Phytophthora capsici, 100 Years Later: Research Mile Markers from 1922 to 2022.

In 1922, Phytophthora capsici was described by Leon Hatching Leonian as a new pathogen infecting pepper (Capsicum annuum), with disease symptoms of root rot, stem and fruit blight, seed rot, and plant wilting and death. Extensive research has been conducted on P. capsici over the last 100 years. This review succinctly describes the salient mile markers of research on P. capsici with current perspectives on the pathogen's distribution, economic importance, epidemiology, genetics and genomics, fungicide resistance, host susceptibility, pathogenicity mechanisms, and management.

Broad Resistance to Post-Harvest Fruit Rot in USVL Watermelon Germplasm Lines to Isolates of Phytophthora capsici Across the United States.

Watermelon is an important cucurbit vegetable crop grown in most of the United States. Phytophthora fruit rot of watermelon caused by Phytophthora capsici has been a major factor, limiting production for the past 15 years in the southeastern United States. The U.S. Department of Agriculture, Agricultural Research Service released five Phytophthora fruit rot-resistant germplasm lines for use in breeding programs. These lines were developed by phenotyping using a local isolate of P. capsici from South Carolina. The present study was undertaken to determine if these resistant lines had broad resistance to diverse P. capsici isolates collected from different states and crops. Five resistant germplasm lines (USVL020-PFR, USVL203-PFR, USVL782-PFR, USVL489-PFR, and USVL531-MDR) and two susceptible cultivars, Sugar Baby and Mickey Lee, used as checks were grown in a field in 2014 and 2015 to produce fruit for evaluation. Mature fruit were harvested and placed in a walk-in growth chamber and inoculated with 20 different P. capsici isolates. The chamber was maintained at 26 ± 2°C and high relative humidity (>95%) using a humidifier. All five resistant germplasm lines were significantly more resistant than the two susceptible checks to all 20 P. capsici isolates. Among the five resistant germplasm lines, USVL020-PFR, USVL782-PFR, and USVL531-MDR had broad resistance. Some P. capsici isolates induced minor lesions and rot on USVL489-PFR compared with the other resistant lines. Variation in virulence and genetic diversity among the 20 P. capsici isolates was also observed. The five watermelon germplasm lines will be useful for developing commercial watermelon cultivars with broad resistance to P. capsici.

Spatial and Temporal Physiognomies of Whitefly and Tomato Yellow Leaf Curl Virus Epidemics in Southwestern Florida Tomato Fields.

Epidemics of tomato yellow leaf curl virus (TYLCV; species Tomato yellow leaf curl begomovirus) have been problematic to tomato production in the southeastern United States since the first detection of the virus in Florida in the late 1990s. Current strategies for management focus on farm-centric tactics that have had limited success for controlling either TYLCV or its whitefly vector. Areawide pest management (AWPM)-loosely defined as a coordinated effort to implement management strategies on a regional scale-may be a viable management alternative. A prerequisite for development of an AWPM program is an understanding of the spatial and temporal dynamics of the target pathogen and pest populations. The objective of this study was to characterize populations of whitefly and TYLCV in commercial tomato production fields in southwestern Florida and utilize this information to develop predictors of whitefly density and TYLCV disease incidence as a function of environmental and geographical factors. Scouting reports were submitted by cooperating growers located across approximately 20,000 acres in southwestern Florida from 2006 to 2012. Daily weather data were obtained from several local weather stations. Moran's I was used to assess spatial relationships and polynomial distributed lag regression was used to determine the relationship between weather variables, whitefly, and TYLCV. Analyses showed that the incidence of TYLCV increased proportionally with mean whitefly density as the season progressed. Nearest-neighbor analyses showed a strong linear relationship between the logarithms of whitefly densities in neighboring fields. A similar relationship was found with TYLCV incidences. Correlograms based on Moran's I showed that these relationships extended beyond neighboring fields and out to approximately 2.5 km for TYLCV and up to 5 km for whitefly, and that values of I were generally higher during the latter half of the production season for TYLCV. Weather was better at predicting whitefly density than at predicting TYLCV incidence. Whitefly density was best predicted by the number of days with an average temperature between 16 and 24°C (T16to24), relative humidity (RH) over the previous 31 days, and vapor pressure deficit over the last 8 days. TYLCV incidence was best predicted by T16to24, RH, and maximum wind speed over the previous 31 days. Results of this study helped to identify the extent to which populations of whitefly and TYLCV exist over the agricultural landscape of southwestern Florida, and the environmental conditions that favor epidemic growth. This information was used to propose an approach to AWPM for timing control measures for managing TYLCV epidemics.

