Inoculation conditions for testing the resistance of sweet potato to soil rot disease using the lesions in tuberous root tissue sections as an index

Abstract

We examined inoculation conditions for testing the soil rot disease resistance of sweet potato, with the aim of establishing a test method that uses the lesions in tuberous root tissue sections. In inoculation tests with mycelial agar disk inoculum of the pathogen to two sweet potato cultivars and one breeding line with different levels of resistance（ ‘Kokei No. 14’, ‘Purple sweet lord’, and ‘90IDN-47’）, no significant differences were found in resistance characteristics. However, when the inoculation test was performed on tuberous root tissue sections that had been dipped into mycelial fragment suspensions of different pathogen concentrations and then buried in vermiculite with different moisture contents at 30℃, a significant difference in resistance could be observed at an inoculum suspension dilution of 1,000 times （v/w） fresh mycelial weight （approximately 105 CFU/ml） and a soil moisture content of 187.0% （v/w）. In this inoculation condition, the inoculated sweet potato tuberous root tissue sections started to exhibit lesions 3 days after inoculation, and severities of the disease particularly became more intense from 6 to 7 days after inoculation. As a result, it was possible to clearly distinguish the levels of resistance in these cultivars and breeding line.