Infectious Yellowing Diseases
of Watermelon in Texas

By Tom Isakeit
Associate Professor and Extension Plant Pathologist
Texas A&M University, College Station

Several different infectious yellowing diseases occur on watermelon in Texas. These diseases can be difficult to diagnose, because the symptoms can resemble pesticide toxicity or nutrient imbalances, and the pathogens themselves are difficult to isolate and study. Insect vectors are involved with many of the pathogens, further complicating diagnosis. A common feature of these diseases is that effective control measures are not available. This article is a review of yellowing diseases of watermelon that suddenly appeared in Texas over the past decade.

Why do these diseases seem to just suddenly appear? Some pathogens may be introduced through the movement of infected seed or transplants, or carried within migrating insects, but others may have been present in the growing area all along, residing in weeds and native plants. Here, these pathogens are inconsequential. But some change in the nature of an insect vector could make it much more efficient for transmitting a pathogen. So the pathogen is then moved into a crop, where it causes substantial disease. The origin of the diseases discussed in this article are not known with certainly, but an understanding of where these diseases come from could provide some insight into control measures.

Lettuce Infectious Yellow Virus (LIYV)
This is a whitefly-transmitted virus that was first detected in 1982 in the Imperial Valley of California and in Arizona. At that time, it caused serious disease not only on cucurbits, but also on lettuce. This virus has a wide host range that encompasses 15 plant families. LIYV became insignificant when the populations of the sweet-potato whitefly shifted from the A biotype to the B biotype (also known as the silverleaf whitefly, which is the predominant biotype in South Texas), because the B biotype is much less effective in transmitting LIYV. (However, the appearance of the B biotype led to the appearance of other, serious virus diseases.)

In the summer and fall of 1991, this virus was reported in Central Texas from stunted watermelon with yellow leaves, but no losses were reported. LIYV has not been detected in South Texas, where whiteflies are a persistent problem, nor has it been detected since 1991 in other parts of the state. Thus, it seems that this virus is no longer a current problem in the state.

Squash Leaf Curl Virus (SLCV)
This whitefly-transmitted virus was first identified in California in 1981. It also occurs in Arizona and Florida. It was confirmed in Texas in 1993, but it was likely present as early as the fall of 1992. Its occurrence in Texas coincided with the population explosion of the B biotype of the sweet-potato whitefly. Its distribution is South Texas, including the Lower Rio Grande Valley, and the Winter Garden areas. A whitefly-transmitted virus — probably SLCV — was identified in smellmelon from Edroy (San Patricio County), and this represents the most northern limit observed. SLCV has not been detected in far West Texas, although this is also an area where whiteflies occur. One possible explanation for this could be the lack of a native plant reservoir for the virus.

SLCV has been devastating to the production of fall-planted watermelons in South Texas, particularly in fields where seedlings are infected soon after emergence. Squash is also quite susceptible, while cantaloupes, honeydew melons, and cucumbers are not severely affected. The virus generally has a narrow host range of cucurbits, although strains in other states have wide host ranges (e.g., beans in California and cabbage in Florida).

Cucurbit Leaf-Curl Virus
This is a new whitefly-transmitted virus of cucurbits, closely related to SLCV. It appeared in 1998 in California, Arizona, Coahuila (Mexico), and the Lower Rio Grande Valley of Texas. In the Lower Rio Grande Valley, this virus is found in mixed infections with another whitefly-transmitted virus, cucurbit yellow-stunt-disorder virus. Such mixed infections can work synergistically, resulting in more severe symptoms. Mixed infections also increase the difficulty of breeding resistant cultivars for disease control.

Cucurbit Yellow-Stunt-Disorder Virus
This whitefly-transmitted virus was first identified in the United Arab Emirates in 1982, and it also occurs in Spain. It was confirmed from the Lower Rio Grande Valley in the fall of 1999, but it was probably present there a year earlier. It was also confirmed in 1999 from far West Texas (Presidio). This virus causes severe yellowing in cantaloupe, which is associated with significant yield losses, but the symptoms and impact on yield are less severe in watermelon. The virus has a host range restricted to the cucurbits.