Development of Phytophthora Fruit Rot Caused by Phytophthora capsici on Resistant and Susceptible Watermelon Fruit of Different Ages.

Watermelon is an important crop grown in 44 states in the United States. Phytophthora fruit rot caused by Phytophthora capsici is a serious disease in the southeastern U.S.A., where over 50% of the watermelons are produced. The disease has resulted in severe losses to watermelon growers, especially in Georgia, South Carolina, and North Carolina during the past few years. Several fruit rot-resistant watermelon germplasm lines have been developed for use in breeding programs. To evaluate the development of Phytophthora fruit rot on fruit of different ages, plants of fruit rot-resistant and susceptible lines were planted at weekly intervals for five consecutive weeks in experiments conducted over three years (2011 to 2013). Flowers were routinely inspected and hand pollinated to ensure having fruit of different ages. In each year, different aged fruit were harvested on the same day and inoculated with a 5-mm agar plug from an actively growing colony of P. capsici. Inoculated fruit were maintained in a room set to conditions conducive for disease development (>95% relative humidity, 26 ± 2°C). After 5 days, lesion diameter and intensity of sporulation was recorded for each fruit. Lesion diameter and sporulation intensity were significantly greater on fruit of susceptible lines compared with resistant lines. Fruit age did not have an effect on either measurement on susceptible (Sugar Baby) or resistant lines (PI 560020 and PI 595203). Our results showed that resistance to Phytophthora fruit rot in watermelon was not correlated with fruit age.

Powdery Mildew Resistant Rootstocks that Impart Tolerance to Grafted Susceptible Watermelon Scion Seedlings.

Powdery mildew (PM) is a major foliar disease causing serious economic losses of cucurbit crops grown in the United States. The pathogen Podosphaera xanthii, which causes PM, is known to infect seedlings, stems, foliage, petioles, and fruit of cucurbit crops. In recent years, grafting watermelon on resistant rootstocks for managing soilborne diseases has been gaining popularity in the U.S.A. However, grafting for managing foliar diseases has not yet received adequate attention. Three greenhouse experiments were conducted during the summer months of 2012, 2013, and 2014 to determine if PM-resistant rootstocks could impart resistance to a susceptible watermelon scion. Susceptible watermelon scion 'Mickey Lee' seedlings were grafted onto 25 watermelon (Citrullus lanatus, C. amarus, C. mucosospermus) and four bottle gourd (Lagenaria siceraria) rootstocks. Grafted plants were inoculated with a 2 × 104 conidia ml-1 suspension of P. xanthii conidia and disease severity was rated 14 days after inoculation. Mickey Lee grafted on six PM-resistant watermelon rootstocks had significantly (P ≤ 0.05) lower PM severity on cotyledons, 2nd true leaf, and upper leaves (rating for 3rd to 7th or 8th true leaf) compared with Mickey Lee grafted on susceptible watermelon USVL677-PMS or bottle gourd USVL848-PMS rootstocks. However, some of the resistant watermelon rootstocks did not impart significant resistance to the scion. The resistant bottle gourd rootstocks USVL482-PMR and USVL351-PMR provided significantly greater levels of resistance, compared with many of the resistant watermelon rootstocks. Grafting watermelon on resistant rootstocks may help mitigate the effects of PM on susceptible scion seedlings.

Development and Evaluation of ELISA and qRT-PCR for Identification of Squash vein yellowing virus in Cucurbits.

Squash vein yellowing virus (SqVYV) causes viral watermelon vine decline. To facilitate detection of SqVYV, enzyme linked-immunosorbent assay (ELISA) and quantitative reverse-transcription polymerase chain reaction (qRT-PCR) diagnostic methods were developed. Both methods were capable of detecting SqVYV in a wide range of cucurbit hosts. ELISA was able to detect virus in infected host tissue diluted to at least 1:2,560, which was sufficient for detection in symptomatic squash and watermelon plants. The qRT-PCR method was capable of reliably detecting as few as 3.4 copies of a cloned fragment of SqVYV genomic RNA with an average cycle threshold (Ct) value of 36.4. The sensitivities and specificities for each detection method were estimated by latent class analysis for a set of inoculated squash and watermelon plants at two sampling scales. The scales were hierarchical, with individual plants representing the upper scale and samples from the plant representing the lower scale. The number of samples per plant varied from 1 to 8, and a plant was diagnosed positive if any of its samples tested positive. For all analyses, a cutoff Ct of 35 was chosen for qRT-PCR, which is approximately 2.5 cycles lower than the lowest Ct value achieved for mock-inoculated plants (presumed to be a false positive). qRT-PCR showed high sensitivities (≥0.99) at both sampling scales for squash and watermelon, whereas the sensitivities for ELISA ranged from 0.58 to 0.76. The specificities for both tests were very similar (≥0.94), with ELISA sometimes outperforming qRT-PCR. These diagnostic methods provide additional tools for the identification of SqVYV and management of SqVYV-induced watermelon vine decline.

