P. Ji

Application of Acibenzolar-S-Methyl Enhances Host Resistance in Tomato Against Ralstonia solanacearum

The chemical elicitor acibenzolar-S-methyl (ASM; Actigard 50 WG), which induces systemic acquired resistance (SAR), was investigated to determine the effect on bacterial wilt of tomato caused by Ralstonia solanacearum on moderately resistant cultivars under greenhouse and field conditions. In greenhouse experiments, ASM was applied as foliar spray and/or soil drench (3 μg/ml) before and as foliar spray (30 μg/ml) after transplanting. The chemical elicitor was ineffective in reducing bacterial wilt incidence on susceptible tomato cultivars Equinox and FL 47 when plants were inoculated with R. solanacearum. However, greenhouse studies indicated that ASM significantly enhanced resistance in cultivars with moderate resistance to bacterial wilt such as Neptune and BHN 466. It appeared that ASM-mediated resistance was partially due to prevention of internal spread of R. solanacearum toward upper stem tissues of tomato plants. The effect of ASM on moderately resistant cultivars was consistent in field experiments conducted in 2002 and 2003 in Quincy, FL, where bacterial wilt incidence was significantly reduced in ASM-treated BHN 466 (in 2002), FL 7514 (in 2003), and Neptune (both years) plants. ASM-treated BHN 466 and FL 7514 produced significantly higher tomato yield than the untreated controls. This is the first report of ASM-mediated control of bacterial wilt under field conditions, which suggests that use of this treatment for moderately resistant genotypes may be effective for control of bacterial wilt of tomato.

Development of an Integrated Approach for Managing Bacterial Wilt and Root-Knot on Tomato Under Field Conditions

A 2-year field study was conducted to develop a field application method using thymol as a preplant soil treatment for controlling bacterial wilt and root-knot nematode on tomato (Lycopersicon esculentum). In addition, acibenzolar-S-methyl (ASM), which induces plant systemic resistance, was applied in conjunction with thymol to determine whether combining these tactics could improve bacterial wilt management. The test sites were artificially infested with Ralstonia solanacearum and Meloidogyne arenaria, and thymol was applied as preplant fumigation through drip irrigation lines under polyethylene mulch at a rate of 73 kg/ha in both 2004 and 2005. ASM was applied primarily as foliar spray at a concentration of 25 mg/liter. Application of thymol significantly reduced incidence of bacterial wilt on tomato in both years of the trial. In thymol-treated plots, 26.0 and 22.6% of the plants wilted in 2004 and 2005, respectively; whereas, in untreated plots, more than 95% of the plants wilted in each year. Number of root-knot nematode juveniles was significantly reduced in field plots treated with thymol and ASM for both years. The combined use of thymol and ASM provided the greatest reduction of root galling among the treatments. Tomato yield (cv. FL47) was evaluated only in the 2005 trial; thymol-treated plots produced significantly higher marketable yield than untreated plots, and the thymol treatment in combination with ASM significantly increased tomato yield compared with thymol or ASM alone. These results indicate that use of thymol and ASM was beneficial in controlling bacterial wilt and root-knot. We developed an effective method for applying thymol through drip irrigation lines for managing these diseases in tomato production.

Sensitivity of Phytophthora capsici on Vegetable Crops in Georgia to Mandipropamid, Dimethomorph, and Cyazofamid

Phytophthora blight, caused by Phytophthora capsici, is a serious disease in vegetable production, and selective use of fungicides continues to be a significant component of disease management programs. The effect of three chemical compounds-mandipropamid, dimethomorph, and cyazofamid-on asexual stages of P. capsici collected from bell pepper and cucurbits in Georgia was assessed in this study. Forty isolates of P. capsici were determined to be sensitive to mandipropamid and dimethomorph based on mycelial growth, zoospore germination, and sporangial production. Concentrations that were 50% effective (EC50 values) of mandipropamid that inhibited mycelial growth, zoospore germination, and sporangial production of the isolates averaged 0.03, 5.70, and 0.02 μg/ml, respectively. EC50 values of dimethomorph in inhibiting mycelial growth, zoospore germination, and sporangial production averaged 0.24, 0.10, and 0.46 μg/ml, respectively. The majority of isolates were either resistant or intermediately sensitive to cyazofamid at 500 μg/ml or lower concentrations based on mycelial growth or sporangial production, although all the isolates were sensitive to this compound based on zoospore germination, with an average EC50 of 0.04 μg/ml. The results indicated that P. capsici populations in Georgia have not developed resistance to mandipropamid and dimethomorph whereas, for the majority of the isolates, certain asexual stages were resistant to cyazofamid.

