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Bleeding Canker on Mesquite in Peru caused by Phytophthora syringae

Mesquite (Prosopis pallida (Wildenow) Kunth) is a drought-tolerant tree widely distributed in the northern Pacific Coast of South America. This species prevents soil erosion, provides shade, conserves prairies, supports bee nutrition, and provides fruits for human and animal consumption. Since the spring of 2004, bark lesions and bleeding cankers were observed on trunks and branches of 70% of declining mesquite trees in some parks at Ica in southern Peru. Badly affected trees were killed by the disease. Isolations were made from the edge of necrotic lesions of the inner bark and roots using PARPH medium (2) and incubated at 22°C for 7 days. A Phytophthora species was consistently isolated from lesions of 10 mesquite trees, and six pure cultures (PS-87-PS-92) were obtained by transferring hyphal tips and characterized. Colonies were stellate on V8 juice agar (VJA; 2 g CaCO₃, 200 ml of V8 juice, and 15 g of agar in 800 ml of distilled water), uniform to slightly radiate on corn meal agar (Oxoid Ltd., London, England), and knotty on PDA (Biokar Diagnostics, Beauvais, France). On VJA at 22°C, the average radial growth rate for the six isolates was 1.7 mm per day. Colonies grew slowly at 5 and 25°C with 0.4 and 0.7 mm per day growth rate, respectively. There was no growth at 30°C. Catenulate hyphal swellings formed on VJA and liquid media (1.5% sterile soil extract). Sporangia were persistent, ovoid to obpyriform, semipapillate with narrow exit pores (<5.0 μm in diameter), 32.3 to 39.7 × 21.0 to 27.2 μm, with a length/width ratio of 1.4:1 to 1.6:1. Sporangia were produced by cutting 5-mm disks from the advancing margin of a colony on VJA and adding disks to 10 ml of 1.5% sterile soil extract for 4 to 5 days at 22°C under fluorescent light. Isolates were homothallic with spherical oogonia, 32 to 35 μm in width with paragynous antheridia, and aplerotic oospores, 26 to 31 μm. These characteristics fit the descriptions of Phytophthora syringae (Kleb.) Kleb. (1). Sequences of the internal transcribed spacer regions on the isolates and comparison with other sequences in GenBank showed that they were identical to P. syringae (Accession No. AJ854297 from Citrus limon). In 2005, two methods were used to inoculate mesquite with two isolates. One method used two 20-mm-diameter branches of five 5-year-old mesquite trees where a 5-mm wound was made with a cork borer and a 5-mm block of the agar culture was placed under the bark and sealed with Parafilm. Another method used 10 4-month-old potted plants that received a 30-ml drench of a 10⁴ zoospores/ml suspension per plant. Controls received clean agar blocks and a sterile water drench for 10 control pots. Two weeks after inoculation, black areas and resinosis were observed around inoculated wounds. Inoculated branches produced cankers of 4.7 to 6.8 cm², 4 weeks after inoculations. Twenty days after inoculation of roots, wilting and root rots of seedlings occurred. No symptoms were found on the control plants. P. syringae was reisolated from the diseased branches and root rots and pure cultures were established. This test was repeated for both methods with similar results. To our knowledge, this is the first report of P. syringae in Peru and the first description of this pathogen on mesquite worldwide.

References: (1) D. C. Erwin and O. K. Ribeiro. Phytophthora Diseases Worldwide. The American Phytopathological Society, St. Paul MN. 1996. (2) S. N. Jeffers and S. B. Martin. Plant Dis. 70:1038, 1986.