Resistance to Southern Root-knot Nematode (Meloidogyne incognita) in Wild Watermelon (Citrullus lanatus var. citroides).

Southern root-knot nematode (RKN, Meloidogyne incognita) is a serious pest of cultivated watermelon (Citrullus lanatus var. lanatus) in southern regions of the United States and no resistance is known to exist in commercial watermelon cultivars. Wild watermelon relatives (Citrullus lanatus var. citroides) have been shown in greenhouse studies to possess varying degrees of resistance to RKN species. Experiments were conducted over 2 yr to assess resistance of southern RKN in C. lanatus var. citroides accessions from the U.S. Watermelon Plant Introduction Collection in an artificially infested field site at the U.S. Vegetable Laboratory in Charleston, SC. In the first study (2006), 19 accessions of C. lanatus var. citroides were compared with reference entries of Citrullus colocynthis and C. lanatus var. lanatus. Of the wild watermelon accessions, two entries exhibited significantly less galling than all other entries. Five of the best performing C. lanatus var. citroides accessions were evaluated with and without nematicide at the same field site in 2007. Citrullus lanatus var. citroides accessions performed better than C. lanatus var. lanatus and C. colocynthis. Overall, most entries of C. lanatus var. citroides performed similarly with and without nematicide treatment in regard to root galling, visible egg masses, vine vigor, and root mass. In both years of field evaluations, most C. lanatus var. citroides accessions showed lesser degrees of nematode reproduction and higher vigor and root mass than C. colocynthis and C. lanatus var. lanatus. The results of these two field evaluations suggest that wild watermelon populations may be useful sources of resistance to southern RKN.

Spatial and Temporal Analysis of Squash vein yellowing virus Infections in Watermelon.

Squash vein yellowing virus (SqVYV) is a whitefly-transmitted ipomovirus infecting watermelon and other cucurbits that was recently introduced to Florida. Effects on watermelon are devastating, with total vine collapse, often near harvest, and fruit rendered unmarketable by brown, discolored flesh. The epidemiology of SqVYV was studied in a 1-ha field of 'Fiesta' watermelon over six growing seasons (I to VI) to characterize the spatial patterning of disease and temporal rate of disease progress, as well as its association with Cucurbit leaf crumple virus (CuLCrV) and Cucurbit yellow stunting disorder virus (CYSDV), two additional whitefly-transmitted viruses that often occur with SqVYV. The field was scouted at regular intervals for the length of the season for incidence of virus and number of whiteflies. Incidence of SqVYV reached 100% during seasons I, II, and V and 20% during season III. SqVYV did not occur during seasons IV and VI. SqVYV progressed in a characteristic logistic fashion in seasons I, II, and V but less so in season III. The rate of disease progress was similar for the three seasons with high disease incidence, with an average value of 0.18. A positive correlation between the area under the disease progress curve and whitefly-days was found, where both progress curves were calculated as a function of thermal time (degree days, base 0°C). SqVYV displayed significant but variable levels of aggregation, as indicated by its fit to the ß-binomial distribution, the binary power law, and ordinary runs analysis. Association analysis indicated that the viruses were largely transmitted independently. Results of this study provide epidemiological information that will be useful in the development of management strategies for SqVYV-induced vine decline, and provide new information for CuLCrV and CYSDV.

The use of latent class analysis to estimate the sensitivities and specificities of diagnostic tests for Squash vein yellowing virus in cucurbit species when there is no gold standard.

