Diana M. Henderson

Bacterial Blight of Leek: A New Disease in California Caused by Pseudomonas syringae

During 1996 and 1997, a new and damaging disease of leek (Allium porrum) was observed on greenhouse-produced transplants and field-grown plants in California. Symptoms were water-soaked lesions at leaf tips, which eventually expanded down the length of the leaf and resulted in brown, elongated, stripe-like lesions with yellow margins. Diseased leaves eventually wilted. A blue fluorescent pseudomonad was consistently recovered from lesions, and biochemical and physiological tests indicated that it was Pseudomonas syringae. Pathogenicity tests established that representative strains of this P. syringae induced disease symptoms in leek that were similar to those observed on leek plants in the greenhouse and field, and that this bacterium caused similar symptoms in onion, chives, and garlic plants. Representative strains were further characterized by fatty acid analysis, repetitive bacterial sequence-polymerase chain reaction (rep-PCR), and rDNA sequencing. Fatty acid analysis confirmed that these isolates were P. syringae, but did not provide a clear pathovar designation. Rep-PCR analysis revealed that all the California leek P. syringae strains had identical DNA fingerprints and that these strains were indistinguishable from those of known strains of P. syringae pv. porri. In addition, the rDNA sequence of the spacer region between 16S and 23S rDNA genes was identical among the California leek P. syringae strains and P. syringae pv. porri. Together, these results established that the new leek disease in California is caused by P. syringae pv. porri. P. syringae pv. porri was recovered from a commercial leek seed lot imported into California, which suggests that the pathogen was introduced in association with seed. Commercial leek production in California is favorable for development of this disease because transplants are produced in greenhouses with high plant densities, overhead irrigation, and mowing of plants.

A new root and crown rot disease of heath in California caused by Cylindrocladium pauciramosum

Heath (Erica capensis Salter) is a woody, evergreen plant used in Cali-fornia as a landscape shrub or ground cover. In 1997, a new root and crown disease was found in commercial nursery plantings of potted heath. A similar disease was found in 1998 on heath transplants being grown as liners. In both situations, roots were necrotic and crown tissue turned brown. Affected plants became gray-green in color, withered, and died. A Cylindrocladium species was consistently isolated from roots, crowns, and lower stems of symptomatic plants. Isolates were characterized by having penicillate conidiophores terminating in obpyriform to broadly ellipsoidal vesicles. Conidia were hyaline, 1-septate, straight with rounded ends, (30-) 45 to 55 (-60) × (3.5-) 4 to 5 μm, placing it in the Cylindrocladium candelabrum Viégas species complex. Ten single-conidial isolates produced perithecia with viable progeny of Calonectria pauciramosa C.L. Schoch & Crous when mated on carnation leaf agar with tester strains of Cylindrocladium pauciramosum C.L. Schoch & Crous (1). Matings with tester strains of all other species in this complex proved unsuccessful. Pathogenicity of 8 representative isolates was confirmed by applying 3 ml of a conidial suspension (3.0 × 105 conidia per ml) to the crowns of potted, 6-month-old, rooted heath cuttings that were subsequently maintained in a greenhouse (23 to 25°C). After 2 weeks, plant crowns and roots developed symptoms similar to those observed in the field, and plants later wilted and died. C. pauciramosum was reiso-lated from all plants. Control plants, which were treated with water, did not develop any symptoms. The tests were repeated and the results were similar. This is the first report of C. pauciramosum as a pathogen of heath, and the first record of this pathogen from North America. Reference: (1) C. L. Schoch et al. Mycologia 91:286, 1999.

Leaf Spot Disease of Spinach in California Caused by Stemphylium botryosum

Beginning in 1997, a new disease of spinach was found in the Salinas Valley, Monterey County, CA. Initial symptoms were leaf spots that were 2 to 5 mm in diameter, circular, and gray-green in color. Spots later enlarged, turned tan in color, and became dry and papery in texture, resembling phytotoxicity due to agrichemicals. Although fungal signs were generally absent from the spots, Stemphylium botryosum was consistently isolated and caused identical symptoms when inoculated onto 20 spinach cultivars. Three isolates did not cause disease symptoms when inoculated onto other crop plants representing 16 different genera and a Chenopodium weed species. A fourth isolate showed similar results with the exception of small leaf spots occurring on inoculated fava bean. Isolates produced a Pleospora herbarum teleomorph after 7 months incubation at 5°C. Preliminary experiments with cell-free culture filtrates indicated that phytotoxins apparently were not produced by these isolates. This is the first report of a foliar spinach disease caused by S. botryosum.

