Virus-tolerant varieties identified
Arizona is second in the nation in melon production. Although California produces the most melons, Arizona produces more melons on desert acres with an extended growing season. Between 36,000 and 38,000 acres of melons are grown in three Arizona counties. Yuma County grows some 9,000 acres of cantaloupes and about 1,000 acres of honeydew melons, about 85 percent of the melons grown in the state. When melon vines in Yuma County began yellowing in 2006, the concern was high. About the same time, similar yellowing of vines was occurring in the adjacent Imperial Valley of California and in Sonora, Mexico. Data from field trials is helping growers select varieties that better tolerate the disease causing the damage.
“At first, we thought it was a nutrient deficiency,” said Kurt Nolte, University of Arizona extension director and agricultural agent for Yuma County. As the yellowing vines became more prevalent, Nolte and associates realized that something other than that was causing the problem. “We sent some samples to Judith Brown at UA Plant Sciences in Tucson,” Nolte said, and Brown identified the problem as cucurbit yellow stunting disorder virus (CYSDV).
The fall crops of both cantaloupe and honeydew melons showed signs of the CYSDV, which is spread by whiteflies as they seek various plants to feed upon, with significant populations present in the desert region in the fall. A regional management approach was developed among faculty, extension specialists and scientists from UA, the University of California, the University of Sonora and USDA. The approach to managing the problem includes identifying rapid diagnostic efforts, finding the most effective whitefly control measures, identifying melon varieties that are least susceptible and developing new, CYSDV-resistant melon varieties.
Nolte has conducted field studies to identify current varieties that are least susceptible to the disease or that tolerate the disease better if infected. “We have two years of data. We can’t say these varieties are resistant, but they are less susceptible,” Nolte said. He has developed a susceptibility index that rates varieties on their susceptibility to the disease.
CYSDV is spread by the same whiteflies that in the 1980s spread yellow tomato leaf curl across Florida tomato fields. The disease is spread when the whiteflies feed on infected plants, then feed on other plants, transferring the virus. CYSDV was previously thought to infect only the plant family Cucurbitaceae, which includes melons, squash, pumpkins, gourds and cucumbers.
Brown said, “I had spent some time working on this disease on the southern coast of Spain where the virus had spread from its origin in the Middle East, and I had seen the symptoms there in the melon crop in the 1990s. We used a detection method called reverse transcriptase polymerase chain reaction, RT-PCR, to identify the virus.” Studies conducted by Brown at the UA lab in Tucson and by William Wintermantel at the USDA lab in Salinas, Calif., determined hosts for the virus. By applying a more sensitive quantitative real-time polymerase chain reaction (PCR) assay, Brown and Wintermantel were able to identify some of the wild hosts that include the common bean, alkali mallow and Malva parviflora, as well as buffalo gourd, a wild member of the Cucurbitacae plant family.
Brown has also recently identified CYSDV on a papaya plant growing in the Phoenix area. She said, “These are not grown commercially, but in backyard gardens. But if the virus can infect papayas, which belong to a completely different plant family than any other host known so far, it leaves open the question of what other plant species may be hosts of the virus.”
CYSDV has been identified in the Rio Grande Valley of Texas, but whiteflies are not so prevalent there because the valley has a different cropping system than found in the hot deserts of the southwestern U.S. Brown said, “Whiteflies feed extensively on the cotton raised in Arizona. When the cotton is harvested the plants are defoliated, and the whiteflies leave the cotton and search for other plants to feed on. As they disperse, they feed on CYSDV-infected weeds and other hosts and transmit the virus to young melon plants.
Identifying less susceptible varieties
Nolte noted that while some varieties are more tolerant, they may not produce as high a yield as desired by growers, so a tradeoff is sometimes necessary in the selection process. A variety scoring higher on the susceptibility scale may be grown because its yield potential is greater.
