Author : Vicha Sardsud

Summary : Inoculation by placing the fungus in a 5-mm diameter and 8-mm deep hole was successfully employed in these studies. Mycelial agar disc induced disease development faster than spore suspension. The mycelia could not penetrate through the well-formed periderm of sweet potato storage roots inoculated 12 days after harvest. On newly harvested storage roots with loosely formed periderm, the mycelia invaded intercellularly and were restricted in the cork layer. A periderm was formed in response to the penetration. This periderm seperated the healthy tissues from the penetrated cells and some cortical cells which collapsed ahead of mycelial penetration. Penetration both inter-and intracellularly through wound was first observed at six hours after inoculation. The fungus penetrated and concentrated on the xylem vessel, anomalous cambium, and vascular cambium layers. The pycnidia were formed at the cork cambium layer at about four days after inoculation and pycnidiospores were released after six days. On resistant varieties, the infected areas were limited by a wound periderm which was formed in response to injury and infection. Moist chamber was needed for disease development. Incubation for 48 hours in moist chamber and exposure under room condition for 24 hours were recommended for disease induction. Disease development was favored at 30 degree C. The storage roots became more susceptible when inoculated at six or more days after harvest than when they were newly harvested or at three days after harvest. Injured surface of storage roots inoculated three days or more after harvest formed a wound periderm which protected the healthy tissues from mycelial penetration. Sweet potato storage roots (var. Minolo) harvested at five months were more susceptible than those harvested after four or six months. From 39 varieties of sweet potatoes harvested after four months, only four varieties were found to be resistant. At five months old, there was no resistant variety and only two varieties were moderately.