Leafspot & Blight of Basil, Ocimum Basilicum, Caused By
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Proc. Fla. State Hart. Soc. 99:249-251. 1986. LEAFSPOT & BLIGHT OF BASIL, OCIMUM BASILICUM, CAUSED BY PSEUDOMONAS CICHORII Stephanie M. Burgess, Ora Bell Lawson, a yellow, butyrous growth on lima bean agar and gave a John W. Miller positive hypersensitive reaction on tobacco and tomato Florida Department of Agriculture & Consumer Services plants, and xanthomonadin pigment was extracted from Division of Plant Industry these isolates (3). This identifies the bacterium as Xanth P. O. Box 1269, Gainesville, FL 32602 omonas sp. Our objectives were to test the pathogenicity of P. cichorii and Xanthomonas sp. on basil, to compare the and A. R. Chase pathogenicity of P. cichorii isolates from basil and other University of Florida, IFAS hosts on Ficus lyrata Warb., to determine the'influence of Agricultural Research 6? Education Center moisture on disease development on basil, and to evaluate 2807 Binion Road, Apopka, FL 32703 the effectiveness of streptomycin and copper-maneb as protectants for disease control. Abstract. A new disease with severe leaf spot symptoms was observed on basil, Ocimum basilicum L. from a nursery in Materials and Methods southeast Florida. The disease is characterized by circular, Expt. 1. Healthy field grown seedlings were pulled from gray to black watersoaked lesions on the leaves and dark- the initial disease site at Pontano Farms. The seedlings gray sunken areas on the stems. The lesions coalesce, become were transplanted to 4-inch pots in a Gainesville irregular, dry, and dark-brown with age. Two bacterial types greenhouse and grown until the start of the experiment. were consistently isolated from the lesions. One type was In order to test the pathogenicity of P. cichorii and Xanth off-white, produced a green fluorescent pigment on King's omonas sp. on basil, five isolates of P. cichorii and two iso Medium B, negative on arginine dihydrolase medium and lates of Xanthomonas sp. from basil were grown for 24 hr positive for the oxidase test, identifying it as Pseudomonas on two lima bean agar (LBA) slants each. The resulting cichorii (Swingle) Stapp. The second type produced a yellow, bacterial colonies were suspended in 0.85% sterile saline butyrous growth on lima bean agar slants and gave a positive and spray inoculated (0.2 fl. oz./plant) on three basil plants hypersensitive reaction on tobacco and tomato. It appears to each using a glass atomizer and pressure pump at 1 psi. be a species of Xanthomonas. Pathogenicity tests on healthy The plants were placed in a mist chamber 24 hr prior to basil plants proved P. cichorii to be the cause of the disease. inoculation (30 sec mist 3 times/hr). Three plants each were High moisture levels greatly increased severity of disease de sprayed with 0.85% sterile saline only to serve as controls. velopment. Streptomycin and copper-maneb provided excel The plants were covered with polyethylene bags, arranged lent disease control when applied as preventative treatments. in three replications in a randomized block design and re turned to the greenhouse. Two days later the bags were Ocimum basilicum commonly known as Sweet Basil, is a removed from the plants and disease ratings were made spice of the family Labiatae. Native to tropical Asia, this by counting the number of lesions per plant. Reisolations herb is widely distributed in tropical areas. Basil is a small were done by crushing lesions in 2 ml of sterile tap H2O, plant, rather bushy and somewhat woody at the base with and streaking the resulting suspension on nutrient agar. glands in the leaves containing an aromatic oil. It is grown Characteristic colonies of P. cichorii and Xanthomonas sp. from seed as an annual herbaceous plant, although it is a were transferred to LBA following 24 hr inoculation perennial species. The foliage is normally removed and period, and the bacteria were identified by the above dried for domestic and commercial food and beverage criteria. flavoring (2). Florida, basil is a very difficult crop to grow because of its sensitivity to high humidity and cold temper atures and also its sensitivity to packing for transport to market areas (Personal communication, Mike Pontano, Pontano Farms, Lake Worth, Fla.). Plants are also grown for the fresh produce market in which leaves are used for spices and in teas. A new disease with severe leafspot and stem symptoms was observed on basil from a nursery in southeast Florida. The disease is characterized by circular, gray to black, watersoaked lesions on the leaves. The lesions coalesce, become irregular in shape, dry, and dark-brown with age (Fig. 1). In some cases, the petioles become necrotic and the disease advances into the stems causing sunken lesions. Two bacterial types were isolated from the lesions. One type was off-white, produced a green fluorescent pigment on King's medium B (4), was negative for arginine dihyd rolase production (7), positive for oxidase (6) and positive for hypersensitivity (5). These results identify the bac terium as Pseudomonas cichorii. The second type produced Fig. 1. Irregular, dark-brown leaf spots on Ocimum basilicum caused by Pseudomonas cichorii. Proc. Fla. State Hort. Soc. 99: 1986. 