Wednesday, February 4, 2015

Pecan scab on dormant twigs

    I spent some time this morning looking over the trees in our pecan scionwood block. While checking on the quantity and quality of the scions we will begin harvesting later this month, I noticed an abundance of pecan scab lesions on the twigs of a Hirschi tree (photo at right). When thinking about pecan scab, most folks picture pecan shucks covered with black spots. However the scab fungus can infect both leaves and twigs.
   During the winter months, you can check your trees for scab lesions on the one-year-old twigs. The amount of scab seen on the twig pictured at right is a good indication that Hirschi is very susceptible to the disease. All of those black lesions will be releasing spores next spring that will start new infections on leaves, nuts, and new shoots.

   For many scab susceptible cultivars, scab lesions are not as large or numerous as what I found on Hirschi. Pictured at left are twigs cut from two northern pecan cultivars. Note the small black spots on theses twigs. Although the number and size of lesions on these two twigs look similar, Chetopa nuts are only slightly susceptible to scab. In contrast, Peruque nuts are moderately susceptible and are often troubled by nut scab. In other words, you can not predict the severity of future nut infections by looking at twig lesions.

    While checking for scab lesions on twigs, I decided to look at Pawnee as compared to Mandan. When Pawnee was released in 1984 it was touted as a scab free cultivar. After testing this cultivar at our location in Kansas, we found scab did infect Pawnee nuts and required multiple fungicide application to control the disease.
  In 2009, the USDA released Mandan and promoted this new cultivar as being scab resistant. If you look at the twigs pictured at right, you will note that both Pawnee and Mandan have scab lesions. This tells me that Mandan is not scab resistant as promoted. However, we have yet to produce enough nuts on our young Mandan trees to determine how susceptible this cultivar is to nut scab.