Squash vein yellowing virus (SqVYV) is the causal agent of viral watermelon vine decline, one of the most serious diseases in watermelon (Citrullus lanatus L.) production in the southeastern United States. At present, there is not a gold standard diagnostic test for determining the true status of SqVYV infection in plants. Current diagnostic methods for identification of SqVYV-infected plants or tissues are based on the reverse-transcription polymerase chain reaction (RT-PCR), tissue blot nucleic acid hybridization assays (TB), and expression of visual symptoms. A quantitative assessment of the performance of these diagnostic tests is lacking, which may lead to an incorrect interpretation of results. In this study, latent class analysis (LCA) was used to estimate the sensitivities and specificities of RT-PCR, TB, and visual assessment of symptoms as diagnostic tests for SqVYV. The LCA model assumes that the observed diagnostic test responses are linked to an underlying latent (nonobserved) disease status of the population, and can be used to estimate sensitivity and specificity of the individual tests, as well as to derive an estimate of the incidence of disease when a gold standard test does not exist. LCA can also be expanded to evaluate the effect of factors and was done here to determine whether diagnostic test performances varied among the type of plant tissue being tested (crown versus vine tissue), where plant samples were taken relative to the position of the crown (i.e., distance from the crown), host (i.e., genus), and habitat (field-grown versus greenhouse-grown plants). Results showed that RT-PCR had the highest sensitivity (0.94) and specificity (0.98) of the three tests. TB had better sensitivity than symptoms for detection of SqVYV infection (0.70 versus 0.32), while the visual assessment of symptoms was more specific than TB and, thus, a better indicator of noninfection (0.98 versus 0.65). With respect to the grouping variables, RT-PCR and TB had better sensitivity but poorer specificity for diagnosing SqVYV infection in crown tissue than it did in vine tissue, whereas symptoms had very poor sensitivity but excellent specificity in both tissues for all cucurbits analyzed in this study. Test performance also varied with habitat and genus but not with distance from the crown. The results given here provide quantitative measurements of test performance for a range of conditions and provide the information needed to interpret test results when tests are used in parallel or serial combination for a diagnosis.

Squash vein yellowing virus Infection of Vining Cucurbits and the Vine Decline Response.

The responses of a diverse group of vining cucurbits to inoculation with Squash vein yellowing virus (SqVYV) were determined. For the first time, Cucurbita maxima, Cucumis dipsaceus, and Cucumis metuliferus were observed to develop necrosis and plant death similar to the SqVYV-induced vine decline in watermelon (Citrullus lanatus var. lanatus). The majority of cucurbits inoculated, however, either exhibited no symptoms of infection, or developed relatively mild symptoms such as vein yellowing of upper, noninoculated leaves. All inoculated plants were sectioned and tested for the presence of SqVYV. The virus was widely distributed in mature, fruit-bearing cucurbits with over 72% of plant sections testing positive for SqVYV by tissue-blot and/or reverse transcription-polymerase chain reaction. Plants of several cucurbits, including a wild citron (Citrullus lanatus var. citroides), were symptomless and had a decreased frequency of virus infection of vine segments compared to susceptible vining cucurbits, indicating a higher level of resistance. However, no significant relationship between the frequency of infection or virus distribution within plants and the symptom response was observed. These results demonstrate that a diverse group of cucurbits may decline when infected with SqVYV, and suggest that widespread distribution of virus within the plant is not the sole cause of decline.

Physiological Effects of Squash vein yellowing virus Infection on Watermelon.

Squash vein yellowing virus (SqVYV) is the cause of viral watermelon vine decline. The virus is whitefly-transmitted, induces a systemic wilt of watermelon plants, and causes necrosis and discoloration of the fruit rind. In the field, SqVYV is often detected in watermelon in mixed infections with other viruses including the aphid-transmitted Papaya ringspot virus type W (PRSV-W). In this study, watermelon plants of different ages were inoculated with SqVYV or SqVYV+PRSV-W in the greenhouse or SqVYV in the field to characterize the physiological response to infection. Symptoms of vine decline appeared about 12 to 16 days after inoculation with SqVYV regardless of plant age at time of inoculation, plant growth habit (trellised or nontrellised), and location (greenhouse or field). However, the presence of PRSV-W delayed the appearance of vine decline symptoms by 2 to 4 days, and vine decline did not develop on plants with no fruit. For all inoculation treatments, more severe symptoms were observed in younger watermelon plants. Physiological responses to SqVYV infection included reduction in plant and fruit weights, alterations in fruit rind and flesh color, reduction in fruit sucrose content, increase in fruit acid content, and changes in plant nutrient composition, particularly increases in Ca, Mg, B, Mn, and Zn and decreases in K and N. These results demonstrate wide-ranging physiological effects of SqVYV infection and provide new insights into watermelon vine decline.

Ecology and management of whitefly-transmitted viruses of vegetable crops in Florida.

