James J. Marois

Integrated Management of Tomato Spotted Wilt on Field-Grown Tomatoes

Epidemics of spotted wilt caused by Tomato spotted wilt virus (TSWV) vectored by Frankliniella occidentalis and possibly other thrips species occur regularly in tomato in the southeastern United States. Field experiments were conducted to determine the effects of UV-reflective mulch, acibenzolar-S-methyl (plant activator), and insecticides on progress of tomato spotted wilt incidence and population dynamics of flower thrips (including F. occidentalis, F. tritici, and F. bispinosa). Whole plots of tomatoes grown on UV-reflective and black polyethylene mulch were divided into subplots of acibenzolar-S-methyl and no acibenzolar-S-methyl, and sub-subplots of insecticide and no insecticide for thrips control. The UV-reflective mulch was more effective than black polyethylene mulch each year in reducing colonization of thrips in May and the consequent primary infections of tomato spotted wilt. Application of acibenzolar-S-methyl further reduced tomato spotted wilt incidence in 2000 and 2002, when disease pressure was great. Reproduction of thrips on tomato was poor in these experiments, but their control in the insecticide-treated sub-subplots prevented secondary spread in both years. The combination of UV-reflective mulch, acibenzolar-S-methyl, and insecticides was very effective in reducing tomato spotted wilt incidence in tomato.

Effect of Fungicide and Timing of Application on Soybean Rust Severity and Yield

Soybean rust, caused by Phakopsora pachyrhizi, is a devastating foliar disease of soybean that may cause significant yield losses if not managed by well-timed fungicide applications. To determine the effect of fungicide timing on soybean rust severity and soybean yield, field trials were completed in Paraguay (four locations), the United States (two locations), and Zimbabwe (one location) from 2005 to 2006. Treatments at each location included applications of tebuconazole, pyraclostrobin, or a combination of azoxystrobin + propiconazole, and in some locations pyraclostrobin + tebuconazole at the following soybean growth stages (GS): (i) GS R1 (beginning flowering), (ii) GS R3 (beginning pod), (iii) GS R5 (beginning seed), (iv) GS R1 + R3, (v) GS R3 + R5, and (vi) GS R1 + R3 + R5. Soybean yields from plots treated with fungicides were 16 to 114% greater than yields from no fungicide control plots in four locations in Paraguay, 12 to 55% greater in two locations in the United States, and 31% greater in Zimbabwe. In all locations, rust severity measured over time as area under the disease progress curve (AUDPC) was negatively correlated (r = -0.3, P < 0.0001) to yield. The effectiveness of any given treatment (timing of application and product applied) was often dependent on when rust was first detected and the intensity of its development. For example, when soybean rust was first observed before GS R3 (two locations in Paraguay), the plants in plots treated with a fungicide at GS R1 had the lowest AUPDC values and highest yields. When soybean rust was first observed after GS R3, plants treated with a fungicide at GS R3 and/or GS R5 had the lowest AUDPC values and highest yields with a few exceptions.

Effects of Silicon Applications on Soybean Rust Development Under Greenhouse and Field Conditions

Soybean rust (SBR), caused by Phakopsora pachyrhizi, is one of the most destructive fungal diseases affecting soybean production. Silicon (Si) amendments were studied as an alternative strategy to control SBR because this element was reported to suppress a number of plant diseases in other host-pathogen systems. In greenhouse experiments, soybean cultivars inoculated with P. pachyrhizi received soil applications of wollastonite (CaSiO3) (Si at 0, 0.96, and 1.92 t ha-1) or foliar applications of potassium silicate (K2SiO3) (Si at 0, 500, 1,000, or 2,000 mg kg-1). Greenhouse experiment results demonstrated that Si treatments delayed disease onset by approximately 3 days. The area under disease progress curve (AUDPC) of plants receiving Si treatments also was significantly lower than the AUDPC of non-Si-treated plants. For field experiments, an average 3-day delay in disease onset was observed only for soil Si treatments. Reductions in AUDPC of up to 43 and 36% were also observed for soil and foliar Si treatments, respectively. Considering the natural delayed disease onset due to the inability of the pathogen to overwinter in the major soybean production areas of the United States, the delay in disease onset and the final reduction in AUDPC observed by the soil Si treatments used may lead to the development of SBR control practices that can benefit organic and conventional soybean production systems.

Winter Survival of the Soybean Rust Pathogen, Phakopsora pachyrhizi, in Florida

Soybean rust (SBR) survival and host availability (kudzu, Pueraria spp.) were assessed from November 2006 through April 2007 at six sites from the panhandle to southwest Florida. Micro loggers recorded both temperature and relative humidity hourly at each location. Periods of drought and cumulative hours below 0°C correlated with kudzu defoliation. Inoculum potential from detached kudzu leaves was evaluated in vitro under various temperature and relative humidity levels. Kudzu leaves with SBR kept at 4°C produced viable urediniospores with the highest germination at all moisture levels over time. Freezing temperatures (-4 and -20°C) drastically reduced spore germination. However, when leaves were incubated at low (<35%) relative humidity, inoculum potential was prolonged. Results from this study demonstrate that both temperature and relative humidity impact P. pachyrhizi in the field and in vitro, and that detached kudzu leaves have the potential to serve as an inoculum source in kudzu stands.

