A note from UGA Pecan Breeder Dr. Patrick Conner on a recent observation of scab on ‘Avalon’:
‘Avalon’ pecan was released in 2016 because of its high yields, good nut quality and excellent scab resistance in trial orchards. In the last five years ‘Avalon’ has become a widely planted pecan cultivar in Georgia and is being sold throughout the Southeast. Because its popularity is due in part to the reported high levels of scab resistance, recent indications of susceptibility to scab in some Georgia orchards are an important consideration for growers contemplating new ‘Avalon’ plantings and in managing existing ‘Avalon’ orchards.
This May I was notified of a five-year-old ‘Avalon’ orchard in northeast Georgia with a significant scab outbreak. The trees had never had fungicide sprays and had just begun to come into fruiting age. No scab had been observed in prior years, but scab was very evident on the first flushes of foliage throughout the orchard in 2025. Interestingly, a nearby orchard of ‘Avalon’ remains scab free. The reported levels of scab were high enough that I originally considered whether there had been a mix up in the nursery trees. However, DNA testing a few trees indicated that they were, indeed, ‘Avalon’. I travelled to the orchard in June, and scab was clearly visible on the leaves of most ‘Avalon’ trees (Fig. 1). After the initial consultation in May, the grower had begun applying fungicides and new foliage was much less infected. There is a fair number of fruit present on most trees, and they had little infection on them. However, they had not sized much, so it is too early to really know how much they will be affected. Talking with growers in southeast Georgia, I have also had reports of scab on ‘Avalon’ in this region. So far, it appears to be easily controlled with a moderate spray program but scab can be significant on unsprayed trees.
So, what has changed since the initial testing of ‘Avalon’ where little to no scab infection was reported? ‘Avalon’ has not “lost” its resistance to scab in the sense that the cultivar itself has changed. The ‘Avalon’ mother tree and test trees in our unsprayed trials remain scab free. However, one of the great challenges of breeding for scab resistance in pecan is that the causal organism, the fungus Venturia effusa, is not a monolithic organism. Rather it is composed of multiple strains of fungi, each with differing capacity to infect different pecan cultivars. Almost certainly these strains are also undergoing sexual reproduction and developing new strains all the time. ‘Avalon’ made it past our first screens and was resistant to the scab isolates residing in our test orchards. This was no small feat, more than 95% of our seedlings fail this initial test. However, no test location is going to have all scab strains represented, and new strains are likely developing all the time, so resistance levels are really just a snapshot of what the cultivar is doing right now to these strains of scab and cannot be considered a guarantee of future performance. A similar situation has recently occurred with the cultivar ‘Kanza’. We have always considered ‘Kanza’ to have very high levels of scab resistance. However, in the last few years scab has been reported to occur in separate orchards in multiple states. With this variability in the pathogen we unfortunately cannot predict what a cultivar is going to look like in terms of resistance five years from now, or in untested orchards. Even worse, the more a cultivar is planted the more likely it is to encounter a scab strain able to overcome its resistance. This is a frustrating condition for the breeder and grower alike, to be sure.
Given this situation, what are the recommendations for planting ‘Avalon’ going forward? Since we can only test new selections in a relatively small way both in terms of locations and years, we recommend they be planted on a trial basis until more can be known about their performance. In the last few years, ‘Avalon’ has become a commonly planted cultivar, and it might be time to slow down a bit until we get some more information. We now know that strains of scab exist that can readily infect ‘Avalon’ in some Georgia orchards. I think it would be unreasonable to expect that those strains won’t move around into other orchards if we continue to plant ‘Avalon’. The speed at which this will occur is less certain. I think the most important unknown factor we need to determine is how easily scab is controlled on ‘Avalon’. Right now, it appears that scab can be controlled with a few sprays. However, I would like to judge fungicide effectiveness on heavily fruiting trees in orchards with known virulent strains of scab. The orchard I visited last week will be an excellent proving ground for this. I would also like to see how well ‘Avalon’ produces in these newly planted orchards. Initial productivity testing of ‘Avalon’ was excellent, but those results need to be replicated in more grower orchards. Ultimately, I think those two factors, scab control and tree productivity, will determine the usefulness of ‘Avalon’ going forward. There are certainly enough trees in the ground that we should have much better handle with this in the next five years.
There are a couple of other points I want to bring up with this new information. First, growers really should consider applying a minimal spray program even on cultivars with reported good scab resistance and in young non-bearing orchards. This will be insurance against developing or spreading virulent strains of scab, while also helping to control minor diseases. Secondly, I would no longer consider ‘Avalon’ as a first-choice cultivar for homeowners or others looking to plant a tree or two and not spray for scab at all. I would go back to something like ‘Elliott’ that has a more proven track record of scab resistance over many years.
Cultivar recommendation in pecan is a difficult proposition because there are so many variable factors that come into play in the ultimate success of a given cultivar. Scab resistance is certainly an important factor, but is far from the only factor, and there are a lot of stops between a few sprays giving excellent control and needing to spray every 7 days to save your crop. Ultimately, success comes down to how many pounds of kernels per acre can be produced and at what cost. These variables are going to be different for different growers in different regions. Finding what works in your orchards will be a balancing act of sticking with the cultivars that have worked in the past and trying to improve as new options come online. The UGA breeding program looks forward to continuing to work with the industry to improve the options available to our pecan growers and provide thjiujem with the information needed to be successful.