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University of Arizona
March 2nd 2014 Vegetable IPM Updates
Insect Management
Weed Science
Insect Management:

Be on the Look-out For a New Whitefly-Transmitted Virus on Cucurbits

Last fall, researchers from the University of California discovered pumpkin plants on the UC Research Center in Holtville that showed signs of viral infection (severe stunting, leaf yellowing, and leaf curling) associated with high populations of whiteflies. Through a series of laboratory tests, they found the plants were infested with a virus similar to Squash vein yellowing virus (SqVYV), a Bemisia whitefly transmitted ipomovirus (family Potyviridae). It is not clear where the ipomovirus originated and fortunately since the initial discovery, subsequent sampling of melons this spring has yielded no further infected plants. So why is this important? If this ipomovirus were to spread throughout the region it could pose a threat to melon production in the desert southwest, particularly on watermelons in years when whitefly populations are high during the spring. SqVYV was first identified from squash and watermelon in Florida in 2005, and had been causing serious decline of watermelon vines since 2003. To date, Florida is the most economically impacted area by SqVYV, but the virus has also been detected in Indiana and South Carolina. As noted above, whiteflies (Bemisia tabaci b-biotype) acquire SqVYV in semi-persistent manner, and appear to retain the virus for only 24 hr after leaving infested plants. In commercial watermelon production, symptoms of SqVYV vine decline are usually first noticed at or just before harvest when whitefly populations are allowed to build up. Symptoms include yellowing, scorched or brown leaves, defoliation and wilting of the vines, and a rapid collapse of mature vines. The interior of the mature fruit rind can be discolored (brown areas under the rind), rendering the fruit unmarketable. Early symptoms on watermelon include yellowing and downward curling of the new growth. Based on reports from Florida, muskmelon (Cucumis species) are not very susceptible to SqVYV. To date, the host range of SqVYV appears to be limited only to cucurbit crops and weeds. For more information on this virus visit these publications: Recommendations for Management of Whiteflies, Whitefly-transmitted viruses, and Insecticide Resistance for Production of Cucurbit Crops in Florida and A new and potentially damaging whitefly-transmitted virus of cucurbits was found this fall 2014 in Imperial County, CA. If you experience vine decline near or during watermelon harvest or observe any unusual symptoms on melons, please contact us.


Remember, When in Doubt . . . . . “SCOUT”

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Disease Resistance and Tolerance

An exceptionally valuable weapon in the battle to manage plant diseases may reside within the plant’s genetic composition. This genetic disease management tool is commonly referred to simply as disease resistance or tolerance. These names are often used interchangeably; however, the definitions of each term denote a significant difference. Resistance can be thought of as the ability of a plant to exclude or overcome the effect of a plant pathogen, whereas tolerance is the ability of a plant infected by a pathogen to grow without dying or sustaining serious injury or yield loss. Therefore, resistance focuses on preventing infection, whereas tolerance allows infected plants to grow without serious injury or yield loss. Disease resistance and tolerance are not all or nothing conditions. For example, resistance can range from its highest level, which is called immunity, through degrees of useful resistance, and finally to its lowest level, where a plant is highly susceptible to a particular pathogen. Also, resistance and tolerance usually are specific to one or at most a few diseases, and not broadly functional against many or all plant ailments. The mechanisms within plants that give rise to disease resistance and tolerance are many and varied, but usually are biochemical or structural in nature. Successful suppression of pathogen activity by a plant is tied to how a particular pathogen gains entrance into a plant to initiate disease and additionally how a plant defends itself from that infection. One key advantage of strong genetic resistance or tolerance is that this disease management tool will be active for the life of the plant, without any input by the grower. In contrast, disease management products such as fungicides may have to be applied several times so as to be in place over the entire growth period of the plant when disease is expected. Also, disease management provided by plant genetics often targets diseases for which no other known effective disease management tools are known. Building disease resistance or tolerance into plants is an ongoing activity of plant breeders, using classical as well as modern genetic manipulation techniques to achieve this goal.

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Weed Science:


Few weeds are as disliked or as difficult to control as sandbur. There are more than 20 species of sandbur but only two are found in this area. Approximately 75% of the sandbur in this region is field sandbur (Cenchrus longines) with the remainder being southern sandbur (Cenchrus Encinitas). The burs are stiffer and longer than those of bur clover but not as stiff as those on puncture vine. Their ability to stick onto almost anything aids in their distribution. They are difficult to remove and often break off in your fingers and cause temporary numbing of the nerves. Field sandbur has a long thin leaf that resembles bermudagrass. The burs are yellow and there are relatively few of them compared to southern sandburs which are denser, have a reddish coloration and a broader leaf. Both have similar growth habits. They are supposed to be summer annuals but many plants will overwinter and come back the following season. They also both are found almost exclusively in sandy soil. No one has ever explained to me why this is. Postemergence grass herbicides don’t work on sandbur. It can be adequately controlled with preemergence herbicides (Prowl, Treflan etc.) but these won’t work on plants that have overwintered. Each bur contains 3 to 6 seeds that germinate in the bur.



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The Vegetable IPM Updates Archive page provides links to updates from previous weeks.

The Vegetable IPM Video Archive page contains a collection of educational videos from current research work in vegetable crops by University of Arizona Researchers.


For questions or comments on any of the topics please contact Marco Pena at the Yuma Agricultural Center.
College of Agriculture, The University of Arizona, Tucson, AZ.

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