From select agent to an established pathogen: The response to Phakopsora pachyrhizi (soybean rust) in North America

The pathogen causing soybean rust, Phakopsora pachyrhizi, was first described in Japan in 1902. The disease was important in the Eastern Hemisphere for many decades before the fungus was reported in Hawaii in 1994, which was followed by reports from countries in Africa and South America. In 2004, P. pachyrhizi was confirmed in Louisiana, making it the first report in the continental United States. Based on yield losses from countries in Asia, Africa, and South America, it was clear that this pathogen could have a major economic impact on the yield of 30 million ha of soybean in the United States. The response by agencies within the United States Department of Agriculture, industry, soybean check-off boards, and universities was immediate and complex. The impacts of some of these activities are detailed in this review. The net result has been that the once dreaded disease, which caused substantial losses in other parts of the world, is now better understood and effectively managed in the United States. The disease continues to be monitored yearly for changes in spatial and temporal distribution so that soybean growers can continue to benefit by knowing where soybean rust is occurring during the growing season.

A Coordinated Effort to Manage Soybean Rust in North America: A Success Story in Soybean Disease Monitoring

Existing crop monitoring programs determine the incidence and distribution of plant diseases and pathogens and assess the damage caused within a crop production region. These programs have traditionally used observed or predicted disease and pathogen data and environmental information to prescribe management practices that minimize crop loss. Monitoring programs are especially important for crops with broad geographic distribution or for diseases that can cause rapid and great economic losses. Successful monitoring programs have been developed for several plant diseases, including downy mildew of cucurbits, Fusarium head blight of wheat, potato late blight, and rusts of cereal crops. A recent example of a successful disease-monitoring program for an economically important crop is the soybean rust (SBR) monitoring effort within North America. SBR, caused by the fungus Phakopsora pachyrhizi, was first identified in the continental United States in November 2004. SBR causes moderate to severe yield losses globally. The fungus produces foliar lesions on soybean (Glycine max) and other legume hosts. P. pachyrhizi diverts nutrients from the host to its own growth and reproduction. The lesions also reduce photosynthetic area. Uredinia rupture the host epidermis and diminish stomatal regulation of transpiration to cause tissue desiccation and premature defoliation. Severe soybean yield losses can occur if plants defoliate during the mid-reproductive growth stages. The rapid response to the threat of SBR in North America resulted in an unprecedented amount of information dissemination and the development of a real-time, publicly available monitoring and prediction system known as the Soybean Rust-Pest Information Platform for Extension and Education (SBR-PIPE). The objectives of this article are (i) to highlight the successful response effort to SBR in North America, and (ii) to introduce researchers to the quantity and type of data generated by SBR-PIPE. Data from this system may now be used to answer questions about the biology, ecology, and epidemiology of an important pathogen and disease of soybean.

Evolution of Bacillus subtilis to enhanced growth at low pressure: up-regulated transcription of des-desKR, encoding the fatty acid desaturase system.

The atmospheric pressure on Mars ranges from 1-10 mbar, about 1% of Earth pressure (∼1013 mbar). Low pressure is a growth-inhibitory factor for terrestrial microorganisms on Mars, and a putative low-pressure barrier for growth of Earth bacteria of ∼25 mbar has been postulated. In a previous communication, we described the isolation of a strain of Bacillus subtilis that had evolved enhanced growth ability at the near-inhibitory low pressure of 50 mbar. To explore mechanisms that enabled growth of the low-pressure-adapted strain, numerous genes differentially transcribed between the ancestor strain WN624 and low-pressure-evolved strain WN1106 at 50 mbar were identified by microarray analysis. Among these was a cluster of three candidate genes (des, desK, and desR), whose mRNA levels in WN1106 were higher than the ancestor on the microarrays. Up-regulation of these genes was confirmed by quantitative reverse transcription-polymerase chain reaction (qRT-PCR) analysis. The des, desK, and desR genes encode the Des membrane fatty acid (FA) desaturase, the DesK sensor kinase, and the DesR response regulator, respectively, which function to maintain membrane fluidity in acute response to temperature downshift. Pressure downshift caused an up-regulation of des mRNA levels only in WN1106, but expression of a des-lacZ transcriptional fusion was unaffected, which suggests that des regulation was different in response to temperature versus pressure downshift. Competition experiments showed that inactivation of the des gene caused a slight, but statistically significant, loss of fitness of strain WN1106 at 50 mbar. Further, analysis of membrane FA composition of cells grown at 1013 versus 50 mbar revealed a decrease in the ratio of unsaturated to saturated FAs but an increase in the ratio of anteiso- to iso-FAs. The present study represents a first step toward identification of molecular mechanisms by which B. subtilis could sense and respond to the novel environmental stress of low pressure.