Bacterial blight of broccoli raab : A new disease caused by a pathovar of Pseudomonas syringae

Bacterial blight is a new disease of broccoli raab or rappini (Brassica rapa subsp. rapa) that has developed on commercially grown crops in the Salinas Valley (Monterey County) in California. Symptoms consist of small, angular, water-soaked flecks on lower foliage that are visible from both adaxial and abaxial sides of the leaves. These flecks expand and become surrounded by bright yellow borders. With time, multiple leaf spots coalesce and result in large, irregular necrotic areas, extensive leaf yellowing, and eventual leaf death. If symptoms develop on the uppermost leaves attached to the inflorescence, the shoot loses market quality and will not be harvested. Pseudomonas syringae was consistently isolated from symptomatic plants, and selected strains caused similar symptoms when inoculated onto broccoli raab test plants. Broccoli raab strains caused leaf spot symptoms on nine other Cruciferous plants, as well as on three grass species (California brome, oat, and common timothy). Conversely, broccoli raab was not infected by P. syringae pathovars coronafaciens, maculicola, and tomato. Broccoli raab strains were positive for coronatine toxin production. Fatty acid analyses indicated that the P. syringae from broccoli raab was most closely related to P. syringae pvs. coronafaciens and maculicola, but its distinct host range suggests that it may be considered a separate pathovar.

Outbreak of Leaf Spot of Saponaria Caused by Alternaria saponariae in California

Saponaria (Saponaria vaccaria [= Vaccaria hispanica]) is a Caryophyllaceae plant that is grown commercially in California as a cut flower. In 1998, a leaf spot disease devastated the commercially grown saponaria in coastal California. The entire saponaria crop was completely unmarketable because of extensive leaf spotting. Symptoms consisted of circular, brown, necrotic leaf spots with diameters up to 8 mm and concentric zones of lighter and darker tissue. Chlorotic borders developed around the spots. Conidia from leaves were obclavate, usually had 7 transverse and 1 to 4 longitudinal septa, and narrowed gradually toward the apex into a blunt-tipped, unbranched beak cell. The spore body measured 69 to 90 (to 119) × 17 to 21 (to 25) μm, with the distinctive beak cell 17 to 53 μm long. Conidia formed short chains on host tissue. The fungus was identified as Alternaria saponariae (Peck) Neergaard (2). For pathogenicity tests, six representative isolates were grown on V8 juice agar under fluorescent tube lighting. Potted saponaria were sprayed with either conidial concentrations (1 × 10e5 conidia per ml) or water. Plants were incubated in a chamber with a humidifier for 48 h and then maintained in a greenhouse (23 to 25°C). After 14 days, leaf spots similar to the original symptoms developed on all inoculated plants, and the pathogen was reisolated. Plants sprayed with water were symptomless. The experiment was repeated and the results were similar. Using the same isolates and method, we inoculated carnation (Dianthus caryophyllus), sweet William (Dianthus barbatus), and saponaria. However, disease developed only on saponaria. While A. saponariae on saponaria was reported previously in California (1), this is the first report to characterize the pathogen and document that isolates are pathogenic on saponaria but not on other commercial Caryophyllaceae hosts. References: (1) K. F. Baker and L. H. Davis. Plant Dis. Rep. 34:403, 1950. (2) P. Neergaard. Aarsberet. J. E. Ohlsens Enkes Plantepat. Lab. No. 3, 1938.

Black Root Rot, Caused by Thielaviopsis basicola, on Tomato Transplants in California

In 1997, greenhouse-produced transplants of tomato (Lycopersicon esculentum) exhibited stunting, yellowing of leaves, and lack of vigorous growth. Roots of affected plants had numerous small (2 to 10 mm long), brown lesions. Isolations from symptomatic roots onto acidified potato dextrose agar and TBM-V8 medium (1) consistently resulted in gray fungal colonies that formed catenulate, cylindrical, hyaline endoconidia and catenulate, subrectangular, thick-walled, dark aleuriospores. The fungus was identified as Thielaviopsis basicola (Berk. & Broome) Ferraris based on colony characteristics and conidial morphology. Pathogenicity was tested by producing endoconidial suspensions of representative isolates and applying them as root drenches to 2-month-old tomato (cv. Early Girl) plants in soil-less, peat-based potting mix. Sterile, distilled water was applied to control plants. After 14 days in a greenhouse, symptoms similar to those originally observed developed and the pathogen was reisolated from lesions on the roots. Control plants developed no disease symptoms. The test was repeated and the results were similar. This is the first documentation of black root rot caused by T. basicola on tomato transplants in California. Disease incidence reached as high as 50 to 60% in certain plantings. Reference: (1) J. N. C. Maduewesi et al. Phytopathology 66:526, 1976.