Reprinted from January 1977 PHYTOPATHOLOGY, Volume 67, No.1 Published by The American Phytopathological Society, Inc. Ecology and Epidemiology 3340 Pilot Knob Road, St. Paul, Minnesota 55121 The Effect of Visible and Near-Visible Radiation on Sporangium Production by Phytophthora cinnamomi G. A. Zentmyer and O. K. Ribeiro Professor and Postgraduate Research Associate, Department of Plant Pathology, University of California. Riverside, CA 92502. Research supported in part by NSF Grant PCM 74-19982. Accepted for publication II June 1976. ABSTRACT ZENTMYER, G. A., and O. K. RIBEIRO. 1977. The effect of visible and near-visible radiation on sporangium production by Phytophthora cinnamomi. Phytopathology 67: 91-95. The effect of light quality and intensity on the production production was stimulated in the near-UV by intensities up to 2 2 of Phylophlhora cinnamomi sporangia was studied using 100 IlW cm- • At 200 IlW cm- there was no significant cultures that were grown in a chemically-defined liquid difference in sporangium production in wavelengths ranging medium. Cultures were irradiated at intensities ranging from from the near-UV to infrared (312 nm-I,350 nm). These 2 2 31lW cm- to 200 IlW cm- , and at wavelengths between 312 results indicate that sporangium production by P. and 1,350 nm. Sporangia were produced at all wavelengths cinnamomi is light-variable, but not light-dependent. and intensities tested, and in darkness'. Sporangium Additional key words: soil extract, pea broth, sporangium morphology, light. Production of sporangia by Phytophthora cinnamomi between 312-440 nm, peak intensities at 405 nm, 370 nm, Rands has been the subject of many experiments, and a broad peak between 356-344 nm) suspended above numerous publications, and considerable controversy. single-strength window-glass tanks that contained 5% Two principal methods have been employed: use of a (w/v) CUS04 solution (6 em deep) to remove the mercury nonsterile soil extract (2, 3, 6, 11, 13, 14, 21, 22) and the line band at 1,025 nm in the infrared; Westinghouse blue axenic method developed by Chen and Zentmyer (7). (315-680 nm, peak intensities at 437 nm and 462 nm); Varied effects of light on the production of sporangia Sylvania green (380-650 nm, peak intensity at 537 nm); by several other Phytophthora spp. have been reported Sylvania red (600-800 nm, peak intensity at 662 nm); G. E. (1,4, 8,9, 10, 18, 19), but the effect oflight on sporangium Daylight (315 nm, 366 nm, 450 nm, 550 nm, and 625 nm); production by P. cinnamomi has not been clarified. Westinghouse Plant-Gro (368-800 nm, peak intensity at Zentmyer and Marshall (22) reported that sporangia were 662 nm, with lesser peaks at 450 nm and 500 nm). The produced in continuous artificial light, in continuous infrared source consisted of an Oscar 25-Waquarium darkness, and in alternating light and dark periods. lamp (700-1,500 nm, peak intensity at 950 nm). Emission Sporangium production was less in continuous light than of near-ultraviolet (UV) from this lamp was eliminated by in the other two systems; these tests were in nonsterile soil placing a sheet of Dupont Mylar W between the lamp and extract, using disks cut from PDA or V-8 agar cultures. the cultUfes. The apparatus used to irradiate cultures at a 2 Manning and Crossan (II), using a nonsterilized hemp very low light intensity (3 p. W cm- ), in the blue, red, and seed decoction, found that sporangia were produced by P. far-red regions of the spectrum was described previously cinnamomi only when cultures were exposed to (16). 2 continuous light or alternating light and darkness; At higher intensities (200 p.W cm- ), single-strength intensity and wavelength were not indicated. Schoulties window-glass tanks that contained distilled water (6 em and Baker (20) recently reported that "uniform light deep) were suspended under the blue, green, red, conditions" are important for reducing variability in Daylight, and Plant-Gro lamps to dissipate the heat and sporangium production by P. cinnamomi. maintain the temperature at26± I C. The near-UV lamps We examined sporangium production by P. had tanks that contained CUS04 solutions as described cinnamomi as part of a current project on the effect of above. The heat from the infrared source was dissipated quality and quantity of light on spore production and by employing a clear plastic sheet (4-mm thick), germination in the genus Phytophthora. This paper combined with a sheet of Dupont Mirror Mylar®(E. 1. summarizes these results. DuPont de Nemours & Co., Wilmington, Delaware), suspended 22 cm below the lamp. There was no MATERIALS AND METHODS interference with the transmittance of infrared wavelengths (800-1,350 nm), through either the plastic or The following l5-W fluorescent sources were used: the mylar sheet. Radiation intensities and spectral Phillips and Sylvania BLB