Foliage and Fruit Susceptibility of a Pecan Provenance Collection to Scab, Caused by Venturia Effusa

Background Pecan ( Carya illinoinensis ) is an important native nut crop in the southern USA. In the Southeast, scab (caused by Venturia effusa ) is a major constraint to production, and can result in significant yield losses. The breadth of sources of resistance are poorly defined, and the genetics of resistance has not been established. The aim of this study was to assess the severity of pecan scab on foliage and fruit of the trees in a collection at Georgia, USA, of 875 native pecan genotypes from 93 families in 19 provenances in various states in the USA and Mexico. Methods The scab responses on foliage and fruit on each pecan tree was visually assessed in 2018 and 2019. Both a most severe measure (most severely infected leaf or fruit), and an overall tree rating was taken. Scab severity data were analyzed using a mixed linear model with means separation to explore provenance and family within provenance scab susceptibilities. Results Significant differences in scab susceptibility between provenances and among families within provenances were detected. Trees from provenances in Illinois, Kansas, Kentucky, Missouri, Mississippi, Tennessee and Texas, USA, showed the greatest resistance (based on mean scab severity) for both foliar and fruit scab. Assessments of scab on foliage and fruit gave similar results, as did the values for the most severe scab in the tree and the overall scab severity score. Of the 875 trees assessed, 201 trees had no detectable scab symptoms on foliage or fruit in either year. An additional 116 trees were free of scab symptoms on foliage but did not produce fruit in either year. Those provenances, families within provenances and trees with greatest resistance to scab most often originated from provenance locations with higher rainfall and consequently would be under selection to adapt to scab. Conclusions Identifying populations with most diverse resistance, and those most consistently resistant trees can serve as a resource to develop a better understanding of the scab resistance mechanisms. These natural sources of scab resistance coupled with molecular and genomics tools will contribute to the accelerated development of new pecan cultivars with durable resistance to scab.