Phytophthora leaf blight caused by Phytophthora nicotianae on natal mahogany Trichilia dregeana

T. B. Pratt, W. R. Graves, R. T. McMillan Jr., R. M. Leahy


Water soaked leaf spots were observed on Natal Mahogany, Trichilia dregeana Harv. & Sound at a south Florida Nursery in the summer. The leaf spots were affecting 5% to 8% of the nursery stock and caused severe cosmetic damage to the crop. The actively expanding leaf spots were necrotic and water soaked. A species of Phytophthora was isolated on modified rye seed agar (RSA) by the Division of Plant Industry and determined to be Phytophthora nicotine van Breda de Hann. The Phytophthora nicotine spot of Natal Mahogany, Trichilia dregeana is typified by black lesions. Ten uninfected leaves of T. dregeana were placed in each of two plastic boxes lined with damp paper towels (incubation chambers ICBC). Five leaves were treated as controls and the other five were inoculated. Leaves misted with water prior to inoculation. Control leaves were inoculated with one, three-millimeter square (3 mm2) of uninoculated RSA. Treated leaves were inoculated with 3-mm squares of seven-day old cultures of the P. nicotianae on RSA. Leaves were misted again, covered and incubated in the ICBC at 25 °C (77 °F). After six days, all of the inoculated leaves that were showing actively expanding leaf spots were necrotic and water soaked. Phytophthora nicotine was reisolated from the inoculated leaves thus proving Koch’s Postulates.

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Proc. Fla. State Hort. Soc.     ISSN 0886-7283