In growth chamber experiments with five concentrations of NH4NO3 and inoculation of lettuce (Lactuca sativa L.) cv. Salinas with Rhizomonas suberifaciens, the causal agent of corky root (CR), symptoms of noninfectious corky root induced by high rates of N were distinct from those of infectious corky root (ICR). Nitrogen toxicity was observed at 350 kg·ha-1 and above, and was not affected by inoculation with R. suberifaciens. There was a curvilinear relationship between concentration of NH4NO3 applied and ICR severity with a maximum at 525 kg·ha-1. In a similar growth chamber experiment with NH4NO3 plus urea, ICR severity decreased and N toxicity increased at increasing N levels (N at 160 to 650 kg·ha-1). In microplots at Davis, Calif., sidedressing with NH4NO3 (N at 170 kg·ha-1) increased ICR severity on `Salinas' lettuce over the nonfertilized control. There was a significant interaction between N fertilization and soil-infestation with R. suberifaciens with respect to head fresh weight: sidedressing with NH4NO3 increased head weight in noninfested plots, but decreased head weight in infested plots. In four field experiments at Salinas, Calif., sidedressing with N at 78 to 213 kg·ha-1, with N as (NH4)1SO4, NH4NO3, urea, or Ca(NO3)2, increased ICR over the control, but there were no significant differences between the forms of N. Head fresh and dry weights were either increased or unaffected by sidedressing with N fertilizers, depending on the residual concentrations of N in the soil. The increase in ICR was likely related to concentrations of soil NO3 rather than NH4.
Ariena H.C. van Bruggen, Philip R. Brown and Art Greathead
Kalpana Sharma, Erica Goss and Ariena H.C. van Bruggen
Imported Hawaiian Ti Cordyline plants (Cordyline fruticosa) ‘Tipsy Pink’ with anthracnose symptoms were found in Gainesville, FL, in 2013. A Colletotrichum spp. was isolated from symptomatic Cordyline plants and Koch’s postulates were fulfilled. The colony color on acidified potato dextrose agar (PDA) was orange with slight shades of pink and light gray aerial mycelium. Sclerotia and setae were absent. Conidia were one-celled, hyaline, guttulate, and cylindrical with round ends. The mean size of the conidia was 14.7 × 5.0 μm and ranged from 12.5 to 17.5 × 3.8 to 7.5 μm. Polymerase chain reaction (PCR) was performed on the internal transcribed space (ITS) and 28S rDNA regions of the isolate, and the sequences were compared with those of Colletotrichum spp. in GenBank. Sequence analysis indicated that the isolate belonged to C. cordylinicola. This is the first report of C. cordylinicola on C. fruticosa in Florida and the United States. Anthracnose symptoms developed on healthy-looking, latently infected Hawaiian Ti plants within 2 to 3 months, and 34% to 44% of the non-inoculated plants became diseased in 3 months. Reactions of several Dracaena and Cordyline species and varieties including Hawaiian Ti to C. cordylinicola were assessed. Several Dracaena and Cordyline species and varieties including Hawaiian Ti exhibited a differential response when inoculated with C. cordylinicola, but none of them was resistant. Hawaiian Ti was the most and lucky bamboo (Dracaena sanderiana) the least susceptible [area under the disease progress curve (AUDPC) = 71 vs. 10 cm2·d–1] to C. cordylinicola. The slope of the log-transformed disease progress regression line was steepest on Hawaiian Ti and D. marginata variety ‘Colorama’ plants, intermediate on varieties ‘Tarzan’ and ‘Magenta’, and least on lucky bamboo [slope = 0.046, 0.044, 0.036, and 0.034 vs. 0.020 log(cm2 + 1)/d, respectively, with a mean se of 0.0006].
Laurie E. Drinkwater, Deborah K. Letourneau, Fekede Workneh, Marita Cantwell, Ariena H.C. van Bruggen and Carol Shennan
Twenty commercial tomato production systems were compared in a multidisciplinary on farm study. The aim was to determine if organic (ORG) and conventional (CNV) systems differed in terms of agronomic criteria or indicators of underlying ecological characteristics. Field level measures of inputs, yields, fruit quality, arthropod abundance and management operations were made. Also, multiple samples within each field were taken to measure soil chemical and physical properties, root pathogen populations, disease incidence, and pest damage levels for multivariate analysis. Management effects on agronomic criteria (yield, fruit quality, pest damage) were small, whereas differences in soil N pools, microbial activity, pathogen populations and arthropod communities between ORG and CNV sites were sufficiently robust to be distinguished from site to site variation. Relationships between management, crop productivity and fruit quality will be discussed.
Kalpana Sharma, Joyce L. Merritt, Aaron Palmateer, Erica Goss, Matthew Smith, Tim Schubert, Robert S. Johnson and Ariena H.C. van Bruggen
International trade in ornamental plants has increased worldwide. Dracaena is the genus most frequently imported into the United States and many pests and pathogens currently not in the United States could be imported with Dracaena plant materials. In 2009, lucky bamboo (Dracaena sanderiana) with anthracnose symptoms was found in Florida. The associated fungus, putative Colletotrichum dracaenophilum, is considered a reportable pathogen that originated from Asia. Imported D. sanderiana plants with anthracnose symptoms were collected from a nursery in south Florida in 2011 and retail stores in north Florida in 2012 and 2013. Five isolates of Colletotrichum spp. were isolated from symptomatic D. sanderiana plants and Koch’s postulates were fulfilled for the first time. Polymerase chain reaction was performed on the internal transcribed space (ITS) and 28S rDNA regions of three original isolates and re-isolates and these sequences were compared with sequences of Colletotrichum spp. in GenBank. Sequence analysis indicated that the Colletotrichum isolates obtained from D. sanderiana in Florida belonged to C. dracaenophilum or the C. gloeosporioides species complex. Latent infections on healthy-looking lucky bamboo developed anthracnose lesions within 2 to 3 months. The reaction of several Dracaena species and varieties to the two Colletotrichum species was tested. Colletotrichum dracaenophilum caused the most severe disease on lucky bamboo, whereas one isolate of the C. gloeosporioides species complex was least pathogenic to all of the Dracaena spp. and varieties. Hot water treatments were not effective at controlling latent infections, but application of Azoxystrobin at 0.075 g a.i./L significantly reduced anthracnose development on both latently infected and inoculated lucky bamboo plants.
Joyce L. Merritt, Ellen Dickstein, Robert S. Johnson, Michael Ward, Robert J. Balaam, Carrie L. Harmon, Philip F. Harmon, G. Shad Ali, Aaron J. Palmateer, Timothy Schubert and Ariena H.C. van Bruggen
The purpose of this study was to evaluate the U.S.-Canadian Greenhouse Certification Program (USGCP) that was initiated in 1998. A survey consisting of 34 questions was designed and 43 out of ≈48 nurseries in Florida participating in the USGCP were visited. Based on the answers to the questionnaire, most of the nurseries were in compliance with the majority of USGCP requirements, growers were satisfied with the program, and there was an economic benefit to participating in the program. The main problems identified were the ambiguous wording of some of the requirements and the impracticality of keeping imported and domestic plants completely segregated. Moreover, many of the respondents did not have a written description of a pest management plan. Chi square statistical analysis showed that there was almost no difference between nursery groups in their responses to the majority of the survey questions, indicating that the USGCP is a successful program for both large and small nurseries. This quantitative assessment of the USGCP is the first assessment conducted for this program and discussed in a peer-reviewed publication.