2016 Volume 82 Issue 4 Pages 301-309
Ceratocystis ficicola causes serious wilt disease in many fig orchards in Japan. To elucidate the process of symptom development in the disease, we macroscopically examined external symptoms and assessed xylem sap flow in fig plants inoculated with C. ficicola. External symptoms were classified as follows: (1) no external symptoms (NS), (2) leaf wilt (LW), and (3) dead (D). Perithecial development of C. ficicola on stem segments was observed within a relatively narrow range, 5 cm above and below the inoculation site of each NS plants. In plants with LW, perithecial development was observed on segments from a wider range, 10 cm above and below the inoculation sites. In D plants, perithecial development was observed on segments from a narrower range than that of the LW plants. Xylem discoloration was observed near the inoculation site, in the range of 5 cm above and 10 cm below the inoculation site, in NS plants. The maximum percentage of xylem discoloration (discolored area/cross-section area) in each NS plants ranged from 1.3 to 16.4%. The range of discoloration extended 15 cm above and 10 cm below the inoculation site, and the maximum value of the xylem discoloration in each LW plants ranged from 16.5 to 52.4%. Discoloration similar to that in LW was observed in each D plants. Water conductivity was evaluated as the percentage of stem dyed pink with acid fuchsin solution absorbed by the roots. Water conductivity by NS plants was similar to that in the controls. Water conductivity was lower in each LW plant than in the NS, and the minimum dyed pink area on cut stem segments was less than 4.6%. The dyed area was barely recognizable in D plants. Leaves of LW or D plants did not have any dyed areas. Symptom development after inoculation of fig trees with C. ficicola can be explained as follows: (1) Xylem discoloration expands from the inoculation site corresponding to the expanding distribution of C. ficicola. (2) Xylem discoloration is correlated with xylem dysfunction. (3) When conductivity of the xylem falls to a certain threshold value in a cross section near the inoculation site, the water supply to leaves decreases, causing leaf wilting. (4) Extensive xylem dysfunction results in tree death.