Yellow Vine
The cause of this disease has not yet been determined, although the cause may be a phloem-inhabiting bacterium. Leafhoppers may be involved in transmission of the pathogen, since weekly insecticide applications provide some degree of control in squash. This disease was first observed in Oklahoma in 1988. It is consistently seen in the Cross Timbers Vegetational Area of Central and North Texas, although it has also been seen in East Texas. The range may be even wider, (it was documented in Tennessee in 1997). It occurs on squash, watermelon, and cantaloupe. Symptoms are generally seen 10 to 15 days before fruit maturity. Leaves turn from green to lime-yellow to bright yellow. Plants decline or wilt and collapse. There is no root rot. A key diagnostic feature is a honey-colored discoloration of phloem when the stem is cut at the crown. (Stem discoloration cause by the Fusarium wilt fungus occurs in the xylem, which is more in the center of the stem.) The severity of the disease in a region varies from year to year. Possible reasons could include differences in vector populations or movement, or differences in the abundance of pathogen hosts. There are no effective control measures, but triploid watermelons are more resistant to yellow vine than diploids.

"High Plains Yellowing" — A New Disease?
This disease was seen on isolated plants in several different fields of different cultivars in one county in the southern High Plains in 1998, 1999, and 2000. The incidence is less than 1 percent, but affected plants are noticeable from a distance because they are bright yellow, in stark contrast to surrounding green plants. There is a progressive yellowing of leaves on a vine, starting with the older leaves. Yellowing may start with the veins of a leaf, or in between the veins, but eventually, the whole leaf turns yellow. The distribution of symptomatic plants in fields suggests that an infectious disease is occurring. To date, there has been no obvious economic loss from it, though,

What Are Control Options?
There are three approaches to controlling yellowing diseases that have an insect vector:

Decreasing pathogen sources may be impossible if the pathogen is widespread in weeds or native plants. This strategy could work only if an insect vector cannot migrate very far, e.g., no further than the weeds around the edge of a field. But vectors such as whiteflies can migrate several miles. In growing areas where multiple crops are possible, the use of a plant-free break between crops can prevent infection of a new crop. Destroying the fall-planted watermelons after harvest to get a plant-free period of four weeks is a key element in preventing infection of the young spring-planted crop with the squash-leaf-curl virus in South Texas.

Decreasing vector populations through the use of insecticides has generally not been successful for virus disease control. Such an approach works only if the virus increases slowly within the field during the season. However, most of the yellowing diseases are associated with initial high populations of insects, and by the time these insects are controlled, the crop is already infected.

Plant susceptibility can be decreased through cultural approaches. These include the use of transplants, since plants infected at a later stage of development are not as severely affected. Cultural approaches can work indirectly by affecting the insect vector. Row covers can effectively exclude insects, but they are not economical for watermelon production. Colored mulches, which alter insect behavior, have given mixed results in experimental trials, as have trials with stylet oils applied to foliage, which interfere with insect feeding. The use of genetic resistance in plants is very effective. Sources of resistance exist for some of the diseases mentioned here, so there is the potential to incorporate them into commercial cultivars. However, some of the viruses can genetically change over time, so plant resistance to these viruses may not be permanent.

For photographs of these diseases, see

http://plantpathology.tamu.edu/texlabn/notice.html
(scroll down and click on 'Vegetables - Watermelons').

For more information, contact Tom Isakeit by telephone at 979 - 862-1340 or e-mail at t-isakeit@tamu.edu.

This article appeared in the December 2000 issue of Vegetable Production & Marketing News, edited by Frank J. Dainello, Ph.D., and produced by Extension Horticulture, Texas A&M University, College Station, Texas.