The Worst Pecan Scab Ever?

By Bill Goff

Like a fish story, where the fish gets bigger and bigger, this year the scab story gets worse and worse.  So I'll go ahead and elevate it to the extreme: this could be the worst scab year ever recorded on pecans in the southeastern US.

That notion isn't so far-fetched.  Paul Bertrand, University of Georgia plant pathologist, publicly stated this spring, I was told, that it was the worst leaf scab year in the 25 years he's been studying scab in Georgia.  Richard Grebel, who has scouted and grown pecans in the Albany, Georgia, area for longer than 25 years said that it was the worst nut scab he had seen.  Richard went on to say that he thought scab could reduce Georgia's crop by as much as 60%.

If it's the worst in 25 years, it could well be the worst ever, since scab strains attacking a particular cultivar get more virulent over time.  Twenty five years ago, the strains of the scab fungus were not as bad on Desirable and Stuart as they are today, so even a very wet year may not have resulted in scab like we are seeing now.

The disease is especially bad on Desirable, bad news for Georgia because that state is overweight, especially in young trees, in that cultivar. If scab gets to the point that Desirable cannot be grown profitably in Georgia, a major upheaval in the pecan industry would result.

There is speculation that perhaps a drop-off in effectiveness of the triazole fungicide group, which includes Orbit, Propimax, and Enable, has occurred following years of use of these chemicals. This chemistry is a part of some co-packs and of the new fungicide Stratego as well.  Maybe, the speculation goes, a strain of the scab fungus that is both virulent on Desirable and somewhat tolerant to these chemicals has developed.  By year's end, researchers should have a better handle on this.  We will all know a lot more about scab after this year is through.

A more likely scenario perhaps is that simply weather conditions have made scab incidence so much worse this year that application frequency and coverage were insufficient to keep up with the progress of the disease.  We had the rainiest May on record in Alabama.
 

Many growers had already sprayed 8 to 10 times with fungicides by mid-July, and still have damaging levels of the disease on Desirable.   Scab got started early and those who delayed, or couldn't get on because of the weather, their budbreak application had severe leaf scab from the very beginning of the season.  The very rainy spring and early summer combined with the high inoculum levels from the early leaf scab (Fig. 1) set up an extremely difficult situation.  It was raining so much so often that growers could not get in the orchards to spray even though they knew they needed to.  Aerial applications sprayed after-the-fact in some cases, as this was all the could be done, were not enough.

Those who cut costs and sprayed less are in jeopardy of losing their entire crop on susceptible varieties.  Compounding the situation is an especially bad year for downy spot disease which attacks the other major cultivar, Stuart.  I heard one report where Elliott, the most scab resistant of the widely grown cultivars, was sprayed little for scab, and didn't have scab, but had so much downy spot on the trees that they would defoliate by late summer.

I would advise growers overweight in Desirable to use this year as a lesson and hedge their bets.  This would be especially important and low-lying wetter areas where scab is worst.  A great many orchards need pollenizers for Desirable anyway.

The combination of Elliott and Cape Fear matches well with Desirable as pollenizers.  Elliott is highly resistant to scab, while Cape Fear is slightly better in most locations on scab than Desirable is.  Introducing these cultivars, which would require a separate strain of scab than Desirable, would lessen vulnerability to the disease.  So you could reduce your scab risk while at the same time improving your pollination.

We have a number of cultivars that are much better than Desirable with regard to scab incidence.  The standard cultivars Sumner and Elliott are good but have some flaws. Sumner is late harvest and is extremely susceptible the black aphids.  Elliott is early budbreak and freeze prone, alternately bears severely, and is quite susceptible to yellow aphids and to downy spot.  For commercial orchards, of the newer selections we are evaluating, McMillan, Carter, Jenkins, and Gafford are worth considering, especially where scab is a limiting factor.


Goff is an extension and research horticulturist at Auburn University