Resistance of Nineteen Major Caladium Commercial Cultivars to Pythium Root Rot

Pythium Isolates

Three P. myriotylum isolates, 97-439B, P1, and P2, were collected from rotting roots of field- or greenhouse-grown plants, and their pathogenecity was tested on three cultivars, 'Florida Elise,' 'Florida Fantasy,' and 'Florida Red Ruffles'. All three isolates were found to be pathogenic and very aggressive on caladium roots. When tissue-culture-derived plants of these cultivars were inoculated with Pythium oogonia at a density of 10,000 per mL, root rotting and lesions appeared within 3 days after inoculation. By the 10th day after inoculation, 30%–75% of the roots rotted. Rotting continued to develop on roots from 10 to 17 days after the inoculation but at a much slower rate. After this period of time, the majority of the roots were rotted. Difference in the mycelial growth rate was noticeable among the isolates, but no significant differences were observed among them in inducing root rot on the three cultivars.

Resistance Evaluation

Two experiments were conducted to evaluate the resistance levels of 19 major commercial cultivars (Table 1). The resistance evaluation was based on artificial inoculation of tissue-culture-derived plants with Pythium oogonia followed by visual assessment of tissue rotting on roots. The tissue-culture plants were initially derived from shoot tips of caladium buds on tubers, and they were grown in 6-pack plastic cells (each with a volume of 67 ml) in greenhouses. When plants were approximately 8 weeks old and 6 to12 inches tall, with 3 to 6 leaves, they were inoculated with Pythium oogonia. Five mL of an oogonium suspension, from the above three isolates and adjusted to 1,000–6,000 oogonia per mL, were applied to the root media (vermiculite, ~50 mL in volume) surface of each plant. Control plants each received 5 mL of sterile water to their root media. To promote root rot after inoculation, continuous moisture saturation of the root media and warm day and night temperatures (90°F–99°F and 79°F–82°F) were provided. Ten days after inoculation, root rotting and plant leaf losses were visually rated for each plant on a scale of 1 to 5. For the root rot rating: 1 = 100%, 1.5 = 95%–99%, 2 = 85%–94%, 2.5 = 84%–55%, 3 = 35%–54%, 3.5 = 15%–34%, 4 = 5%–14%, 4.5 = 1%–4% of root tissue on the root ball surface rotted, and 5 = all root tissue appeared healthy without any rotting, compared to the control plants. To help categorize the levels of root rot resistance, the following scale was also used: =4.5: resistant; <4.5 but =4: moderately resistant; <4 but =3: susceptible; and <3: very susceptible. The scale for rating plant leaf losses was: 1 = all leaves wilted or lost; 2 = most (>50%) leaves wilted or lost; 3 = some leaves wilted (~30%) and showing chlorosis; 4 = almost normal, but showing chlorosis and some stunting; and 5 = all leaves growing normally, compared to the control plants. After evaluation, re-isolations of Pythium were made from representative roots of rotted plants to verify the presence of the causal disease agent. The 19 cultivars showed similar levels of resistance in both experiments; their data were averaged and are shown in Table 1.

Cultivar Differences

The majority of the cultivars (15 out of 19) were susceptible or highly susceptible to Pythium root rot. Within 10 days after inoculation, seven cultivars—'Fannie Munson,' 'Gingerland,' 'Gray Ghost,' 'Red Flash,' 'Red Frill,' 'Texas Beauty,' and 'White Queen'—had 35% to 94% of their root tissue rotted and ratings of 2.9 to 2.1. Eight cultivars—'Aaron,' 'Carolyn Whorton,' 'Florida Elise,' 'Florida Fantasy,' 'Florida Red Ruffles,' 'Florida Sweetheart,' 'Miss Muffet,' and 'Pink Beauty'—lost 15% to 35% of their roots to rotting and had ratings of 3.8 to 3.0. 'Red Frill' seems to be extremely susceptible to Pythium root rot (Figure 1); it lost more than 85% of its roots to rotting by 10 days after inoculation. Four cultivars—'Candidum,' 'Candidum Jr.,' 'Freida Hemple,' and 'White Christmas'—had moderate levels of resistance to Pythium root rot. Plants of these cultivars had only 5%–14% of their roots rotted by the 10th day after inoculation. These plants were maintained for an extended period (6–7 weeks after inoculation) and were able to maintain ratings of 2.1 to 2.9 for root rot and leaf losses and kept 10% to 50% of their roots healthy. Control plants for each cultivar did not show signs of rotting during the 10 day period of evaluation.

In addition to reddish lesions and rotting on roots, several leaf symptoms appeared soon after inoculation. These symptoms included tan, necrotic blotches on leaves, especially along leaf edges; yellowing on entire leaves; epinastic petioles; and leaf wilting, collapse, and losses (defoliation) (Figure 2).

These symptoms occurred as early as 3 days after inoculation and became more severe 7 days after inoculation. When a rating of plant leaf losses was made on a scale of 1 to 5, leaf losses were highly correlated to the severity of root rotting. Regression analysis showed a linear relationship between plant leaf loss and root rot severity. The four cultivars that had moderate levels of resistance to root rot maintained an average leaf loss rating of 4.0, while the eight susceptible and the seven very susceptible cultivars maintained an average leaf loss ratin of 3.4 and 2.8, respectively (Table 1).

Literature Cited

Deng, Z. D., B. K. Harbaugh, R. O. Kelly, and T. Seijo. 2005. "Pythium root rot resistance in commercial caladium cultivars." HortScience 40(3):549–552.

Hartman, R. D. 1974. "Dasheen mosaic virus and other phytopathogens eliminated from caladium, taro, and cocoyam by culture of shoot tips." Phytopathology 64:237–240.

Ridings, W. H. and R. D. Hartman. 1976. "Pathogenicity of Pythium myriotylum and other species of Pythium to caladium derived from shoot-tip culture." Phytopathology 66:704–709.