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Arizona Veg IPM: aphid identification, downy mildew, herbicide codistillation

Arizona Veg IPM: aphid identification, downy mildew, herbicide codistillation

The proper identification of winged-aphid species found on leafy vegetables in the desert is important for cost-effective pest management. Rainfall and higher humidity favor the development of downy mildew in desert-grown lettuce and suggest increased inspection of fields for the disease. Herbicide codistillation is when a herbicide evaporates or changes from a liquid to a vapor with water. 

The latest Arizona Vegetable Integrated Pest Management Update from the University of Arizona (UA) Cooperative Extension in Yuma, Ariz., released Nov. 16, 2011.

Aphid identification in leafy vegetables

By John Palumbo, UA Research Scientist and Extension Specialist

The proper identification of winged-aphid species found on leafy vegetables in the desert is important for cost-effective pest management. This is the time of the growing season when we often observe winged (alate) aphids in desert lettuce and cole crops.

Most of the important aphid species found on local crops do not over summer here because of high temperatures. They typically begin migrating onto desert crops beginning in November; often blown in with gusting winds.

My experience over the past 20 years suggests this is due in part to cooler weather and changes in prevailing winds that now begin to blow into the area from the north and northwest.

Once the aphids reach the desert valleys, they typically move from crop to crop until they find a suitable host to feed and colonize. It is not uncommon to find winged aphids on lettuce or broccoli that are specific pests of small grains (i.e., corn leaf aphid) or alfalfa (i.e., pea aphid).

Since these aphid species will not colonize lettuce, it is important to distinguish them from the key aphid pests commonly found on lettuce that colonize and require management to prevent problems at harvest (green peach aphid, foxglove aphid, lettuce aphid). Also, you are likely to find cowpea aphid in lettuce as it is common in alfalfa at this time.

Experience has shown that although small colonies may be found on lettuce the populations rarely increase in lettuce crops.

The bottom line is proper aphid identification can save pest control advisers (PCAs) time and money and prevent unnecessary insecticide applications.

A simple pictorial key provides information to assist PCAs with identifying winged aphids important in lettuce and other leafy vegetables. Click on the following link to view the Aphid ID tool.

Contact Palumbo: (928) 782-3836 or

Downy mildew

Downy mildew

By Mike Matheron, UA Extension Plant Pathologist

Two rainfall events during November in Yuma County have raised the relative humidity levels considerably. As of Nov. 14, the number of days this month with maximum relative humidity levels of 90 percent or more recorded at the Yuma Agricultural Center, South Yuma, North Gila, and Roll AZMET stations was 5 percent, 6 percent, 8 percent, and 9 percent, respectively.

These environmental conditions favor the development of downy mildew and suggest increased inspection of lettuce fields for the possible appearance of the disease.

Downy mildew is best managed by having a fungicide in place before disease symptoms become apparent. Good levels of disease suppression can also be obtained by the initiation of fungicide applications at the very first sign of the disease.

However, there is a lag time between infection by the pathogen, Bremia lactucae, and the appearance of visible symptoms. This incubation period can range from three days to longer than a week; depending on the temperature, relative humidity, and susceptibility of the lettuce variety.

By the time lettuce downy mildew lesions are observed, many more are likely present but have not matured to a sufficient extent to be visible.

Data from trials conducted on lettuce downy mildew in 2009 and 2010 at the Yuma Agricultural Center demonstrated that disease severity could be reduced from 70 percent to 86 percent - compared to untreated plants - with treatment programs containing Acrobat, Aliette, Curzate, Presidio, Previcur Flex, Prophyt, Revus, and Tanos.

In the latest field trial completed in 2011 where individual products were evaluated, the reduction in disease incidence was the highest on plants treated with Forum, Curzate, Presidio, Manzate, Actigard, and Reason.

Somewhat lower but still significant efficacy compared to non-treated plants was demonstrated by Revus, Tanos, Previcur Flex, and Cabrio.

Several different modes of action are represented by these compounds facilitating alternation among different chemistries for effective disease management plus pathogen resistance management.

Contact Matheron: (928) 726-6856 or


‘Lift-off’ or codistillation of GoalTender and Goal 2XL (Oxyfluorfen)

By Barry Tickes, UA Area Agriculture Agent

Herbicide codistillation is when a herbicide evaporates or changes from a liquid to a vapor with water. This can occur from soil, water, or plant surfaces and can be responsible for the substantial loss of some herbicides.

When codistillation occurs with GoalTender or Goal2XL, the concern is not herbicide loss but crop injury.

Codistillation can occur with several herbicides. It is affected by many factors including temperature, moisture, organic matter, soil pH, and other variables.

In general, codisillation is greatest when temperatures, moisture, and pH are high and organic matter is low.

One of the herbicides used in this region most affected by codistillation is Eptam. A study conducted several years ago in Brawley, Calif. found more than 80 percent of the Eptam applied in irrigation water was lost by codistillation. Most of this was from the soil after it had reached the field.

In our trials, we have found codistillation may help GoalTender and Goal 2XL (oxyfluorfen, also sold as Galligan, Oxi Flo and others) kill weeds but it also can increase crop injury. Goal can move into plants in the vapor phase once it has lifted off and weed control and crop injury are enhanced. We have seen this when Goal is chemigated through sprinklers.

Goal is primarily a contact-type herbicide and moves little in the plant. It is rare for contact-type herbicides to work better when overhead water is applied but this seems to be the case with this herbicide.

“Lift-Off” or codistillation of Goal has earned it the reputation by some as a herbicide that is prone to causing injury to sensitive crops grown in adjacent fields. In many cases, this potential is exaggerated.

Lift-Off of Goal differs from the usual off target drift that can occur with other herbicides. In this case, it is the movement of the herbicide with water vapor. Moisture must be present and must evaporate. The vapor normally stays in the field. It is common for a band application to the furrows, for example, to move across the bed top. Significant movement out of the field normally only occurs with wind.

GoalTender is not as volatile as Goal 2XL and is less prone to codistillation.

Contact Tickes: (928) 580-9902 or

Row cover video

By Marco Pena, UA Research Specialist

Some organic growers use polypropylene fabric row covers as insect barriers and others use it for frost protection.

John Palumbo is conducting research at the Yuma Ag Center with this material for insect control and in some cases the fabric is used as a cage. Plots are infested with Bagrada bugs to evaluate the damage that could occur to the crop.

The following video shows the procedures to establish a trial:

Contact Pena: (928) 782-3836 or

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