Both cantaloupe and honeydew varieties were studied, with 40 varieties of cantaloupes and 20 varieties of honeydews evaluated. A susceptibility index was developed with zero indicating complete resistance to CYSDV and 100 being very susceptible. All 40 varieties of cantaloupes had some degree of susceptibility, and honeydew melons were found to have a much higher susceptibility to CYSDV than cantaloupes. The varieties listed above had more tolerance to the disease than other varieties, and growers have begun planting the more tolerant varieties.
Spring crop planting is usually completed by late February, with fall crop planting in August. Fall crops growing during the times that whiteflies are present are most susceptible. Nolte noted that growers must be extremely vigilant in whitefly control, and they must keep the fields free of weeds. Nolte’s field studies indicated that newly emerged plants were most susceptible to the disease, and if plants could remain disease-free to grow to a larger size, they were able to better tolerate the disease with less impact on the crop. With research indicating more hosts for the disease than originally thought, keeping the fields free of plants that may serve as hosts becomes even more important.
Growing in the desert
Hitting the market at just the right time is significant to marketing any produce, and the extended growing season for cantaloupes and honeydew melons in Yuma County is of major importance. Its role in melon production gained prominence in the early 1920s when buyers realized the area offered an extended growing period.
Nolte said, “Arizona melon production began in earnest during the early 1920s when buyers recognized that Arizona, and the Yuma area in particular, provided the earliest melon harvesttime of mid-May, and an extended harvest period of early November. It’s because of this advantageous harvest window that provides Arizona melon growers with a production niche that still exists today.”
Ongoing melon disease problems occur in Yuma County and the adjacent areas and include diseases such as downy mildew, charcoal rot, powdery mildew and Pythium root rot. “These diseases really have not caused a significant reduction in yield or quality,” Nolte said. “The problem with CYSDV is that it severely affects melon yield and quality, and we have only partial management strategies available for producers.”
Brown said, “It’s important for us to understand and be able to tell growers when the whiteflies are flying. If they just apply chemicals when the plants emerge, and the whiteflies aren’t there, they are putting unneeded chemicals into the environment and wasting money.”
The desert region of Arizona and the Imperial Valley of California have increased food production greatly in the past decade to meet an increasing demand for domestically grown fruits and vegetables. Brown said, “The value and extent of the food crops in the desert growing region is often underestimated. Since we still do not know the extent of the host range nor is the epidemiology of CYSDV understood, it is imperative that we learn more about whitefly vector dynamics in relation to the virus hosts.” She emphasized the concurrent need to help producers with sustainable strategies for management such as the development of tolerant or resistant varieties.
Brown pointed out the need for regional whitefly management that takes into account knowledge of local and regional distributions, including when whiteflies disperse between wild and cultivated hosts. She said, “This information can be learned from trapping data that includes GIS and mapping, which are underway. Implementing a host-free summer period is being addressed in Arizona, California and Sonora, Mexico, which are affected by CYSDV pandemic.”
Looking for disease-resistant varieties
Developing any new variety is a lengthy process. As part of the regional team approach to addressing the CYSDV problem, Jim McCreight, a research horticulturist at USDA Agricultural Research Service, Salinas, Calif., is working with two varieties that show resistance to CYSDV.
Research on a melon variety from Zimbabwe identified as TGR1551 started in Texas when the virus was discovered there about a decade ago. McCreight has used a variety identified as PI 313970 from India which has been crossed with the Top Mark variety. Field trials are being conducted at the University of California Field Station at Holtville in the Imperial Valley.
McCreight said, “To date we have evaluated approximately 300 accessions from India. PI 313970 appears to be a useful source of resistance, but it produces a non-dessert type melon, cucumber shaped and sized fruit that is not sweet. It will take considerable time to recombine resistance with western U.S. shipping type cantaloupe fruit qualities for size, color and flavor. We believe we may need both varieties as sources of resistance.”
Nancy Riggs is a freelance writer and frequent contributor. She resides in Mount Zion, Ill.