lamps (spectral emissions distribution of all lamps used in this study were measured with an ISCO Model SR spectroradiometer using a Copyright © 1977 The American Phytopathological Society, 3340 remote probe attachment, calibrated against a 2,800° K Pilot Knob Road, St. Paul, MN 55121. All rights reserved. spectral standard lamp. Measurements were made at 10- 91 92 PHYTOPATHOLOGY [Vol. 67 25 nm (380-1 ,550 nm) wavelength intervals, by placing the acetic acid, and formalin (10: 1: I). Numbers of sporangia remote probe directly under the lid ofthe petri dish. Since per milliliter were counted using an eelworm counting the spectroradiometer does not measure wavelengths slide (Gelman Hawksley Ltd., Sussex, England). shorter than 380 nm, a McPherson spectrophotometer In some experiments, after the fungal colonies were was used in conjunction with a Hamamatsu type R106 washed with salt solution, 6 ml of a nonsterile soil extract photomultiplier to measure radiation in the UV and near­ was added to each petri dish before they were placed UV regions of the spectrum. Dark controls were placed in a blackened three-ply wooden box covered with two layers of heavy-duty aluminum foil. The intensities chosen throughout this study are within E the range of those recorded for the visible and near-visible E spectrum in an avocado grove in San Diego County, !':!'" California, over a period of I year. z>­ o For sporangium production, cultures of P. cinnamomi ...J o (isolates Pc97 Al and Pc40 A2, University of California, U Ul Riverside, stock culture designations), were grown for 5 ::J Z days on a synthetic agar medium (15). Preliminary tests ::J'" indicated no interaction between this medium and the "­.... <l ~ light sources employed. Disks 5 mm in diameter were Z <l transferred from the edge of the resulting colonies to 60- 0: o Q. mm diameter plastic petri dishes that contained 6 ml of Ul liquid synthetic medium. All dishes were incubated in o z darkness at 25 ± I C until colony diameters were uniformly between 22-25 mm in diameter (48-60 hours). The medium was then decanted from each dish and replaced with 10 ml of salt solution (7). Four washings at Fig. 2. Influence of light quality and intensity on sporangium one wash per hour were carried out, using 10 ml of salt production by Phylophlhora cinnamomi: number of sporangia solution per petri dish for each wash. After the final wash, produced in colonies initially grown in a chemically-defined 6 ml of salt solution (7) was dispensed into each dish and medium and irradiated at equal intensities of2oo p. W cm -2 under the cultures were placed (in quadruplicate) for 48 hours at specific regions of the spectrum. Lab light (G. E. Daylight 2 X 40- 2 26 ± 1 C under the different light sources described above. W at 1,200 p.W cm- ), was included for comparison. No A continuous light cycle was employed. One replicate significant differences were found among spectral regions (P = from each light condition was then removed and placed at 0.05). 9 C for 20 minutes to check zoospore differentiation and release from sporangia, and the other replicates _ 2500r-,,------------------------__________-, immediately were fixed in a solution of etha~ol (50%), E c ." ,; C E 2200 E N -'" 2000 z>­ b 3 1800 o be u CI) 1600 ::J z '"::J 1400 ...."- <l 1200 z « "a: 1000 a.0 CI) 0 800 z NEAR 8LUE DAY- UV LIGHT SPECTRAL REGIONS SPECTRAL REGIONS Fig. 1. Influence of light quality and intensity on sporangium Fig. 3. Influence of light quality and intensity on sporangium production by Phylophlhora cinnamomi: a number of sporangia production by Phylophlhora cinnamomi: number of sporangia produced in colonies initially grown in a chemically-defined produced in colonies injtially grown in II chemically-defined medium and irradiated at equal intensities of 100 p.W cm-2 under medium and irradiated al 45 jJ.W cm-1 in the. near-ultraviolet; 3 specific regions of the spectrum. See text for details of spectral p.W cm-2 in the blue and rar-red; 20 I'W cm-2 in the Daylight emission of each specific spectral region. Lab light (G. E. region; 421' W cm-1 in lbe Plant-Oro region; a nd r, 200 p.W cm- 2 2 Daylight 2 X 40-W at 1,200 p.W cm- ) , was included for in laboratory light (G. E. Daylight 2 X 40-W). Bars having the comparison. Bars having the same letter do not differ same letters are not significantly different from each other (P = significantly from each other (P = 0.05). 0.05). • January 1977] ZENTMYER AND RIBIERO: PHYTOPHTHORAjLIGHT EFFECTS 93 under the different light sources. In other experiments, all other wavelengths and in darkness (Fig. 3). At the high 2 the fungus was grown in 6 ml of either a dilute pea broth intensity (200 J.l.W cm- ), there were no significant or V-8 juice broth (7), before being washed with the salt differences in the production of sporangia at the different solution (7).