Friday, March 30, 2012

Where does pecan scab overwinter?

   Pecan scab is the most serious disease attacking pecan leaves and nuts. Every fall infected leaves and nut shucks fall to the ground often getting ground up by the action of pecan harvesters. So where does pecan scab come from every spring with the start of a new growing season?
    If you look carefully at the twigs of scab susceptible pecan cultivars, you'll usually spot overwintering scab lesions on the bark and even on bud scales. In the photo at right, the red arrows point to black, sunken scab lesions. Some lesions are large and quite noticeable while others are much smaller. All can produce the spores that lead to scab infections of new leaves and nuts.

   At the top of the twig pictured at left is last year's peduncle. The peduncle is the stem like structure that holds all the nuts in a cluster. Look closely at the photo and you can see where three nuts were attached last year. Also note that this peduncle is loaded with scab lesions (click on the photo to enlarge it for a closer look). Old peduncles are one of the primary sources of scab spores at the start of a new season.
   In Kansas, we have found that it cost effective to concentrate control measures on protecting the nuts from contracting scab. Our first fungicide spray is usually applied soon after pollination is complete. Additional applications may be necessary during wet summers.