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

Using Insecticides to Control Whiteflies in Spring Melons

In our last update we discussed whitefly management, but now that daytime temperatures are forecasted to be in the 90’s for the next 10 days, I thought that it would be good to review your insecticide options for whitefly management. Several factors play a role in determining which insecticide(s) you may want to apply. If the grower applied a neonicotinoid at planting (e.g., imidacloprid) the soil residual is likely long gone and it is likely no longer providing control of nymphs and newly-emerging adults. Thus, if whitefly population abundance is sufficiently high enough to justify control, a foliar insecticide is recommended. When nymphs can easily be found on the crown and mid-vine leaves, an IGR or IGR-like product is recommended. This would include: Vetica, Courier, Oberon, and Knack. None of these products will provide good adult knockdown, but if applied correctly, will eventually suppress adult populations by preventing the development of the nymphs within the field. Experience has shown that these products can provide 14-21 days of residual control of nymphs. Control of adults infesting plants from outside sources will require a different approach. A Vydate or Lannate combined with a bifenthrin or Danitol tank-mixture is an option, but will likely only provide adult knockdown, with limited residual control (3-5 d). Among the neonicotinoids, Assail and
Venom/Scorpion have shown the most consistent residual adult control (7-10 d) in experimental trials and will also provide decent control of nymphs on treated leaves. Because neonicotinoids are used on many crops grown throughout the year be sure to consider resistance management statements on the label as well as the UA Cross-commodity Guidelines before you apply them. A newer mode of action, Exirel (anthrillic diamide) can provide knockdown and residual control (14 d) of both adults and nymphs following foliar application. Because Exirel works via translmaniar movement in treated leaf tissue, good spray deposition on all melon foliage is important for maximizing control of both adults and nymphs. This brings up another factor that is important to consider – presence of pollinators in or near the field. Be sure to check the label carefully for the Environmental Hazards statement, specifically for language and restrictions on honey bee safety. In some cases, products can be used effectively and safely through application timing and rates, whereas in other cases, some products should not be used when plants are flowering and pollinators are actively working fields. You must read the label carefully. The proximity to harvest may limit your choices as well. The PHIs for whitefly products vary anywhere from 0-7 days. Also, the presence of worms near harvest may influence your choice of products. If you’re using a whitefly specific product (e.g., Courier, Oberon, Knack, Assail, Venom/Scorpion) you might consider adding a pyrethroid for cabbage looper control, or a Lep material (e.g., Radiant, Intrepid, Coragen, Belt) for control of both looper and beet armyworm. Products such as Vetica and Exirel are efficacious against both whiteflies and Lep species. Visit these publications for information on products available for Whitefly Control on Desert melons and Lepidopterous Larvae Control on Desert Melons.

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

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Powdery mildew

Spring is a time of transition for agriculture in the desert southwest. Cool season crop harvest is wrapping up and spring and summer crops are being planted and grown. This is also powdery mildew season. Powdery mildew can develop on commercial crops, such as late-season lettuce, wheat or melons, as well as landscape plants. It is not too early to begin considering management options for powdery mildew on melons. The disease generally is favored by a lack of rainfall, moderate temperature and relative humidity levels, reduced light intensity for leaves within the plant canopy, fertile soil, and succulent plant growth. The overall risk of powdery mildew increases as more of these factors become established in a melon field. Lack of rainfall and fertile soil are givens in our desert melon production fields. Spores of the melon powdery mildew pathogen, Podosphaera xanthii, can germinate to initiate disease at temperatures ranging from 72 to 88°F, and optimally at about 82°F. These moderate temperatures as well as reduced light intensity within the leaf canopy and succulent plant growth all become increasingly prevalent as the melon plantings grow rapidly during April and May. Another factor to consider when determining powdery mildew risk is the inherent susceptibility of the melon cultivar being grown. Those varieties known to be susceptible to powdery mildew will require implementation of a rigorous disease management program involving applications of fungicides with differing modes of action throughout the period of high disease risk. On the other hand, melon varieties that have moderate to high levels of genetic resistance to the pathogen will require less fungicide inputs. To achieve maximum levels of disease control, powdery mildew fungicide application programs must be initiated before the visible detection of the fungus. Good levels of disease control can also be attained by waiting to begin fungicide applications until no later than the very first sign of disease in the field. These initial infection sites are often on the underside of leaves, so frequent and comprehensive examination of the melon planting is required.

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Weed Science:


Pigweeds are some of the most common summer annual broadleaf weeds in the low deserts. Although they are often lumped together, there are 4 different species of pigweed that are common here and more than 10 species that occur as weeds in California and Arizona. Their growth habits and response to herbicides are similar. It is easy to identify them by physical characteristics but one species of pigweed can hybridize with another and become less distinguishable.

Palmer Amaranth: (Amaranthus palmeri) is probably the most common pigweed species found in this region. It is very aggressive and fast growing and can become 6 feet tall or higher if uncontrolled. It has one thick stem and several lateral branches. The leaves are lance shaped, hairless and have distinctive white veins on the underside. It has flowering tassels that become stiff and spiny. This species has become resistant to Glyphosate in many parts of the county.

Redroot Pigweed: (Amaranthus retroflexus) is probably the second most common pigweed species. It is shorter and the seed heads are smaller, in clusters and have stiff spine-like scales. It has leaf hairs on the margins and the veins are often reddish. The lower stems are often reddish. This species will hybridize with Palmer Amaranth and become less distinguishable.

Tumble Pigweed: (Amaranthus albus) is very different from Palmers or Redroot. It grows lower to the ground and has many branches that turn upright. The leaves are much smaller and narrower. The numerous stems are light green rather than red. The seed heads are small, spiny and at the base of the leaves rather than in long terminal spikes. When mature, the branches are sticky, stiff bristles that break off at the ground and tumble with the wind.

Prostrate Pigweed : (Amaranthus blitoides) is very similar to Tumble Pigweed but the stems are more prostrate, grow close to the ground and form mats. The stems and leaves are smaller and reddis

This question comes up every year. The answer is that it is unlikely. Glyphosate binds strongly to soils, especially those that are fine textured. Glyphosate is a systemic herbicide and it must be absorbed into the plant to work. If it is bound to soil it is no longer active. This is still the case if treated soil is deposited onto the foliage. Dirty water that contains soil particles also reduces the activity of glyphosate. We have conducted trials in the greenhouse where we blew treated soil unto foliage and seen no symptoms. This is not the same for all herbicides. Oxyfluorfen (Goal, Galigan), for instance, does not adhere well to soil and it can “lift off” readily from the soil with water and injure crops it comes in contact with.

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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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