Friday, August 23, 2013

The many faces of pecan scab

Scab lesions on Giles pecan shucks
   Most growers recognize the black scruffy spots on pecan shucks of as being caused by the pecan scab fungus, Cladosporium caryigenum. This year we are seeing plenty of scab develop of susceptible pecan cultivars (photo at right). 
    However, scab lesions can develop on several plant parts other than on pecan shucks. Depending on the location of the infection, scab lesions take on several different looks. While photographing scab infections on Giles nuts, I stopped to record the appearance of scab on other parts of the tree.
Scab lesions on Giles leaf

     First, I looked at the leaves. The scab fungus had created lesions on the midrib on the underside of a leaflet (red arrows at left). These lesions were small, black, and limited to growing on just the midrib. Scab also infected the leaf rachis (inside red circles at left). These infection sites were not as intensely black as the midrib lesions and contained brown scruffy areas. Lesions on the rachis were much larger in size than the lesions found along the leaflet midrib.
Scab lesions on Giles stem

    On current season's wood, I found very-small, black depressions in the bark (red arrows, at right). Because of their small size, stem lesions are easily overlooked but these infection sites are extremely important for the spread of scab the following spring. Leaves and shucks fall to the ground every winter but stems and stem scab lesions remain in the tree's canopy ready to help spread scab to next year's nut crop.