Detection of Ralstonia solanacearum in Irrigation Ponds and Aquatic Weeds Associated with the Ponds in North Florida

The discovery of exotic Ralstonia solanacearum biovar 1 strains on geranium in north Florida led to a concern that this strain may have become established. Therefore, we monitored irrigation ponds and potential alternative aquatic weeds from 2002 to 2005 for the presence of this strain. We report that this strain, possibly originating from the Caribbean, has become established in several ponds in Gadsden County, FL. Cladistic taxonomy was used to subclassify the bacterium at the species level into four groups or phylotypes based on multiplex polymerase chain reaction of the internal transcribed spacer (ITS) region. The bacterium was further divided into sequevars by sequencing the endoglucanase gene (egl). The strains were determined to belong to phylotype II/sequevar 4 NPB (nonpathogenic on banana) that was recently reported in Martinique. Partial sequencing of the egl followed by phylogenetic analysis placed the new Caribbean strains in a different clade than the typical Florida endemic strains. Pulsed-field gel electrophoresis (PFGE) revealed different haplotypes upon comparison of the collected pond strains and the Floridian strains. Based on PFGE polymorphism, egl sequencing, and phylogenetic analysis, the Caribbean strains were shown to be identical to the strain isolated from infected geranium plants. Experiments were undertaken to monitor R. solanacearum in irrigation ponds and associated weeds. R. solanacearum was detected in surface-disinfested common aquatic weeds growing in the irrigation ponds, including Hydrocotyle ranunculoides (dollar weed) and Polygonum pennsylvanicum (Pennsylvania smart weed). Both weeds were latently infected and showed no signs of wilt when collected. Two different Hydrocotyle spp. were artificially inoculated with R. solanacearum under greenhouse conditions and both developed symptoms 14 days post inoculation (dpi) and the bacterium was recovered from the tissues 42 dpi. There was a positive correlation between ambient temperature and R. solanacearum populations in irrigation water, as previously shown by other researchers.

First report of black leaf spot of banana caused by Deightoniella torulosa in Georgia.

Black spots were observed on the leaves of bananas (Musa spp.) grown at the University of Georgia Bamboo Farm and Coastal Gardens in Savannah, GA in November 2007. Symptoms occurred on more than 60 plants, representing 16 of 34 cultivars of bananas investigated. Most lesions were less than 10 mm in diameter and tan to black. However, larger oval lesions more than 20 mm across with black borders and yellow halos also occurred. Lesions were more prevalent on older leaves. On young leaves, lesions first appeared along the leaf margin near the tip of the leaf on one side of the central vein. Lesions expanded to the entire leaf as the disease progressed, but were more prevalent along leaf margins. Thirty-two diseased leaf samples, two from each cultivar, were incubated at 25°C in the dark and conidia were produced on the lesions 2 days after incubation. Pure cultures of the fungus were obtained from five leaf samples by single-spore culturing on potato dextrose agar medium and identified on the basis of morphological characteristics. Conidia on V8 agar are straight or slightly curved, obpyriform to obclavate, and olive to brown with 3 to 13 septa. Conidiophores are brown and swollen at the apex. The fungus was identified as Deightoniella torulosa (Syd.) Ellis on the basis of morphological characteristics described previously (1,2). Pathogenicity studies to fulfill Kochs postulates were conducted on banana cvs. Dwarf Namwah and Dwarf Nino under greenhouse conditions (25 to 27°C). Six plants of each cultivar were used in one experiment and the experiment was repeated one more time. Banana leaves were inoculated by spraying with a suspension of conidia from a pure culture. Symptoms developed as small black lesions on the leaves of both cultivars within 1 week of inoculation. As the disease progressed, some of the small lesions expanded to form larger oval lesions. Symptoms were identical to those on the field samples and were identified as the black spot disease as described on abaca and banana (2). The fungus was reisolated from symptomatic leaves and the identity was confirmed. No symptoms were observed on noninoculated control plants. The black spot disease has been reported in Florida attacking banana and plantain (3). To our knowledge, this is the first description of the presence of the disease on field-grown banana in Georgia. In recent years, increasing efforts have been made in Georgia in the search of banana cultivars suitable of commercial production in the coastal and southern areas of the state. Black spot of banana is an important disease and its occurrence deserves consideration in evaluating banana cultivars and developing disease management approaches for banana production in Georgia. References: (1) M. B. Ellis. Mycol. Pap. No. 66. CAB International Mycological Institute, Wallingford, UK, 1957. (2) R. H. Stover. Banana, Plantain and Abaca Diseases. Commonw. Mycol. Inst., Kew, Surrey, UK, 1972. (3) C. Wehlburg et al. Bull. 11. Fla. Dep. Agric. Consum. Serv. Div. Plant Ind., 1975.

