Kernel Breakdown Appears when Hybrid Castanea Cultivars Are Pollinized by Castanea mollissima

Abstract

Chestnut rots are caused by various fungal species which infect the nuts during the growing season and may be indirectly involved with insect infestation of chestnuts. Michigan-grown chestnuts appeared to have an internal decay or kernel rot problem. Chestnuts from the Castanea sativa. C. crenata hybrid cultivar 'Colossal', widely planted throughout Michigan, have been the most susceptible while chestnuts from C. mollissima trees do not appear to be affected. Thirty-to-forty percent of the chestnuts showed decay at harvest in some orchards growing 'Colossal' trees while others showed very little or none. A fungus or other infectious agent has not been isolated or observed microscopically despite many attempts over several years to culture or observe a causal agent. We tested the hypothesis that the decay, which has been termed internal kernel breakdown (IKB), is a physiological problem promoted by particular pollen sources. Female flowers from 'Colossal' trees planted in three isolated plots in mid-Michigan were exposed to four pollen sources: 'Nevada' (C. sativa. C. crenata); 'Precoce Migoule' (C. sativa. C. crenata); 'Okei' (C. crenata. C. pumila var. pumila); and 'Benton Harbor' (C. mollissima), and a no-pollen control. After the chestnuts were harvested, the kernels were observed for IKB symptoms. No symptoms were found in chestnuts when 'Nevada' or 'Precoce Migoule' were the pollen sources; however, approximately thirty-five percent of trees pollinized by 'Benton Harbor' showed IKB symptoms. Only one percent of the chestnuts pollinized with 'Okei' pollen showed IKB. Re-evaluating the Michigan chestnut orchards, we found that orchards without IKB-afflicted 'Colossal' chestnuts had few or no C. mollissima pollen sources, and those orchards with 'Colossal' chestnuts showing IKB were primarily utilizing pollen from C. mollissima seedlings or cultivars.