Distribution of Viruses Infecting Cucurbit Crops and Isolation of Potential New Virus-Like Sequences from Weeds in Oklahoma

Ali, A., Mohammad, O., and Khattab, A. 2012. Distribution of viruses infecting cucurbit crops and isolation of potential new virus-like sequences from weeds in Oklahoma. Plant Dis. 96:243-248. Field surveys were conducted from 2008 to 2010 to detect and determine the incidence of viruses in the major cucurbit-growing areas of Oklahoma. In total, 1,049 symptomatic leaf samples (890 from cucurbits, 109 from weed species, and 50 from crop plants [agricultural crops]) were collected from 90 fields in four counties (Atoka, Blaine, Jefferson, and Tulsa) of Oklahoma. Samples were tested against seven viruses, including Cucumber mosaic virus (CMV), Cucumber green mottle mosaic virus (CGMMV), Melon necrotic spot virus (MNSV), Papaya ringspot virus-watermelon strain (PRSV-W, formerly known as Watermelon mosaic virus-1), Squash mosaic virus (SqMV), Watermelon mosaic virus-2 (WMV-2), and Zucchini yellow mosaic virus (ZYMV), using dot-immunobinding assay (DIBA). Results showed the highest incidence for PRSV (51%), followed by WMV-2 (14%) and ZYMV (10%) among the collected samples. SqMV, MNSV, and CMV were detected in 3.8, 3.3, and 1.1% of the samples, respectively. None of the samples collected during surveys reacted positively against the antiserum of CGMMV. Mixed virus infections were common involving two (5.18%) or three (4.61%) viruses in various combinations. New weed host species were found to be infected with PRSV when confirmed by both DIBA and reverse-transcription polymerase chain reaction (RT-PCR). Some weed species contained possible new viruses when analyzed by random RT-PCR, followed by cloning, sequencing, and BLAST analysis with sequences in GenBank. Cucurbits (cucumber, cantaloupe, pumpkin, watermelon, and zucchini squash) are economically important cash crops worldwide, including the United States. Virus diseases are a major constraint in commercial cucurbit production (21,27), causing sporadic epidemics. More than 39 different viruses have been reported to cause cucurbit diseases and many are responsible for economic losses in the quality and quantity of cucurbit crops (15,18,27). In the United States, cucurbit crops are grown in several states and are commonly infected by a number of viruses. Several of these viruses, particularly the Potyvirus spp. Papaya ringspot virus-watermelon strain (PRSV-W, formerly known as Watermelon mosaic virus-1) (29), Watermelon mosaic virus-2 (WMV-2) (30), Zucchini yellow mosaic virus (ZYMV) (19,20), and Cucumber mosaic virus (CMV) (9), are the most frequently encountered and economically important (24,27). These viruses have been shown to infect cucurbit crops in major cucurbit-growing areas of California and Connecticut (25,28), Florida (1,31), Hawaii (35), Illinois (14), New Jersey (7), New York (25, 28), South Carolina (34), and Texas (5,11,12,22). These viruses are transmitted by many aphid species in a nonpersistent manner. This makes them difficult to control by insecticidal sprays because the aphids are capable of rapidly transmitting the viruses before the insecticides can kill them (36). These viruses are a significant and continuous threat to cucurbit production in these regions. In Oklahoma, cucurbit crops, including cucumber, melon, squash, pumpkin, and watermelon, are grown on about 5,088 acres annually (26). Among these, watermelon is a very important cash crop (grown on approximately 4,500 acres) and currently generates $3.5 million annually in farm income. Several virus-like mosaic diseases have been observed in Oklahoma for the last several years that affect cucurbit crops, particularly watermelon and pumpkin. Natural incidence of PRSV-W, WMV-2, ZYMV, CMV, and Squash mosaic virus (SqMV) with pumpkin has been reported in field trials located at the Vegetable Research Station Bixby and Wes Watkins Agricultural Research and Education Center in Lane, OK (6). However, there is no detailed information on the incidence, ecology, spread, and distribution of these viruses in watermelon and other cucurbit crops in the commercial grower’s fields. Information on the identity and distribution of viruses and potential reservoir hosts in major cucurbit production areas is important in developing and implementing effective control strategies for virus diseases in local areas. In addition, accurate virus identification and detailed information about prevalent viruses in a particular region is important for growers. The purpose of this study was to determine the incidence and distribution of the most prevalent viruses infecting cucurbit crops in the grower fields, and identify their potential reservoir hosts in the major growing areas of Oklahoma. Materials and Methods Seed, antisera, and positive control of viruses. Squash seed was kindly provided by Marilyn Roossinck, the Samuel Roberts Noble Foundation, Ardmore, OK. Seven kits that included polyclonal antiserum and positive controls of CMV, Cucumber green mottle mosaic virus (CGMMV), Melon necrotic spot virus (MNSV), PRSV-W, SqMV, WMV-2, and ZYMV were obtained commercially (AC Diagnostics, Arkansas, and Agdia, Inc., Indiana). Positive controls of all seven viruses were supplied in lyophilized form with the kits and were resuspended in the sampled buffer as recommended by the manufacturers’ instructions (AC Diagnostics and Agdia, Inc.). Survey area and sample collections. Surveys of cucurbit crops (cantaloupe, cucumber, pumpkin, squash, and watermelon) and weeds within adjacent fields were conducted during the growing seasons of 2008, 2009, and 2010 in four counties of Oklahoma that included Atoka, Blaine, Jefferson, and Tulsa (Fig. 1). Atoka County is located in the south, Blain County in the west, Jefferson County in the southwest, and Tulsa County in the east of Oklahoma. Surveys were conducted from July to August of the growing seasons and only symptomatic samples were collected randomly throughout the field in each county. Selected fields were at a disCorresponding author: A. Ali, E-mail: [email protected] Accepted for publication 6 September 2011. http://dx.doi.org/10.1094 / PDIS-05-11-0419 © 2012 The American Phytopathological Society