Whitefly-Transmitted and Yellowing Viruses in Watermelon and Other Cucurbit Crops
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Whitefly-transmitted and Yellowing Viruses in Watermelon and Other Cucurbit Crops Cucurbits, including watermelons, cantaloupes viruses have been present in the U.S. for many and other melons, cucumbers, pumpkins, and years, but within the past two decades, the squashes, are important agricultural crops in number and distribution of these viruses has many U.S. states. Many of these crops are grown greatly expanded. This increase has largely in the field, while some, particularly cucumbers, coincided with an increase in the movement of may also be grown in enclosed structures, plant material, including grafted transplants, such as high tunnels and greenhouses. Florida, which play an important role in the management Texas, Georgia, and California lead the U.S. in of some diseases. Because transplants may watermelon production. However, watermelon remain symptomless up to 3 weeks after is produced commercially on some level in many infection, infected transplants may unknowingly states, including throughout the Southeast. Other be moved among regions and lead to accidental cucurbit crops, such as cucumbers, pumpkins, introduction of viruses. In addition, yellowing and squashes, are also produced commercially viruses generally produce symptoms that mimic in these states and contribute to the agricultural nutritional deficiencies, and this has historically economy of the region. caused virus infections to go undetected. A variety of diseases, disorders, and pests In addition to yellowing viruses, other may damage a crop during any given growing whitefly-transmitted viruses can also lead to season, leading to reduced or poor-quality severe losses in cucurbit production. These yields. Cucurbits, like all crops, can be infected viruses often occur in fields together with by disease-causing organisms known as yellowing viruses. Mixed infections (where pathogens, including fungi, bacteria, and viruses. different viruses infect the same plant) can alter Viruses can cause many types of symptoms on the appearance of symptoms, which makes plants, including stunting, leaf curling, necrosis field identification challenging. Many of the or vine collapse, mottles or mosaics, bumpiness whitefly-transmitted viruses that originally on leaves and fruit, and chlorosis (yellowing). emerged in a specific region of the U.S. have In cucurbit crops, most of the viruses known now been accidentally introduced to most other to cause economically important diseases are production regions. For example, cucurbit transmitted by insects, and many induce foliar yellow stunting disorder virus (CYSDV) and yellowing symptoms. These are known as cucurbit leaf crumple virus (CuLCrV) were “yellowing viruses.” Most yellowing viruses are transmitted by whiteflies, although at least one is transmitted by aphids. A few yellowing A B Figure 2. Greenhouse whitefly (Trialeurodes vaporariorum) adults and Figure 1. Sweetpotato whitefly Bemisia( tabaci) adults (A) and nymphs (B). nymphs. Photo: W. Cranshaw, Colorado Photos: D. Riley, University of Georgia, Bugwood.org. State University, Bugwood.org. established in California and Arizona but are lens or microscope, sweetpotato whitefly nymphs now prevalent in the Southeastern U.S. Similarly, are smooth (Figure 1B), whereas greenhouse squash vein yellowing virus (SqVYV) was whitefly nymphs are oval and have long, waxy, originally identified in Florida but is now found in hair-like structures (Figure 2). Knowing the type California and Arizona. In both cases, movement of whitefly present in crops can help determine of infected plant material is suspected as the what viruses may be present. source of introduction. As previously mentioned, watermelon The two types of whitefly known to spread and other cucurbit producers face a variety viruses among cucurbit crops in the U.S. are the of challenges throughout the growing season. sweetpotato whitefly (Bemisia tabaci) and the Diagnosis is the first step in disease management. greenhouse whitefly (Trialeurodes vaporariorum). Identifying the cause of a problem is necessary The sweetpotato whitefly (Figure 1A) is white, before appropriate management methods can be often with a pale-yellow body, and its wings taken. This publication provides information on are angled like the roof of a house, but with a the yellowing viruses that can infect watermelon distinct gap between the wings. In contrast, the and other cucurbit crops as well as descriptions of greenhouse whitefly (Figure 2) is slightly larger the symptoms associated with the diseases caused than the sweetpotato whitefly, and its wings by those viruses. Several plant pathology and are more horizontal with only a slight angle entomology terms are used in