Friday, September 30, 2011

Pecan scab in a dry year

    While I was walking over our pecan grove checking cultivars for shuck split, I was surprised to find pecan scab. I thought that a summer of heat and drought would eliminate scab disease from the face of the earth. No such luck! Here is a photo of scab infected 'Giles' pecan tree taken this year. You can see scab lesions (black spots) on the shucks, leaflet midribs, and leaf rachii. Infections on all those plant parts means we actually had multiple scab infection periods throughout the season. How do I know this by just looking at a photo?  Here is my thinking.

     The scab fungus infects pecan tissues that are rapidly expanding. As new leaves develop in the Spring, the leaf rachis grows rapidity first, followed closely by expanding leaflets. Now look at the photo at right. Note that scab lesions appear on the oldest leaflets at the base of the leaf while the younger leaflets near the end of the leaf are largely scab free. This tells me that we had a scab infection period early in the growing season but as conditions turned hot and dry new scab infections ceased.

      So if the hot dry weather stopped the spread of scab, why can I find scab on this year's nut crop (photo at left)? The answers lies in this summer's rainfall events. During the second week of August we finally received some much needed rain. Up until that point, the nuts of most cultivars had hardly grown in size. It was like the trees were waiting for a rain to develop normal sized nuts. When the rains came, we received multiple showers over 6 days. The trees responded with a burst of growth in nut size. The combination of a wet week and rapidly growing nuts provided ideal conditions for the spread of scab. However, since this infection period was so late in the season, the amount of scab observed this year will have little impact on shuck split and kernel quality. But remember, scab will be back next year and we should be prepared to control this important disease.