While watermelons and pumpkins continue to be major cash crops for Alabama vegetable growers, they often are threatened by plant viruses that cause sporadic but frequently severe production problems.
A study being conducted by the Alabama Agricultural Experiment Station (AAES) is helping to identify potential viral problems in these crops in hopes of developing better prevention and treatment strategies.
In 1999, more than 8,000 acres of watermelons were planted in Alabama with a value of almost $4 million. Pumpkins also are fast becoming an important vegetable crop in the state, with almost 1,000 acres planted in 1999.
Plant viruses, say researchers, are a persistent threat to the production of both watermelons and pumpkins throughout Alabama. Although severe outbreaks of viral diseases tend to occur on a sporadic basis, many plant viruses are ubiquitous in nature and can result in severe outbreaks under the right conditions.
Management of viruses usually is limited to the availability of resistant varieties. However, commercially acceptable varieties that also are resistant to a particular virus or several commonly occurring viruses often are not available.
To learn more about the potential for viral outbreaks, AAES researchers systematically surveyed commercially grown fruit and vegetable crops in Alabama for viruses known to be important pathogens to each of the surveyed crops.
In 1998 and 1999, a survey was conducted to identify viruses in commercially grown watermelons, with an extension of the survey in 1999 into several fields of commercially grown pumpkins.
Each field was evaluated on a whole-field basis for the occurrence of virus-like disease symptoms, such as yellowing (chlorosis) and mosaic or mottled leaves (light green/dark green patches that may or may not have a blistered appearance) and deformation and stunting of leaves and stems.
Twenty to 25 plants in each field were tested for the occurrence of four viruses commonly identified in pumpkin and watermelon crops: cucumber mosaic virus (CMV), papaya ringspot virus (PRSV), watermelon mosaic virus (WMV) and zucchini yellow mosaic virus (ZYMV).
These plants were selected randomly across the field. Each plant represented a single sample and each sample consisted of three leaves collected at different locations along the vine of each plant. Samples were tested for each of the viruses.
In 1998, watermelon fields located in south (Geneva and Houston counties), central (Autauga, Chilton and Elmore counties) and north (Blount County) Alabama were surveyed. Three of the viruses — PRSV, WMV and ZYMV — were detected among the samples tested.
PRSV and ZYMV were detected in Geneva County while samples from Houston County contained only ZYMV. In central Alabama, WMV (Autauga and Chilton counties) and ZYMV (Chilton County) were detected, with no virus detected among samples collected from Elmore County.
The combination of PRSV and WMV occurred in many samples from Blount County with none of the samples containing ZYMV.
The 1999 survey included watermelon crops in Geneva and Blount counties with an additional survey of two commercial pumpkin fields in Blount County. All four viruses were detected in this survey with PRSV and WMV being the predominant viruses.
Four of the seven fields surveyed in Geneva County had WMV as the sole virus detected among the samples with no less than 70 percent infection in four of the fields containing WMV. Samples from two of the fields contained no virus infection while one sample was infected with CMV.
In the 1999 survey of pumpkins and watermelons grown in Blount County, PRSV and WMV were detected in samples from each field with a total of only three samples containing ZYMV. PRSV infection ranged from 10 to 86 percent among the different fields while that of WMV was consistently higher, ranging from 86 to 100 percent. Many of the plants contained a mixed infection of PRSV and WMV.
Watermelon plant growth varied greatly from one field to another, according to the research. Many fields were not irrigated and, as a result, plants were a pale green and leaves were slightly to severely wilted. Watermelon plants receiving irrigation tended to be large and dark green.
Regardless of the condition of the plants, virus infection was widespread. In non-irrigated fields, virus symptoms consisted of a mild mosaic of leaves. A mosaic symptom also occurred in plants in irrigated fields. But, because the plants were significantly more vibrant in growth, symptoms tended to be more pronounced.
Ironically, there was less virus infection in two fields that contained excessive amounts of weeds. The low amount of infection in the weedy fields likely resulted from a lack of access to the watermelon plants by the aphids that vector each of these viruses, according to the research.
Pumpkin fields tended to be well maintained and plants showed optimal growth habits. Infected pumpkin plants were severely affected by PRSV and WMV, particularly those plants that contained a mixed infection of both viruses. When a mixed infection occurred, pumpkin plant leaves had a severe mosaic symptom with much blistering and deformation.
Of particular interest in the research was the lack of CMV infection in pumpkins and watermelons in Blount County. CMV has devastated fresh-market tomatoes in Blount and St. Clair counties. Yet, only a single sample was shown to contain this virus even though pumpkins and watermelons are susceptible hosts.
An extreme example of this peculiar virus system occurred in one field where tomato plants severely affected by CMV were immediately adjacent to pumpkin plants. However, none of the pumpkins showed any CMV infection. This clearly illustrates, according to the researchers, the complexity of the biology of these viruses in agricultural crops grown in Alabama.