The latest Arizona Vegetable Integrated Pest Management Update from the University of Arizona (UA) Cooperative Extension in Yuma, Ariz., released March 20, 2012.
Winter weather: impact on produce pests this season
By John Palumbo, UA Research Scientist and Extension Specialist
Spring officially arrived March 20 and the produce season is quickly winding down. I have contemplated how the weather this past winter influenced pest pressure on winter vegetables.
There is no doubt that temperatures throughout Yuma County were considerably warmer this winter relative to last year, particularly during January and February. Based on observations at the Yuma Agricultural Center (YAC), aphid and thrips numbers have been lower this year compared to normal during this period.
As noted in earlier IPM Updates, abundance of these two pests has varied throughout the region. Reports of heavy thrips pressure are still coming in from the Dome Valley and Wellton areas in Yuma County, Ariz. A few more pest control advisers have indicated finding lettuce (red) aphids in late lettuce crops.
Currently, thrips numbers are building exponentially on late lettuce trials at the YAC. This is expected given the warm and dry growing conditions. Surprisingly, cabbage lopper, beet armyworm, corn earworm, and diamondback moth numbers have been higher this spring than I can ever recall.
This is likely a result of the warm spring nighttime temperatures, plus the lack of sprays needed for aphid and thrips in January and February. This condition would have provided ideal conditions for larval development and abundance.
Also found were low numbers of adult whiteflies on late cabbage and early melons at the YAC. The lack of freezing weather this winter may have increased overwintering survival. We’ll find out in a couple of months.
Finally, I’ve had a number of reports of seed corn maggots attacking melons and cotton.
The weather condition in general would not be considered especially conducive to maggot outbreaks, but many reports suggested stand problems were found in fields where preventive insecticides were not used and crops were planted into heavy crop residue or prepared with manure.
Adult seed corn maggots are quite heavy at the YAC, but no stand problems have been noted to date.
For a detailed summary of the winter weather data described above, click on this link: Winter Weather Conditions Yuma County. 
Remember: when in doubt - scout.
Contact Palumbo: (928) 782-3836 or [email protected] .
Lettuce powdery mildew
By Mike Matheron, UA Extension Plant Pathologist
The final stretch of the lettuce production season in the desert southwest is upon us. This is also the time of year when powdery mildew can be an economic concern in lettuce fields.
This disease, caused by the fungus Golovinomyces cichoracearum, can become prominent on lettuce plants at or nearing maturity.
The disease is first observed as very small spots of white fungal growth on upper and lower leaf surfaces of the oldest leaves. From these initial infection sites, the fungus grows on the leaf surface and releases vast quantities of spores carried in the air which upon landing on lettuce leaves initiate additional infections under favorable temperature and moisture conditions.
The most favorable temperature range for spore germination is 65 degrees to 77 degrees Fahrenheit. Relative humidity at 85 percent or above is required for infection, growth, and sporulation by the pathogen. Low light intensity also favors powdery mildew development.
These requirements are often all met for one to several hours daily, especially on lettuce leaves near or at the soil surface in a maturing lettuce planting. As little as four days are needed from infection to production of a new crop of pathogen spores.
Depending on environmental conditions and the particular susceptibility of the lettuce variety, preventative applications of fungicides may be needed to prevent economic loss to the crop.
The oldest leaves where initial powdery mildew infections develop will not be harvested. However, these leaves serve as factories for spore production and launching sites for spore release which can lead to infection of the marketable portion of the lettuce plant.
In recent field trials, fungicides that provided excellent control of powdery mildew on lettuce included Microthiol Disperss (wettable sulfur), Procure (triflumizole), Quintec (quinoxyfen), and Rally (myclobutanil).
Initiating fungicide treatments before or at the very latest at the first sign of infection on the oldest leaves will result in the best levels of disease control.
Contact Matheron: (928) 726-6856 or [email protected] .
Melon weed control
By Barry Tickes, UA Area Agriculture Agent
The list of herbicides available for use in melons is very limited. Fortunately, most of the types of melons produced in the Yuma County area grow rapidly. Non-chemical weed control practices including cultivation and plastic mulch can be effective.
Shallow cultivation and non-selective herbicides with no soil activity can be effective if used after bed formation and before planting. Contact (Gramoxone, Aim) and systemic (glyphosate) herbicides can be used.
Prefar can be applied before or after planting but before weeds emerge. Prefar sticks very well to the soil and must be incorporated with as much overhead water as possible. It does not work well when drip or furrow irrigations are used.
There are more herbicide options after the crop has emerged and has developed three or four leaves. Dacthal and Trifluralin can be used as layby applications before weeds have emerged. Both can cause unacceptable stunting of the crop if applied too early.
Prowl is not safe to melons at normal use rates and is not registered.
Sandea is registered and can be effective on nutsedge and several broadleaf weeds. It is safe to most melon types after four leaves and before blooming. Carryover in the soil is a consideration if sensitive crops are to follow.
The grass herbicides, Select, Select Max, Poast, and the related generics are registered and effective on most grasses. Poast will not control sprangletop or annual bluegrass. The highest rates of Select, Select Max, and generics of Clethodim will work.
Temporary crop injury from Select Max can occur where the sprayer overlaps or slows down at the end of rows. This normally only occurs where a two times-or-greater rate is applied.
Contact Tickes: (928) 580-9902 or [email protected] .