Friday, June 3, 2011

Planning for scab control

    After several years of wet summers, pecan scab is now on every northern pecan grower's mind. This disease infects the sucks of expanding nuts decreasing nut size, reducing kernel fill, and causing the shucks to stick tightly to the shell at harvest. The photo at left shows nuts that were severely infected with scab early in the season (top left), nuts that suffered little or no infection (bottom right) and varying degrees of infection in between.  We do have cultivars that have resistance to this disease (ie. Kanza, Lakota, Oswego, Major, Shepherd) but native pecans and scab-susceptible, improved cultivars require a fungicide program to prevent scab infections on nuts.

     In northern pecan groves, our pecan scab management program focuses on controlling scab infections on nuts. Pecan scab overwinters as sunken lesions on pecan stems that can lead to infection of  expanding leaves early in the season. These early infections on leaves are rarely troublesome in our area and usually occur only on the most scab susceptible cultivars (a Colby leaflet with dark scab lesions is shown above).
      We start our scab control program when pollinated nuts show the first sign of nut enlargement. This actually occurs a couple of weeks after pollination when the egg inside the pistillate flower becomes fertilized and the embryo starts to grow. It is interesting to note that pecan nut casebearer moths seem to lay their eggs only on nuts that have viable, growing embryos. Because these two pests attack nuts at the same time in the life cycle of the pecan, we usually combine our first fungicide treatment with an insecticide to control both scab and casebearer.
      Depending on weather conditions a second fungicide application may be required 2 weeks after the first application. Last year we sprayed our native pecans twice and high-value, scab-susceptible cultivars 3 times. I'll be posting this year's scab spray program as the season progresses.