Pythium root rot, caused by Pythium myriotylum (Ridings and Hartman 1976), is a very damaging disease to caladium plants (Caladium × hortulanum). Use of disease-resistant cultivars has been an effective and economically viable strategy for integrated management of major diseases in numerous crops. This strategy could be used to control pythium root rot in caladium if resistant cultivars could be identified or developed. In an evaluation of 19 major commercial cultivars, 'Candidum', 'Candidum Jr.', 'Freida Hemple', and 'White Christmas' were found to have moderate levesl of resistance to Pythium. To find more resistant cultivars, 23 additional commercial cultivars were screened using the same Pythium isolates, inoculation and evaluation procedures as described earlier (Deng et al. 2004).

Eight cultivars, 'Florida Cardinal', 'Marie Moir', 'Pink Symphony', 'Postman Joyner', 'Rosalie', 'Rosebud', 'Triumph de L'Exposition', and 'White Wing', were highly susceptible to pythium root rot; 35–94% of their root tissue became rotted within 10 days after inoculation and they had a rating of 2.9–1.7. 'Rosalie' and 'White Wing', two lance-leaved cultivars, were the most susceptible to pythium root rot; they lost more than 85% of their roots to rotting within 10 days after inoculation. Twelve cultivars, 'Buck', 'Florida Calypso', 'Jackie Suthers', 'June Bride', 'Kathleen', 'Lance Whorton', 'Mrs. Arno Nehrling', 'Pink Cloud', 'Pink Gem', 'Scarlet Beauty', 'Florida Sunrise', and 'Florida White Ruffles', were susceptible to Pythium infection. They had 15–35% of their root tissue rotted within 10 days after inoculation and a rating of 3.9–3.0. Three cultivars, 'Etta Moore', 'Apple Blossom', and 'Florida Blizzard', showed moderate levels of resistance to pythium root rot. They scored a root rot rating of 4.0–4.3, and only 5–14% of their root tissue was rotted 10 days after inoculation. The inoculated plants of 'Etta Moore' and 'Florida Blizzard' were maintained for 2 more weeks after the 10^th day evaluation, and more than 50% of their root tissues still appeared healthy by the end of this extra period of time, while the inoculated plants of 'Florida Cardinal' and 'Marie Moir', two very susceptible cultivars, had all their roots rotted due to Pythium infection. P. myriotylum was re-isolated from the rotting roots from the inoculated plants. Control plants that received 5 mL of sterile water in the experiments did not show rotting symptoms during the evaluation.

Pythium infection caused several leaf symptoms on caladium plants, including discoloration and necrotic blotches on blades, epinasty and wilting on petioles, and whole leaf collapse (Deng et al. 2004). These symptoms were observed again on the 23 cultivars screened and they appeared as early as 3 days after Pythium inoculation. When the severity of these leaf symptoms was rated on a scale of 1 to 5, those three cultivars showing moderate root rot resistance maintained an average leaf loss score of 3.9, the 12 susceptible cultivars had a average score of 3.2, and the eight very susceptible cultivars had an average score of 2.6 (Table 1).

A linear relationship between root rotting scores and leaf loss scorings was present in the 19 major caladium cultivars evaluated previously (Deng et al. 2004). This relationship was also observed with the 23 cultivars in this study. This relationship highlights the importance of effective management of pythium root rot in caladium pot plant and field tuber production, as root tissue rotting caused by Pythium infection could result in rapid and dramatic losses of plant leaves, and consequently lower aesthetic values of pot or landscape plants and tuber yield of propagation fields.

Commercial production of caladium tubers in Florida generally starts in March each year and extends over a 7- to 8-month period, during which the presence of Pythium in combination with high temperatures and high soil moisture contents could cause significant losses to the caladium growers (Ridings and Hartman 1976). The three cultivars identified in this screening, plus the four reported previously (Deng et al. 2004), may serve as an integral part of the management strategy to control this soil-borne disease and reduce its damage.

The majority (35 out of 42) of the cultivars evaluated in this study and in the previous one (Deng et al. 2004) were susceptible or very susceptible to P. myriotylum. This highlights the need for development of new resistant cultivars for root rot disease control. Such need is particularly strong in several leaf shape/color groups, as four out of the seven cultivars of moderate resistance are in the fancy white cultivar group, one in the fancy red or pink and lance multicolor group each, and no resistant cultivars identified in the rest other four groups. The seven cultivars of moderate resistance may serve as an important source of resistance for breeding for new resistant cultivars or for further improvement of caladium resistance to P. myriotylum.

One of the recent releases, 'Florida Blizzard' (Harbaugh et al. 2002), has moderate resistance to Pythium (Table 1). Comparison of its resistance level to that of its parents ('Aaron' × 'White Christmas') reveals that 'Florida Blizzard' has a resistance level higher than the mid-parent value and is as resistant to Pythium as its resistant parent 'White Christmas'. This seems to indicate the potential of developing new cultivars that could be as resistant as these identified cultivars.