Toward Breeding for Resistance to Fusarium Tuber Rot in Caladium: Inoculation Technique and Sources of Resistance

Fusarium tuber rot, incited by Fusarium solani, is the major cause of losses of tuber quality and quantity in caladium (Caladium ·hortulanum) during storage and production. To develop a reliable inoculation method for evaluating cultivar suscepti- bility to Fusarium tuber rot and identifying sources of resistance, the effect of temperature on the mycelial growth of F. solani in vitro and on tuber rot in vivo was examined.TheoptimaltemperaturewasthenusedtostudytheaggressivenessofF.solani isolates. The effect of temperature (13, 18, 23, 28, and 33 8C) on radial mycelial growth of nine F. solani isolates in vitro was determined, and all responded similarly to temperature variables, with optimal growth predicted to be at 30.5 8C. The relationship of these temperatures to disease development was then determined for the most aggressive F. solani isolate 05-20 and it was found that disease development in inoculated tubers was greatest at low temperatures (13 and 18 8C). Cold damage to tubers was observed at 13 8C; therefore, 18 8C was chosen for all future disease screening. The aggressiveness of nine isolates was tested on two caladium cultivars. Significant differ- ences among isolates were observed for the diameter of rotted tissue in both cultivars, indicating that choice of isolate was important for screening. Isolates 05-20 and 05-257 were highly aggressive on both cultivars. Tubers of 17 commercial caladium cultivars were inoculated withthree isolates (04-03, 05-20, and 05-527) and incubated at 18 8C. The interaction between isolates and cultivars was highly significant (P < 0.0001), indicating that cultivars were not equally susceptible to different pathogenic isolates of F. solani. Lesion diameters differed significantly (P < 0.0001) among cultivars/isolates and ranged from 9.5 mm ('Rosebud' and 'White Christmas' for isolate 04-03) to 23.9 mm ('Carolyn Whorton' for isolate 05-20). The cultivars were ranked for susceptibility to tuber rot within each isolate and the normalized total rank for the three isolates was used to place cultivars into four categories: resistant ('Candidum', 'Rosebud', 'White Christmas', 'Florida Sweetheart', and 'Aaron'), moderately resistant ('White Wing' and 'Red Flash'), susceptible ('Candidum Jr.', 'White Queen', 'Red Frill', 'Florida Cardinal', 'Miss Muffet', and 'Postman Joyner'), and highly susceptible ('Fannie Munson', 'Gingerland', 'Frieda Hemple', and 'Carolyn Whorton'). The availability of these sources of host plant resistance, aggressive isolates, and resistance assessment techniques will facilitate the development of new Fusarium-resistant caladium cultivars.