A variety of fresh market vegetables, including watermelon and tomato are economically important crops in Florida. Whitefly-transmitted Squash vein yellowing virus (SqVYV) was first identified in squash and watermelon in Florida in 2005 and shown to cause a severe decline of watermelon vines as crops approach harvest. Florida is most economically impacted by SqVYV, although the virus has been detected more recently in Indiana and South Carolina. The origin and evolutionary history of SqVYV, one of the few members of the genus Ipomovirus within the family Potyviridae, are not known. Sequence diversity of SqVYV isolates collected at different times, from different locations and from different plant species is being analyzed for insights into the origin of the virus. More recently, Cucurbit leaf crumple virus (CuLCrV) and Cucurbit yellow stunting disorder virus (CYSDV), also whitefly-transmitted, have been detected in watermelon in Florida. Tomato yellow leaf curl virus (TYLCV) was first detected in south Florida tomato crops in 1997. Several surveys have been conducted in the region to identify alternative hosts for these four viruses. Cucurbit weeds including Balsam-apple (Momordica charantia), creeping cucumber (Melothria pendula) and smellmelon (Cucumis melo var. dudaim) provide reservoirs for SqVYV, CuLCrV and/or CYSDV. Green bean (Phaseolus vulgaris) also can be a reservoir for CuLCrV. No wild hosts of TYLCV have been reported in Florida. The effectiveness of insecticides and silver plastic mulch to manage whiteflies and mitigate TYLCV has been demonstrated and is currently being evaluated for SqVYV, CuLCrV and CYSDV. In addition, potential sources of SqVYV resistance have been identified in greenhouse and field screening of watermelon germplasm. Further studies to refine these sources of resistance are underway. Lastly, a comprehensive map of 33,560 hectares (82,928 acres) of vegetable fields in the three counties comprising the majority of the southwest Florida vegetable production area has been developed to identify 'hot spots' and reservoir crops for viruses and whiteflies, and will be useful in evaluation of management strategies to decrease virus incidence in commercial fields.

Sensitivity of Isolates of Phytophthora capsici from the Eastern United States to Fluopicolide.

Fluopicolide, a pyridinylmethyl-benzamide fungicide, was registered in the United States in 2008 to control diseases caused by Oomycete pathogens, such as Phytophthora capsici, on cucurbit and solanaceous vegetables. The main objective of this study was to determine baseline sensitivity to fluopicolide in isolates of P. capsici from the southeastern and midwestern United States. A total of 69 isolates from Florida (14 isolates), Georgia (14 isolates), Michigan (24 isolates), North Carolina (3 isolates), and South Carolina (17 isolates) that had not been previously exposed to fluopicolide were grown on fungicide-amended medium to determine sensitivity of mycelia, sporangia, and zoospores to the fungicide. All isolates of P. capsici tested (range of 54 to 69 isolates per assay) were sensitive to fluopicolide in all four assays. The median EC50 fluopicolide concentration was 0.22, 2.08, 0.048, and 0.10 mg/liter in the mycelial growth, zoospore germination, sporangia production, and zoospore production assays, respectively. For mycelial growth and zoospore germination, isolates from Michigan had a higher mean EC50 value than isolates from the four southeastern states. This is the first report of variation in baseline sensitivity to a fungicide by P. capsici isolates from different regions of the United States. In the sporangia production and zoospore production assays, isolates from different states did not differ in sensitivity. Single rates of fluopicolide were tested with additional isolates to validate discriminatory rates for monitoring sensitivity. A concentration of 0.3 or 1.0 mg/liter is recommended for mycelial growth, and 0.1 mg/liter is recommended for sporangia and zoospore production.

Distribution of four viruses in single and mixed infections within infected watermelon plants in Florida.