First Report of Root Rot Caused by Fusarium solani on Benincasa hispida in the United States

Root rot was observed on wax gourd (Benincasa hispida (Thunb.) Cogn.) cv. Black Giant in August 2010 in a commercial vegetable farm in southern Georgia. Approximately 5% of the plants were affected and infected roots turned light to dark brown with partial or entire roots affected and the lower leaves became wilted. Symptomatic roots from six plants were surface sterilized with 0.6% sodium hypochlorite and plated on potato dextrose agar (PDA) medium. Pure cultures had white mycelia and spore masses and were obtained from all six plants by subculturing hyphal tips onto PDA. One- to two-celled, oval- to kidney-shaped microconidia and cylindrical macroconidia with two or three cells plus apical and basal cell were produced, which averaged 12.5 × 4 μm and 28 × 4.5 μm, respectively. Microconidia were abundant and macroconidia were sparse on PDA. Single-spore isolates were obtained and identified as a Fusarium sp. by PCR analysis with primers ITS-Fu-f and ITS-Fu-r (1). Genomic DNA of two isolates obtained from different plants was extracted and a portion of the translation elongation factor 1-α (TEF) gene of the isolates was amplified and sequenced (3). When compared with sequences available in the GenBank database, DNA sequences of the two isolates (GenBank Accession No. JF928376) shared 100% sequence identity with F. solani strain FRC S1734 (GenBank Accession No. DQ247527). The fungus was identified as F. solani (Mart.) Sacc. based on molecular analysis and morphological characteristics (2). Oat grains were separately infected with two isolates, BG2a and BG6, and used to inoculate healthy, 3-week-old wax gourd seedlings (cv. Black Giant) under greenhouse conditions (14-h photoperiod, 24 to 30°C). Each seedling was grown in a 10-cm pot containing a commercial potting mix, and five healthy plants were inoculated with each isolate by placing 15 infected oat grains around each plant at a depth of 5 cm in the soil. Five plants treated with noninfected oat grains served as controls. Symptoms identical to those on field samples developed on all inoculated plants 3 weeks after inoculation but not on the control plants. F. solani was reisolated from inoculated symptomatic plants and the identity was confirmed, which completed Kochs postulates. The experiment was repeated one more time under similar conditions. To our knowledge, this is the first report of root rot caused by F. solani on wax gourd in the United States. Wax gourd is an important specialty crop in the southeastern United States and the occurrence of this disease needs to be taken into account in wax gourd production. References: (1) K. A. Abd-Elsalam et al. Afr. J. Biotechnol. 2:82, 2003. (2) C. Booth. Fusarium Laboratory Guide to the Identification of the Major Species. CMI, Kew, England, 1977. (3) D. M. Geiser et al. Eur. J. Plant Pathol. 110:473, 2004.