The latest Arizona Vegetable Integrated Pest Management Update from the University of Arizona (UA) Cooperative Extension in Yuma, Ariz., released March 7, 2012.

Lettuce aphids showing up

By John Palumbo, UA Research Scientist and Extension Specialist

Consistent with the warm weather lately in the Yuma County, Ariz., area, lettuce aphids, Nasonovia ribisnigri, (a.k.a. red aphid), are showing up in desert lettuce. In the last week, we positively identified lettuce aphid on head and romaine lettuces in the Dome Valley and Wellton.

In addition, we have picked up a couple of alate (winged) lettuce aphids at the Yuma Agricultural Center. No colonization of apterous (wingless) aphids has been observed yet.

However, previous experience has taught us that daytime high temperatures in the high 70s and low 80s are ideal for population growth. We anticipate lettuce aphids will colonize rapidly throughout March.

There several things to remember about lettuce aphids relative to the other aphids commonly seen.

First, immature nymphs are small and have a red appearance. The apterous adults are typically a large brown-colored aphid with dark bands running across the abdomen.

Second, among the crops we grow locally, lettuce aphids are only found on the lettuce types (Lactuca spp.). Lettuce aphids prefer to colonize the terminal growth, and can often be found in heads or hearts.

Green peach aphids are often found on the frame leaves in high numbers before moving into the heads. Sampling should focus in the terminal growth of young plants, and in the heads and hearts of older plants.

Third, they can reproduce prolifically by producing many more winged alates than other species which can quickly lead to widespread abundance throughout a field or a growing area.

Finally, as the saying goes, “he who hesitates is lost.” If lettuce aphids are found on lettuce, respond quickly with an insecticide treatment.

The product of choice is Movento at five ounces per acre. Due to its systemic activity, Movento will reach aphids in the protected terminal growth.

Be sure to include a penetrating adjuvant for best results at a rate of at least 0.25 percent v/v. It normally requires 7-10 days of activity before a significant reduction in the infestation is observed.

For more information please refer to the bulletin: "Lettuce Aphid on Late Season Produce"

Remember - when in doubt -scout.

Contact Palumbo: (928) 782-3836 or jpalumbo@ag.arizona.edu.

Powdery mildew on celery

By Mike Matheron, UA Extension Plant Pathologist

Powdery mildew was detected on celery the week of Feb. 27 in the Yuma Valley by an observant pest control adviser. Small and inconspicuous colonies of the fungal pathogen were present on leaves and stalks.

Powdery mildew on celery was first reported in North America in California in 1994. The pathogen, Erysiphe heraclei, has also been reported on celery in several European countries and in South America (Chile).

Erysiphe heracleiis known to occur on carrot and parsley in the U.S. Research in California has shown the celery pathogen can cause powdery mildew on parsley as well.

As with powdery mildew on other crops, the initial infection sites are small and often not easily seen. However, under favorable disease development conditions, the fungus continues to grow and produces massive numbers of spores which will initiate more infections to rapidly escalate disease development.

The most favorable temperature range for spore germination is about 65-77 degrees F. Moderate levels of relative humidity are required for infection, growth, and sporulation by the pathogen.

Finally, low light intensity favors powdery mildew development. These conditions can all be found within a maturing crop of celery this time of year.

Contact Matheron: (928) 726-6856 or matheron@ag.arizona.edu.

Herbicide resistance

By Barry Tickes, UA Area Agriculture Agent

Although herbicide resistance has become a very serious and growing problem in many southern and central states, Arizona is one of only four states with no cases of herbicide resistance documented.

In Arizona, this is a problem most commonly associated with insects rather than weeds. It is often misunderstood. Weeds inherit characteristics that determine how they respond to herbicides.

When we conduct herbicide trials, we often list response for individual weed species as control, partial control, no control, or as a percent control. When there is no control a species is tolerant to that particular herbicide in the area and under the conditions tests were conducted.

When weeds are controlled, we say they are susceptible to the herbicide. When weeds change from susceptible to tolerant, they have developed resistance to that particular herbicide.

The use of selective herbicides began on a large scale in the U.S. in the late 1950s and early 1960s. The first case of weed resistance was documented in 1968 in Washington State and involved the resistance of common groundsel to Atrazine and Simazine.

Over the next 25 years, 110 weed species were reported resistant. The International Survey of Herbicide Resistant Weeds reported 185 resistant species in 2008 and 200 resistant species in 2011.

States bordering Arizona have all documented at least one weed species with herbicide resistance. Twenty-four resistant biotypes were documented in California, four in Colorado, two in New Mexico, and one in Utah.

The closest incidence to the Arizona low deserts has been the resistance of littleseed canarygrass, Phalaris minor, to the ACCase inhibitors Sethoxydim, Clethodim, and Fluazifop in California’s Imperial Valley. This was documented in 2001.

Weed populations may look uniform but in fact are very diverse on a genetic level. Resistance to a particular herbicide occurs when there are naturally occurring variant individuals in a weed species population “tolerant” to the herbicide’s effects.

Two principals important to understand are: the resistant variants are not caused by the herbicide but occur naturally in the weed population; and individual weeds do not change to become resistant - rather the weed population changes to become immune over time.

What happens is that if the same herbicide is used continuously for a prolonged period of time, the susceptible biotypes will die out while the resistant biotypes will survive and reproduce - passing on resistance to the next generation.

Contact Tickes: (928) 580-9902 or btickes@ag.arizona.edu