ABSTRACT
Mango (Mangifera indica L) is one of the most important cash crops of northern Peru. Since 2003, adult mango trees (cvs. Criollo and Kent) located in Piura Province developed symptoms of dieback characterized by the death of twigs and branches in the tree canopy. Additional disease symptoms involved darkened, elongated lesions on the peduncle, causing an early maturation of the fruit, and in advanced symptoms, stem-end rot of fruits. Symptoms were frequent in the spring months (September to November) when the lesions expand rapidly. Diseased tissues from branches and fruits were collected and small pieces of necrotic tissues were surface disinfected and plated onto potato dextrose agar (PDA) with 0.5 g L-1 streptomycin sulfate. Plates were incubated at 25°C in the dark. All affected tissues consistently developed colonies with a white mycelium, moderately dense, and becoming olivaceous gray after 5 to 6 days. Pycnidia were produced on sterile mango twigs placed on the surface of potato carrot agar (PCA) after 10 days. Conidia were hyaline, guttulate, aseptate, measuring (15-) 18.5 (-22.5) × (4-) 5.2 (-7.5) µm. Conidia became olivaceous and developed one or two septa before germination. Isolates were identified as Neofusicoccum parvum (Pennycook & Samuels) Crous, Slippers, & A.J.L. Phillips (1). DNA sequences of the rDNA internal transcribed spacer region (ITS) and part of the translation elongation factor 1-alpha (EF1-α) genes were used to confirm the identification through BLAST searches in GenBank (ITS: 99% identity to Accession No. EU080928; EF1-α: 98% identity to Accession No. AY343367). Representative sequences of the studied DNA regions were deposited at GenBank (ITS: Accession No. FJ528596; EF1-α: Accession No. FJ528597). Pathogenicity tests were conducted on 18-month-old potted mango plants cv. Kent with two N. parvum strains (A4 and A5). A mycelial plug (3 cm in diameter) taken from the margin of an actively growing colony of each isolate was put in a wound made with a cork borer of the same diameter on the stem of each plant. Inoculation wounds were wrapped with Parafilm. Controls were inoculated with sterile PDA plugs. Ten replicates for each isolate were used with an equal number of control plants. Plants were maintained in a greenhouse with a temperature range of 22 to 28°C. After 4 weeks, mango plants showed necrotic stem lesions originating from the inoculation point affecting also the branches of the inoculated plants. No differences in lesion area between strains were obtained. No lesions developed in the control plants. Reisolations from necrotic tissues were successful and both isolates were morphologically identical to those used for inoculations. N. parvum was isolated from all symptomatic trees in all surveyed areas. This pathogen has already been reported on mango (2) and currently represents a serious problem in the mango-producing areas of Peru. To our knowledge, this is the first report of N. parvum affecting mango in Peru. References: (1) P. W. Crous et al. Stud. Mycol. 55:235, 2006. (2) B. Slippers et al. Mycologia 97:99, 2005.
ABSTRACT
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.
