A. S. Csinos

Management of Tomato spotted wilt virus in Flue-Cured Tobacco with Acibenzolar-S-Methyl and Imidacloprid

Tomato spotted wilt virus (TSWV) continues to be a major production constraint in flue-cured tobacco in Georgia. Pretransplant applications of acibenzolar-S-methyl (Actigard) and imidacloprid (Admire and Provado) were evaluated in field trials, singly and in combination, in four locations in 1999. Acibenzolar-S-methyl was also evaluated for its effect on growth and yield, potential phytotoxicity, and activity against tobacco blackshank incited by Phytophthora parasitica var. nicotianae. Acibenzolar-S-methyl alone and with imidacloprid significantly reduced TSWV incidence in all four locations, but the disease incidence in the imidacloprid-treated plots was lower in only one of the four sites. Applications of acibenzolar-S-methyl starting at 4 weeks posttransplant had no effect on TSWV incidence, size, or yield of tobacco compared with nontreated plots. Acibenzolar-S-methyl did not reduce blackshank in a field trial. In the greenhouse, rates of acibenzolar-S-methyl ranging from 0.25 to 8 g a.i. per 7,000 plants showed reductions in growth that did not seem to be related to rate. Pretransplant applications of acibenzolar-S-methyl were critical to the suppression of TSWV, while initial applications made posttransplant had no effect, suggesting that plants must be protected prior to introduction into the field.

Biological and Molecular Analyses of the Acibenzolar-S-Methyl-Induced Systemic Acquired Resistance in Flue-Cured Tobacco Against Tomato spotted wilt virus

Tomato spotted wilt virus (TSWV) is an economically important virus of flue-cured tobacco. Activation of systemic acquired resistance (SAR) by acibenzolar-S-methyl (ASM) in flue-cured tobacco was studied under greenhouse conditions by challenge inoculation with a severe isolate of TSWV. ASM restricted virus replication and movement, and as a result reduced systemic infection. Activation of resistance was observed within 2 days after treatment with ASM and a high level of resistance was observed at 5 days onward. Expression of the pathogenesis-related (PR) protein gene, PR-3, and different classes of PR proteins such as PR-1, PR-3, and PR-5 were detected at 2 days post-ASM treatment which inversely correlated with the reduction in the number of local lesions caused by TSWV. Tobacco plants treated with increased quantities of ASM (0.25, 0.5, 1.0, 2.0, and 4.0 g a.i./7,000 plants) showed increased levels of SAR as indicated by the reduction of both local and systemic infections by TSWV. The highest level of resistance was at 4 g a.i., but this rate of ASM also caused phytotoxicity resulting in temporary foliar spotting and stunting of plants. An inverse correlation between the TSWV reduction and phytotoxicity was observed with the increase of ASM concentration. ASM at the rate of 1 to 2 g a.i./7,000 plants activated a high level of resistance and minimized the phytotoxicity. Use of gibberellic acid in combination with ASM reduced the stunting caused by ASM. Present findings together with previous field experiments demonstrate that ASM is a potential option for management of TSWV in flue-cured tobacco.

Effect of acibenzolar-S-methyl and imidacloprid on suppression of tomato spotted wilt Tospovirus in flue-cured tobacco

Abstract Tomato spotted wilt virus (TSWV) has become a major constraint to tobacco production in Georgia. No single control measure has been found effective for minimizing the impact of the disease. Foliar applications of acibenzolar-S-methyl (Actigard ® ) were tested in replicated field trials singly and in combination with imidacloprid (Admire ® and Provado ® ) for their effect on reducing the impact of TSWV in tobacco. Results showed that Actigard significantly reduced final disease incidence in one trial in 1997 and another trial in 1998 in Georgia. Imidacloprid, when applied alone, suppressed disease incidence in one of four trials. Significant reduction in incidence occurred at three of the four locations when both were applied together. The fourth location had very low levels of incidence and no differences were detected among treatments. The percent ELISA positive plants was higher than the percent of symptomatic plants for three of the four locations. Actigard reduced percent ELISA positive plants for two of the four locations and imidacloprid alone and with Actigard reduced percent ELISA positive plants at all four locations. Use of plant defense activators such as Actigard alone or in combination with imidacloprid potentially offers an alternative and effective management tool for spotted wilt suppression on tobacco. This is the first report of effective suppression of an economically important plant viral disease using Actigard.

Methyl bromide alternatives in tobacco, tomato and pepper transplant production.