Whitefly-transmitted Squash vein yellowing virus (SqVYV) and Cucurbit leaf crumple virus (CuLCrV) and aphid-transmitted Papaya ringspot virus type W (PRSV-W) have had serious impact on watermelon production in southwest and west-central Florida in the past 5 years. Tissue-blot nucleic acid hybridization assays were developed for simple, high-throughput detection of these three viruses as well as Cucurbit yellow stunting disorder virus (CYSDV), which was first reported in Florida in 2008. To determine virus distribution within plants, we collected 80 entire plants just before or during the harvest period in a systematic sample, 20 each on 11 April, 18 April, 26 April, and 3 May 2007, from a fruiting commercial watermelon field near Immokalee, FL showing symptoms of infection by SqVYV, CuLCrV, and PRSV-W and, possibly, CYSDV. This was followed by a sampling of five plants collected at harvest showing symptoms of virus infection on 11 October 2007 in a different commercial planting located in Duette, FL. Tissue prints were made from cross sections of watermelon plants from the crowns through the tips at 0.6-m intervals on nylon membranes and nucleic acid hybridization assays were used for virus detection. Results from testing crown tissue showed that SqVYV, CuLCrV, and PRSV-W were present in ≈37, 44, and 54%, respectively, of the 80 plants collected over the four sampling dates from the first field. For individual vines diagnosed with SqVYV, the distribution of SqVYV in vine tissue decreased proportionately with distance from the crown. The probability of detecting SqVYV was 70% at the base of the vine compared with 23% at the tip of the vine. In contrast, CuLCrV tended to be more evenly distributed throughout the plant, with ≈10% higher probability of detection at the growing tip relative to the crown of the plant. The distribution of PRSV-W resembled that of SqVYV but with ≈20% higher probability of detection at the tip of the vine. Similar trends were detected in the smaller sampling; however, CYSDV was also detected in three of the plants. Overall, the results indicated that SqVYV and PRSV-W were distributed differently than CuLCrV in watermelon plants, and this difference has implications on how samples should be collected and may affect vector acquisition and transmission of these viruses.

Grafting for Management of Southern Root-Knot Nematode, Meloidogyne incognita, in Watermelon.

Four bottle gourd (Lagenaria siceraria) cultivars, one squash (Cucurbita moschata × C. maxima) hybrid, five wild watermelon (Citrullus lanatus var. citroides) germplasm lines, and one commercial wild watermelon (C. lanatus var. citroides) cultivar were evaluated as rootstocks for cultivated watermelon (C. lanatus var. lanatus) in fields infested with the southern root-knot nematode (Meloidogyne incognita) in Charleston, SC in 2007 and 2008, and in Quincy, FL in 2008. Commercial watermelon 'Fiesta' (diploid seeded) and 'Tri-X 313' (triploid seedless) scions were grafted onto the rootstocks in 2007 and 2008, respectively. In 2007, the plants grafted on rootstock from the wild watermelon germplasm line RKVL 318 had significantly less (P < 0.05) root galling than nongrafted 'Fiesta' watermelon or plants with the squash hybrid or bottle gourd rootstocks. In 2008, 'Fiesta' plants with rootstocks from all five wild watermelon germplasm lines and the commercial watermelon rootstock had significantly less (P < 0.05) root galling than plants with the squash hybrid or bottle gourd rootstocks. Root galling of the squash hybrid and bottle gourd rootstocks was severe (78 to 99%) in both years. Root galling for nongrafted 'Fiesta' and 'Tri-X 313' watermelon was 36 and 50%, respectively. Root galling for the wild watermelon germplasm lines ranged from 11 to 34% and 36 to 44% in 2007 and 2008, respectively. Wild watermelon germplasm lines derived from C. lanatus var. citroides were identified that may be useful as resistant rootstocks for managing root-knot nematodes in watermelon.

Squash vein yellowing virus Detection Using Nested Polymerase Chain Reaction Demonstrates that the Cucurbit Weed Momordica charantia Is a Reservoir Host.

Squash vein yellowing virus (SqVYV) is a recently described ipomovirus from cucurbits in Florida that induces the relatively unusual symptoms in watermelon of plant death and fruit rind necrosis and discoloration, commonly known in Florida as watermelon vine decline. In this report, SqVYV infection of Momordica charantia (Balsam-apple), a common cucurbit weed, collected in 2005 and 2007 from within or adjacent to fields of declining watermelon, is demonstrated through the use of nested polymerase chain reaction (PCR). M. charantia plants located in or around fallow watermelon fields between spring and fall 2007 watermelon crops were also infected with SqVYV, indicating that this weed can serve as an oversummering host for this virus. Furthermore, whiteflies were able to acquire SqVYV from infected M. charantia and transmit it to squash and watermelon. Nested PCR was 10 to 1,000 times more sensitive than non-nested PCR for SqVYV detection in several cucurbit hosts, including M. charantia and watermelon. Melothria pendula (creeping cucumber), another common cucurbit weed, was experimentally infected with SqVYV. These results suggest that improved management of M. charantia and other cucurbit weeds needs to be incorporated into watermelon vine decline management plans to reduce sources of SqVYV and other cucurbit viruses.