Tuesday, June 28, 2016

Monitoring pecan scab and making the decision to spray

     Last week I scouted the pecan grove for pecan scab. For the most part, our trees look pretty clean. I had a hard time finding scab lesions on any part of this year's new growth. However, when I walked over to trees of some extremely scab-susceptible cultivars, new scab lesions were present on some of the foliage (photo above). Dooley, Hirschi, and Maramec were covered with scab last year but this year, with a drier than normal month of June, scab lesions were confined to only certain leaves and on the upper leaflets of those leaves. This kind of uneven distribution of scab lesions on foliage can be explained by the simple fact that the scab fungus prefers to attack rapidly expanding new plant tissue. For a short period during the leaf expansion phase of spring growth, weather conditions became perfect for the release of pecan scab spores. When those spores landed on leaflets that were still expanding, they were able to infect the new tissue and create a fresh scab lesion. 

   When most growers think about pecan scab, a mental picture of blackened pecan shucks and small pecans comes to mind. However, scab can cause major problems with the foliage. Scab lesions can form on the rachis of the leaf and effectively cut off water and nutrient supplies to a leaflet. In the photo at right, The terminal leaflet of one leaf has fallen off due to scab while on the other leaf several leaflet have dried up and dropped off the rachis.
     Under growing conditions in SE Kansas, foliar scab is primarily a problem on severely susceptible pecan cultivars (Dooley, Hirschi, and Maramec).  Scab is so bad on these cultivars, I've begun the process of eliminating these cultivars from our grove.  

   The main reason for scouting for scab last week was to see if the fungicide we applied with our casebearer spray was effective for keeping scab lesions from forming on nuts. The photo at left shows a cluster of Dooley nuts. As of last week, small scab lesions had formed of leaves, leaf rachii, and the pedicle of the nut cluster. The nuts remained free of scab.
   On Monday, June 27th, we received an inch of rain from two separate rain showers (early morning and late afternoon). Our temperatures have remained warm in spite of the rain and the increased humidity has made ideal conditions for the spread of scab.

    Today, the day after the rain showers,  we started up the sprayer first thing in the morning to apply a fungicide to our trees (photo above). Since it looked like we had a short window of opportunity to spray this week (additional showers are forecast for Thursday, June 30th thru Sunday July 3rd), it was very important to get a systemic fungicide on our pecan trees to protect our crop. We applied Quilt Xcel at the rate of 19 oz/acre.