Abstract Metam-sodium, chloropicrin, 1,3-dichloropropene (1,3-D), 1,3-dichloropropene plus 17% chloropicrin (1,3-D+C-17) and 1,3-dichloropropene plus 35% chloropicrin (1,3-D+C-35) were evaluated alone and together under a polyethylene cover as alternatives for methyl bromide in tobacco, tomato and pepper transplant production over a two-year period. Eleven different weed species, 10 genera or species of fungi, two arthropods and four species of nematodes were evaluated in three different sites. All of the fumigant candidates were successful in controlling some of the pests. However, metam-sodium alone and in combination with 1,3-D+C-17, chloropicrin, and 1,3-D+C-35 were the most efficacious. Seventy-nine different parameters were measured over the two-year period including plant stands, vigour, plant heights and control of weeds, fungi, arthropods and nematodes. Metam-sodium and methyl bromide plus 2% chloropicrin (MeBrC) were not significantly different ( P =0.05) from each other in 76 of the 79 parameters evaluated; MeBrC was significantly better than metam-sodium for three of the 79 parameters. The combination of metam-sodium plus 1,3-D+C-17 was not significantly different from MeBrC for 74 of the 79 parameters tested. MeBrC was more efficacious than metam-sodium plus 1,3-D+C-17, for two of the 79 parameters, and metam-sodium plus 1,3-D+C-17 was significantly better than MeBrC for three of 79 parameters.

Methyl bromide alternatives in a bell pepper–squash rotation

Abstract Field studies were conducted to evaluate potential methyl bromide alternatives against multiple pests in a bell pepper (Capsicum annum L.) – squash (Cucurbita pepo L.) cropping sequence. Early in the growing season, the most effective treatments in suppressing purple nutsedge (Cyperus rotundus L.) emergence through the polyethylene mulch were methyl bromide, methyl iodide, and chisel-injected 1,3-dichloropropene plus chloropicrin [1,3-D+C35 (chisel)]. However by the end of the season in 1999, only methyl bromide treatment had less purple nutsedge emerging through the polyethylene than the nontreated control. Each soil-applied treatment resulted in nematode-susceptible pepper plants with lower root-gall indices [Meloidogyne spp. (root knot nematode)] than the nontreated control, while there were no differences among treatments with the nematode-resistant pepper cultivar. Total fungi isolated from soil was lower in all treated plots relative to the nontreated control, with the exception of methyl iodide. However, methyl bromide was the only treatment that was consistently effective against Pythium spp. and Fusarium spp. A treatment of metham prior to planting squash was beneficial in reducing root-gall indices in plots treated with 1,3-D+C35 (chisel) and methyl bromide prior to the pepper crop. Methyl bromide, methyl iodide, and 1,3-D+C35 (chisel) applied before pepper resulted in squash with lower root-gall indices than the nontreated control. Glyphosate applied between the first and second crop eliminated exposed weed foliage through the polyethylene mulch, possibly muting the effects of the second crop treatment on weed densities. Results of this study indicate that there are some potential methyl bromide alternatives available to growers for use in pest control, however there does not appear to be one broad-spectrum pesticide that will replace methyl bromide. Also, an effective control for nutsedge species within the pepper–squash cropping system is still elusive.

Alternative fumigants for methyl bromide in tobacco and pepper transplant production

Abstract Tobacco and pepper are high-value cash crops in the southeastern USA that require vigorous transplants free of pathogens and insects. Typically soil seedbeds are treated with methyl bromide prior to seeding. Soil fumigants metam-sodium, dichloropropene, chloropicrin and dazomet covered with polyethylene film were evaluated at several rates alone and in combination as alternatives for methyl bromide soil fumigation of tobacco and pepper seedbeds. The studies were conducted over a three-year period, with materials applied in the fall prior to seeding at the beginning of the following year. Nematode and insect pressures were low in each of three sites, but the tests extensively evaluated weeds and soilborne fungi management. Metam-sodium at 935 L ha−1 performed well in all three tests. The combination of metam-sodium (468 L ha−1) plus dichloropropene + 17% chloropicrin (126 L ha−1) provided good control of most of the pests and had high plant yield and vigor when covered with a polyethylene film immediately after treatment. A similar treatment not covered with polyethylene film but sealed with a mechanical soil cultipacker provided poor control of weeds. Stunting of tobacco and pepper was noted, especially in plots treated at the highest rates of metamsodium plus dichloropropene and chloropicrin. These treatments had a pungent odor associated with the treatment, which persisted for several weeks after polyethylene film removal. Many of the treatments, especially metam-sodium and metam-sodium in combination with dichloropropene and chloropicrin, compared well with methyl bromide fumigation for seedbed pest control.

Development of an improved isolation approach and simple sequence repeat markers to characterize Phytophthora capsici populations in irrigation ponds in southern Georgia.

