Wednesday, September 16, 2015

Impact of scab on Peruque pecan

    Peruque is an old pecan cultivar originating in the Mississippi floodplain outside of St. Charles, Mo. Unfortunately, Peruque is scab susceptible and this year scab was especially difficult to keep under control. This morning, I passed by a Peruque tree and noticed all the nuts blackened by the scab fungus. I pulled off an handful of nuts to show you how the severity of scab infection impacts nut size and kernel fill.

    The photo above shows a series of Peruque nuts with shucks infected by pecan scab. With increasing infection coverage of the shuck, nut size is reduced. Scab does not shrink full-sized nuts to a smaller size. Rather, scab infection that occurs during the early portion of the nut sizing period inhibits further fruit expansion. 

    Using the same five nuts, I cut each in half to reveal the kernels inside (photo above). Although all five nuts look like they have well developed kernels, scab did impact kernel fill. Look at the thickness of the inner wall partition in each nut. The nuts with the least amount of scab has thin, compacted partition. As the amount of scab increases, the thickness of the partition increases. In other words, increasing scab decreases the thickness (and plumpness) of the kernel halves. In the end, heavily scab infected nuts will have far less percent kernel.
    The key to controlling scab on Peruque is to start applying fungicides right after pollination and then continue making regular applications all during nut expansion  (late June through July). The early sprays are the most important. As you can see from the nut photos above, light infections caused by infections that start later in the season have far less impact on nut size and kernel quality than infections that got started earlier in the year and covered the entire shuck surface.