Friday, July 17, 2015

Pecan scab sporulation

    Yesterday morning I walked through the orchard checking on the spread of pecan scab. At this point in the season, only cultivars that have a history of severe scab susceptibility are supporting a large number of scab lesions on the fruit. I found bad scab infections on Maramec, Hirschi, Dooley and the clone pictured at right, USDA 75-8-5.
    I collected several nuts from 75-8-5 and placed them in a plastic zip-lock bag to bring inside to photograph. I especially wanted to look at a scab lesion up close, using a dissecting microscope, to see if I could see the scab fungus sporulate. I held the nuts in the plastic bag overnight to provide the high humidity conditions needed to promote the development of pecan scab spores.   

    The photo at left shows the scab lesion located in the center of the nut pictured above. Under magnification the surface of the nut looks like a field of tan-colored pebbles on a base of green shuck. The scab lesion is black and appears somewhat fuzzy.
   In response to the high humidity, that formed inside the plastic bag, the scab fungus developed structures known as conidiophores or translucent stalks that stick up from the surface of the lesion.  These conidiophores give this scab lesion its fuzzy appearance.
    On the ends of each conidiophore, a chain of oval-shaped conidia form. These conidia are the spores are released into the air, spreading the fungus throughout the orchard.
    After this simple experiment in forcing sporulation from a scab lesion, I now know how quickly and easily scab can create new conidiophores and conidia out in the field. I am more than ever convinced that grafting scab resistant pecan cultivars is the best way to fight this disease