First Report of Soybean Rust Caused by Phakopsora pachyrhizi on Phaseolus spp. in the United States

Phakopsora pachyrhizi Syd. & P. Syd., the cause of soybean rust, was first observed in the continental United States in November 2004 (2). During the growing season of 2005, P. pachyrhizi was confirmed on soybean (Glycine max) and/or kudzu (Pueraria montana) in nine states in the southern United States. It is known that P. pachyrhizi has a much larger host range within the Fabaceae family. On 29 September 2005 in Quincy, FL, 45 entries of mostly large-seeded legumes were planted next to soybeans that were infected with P. pachyrhizi. Several seeds of each entry were planted on one hill. Soybean plants growing adjacent to these potential hosts had 15 to 25% of the leaf area affected, 95% incidence, and 73% defoliation on 16 November. On 7 December 2005, all the plants of Phaseolus coccineus L. (scarlet runner bean, PI311827), Phaseolus lunatus L. (lima bean, PI583558), and two Phaseolus vulgaris L. (kidney bean) cvs. Red Hawk and California Early Light Red Kidney (CELRK) were found to have leaves with suspected rust lesions. These plants were at physiological maturity but had not senesced. None of the hosts had been inoculated other than from spores produced by the adjacent rust-infected soybean plants or from unknown locations. On the basis of microscopic examination, suspected infected leaves from plants of the Phaseolus spp. had rust pustules characteristic of P. pachyrhizi uredinia. Uredinia were counted within a randomly selected 2-cm2 area of one leaf of each sample. The mean and range of uredinia per lesion for Phaseolus coccineus was 29 uredinia with a range of 0 to 3 uredinia per lesion, Phaseolus lunatus had 2 uredinia with 0 to 1 uredinium per lesion, Phaseolus vulgaris cv. Red Hawk had 22 uredinia with 0 to 5 uredinia per lesion, and Phaseolus vulgaris cv. CELRK had 43 uredinia with 0 to 4 uredinia per lesion. Polymerase chain reactions using two sets of primers (Ppa1/Ppa2 and Pme1/Pme2) were performed on DNA extracted from leaves of the three species with sporulating rust pustules (1). The results of these tests and further tests conducted by the USDA/APHIS confirmed that P. pachyrhizi was the causal organism for the observed rust. References: (1) P. F. Harmon et al. On-line publication. doi:10.1094/PHP-2005-0613-01-RS. Plant Health Progress, 2005. (2) R. W. Schneider et al. Plant Dis. 89:774, 2005.

Characterization of kudzu (Pueraria spp.) resistance to Phakopsora pachyrhizi, the causal agent of soybean rust.

Kudzu (Pueraria spp.) is an accessory host for soybean rust (SBR) (caused by Phakopsora pachyrhizi) that is widespread throughout the southeastern United States. An expanded survey of kudzu sites was conducted in 2008 to determine the proportion of natural resistance in the north-Florida kudzu population. Of the 139 sites evaluated, approximately 18% were found to be free of SBR infection, while 23% had reduced sporulation. Ten accessions of kudzu from north-central Florida were characterized for their response to challenge by a single isolate of P. pachyrhizi under laboratory conditions. Three outcomes were observed: tan lesions with profuse sporulation (susceptible); reddish-brown lesions with delayed, reduced sporulation (resistant); and an immune response in which no lesions developed (immune). Of the 10 accessions, 6 were susceptible, 3 were immune, and 1 was resistant. Cytological examination revealed that resistant interactions were typified by early onset of a multicell hypersensitive response (HR) while typical immune interactions were the result of cell wall depositions that blocked penetration in combination with early onset of the HR. Quantitative real-time polymerase chain reaction was performed to determine the extent of colonization. After 15 days, there was 10-fold less P. pachyrhizi DNA present in resistant compared with susceptible kudzu, while the amount of P. pachyrhizi DNA present in the immune kudzu was below the detection level. Susceptible kudzu had approximately half the amount of P. pachyrhizi DNA present when compared with a susceptible soybean cultivar.

Influence of Flower Thrips on Fusarium Hardlock Severity

Cotton (Gossypium hirsutum) fiber is sometimes affected by hardlock, which is characterized by a failure of the fiber to expand outward from the boll at maturity. Because affected fiber is inaccessible to mechanical harvesters, yield loss can be considerable. Hardlock has been linked to infection by Fusarium verticillioides. The involvement of flower thrips (Frankliniella spp.), which are commonly found in cotton flowers, was explored. At 1100 h, approximately 10% of cotton flowers contained thrips that were carrying F. verticillioides. The effect of thrips and/or Fusarium in flowers and bolls was explored under greenhouse conditions. Exposing flowers to Fusarium and thrips resulted in bolls with the most severe symptoms. Exposure to either Fusarium or thrips alone resulted in more hardlock than was noted in the control group. The impact of thrips was also evaluated under field conditions. Field plots were treated with insecticides, a fungicide, both, or left untreated. Insecticides reduced thrips numbers and reduced hardlock severity. The fungicide had no impact on thrips numbers and was less effective at reducing hardlock. Combining insecticide and fungicide applications was no more effective than using insecticides alone, although it more frequently increased yield. The untreated control plots generally had the most severe hardlock and lowest yields. Reducing hardlock severity resulted in higher yields, although not consistently. These studies suggest that thrips increase the severity of hardlock, and reducing their numbers may diminish hardlock severity.