Sweetpotato storage root rots: flooding-associated bacterial soft rot caused by Clostridium spp. and infection by fungal end rot pathogens prior to harvest

Abstract

Sweetpotato production in the southern United States is being threatened by a soft rot that develops in storage roots when fields are flooded and by an important post-harvest disease caused mainly by the fungi Fusarium solani and Macrophomina phaseolina. To identify the pathogens responsible for development of the soft rot, samples were collected from storage roots with soft rot from intentionally flooded fields and decayed tissue was streaked on plates of nutrient dextrose agar plus 0.05% cysteine and incubated anaerobically. Two distinct groups of Gram positive strict anaerobic bacteria were re-isolated from rotting storage roots. Endospores were observed in all isolates by differential staining. Genomic DNA was extracted from representative isolates of each group, LSU-B1 and LSU-B7, and the 16s ribosomal RNA region was amplified and sequenced. BLASTn analysis of the 1425 bp sequence of LSU-B1 resulted in 99% homology with Clostridium puniceum strain BL 70/20 from rotting Irish potatoes. Isolate LSU-B7 generated a sequence 1376 bp long, which resulted in 99% homology with C. saccharobutylicum strain P262. To determine how and when end rot pathogens enter sweetpotato storage roots, two greenhouse experiments were designed using tissue culture-derived plants free of F. solani and M. phaseolina. In one experiment, plants were grown in autoclaved soil and one month after transplanting, plants were inoculated at the soil line with either non-infested toothpicks or with toothpicks infested with each fungus alone or combined. In the other experiment, plants were grown in non-infested soil or in soil infested with each fungus alone or combined. Isolations were attempted from different parts of the plants. F. solani and M. phaseolina were recovered from roots, storage roots, and plant stems below the soil line, at the soil line, and five centimeters above the soil line in both experiments. This suggests that these fungi are capable of invading the sweetpotato plants and storage roots from infested soil, and systemically colonize the plant from infected plant propagation material, eventually reaching the storage roots. These findings indicate that infection with F. solani and M. phaseolina can occur prior to harvest adding crucial information to end rot disease control.