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Arizona Veg IPM: produce pests, lettuce powdery mildew, shepardspurse

Arizona Veg IPM: produce pests, lettuce powdery mildew, shepardspurse

Cold Arizona temperatures likely lowered aphid and thrips numbers in winter vegetables; As the winter lettuce season end nears, powdery mildew can be an economic concern in some lettuce fields; The weed shepardspurse has become increasingly widespread in recent years. 

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

Winter weather: impact on produce pests this year

By John Palumbo, UA Research Scientist and Extension Specialist

Spring has officially arrived and one question I've been contemplating is how did the cold weather this past winter impact pest pressure on winter vegetables?

There is no doubt that temperatures throughout Yuma County were considerably cooler this winter relative to last year, particularly during January and February. Based on my observations at the Yuma Agricultural Center, I would say aphid and thrips numbers were lower than what we typically see during this period.

It is quite reasonable to assume that the lower winter temperatures, coupled with lower rainfall amounts, likely kept aphid numbers relatively low.

The cooler temperatures may also explain why, in some cases, Movento appeared to provide inconsistent activity against aphids. The systemic activity of the compound is influenced by the growth of the plant. With the cooler temperatures and multiple freezes experienced this winter, lettuce plant growth and aphid activity were likely negatively influenced.

Thrips numbers are beginning to rapidly increase now, but during January and February the numbers remained very low. Although we saw less rainfall this winter, the cooler temperatures clearly suppressed population growth.

In addition, it appeared to be windier this winter but average wind speeds varied within Yuma County. Wind can influence insect movement, several abiotic-biotic factors, and the ability of pest control advisers to spray crops.

Finally, the hard freezes experienced this winter certainly impacted plant growth, quality, and insect management.

This was especially evident in the Gila Valley where the AZMET weather station recorded seven days when temperatures dropped below 32 degrees F. for an average of about five hours per freeze. This is significant since no freeze events were recorded in the last growing season. In contrast, Roll had 30 days of below 32 degree F. temperatures, compared to 19 last year.

Looking forward, the impact these freezes will have on key pests of spring melons (i.e., whiteflies and cabbage loopers) remains to be seen.

For a detailed summary of the winter weather data described above, click on this link -  Winter Weather Conditions Yuma. For a discussion on the impact of weather on insects, click on this link - Weather and Insects.

Contact Palumbo: (928) 928-782-3836 or


Lettuce powdery mildew

Lettuce powdery mildew

By Mike Matheron, UA Extension Plant Pathologist

As we enter the final stretch of this lettuce production season in the desert southwest, powdery mildew can be an economic concern in some lettuce fields. Powdery mildew, caused by the fungus Golovinomyces cichoracearum, is primarily a disease of 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 releases numerous spores which are 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 to 77 degrees F. Relative humidity at or above 85 percent 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 a fungicide may be needed to prevent economic loss to the crop.

The oldest leaves where initial powdery mildew infections develop will not be harvested. However, the leaves serve as nurseries and launching sites for spore production and 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 disease control.

Contact Matheron: (928) 726-6856 or



By Barry Tickes, UA Area Agriculture Agent

We continually select for those weed species that thrive under our production practices and escape current control techniques. Shepardspurse is one of several weed species around for a long time but that growers and pest control advisors have seen become increasingly widespread in recent years.

The botanical name for shepardspurse is Capsella bursa and it is in the mustard or crucifer family. This is a large family of plants and includes many common weeds including London rocket, sahara mustard, and black mustard. It also includes many of the crops grown here including broccoli, cauliflower, cabbage, bok choy, kale, arugula, kohlrabi, mustard greens, daikon, and others.

Almost all of the plants in this family are valued for nutritional or medicinal properties. Many of the weeds are used for medical purposes. Shepardspurse was frequently used during the World War I to control bleeding. It can be difficult to control shepardspurse selectively in many brassica crops because of its similarity.

It is the physical characteristics of the weed that is largely responsible for its spread. The foliage of shepardspurse is characteristically a low growing rosette that is often covered and can be difficult to reach with herbicides.

The seed head, on the other hand, grows 12 to 20 inches above the vegetative parts and has been reported to produce as many as 50,000 seeds per plant in a single season. The seeds are very small and can float in water and blow in the wind.

The seeds germinate at very shallow depths. Soil active herbicides can fail if they are not concentrated at the soil surface. Shepardspurse proliferated in alfalfa during the years when 2, 4-DB (Butoxone, Butyrac) was the primary broadleaf herbicide used because 2, 4-DB is ineffective on this weed.

In vegetable crops, shepardspurse can be controlled with Kerb on the crops it is registered and if it is concentrated at the surface. It is less effective if some of the herbicide leaches below the germinating seeds. Prefar, Dacthal, Balan and Prowl are mostly ineffective on this weed.

Contact Tickes: (928) 580-9902 or

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