Control of wilt and root diseases of Asclepias tuberosa

Chlorosis and wilt, followed by weak flowering or death, have been occurring in Asclepias tuberosa plants grown in Israel. Some roots have been rotted and tuberous roots had dark lesions. Binucleate Rhizoctonia and Pythium intermedium were detected as the primary pathogens. The objective of the present study was to optimize the chemical control of these pathogens: seed or post-emergence treatments and chemical control of Rhizoctonia were investigated in two separate field experiments. Inoculation of plants with both pathogens significantly reduced the number and weight of flowers as compared with the non-inoculated plants. However, seed treatment or post-emergence application of fungicides had no significant effect on the yield. The percentage of surviving plants, one year after planting, was higher in the non-inoculated plants, with no significant difference between the seed and post-emergence treatments. The recovery of Rhizoctonia from dead plants was higher than the recovery of Pythium, in all treatments, except in the case of seed treatment without inoculation. The various fungicides applied to the foliage of non-inoculated plants in experiment II had no significant effect on the number and weight of flowers, as compared with non-treated control. However, treatment with fludioxonil (Celest) resulted in more flowers than treatment with tolclofos-methyl (Rizolex), in the non-inoculated plots. There was no difference between these treatments in the inoculated plots. This may be explained by the relatively low efficiency of the inoculation. The percentage of surviving plants, one year after planting, was similar in all fungicide treatments and in the control, without significant differences between inoculated and non-inoculated plots. The recovery of Rhizoctonia from dead plants was lower in all treatments than the recovery of Pythium, in spite of the artificial inoculation with Rhizoctonia in part of the plots. It is possible that the climatic conditions during the growing season were more favorable to Pythium, or that the effects of the various treatments will be expressed only during the second year of growth