these descriptions. (frequently overlapping one another), more like These terms are formatted in bold and italics the shape of a fighter jet. Nymphs (immature at their first mention within the text and are whiteflies) of these species are also easy to defined in the Glossary of Plant Pathology and differentiate. When viewed through a magnifying Entomology Terms at the end of this publication. General management methods that can be used to CYSDV can infect beans, lettuce, and numerous prevent or reduce diseases caused by these viruses weeds (Wintermantel et al., 2009). In the U.S., are described in the Management section. CYSDV is known to infect cucurbit crops in Arizona, California, Florida, Georgia, and Texas, Viruses that Cause Yellowing in and has recently been reported in South Carolina Watermelon and Other Cucurbit Crops (Kousik and Adkins, 2020). Symptoms: CYSDV, like other criniviruses, has a WHITEFLY-TRANSMITTED VIRUSES long latent period, and symptoms will not appear Cucurbit yellow stunting disorder virus (CYSDV; until 3 weeks after plants become infected. As genus Crinivirus) is transmitted by at least two such, symptoms may appear well after whitefly biotypes of the sweetpotato whitefly. Biotype B is populations have been reduced. Symptoms begin common in vegetable production regions in the as a mottle but then develop into an extensive U.S., whereas biotype Q is emerging in parts of yellowing between the veins of leaves (Figures 3 the southeastern U.S. Whiteflies acquire CYSDV and 4). Symptoms initially appear on older leaves when feeding on infected plants and can retain while younger leaves remain green, but over time, and transmit the virus for 7 to 9 days without symptoms spread down vines toward newer having to feed again on an infected plant. CYSDV growth. CYSDV infection can limit plant vigor and can infect all major cucurbit crops, including reduce fruit sugar content, resulting in fruit that watermelons, melons, squashes, pumpkins, are often not marketable. and some wild cucurbits such as buffalo gourds Management: clean transplants, timing of (Cucurbita foetidissima). In addition to cucurbits, planting, vector management, and weed host management A B Figure 3. Symptomatic melon leaves from plants infected with cucurbit yellow Figure 4. Mild foliar yellowing on a stunting disorder virus. Early stages of symptom development, a yellow-green watermelon leaf caused by cucurbit mottle, are visible in 3A. More advanced stages of symptom development in yellow stunting disorder virus which the entire leaf exhibits interveinal chlorosis with veins remaining green are infection. Photo: W. M. Wintermantel, visible in 3B. Photos: W. M. Wintermantel, USDA-ARS. USDA-ARS. Cucurbit chlorotic yellows virus (CCYV; genus can lead to reduced fruit sugar content and yield Crinivirus), like CYSDV, is transmitted by at in watermelons and melons and reduced yield in least two biotypes of the sweetpotato whitefly, cucumbers. biotypes B and Q. The virus is acquired when Management: clean transplants, timing of whiteflies feed on infected plants, and once planting, vector management, and weed host acquired, whiteflies can transmit CCYV for a management period of 11 to 14 days without having to feed again on an infected plant. CCYV can infect Beet pseudoyellows virus (BPYV; genus all cultivated cucurbits, including cucumbers, Crinivirus) is transmitted by the greenhouse pumpkins (processing), melons, and watermelons. whitefly but not by the sweetpotato whitefly. Once In addition to cucurbits, CCYV has been found to the whitefly acquires the virus, it can transmit it infect a number of other plants, including beets, to new plants for up to 6 days. BPYV is capable lettuce, spinach, and some weeds (Okuda et al., of infecting an unusually wide range of host 2010). CCYV was recently identified in California plants, including members of the cucurbit family (Wintermantel et al., 2019); due to similarity in and a large number of ornamentals, small fruits, symptoms with other yellowing viruses, it may be vegetables, and weeds. The virus is believed to present yet undetected in other areas of the U.S. be widely distributed throughout the world and Symptoms: Symptoms on cucumbers, melons, is found in association with infestations of the watermelons, and some other cucurbits begin greenhouse whitefly. as chlorotic spots that progress to extensive Symptoms: Like its close relatives, CYSDV yellowing of leaves, while leaf veins typically and CCYV, symptoms of BPYV typically do not remain green. Symptoms initially appear on appear until 3 to 4 weeks after infection. On older leaves near the crown, but over time most cucurbits, symptoms begin as a mottle and spread toward new growth, with the newest develop into extensive interveinal chlorosis. These leaves often remaining green until late in symptoms