New Celery Disease Appears in California
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Above, early blight lesions are circular and gray and often are covered with a fuzzy, gray fungal growth. Late blight lesions, shown below, tend to be large and irregular in shape, often exceeding 2 or 3 cm (1 inch) in diameter. I On field grown celery, bacterial blight lesions are usually found on the lower, older leaves. Severe bacterial blight symptoms, shown here, were observed in fields that had been sprinkler-irrigated through production. New celery disease appears in California Steven T. Koike D Elizabeth L. Little o Andrew L. Bishop o Robert L. Gilbertson A bacterial disease of celery, in transplant greenhouses favor the diameter - water-soaked spots (le- caused by Pseudomonas syringae outbreak of plant diseases on celery sions) visible on the upper and lower pv. apii, has been identified for transplants. For example, celery late leaf surfaces. As these lesions ex- the first time in California. Bacte- blight, caused by the seedborne fun- panded, they were delimited by leaf rial blight develops on green- gus Septoria apiicola, can reach high veins, which gave them an angular ap- house-grown transplants and has levels on greenhouse-grown celery. pearance. In most cases, the lesions Plants infected at this early stage be- turned dark brown; however, under spread into production fields come significant inoculum sources dry conditions they bleached out and throughout coastal regions of the that later may cause economic losses. appeared as light tan necrotic spots. state. Although this disease can In 1989, unfamiliar disease symp- Lesions sometimes coalesced to cover be confused with fungal celery toms were observed on leaves of field- large areas of an infected leaf but were blights caused by Septoria grown celery in Santa Barbara County rarely observed on leaf petioles. No apiicola and Cercospora apii, it that did not match those associated fungal fruiting structures were ob- has several distinguishing charac- with the well-known fungal blight dis- served within lesions, but when exam- teristics. eases. The following year, similar ined with a light microscope, tissues symptoms were detected on field- from lesion margins consistently re- In California, celery is almost always grown celery and greenhouse celery vealed large amounts of bacteria. On planted in the field as transplants, transplants in Monterey County. Be- field-grown celery, lesions usually oc- which are produced in commercial cause of these observations, we con- cur on the lower, old leaves and only vegetable transplant greenhouses. ducted studies to identify the cause of rarely on new developing foliage. Transplants compensate for the un- the disease and to determine its inci- even germination of celery seed and dence in the field. Pathogenicity of the bacterium the slow growth of celery seedlings, The initial symptoms, found both We collected celery leaves with wa- yielding a more uniform celery crop in on transplants and in the field, were ter-soaked lesions from various loca- the field. However, certain conditions small - less than 1 cm (0.4 inches) in tions in California, then examined 32 CALIFORNIA AGRICULTURE, VOLUME 48, NUMBER 2 them for bacterial streaming by sec- sterile cotton swabs into the culture Runner, generated a stratified random tioning tissue from lesion margins, and rubbing the swabs onto leaves sampling pattern based on field di- placing sections in a drop of sterile that had expanded one-half to three- mensions and plant spacing. Field water on a glass slide and viewing quarters. For negative controls, plants Runner divided each field into uni- them with a light microscope. Bacteria were inoculated with nutrient broth form sectors and directed the operator were isolated from lesions by macerat- plus Carborundum; as a positive con- to appropriate sampling sites in each ing lesion tissues in sterile distilled trol, the ATCC P. syringae pv. upii sector. At each site, we rated 20 plants water and streaking the fluid onto nu- strain was used. We also tested se- for the presence (one or more lesion trient agar, Kings’ medium B for lected bacterial strains isolated from per plant) or absence (no lesions) of pseudomonads and a semiselective diseased celery leaves, including P. the disease and entered the data into medium for Xanfhomonks. Pseudomonas syringae, P. fluorescens and a number of the computer. Field Runner then com- strains were identified to species using yellow-pigmented bacteria, for their pleted statistical analyses to determine LOPAT tests (levan production, pathogenicity on celery. Plants were incidence and distribution. We also Kovacs’ oxidase, potato rot, arginine maintained in a greenhouse, and 7 to collected leaf samples from selected dihydrolase and tobacco hypersensi- 10 days after inoculation, we rated dis- sites within each field, examined them tivity). For these biochemical and ease severity on inoculated leaves on for bacterial streaming and cultured physiological tests, strains were grown the following scale: them to detect P. syringue. on nutrient agar amended with 5% su- 0 = no disease reaction crose for 24 hr before testing. The hy- + = localized necrosis or chlorosis Bacterium isolated persensitive response was tested by in- around area of inoculation Bacterial streaming was observed filtrating a lo9 cfu/ml bacterial ++ = water-soaked brown lesions from tissues of all lesions examined, suspension into fully expanded leaves developing at and around the point of and a bacterium was consistently iso- of Nicofianu fabucum ‘Havana.’ Two inoculation lated on nutrient agar and Kings’ me- strains of P. fluorexens were used as +++ = large expanding brown wa- dium B. On Kings’ medium B, this negative controls, and an American ter-soaked lesions with entire area be- bacterium was fluorescent; using the Type Culture Collection strain of P. coming necrotic LOPAT biochemical and physiological syringue pv. upii (ATCC #8722) was Isolations were made from inocu- tests, we identified it as P. syringue used as a positive control. lated leaves as described earlier. (table 1).The isolates induced a hyper- The ability of selected bacterial iso- sensitive response in tobacco, which lates to cause disease in celery was Disease incidence indicates a plant pathogenic strain of tested asofollows: Isolates were inocu- To determine the extent of the dis- P. syringue. A number of other bacteria lated into nutrient broth and grown in ease in production fields, we evalu- were occasionally isolated from le- shake culture at room temperature for ated three mature celery fields in the sions, but not with the frequency and 24 hr. After shaking, Carborundum, an Salinas Valley in November 1989 for consistency of the P. syringue. abrasive powder used to make minute disease incidence and distribution. All Only the P. syringue strains were wounds in leaf tissue, was added to three fields had been transplanted pathogenic (table 1).On inoculated the culture. Ten- to 12-wk-old celery with celery grown in greenhouse fa- celery leaves these strains induced plants were inoculated by dipping cilities. A computer program, Field small water-soaked spots that turned dark brown and necrotic and eventu- ally coalesced. Bacterial streaming was observed from these lesions, and P. syringue was reisolated from lesion margins. Leaves inoculated with nutri- ent broth did not develop symptoms, whereas leaves inoculated with the ATCC P. syringue pv. upii strain devel- oped water-soaked brown lesions that were identical to those induced by the P. syringue strains isolated from Cali- fornia celery. The results of the Field Runner analysis suggested a high disease inci- dence. The three fields surveyed had mean disease incidences of 70%, 34% and 81% (table 2); the disease was ran- domly distributed in all three fields. However, in each field the symptoms were limited to the older foliage, and CALIFORNIA AGRICULTURE, MARCH-APRIL 1994 33 each plant generally had only one to spaced, watered with overhead booms other abiotic problems tend to affect three lesions. P. syringae was consis- at high pressures and mowed numer- only the upper leaf surfaces. Bacterial tently recovered from leaves having ous times to maintain a compact blight lesions do not contain fungal these symptoms. growth habit. These practices create an structures and fruiting bodies, and environment that is conducive to the they are generally not found on peti- Impact of northern bacterial blight development and spread of bacterial oles. This is the first report of a foliar blight. Late blight of celery, an important bacterial disease of celery in Califor- The impact of bacterial blight on the disease in California caused by the nia, and our results indicate that this celery industry is significant: It re- fungus Septoria apiicola, differs from new disease is northern bacterial duces the quality of transplants and bacterial blight in several ways. S. blight, caused by P. syringae pv. apii. incr'eases greenhouse disease manage- apiicola forms profuse numbers of Southern bacterial blight, caused by P. ment costs. In addition, severely in- small, spherical fruiting bodies called cichorii, has been a serious disease of fected transplants may show reduced pycnidia, which are scattered through- celery grown in Florida. But P. cichorii growth in the field, serve as inoculum out late blight lesions and can be eas- was not isolated in this study. sources for subsequent disease spread ily seen with a hand lens. Late blight Northern bacterial blight is com- in the field and lead to increased trim- lesions tend to be large and irregular mon in New York, Michigan and ming and harvesting costs. in shape, often exceeding 2 or 3 cm Canada, where it can cause wide- Symptoms of bacterial blight may (about 1 inch) in diameter. Further,S. spread blighting. The disease now ap- resemble those of the celery fungal apiicola readily infects celery petioles. pears to be distributed throughout the blights that are common in California, Early blight disease of celery, coastal regions of California as well.