Friday, June 27, 2014

Pecan scab present and ready to spread

    The other day, a grower called to ask if I had seen any signs of pecan scab this year. If you remember, we had a serious problem with scab last year and lost a large part of our 'Giles' pecan crop to the disease.
     During our conversation, he mentioned that he had looked over his nut crop and found no trace of the disease. The photo at right shows a cluster of 'Giles' nuts I cut from a tree this afternoon.   Everything looks clean and healthy. However, a closer look over the entire shoot revealed that scab was very much present on the tree.

    Below the nut cluster pictured above and on last year's wood, I found a series of small scab lesions (photo at left). These black, scruffy lesions are one of the primary locations the disease overwinters on the tree. Under conditions of high humidity and warm temperatures, scab spores can be released from these lesions and infect expanding leaves or nuts.

    I looked carefully over each of the leaves on the entire fruiting shoot and discovered the terminal leaflets of one of the leaves were spotted with scab lesions (photo at right). The fact that only three leaflets on the shoot had numerous scab lesions tells me that we've had a single, very-brief scab-infection-period this Spring so far.
    How do I know that? First, I know that the scab fungus most effectively colonizes plant parts that are rapidly expanding. Second, I know that the terminal three leaflets are the last leaflets to expand when a new pecan leaf unfurls in the spring. Sometime this past Spring, weather conditions promoted the release of scab spores from old lesions. At the same time, these three leaflets were at the perfect point in their expansion to become infected by the disease. Weather conditions must have dried up quickly after this initial infection period because later expanding leaves show no sign of scab.
    The nuts pictured at the top of this post have stayed scab free so far this year because we applied a fungicide along with our casebearer spray and the nuts have yet enter their period of greatest susceptibility--rapid fruit enlargement. However, we must remain on our guard because scab can spread quickly under the right weather conditions.