Author：Ann, P. J.^1,*, Tsai, J. N.¹, Wong, I. T.¹, Huang, J. H.¹, Lin, C. P.¹, Wang, S. T.², and Yang, C. K.³

Abstrct：

    In the spring of 2007, a serious disease on amaranth was noticed in several farms in the major amaranth production area in central Taiwan. Abundant oospores were found in the disease tissues. A species of Phytophthora was consistently isolated from disease tissues. Morphological characteristics of this organism did not match any reported Phytophthora species and the organism was named Phytophthora amaranthi. Pathogenicity tests and molecular characterization confirmed the identity of the organism as a new pathogen of amaranth and a new species of Phytophthora. The disease generally occurred from November to June of the next year with the most severe periods from December to next April. A total of 116 isolates from 59 fields were obtained. The pathogen was with host specificity; it only attacked amaranth species but not any other vegetable crops. In the fields, the Phytophthora attacked root, basal stem and leaf of the host and caused root and basal stem rot, and leaf blight, respectively. The ranges of temperatures for supporting pathogen growth were from 8 to 32℃ and for inducing disease from 12 to 28℃, with optimum growth and disease development at 20-24℃. A large number of oospores were formed in all the diseased tissues under all the temperatures suitable for disease development. The pathogen could survive as oospores in the soil for more than 2.5 years. All the isolates were slightly resistant to metalaxyl. They can growth at media amended with 10 ppm chemicals. The effects of chemicals on disease control were evaluated. The test chemicals included neutralized phosphorous acid (NPA), Fosetyl-aluminum, oxine copper and dimethomorph. Resulted showed that the best chemical for control of leaf blight was dimethomorph (4000x), followed by Fosetyl-aluminum (500x) and NPA (500 and 1000x), oxine copper was the worst (1500×). While for inhibition of root and basal stem rot in the green house and field studies, NPA (100 & 200x) and Fosetyl-aluminum (200x) were the best. The soil disinfection with steam for 30 minutes or 60 minutes can completely kill the pathogen in the soil depth of 20 cm and significantly reduced the disease incidence of seedling dumping-off and Phytophthora basal stem root. Meanwhile the effect of soil steaming on suppression of Phytophthora disease was better than the Pythium diseases. Result of field study showed that soil drenching with 1000 ppm and 2000 ppm NPA for 3 times could effectively inhibit Phytophthora disease as well as white rust disease of amaranth in the field, meanwhile the control effect of phosphorous acid could last to next plantation.

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¹ Plant Pathology Division, Taiwan Agricultural Research Institute (TARI), ROC
² Fengshan Tropical Horticultural Experiment, TARI, Kaohsiung, Taiwan, ROC
³ Agricultural Engineering Division, TARI, Taichung, Taiwan, ROC.
* Corresponding Author: pjann5039@gamil.com

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