first mature apple scab ascospores have been detected in leaves collected from abandoned orchards. While conditions in the Hudson Valley region have been ideal in recent weeks (daytime temperatures above freezing with enough moisture to keep leaf litter wet) to favour ascospore development in overwintering scab lesions, most regions in Ontario have experienced slightly more seasonal weather. However, the long-term forecast looks to be calling for quite the spring-like temperatures and precipitation in the upcoming days.
What could this early maturation of ascospores mean for apple scab control? The important thing to remember is that scab infection becomes a risk only when there is green tissue present. As trees push out of dormancy over the next few weeks, be prepared to begin a tight spray schedule with protectant fungicides (e.g., mancozeb, metiram and/or captan) as soon as this green tissue appears and conditions favour the release of any mature ascospores. Note that protectant fungicides do not provide effective post-infection or anti-sporulant activity. That means if sprays are applied in less than ideal conditions, ie., windy, alternate rows, or washed off in rain, the risk of scab infection is increased.
Keep in mind, apple scab management can begin before green tissue is even present as a means to reduce the amount of inoculum you’re starting out with in the orchard this season. Applying urea fertilizer at 45 kg/ha (mixed with 1,000 L water/ha) and/or shredding leaf litter with a flail mower are two practical and inexpensive methods to minimize scab pressure. Ideally, these practices should be done before green tip if possible, but there may still be some benefit if this is delayed shortly after.
For Dr Rosenberger’s full post, click here.
For more information on apple scab and other orchard pests, visit Ontario AppleIPM.