ABSTRACT Phytophthora capsici, the causal agent of Phytophthora blight, is a major concern in vegetable production in Georgia and many other states in the United States. Contamination of irrigation water sources by P. capsici may be an important source of inoculum for the pathogen. A simple method was developed in this study to improve the efficiency of recovering P. capsici from fruits used as baits in irrigation ponds. In contrast to direct isolation on agar plates, infected fruit tissues were used to inoculate stems of pepper seedlings, and the infected pepper stems were used for isolation on agar plates. With isolation through inoculation of pepper stems, the frequency of recovering P. capsici from infected eggplant and pear fruits increased from 13.9% to 77.7% and 8.1% to 53.5%, respectively, compared with direct isolation on agar plates. P. capsici was isolated from seven out of nine irrigation ponds evaluated, with most of the ponds containing both A1 and A2 mating types and a 4:5 ratio of A1 to A2 when isolates from all ponds were calculated. All P. capsici isolates were pathogenic on squash plants, and only a small proportion (8.2%) of the isolates were resistant or intermediately sensitive to mefenoxam. Simple sequence repeats (SSRs) were identified through bioinformatics mining of 55,848 publicly available expressed sequence tags of P. capsici in dbEST GenBank. Thirty-one pairs of SSR primers were designed, and SSR analysis indicated that the 61 P. capsici isolates from irrigation ponds were genetically distinct. Cluster analysis separated the isolates into five genetic clusters with no more than two genetic groups in one pond, indicating relatively low P. capsici genetic diversity in each pond. The isolation method and SSR markers developed for P. capsici in this study could contribute to a more comprehensive understanding of the genetic diversity of this important pathogen.

Response of Peanut, Pepper, Tobacco, and Tomato Cultivars to Two Biologically Distinct Isolates of Tomato spotted wilt virus

Spotted wilt disease, caused by Tomato spotted wilt virus (TSWV), is an economically important disease in peanut, pepper, tobacco, and tomato in the southeastern United States. However, very little is known about the biological variability existent in the virus population. Fourteen isolates of TSWV collected in Georgia were evaluated for symptom severity. The majority of the isolates produced severe systemic necrosis. One mild (GATb-1) and one severe (GAL) isolate were further examined because of the distinct differences in their virulence and symptomatology on tobacco. GATb-1 caused a few chlorotic spots and mild systemic symptoms, whereas GAL produced a large number of local lesions and severe systemic necrosis. Distinct differences in the response of selected commercial cultivars of peanut, tobacco, and tomato to GATb-1 and GAL infection were observed. GAL was lethal to a widely grown tobacco cultivar, K326. Georgia Green, a field resistant peanut cultivar, and C11-2-39, a breeding line with the highest level of known resistance to TSWV, were more susceptible to GAL than to GATb-1. BHN 444, a newly released TSWV-resistant tomato cultivar, showed a resistant reaction, whereas Stiletto, a newly released TSWV-resistant pepper cultivar, was susceptible to both GATb-1 and GAL isolates. Information on the biological diversity of TSWV may be useful in developing more durable TSWV-resistant crops.

Tomato spotted wilt virus epidemic in flue-cured tobacco in Georgia

Spotted wilt, caused by tomato spotted wilt virus (TSWV), was first diagnosed in flue-cured tobacco (Nicotiana tabacum) in Georgia in 1986. By 1988, tobacco fields in 28 of 48 tobacco-producing counties had plants infected with TSWV, but the incidence was less than 1%. In 1989, spotted wilt was observed in all tobacco-growing counties in Georgia. Average incidence was low with 5−7% of plants examined showing symptoms. However, in some fields, incidence was more than 20% and losses in plant stand were observed. First symptoms appeared shortly after tobacco seedlings were transplanted to the field in March, and numbers of symptomatic plants increased through initiation of harvest in June (.)

Sequence Characteristics of Natural Populations of Tomato Spotted Wilt Tospovirus Infecting Flue-cured Tobacco in Georgia

Using primers from conserved regions, the nucleocapsid (Nc) gene sequences of naturally occurring tomato spotted wilt tospovirus (TSWV) isolates from flue-cured tobacco (Nicotiana tabacum) grown in several Georgia counties were amplified by immunocapture-reverse transcription-polymerase chain reaction. The resulting amplicons were cloned and sequenced. Sequence analyses showed a high degree of sequence conservation among the Nc genes of the tobacco isolates, and those reported from other parts of the world. However, distinct differences that were unique to these tobacco isolates as well as the previously studied peanut, tomato and pepper isolates from Georgia were present. The Georgia isolates formed a distinct cluster that was clearly resolved from the rest of the TSWV isolates based on sequence phylograms.