249 Expt. 2. The pathogenicity of P. dchorii isolates from Table 2. Effect of strain source on severity of pseudomonas leaf spot of Ficus lyrata caused by Pseudomonas dchorii. basil and other hosts was tested on F. lyrata. Single 4-inch tall plants obtained from commercial producers were were Strain established in steam-treated Canadian peat and pine bark Mean percentage of foliage (1:1 by volume). The medium was amended with 10 lbs. designation source with symptoms Osmocote (19:6:12 slow-release fertilizer from Sierra Control 0 ax Chemical Co., Milpitas, CA 95035 USA), 7 lbs. dolomite P14 Chrysanthemum morifolium 7.8 ab and 1.5 lbs. Micromax (micronutrient source, Sierra P110 Chrysanthemum morifolium 27.5 c Chemical Co.) per cubic yard of medium. Tests were per P80 Ficus lyrata 15.6 abc formed in a glasshouse with temperatures from 18 to 32° P105 Ficus lyrata 10.0 ab P109 Ficus lyrata 16.1 abc C and a maximum light level of 1500 ft-c. Plants were P89 Gerberajamesonii 20.6 be grown for 3 to 5 weeks and then placed under an intermit P28 Hibiscus rosa-sinensis 16.7 abc tent (5 sec every 30 min from 0800 to 2000 hr) aqueous P115 Ocimum basilium 16.7 abc mist. About 24 hr later, plants were removed, sprayed P101 Schefflera arboricola 17.8 abc ATCC type culture 10857 5.8 ab (three per treatment) to runoff with SDW alone or a bacte P78 rial suspension (1 x 108 colony forming units/ml), covered xMeans separated by Duncan's new multiple range test. with polyethylene bags and replaced under mist. The bags were removed after 3 days. At 14 days after inoculation, each of the four treatments were placed in mist chambers plants were examined for symptoms and suspect lesions and three plants of each treatment were put on were removed and treated as described above. Growth of greenhouse benches. The bags were removed after three bacteria on KMB was similar to that of the suspect bac days and readings were made using a rating system like terium used to inoculate plants and was considered a posi that in experiment 3. tive reisolation (A. R. Chase, unpublished). Expt. 3. The most virulent isolate of P. dchorii from Results and Discussion experiment 1, P86-919, was grown for 24 hrs. on six LBA slants to study the effect of moisture on disease develop Expt. 1. All isolates of P. dchorii caused circular to ir ment on basil. Bacterial cells were suspended in 0.85% regular dark gray lesions on the leaves of healthy basil sterile saline and spray inoculated on 12 basil plants (0.2 plants (Table 1). These symptoms were indistinguishable fl. oz./plant) in the same manner as used in experiment 1. from those caused by natural infection. No stem infection The plants had been misted for 24 hrs. prior to inocula was induced presumably because the stems were old and tion. The plants were separated into two groups; six in the woody at the time of inoculation. One isolate, P86-919, was mist chambers and six in the greenhouse with 2 sterile much more virulent than the others and was selected for saline controls and three replications in each group. The use in future tests. The two isolates of Xanthomonas were plants in the mist chamber were misted three times per hr not pathogenic on basil. Although a few lesions developed for 30 seconds duration each time. All the plants were following inoculations with isolate P86-1648 of Xanth covered with polyethylene bags. Bags were removed 2 days omonas (Table 1), this bacterium was not recovered in reiso later and disease readings made based on a scale of 0-10, lation attempts. Xanthomonas sp. and P. dchorii were reiso- where 0 was no infection, 1 was 1-10% of the leaves in lated from all plants that had been inoculated only with P. fected, and 10 was 90-100% of the leaves had lesions. dchorii. This indicates that Xanthomonas survives epiphyti Expt. 4. The effectiveness of streptomycin and copper- cally on the host plant rather than causing disease. maneb as protectants for disease control was tested. Six Expt. 2. Results from the test on strains of P. dchorii plants were sprayed with streptomycin at 100 ppm, six with from different hosts show that although strain virulence copper-maneb at 1.5 lbs. each/100 gal (Kocide 101 at 1 1/8 does differ somewhat, there was no evidence of host tsp/gal + Manzate 200 at 2 3/4 tsp/ga/), six were inoculated specificity (Table 2).