The latest Arizona Vegetable Integrated Pest Management Update from the University of Arizona (UA) Cooperative Extension in Yuma, Ariz. released Dec. 14, 2011.

Green peach aphids in desert lettuce

By John Palumbo, UA Research Scientist and Extension Specialist

In a recent update, we anticipated that pest control advisers would soon find aphids infesting local lettuce and cole crop fields. Last week (week of Dec. 5) we readily found winged (alate) aphids plus small colonies of apterous (wingless) green peach aphids, Myzus persicae, on untreated lettuce plants at the Yuma Agricultural Center.

The timing of aphid colonization in lettuce and other leafy vegetables varies by species, and depends largely on temperature, rainfall, and planting dates. Previous studies suggest that green peach aphids are historically most severe in late October and early-mid November plantings of lettuce. By late February, green peach aphid populations generally begin to rapidly decline as temperatures increase.

However, this is not always the case on other crops including cabbage and spinach where heavy green peach aphid infestations can occur in late March and early April. Other aphids including foxglove and lettuce 'red' aphid typically cause problems on later lettuce plantings (i.e., late November and December) because these species are better suited to warmer winter temperatures.

There are exceptions to the rule and aphids can occur when least expected.

For more information on aphid population dynamics on desert lettuce see the aphid dynamics report.

It's important to also note that in our recent inspection of lettuce plants, field plots treated with a soil at-planting application of imidacloprid (Admire Pro, 7 oz) were aphid free. This is not surprising since imidacloprid at the 0.25 pounds active ingredient per acre rate still provides long residual control of most aphids (less consistent on foxglove and lettuce aphid).

Research trials from last spring showed imidacloprid soil applications in head lettuce provided protection against aphid contamination nearly season long. In many cases, an application of a foliar alternative is generally required near harvest to prevent unacceptable aphid contamination.

The trick is to determine when the imidacloprid residual begins to decline and aphids begin to colonize. Of course, this will require close examination of heads well in advance to harvest. Remember - "When in doubt – scout."

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

Plant pathogen resistance to fungicides

Plant pathogen resistance to fungicides

By Mike Matheron, UA Extension Plant Pathologist

Plant pathogens are similar to other living organisms with a degree of genetic variability within the genes that govern physical structure and internal biochemical activities. Any selection pressure imposed on a population of an organism can result in visible and invisible changes within that population.

Selective breeding is a tool used to express the genetic diversity within a population of an organism, as demonstrated by the proliferation of dog breeds or varieties of agricultural crops when compared to the original ancestral forms.

Other selection pressures can result in unwanted changes within a population including the development of resistance to antibiotics used to treat animal diseases and plant health chemistries used to treat plant diseases.

In the Yuma area, plant health products are used primarily against diseases caused by fungi. Specific recommendations have been established by an organization called the Fungicide Resistance Action Committee to manage the development of fungicide resistance within a target plant pathogen population.

These resistance management strategies include the following points.

First, do not use a single mode of action in isolation. Instead, apply the material as a mixture or in alternation with one or more fungicides with different modes of action within a treatment program.

Second, restrict the number of applications of a particular mode of action within a season and only make applications when necessary.

Third, do not apply less than the manufacturer’s recommended dose.

Fourth, target fungicide applications for disease prevention - not eradication.

Fifth, use an integrated approach to disease management.

By utilizing as many of these resistance management strategies as possible plus using disease-resistant cultivars, biological control agents, crop rotation, and other beneficial cultural practices, the end result can be a high level of disease control, lower amounts of total fungicides needed, and decreased selection of fungicide-resistant components within the pathogen population.

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

When do seeds germinate?

When do seeds germinate?

By Barry Tickes, UA Area Agriculture Agent

The length of time for weed seeds to germinate can vary considerably even within the same field due to variations in soil and microclimatic conditions. Every year can be different.

We conducted a test to determine how long it would take for various weed seeds to germinate after exposure to moisture. Seven summer and winter annual weeds, two grasses, and five broadleaf weeds were chosen for this test.

Seeds for the weeds were placed in teabags and buried one-quarter inch below the soil surface and in the seed row of newly planted lettuce fields prior to the germination irrigation. Teabags were pulled every 24 hours and evaluated for germination.

This procedure was repeated in sprinkler and furrow irrigated fields at eight locations in Roll, Bard, the Gila Valley, and the Yuma Valley. The tests began in August and were conducted each month until February.

The time to germination for the summer annual grass (barnyardgrass) was 24 hours in August and September, became longer in October (48 hours), and still longer in December (96 hours). It did not germinate in January.

The winter annual grass (cannarygrass) took 168 hours to germinate in September and dropped to 96 hours in December.

The summer annual broadleaves took from 24 hours (purslane) to 96 hours (pigweed and nightshade) to germinate.

The winter annual broadleaf weeds (lambsquarters and Shepardspurse) took from 72 to 168 hours to germinate.

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

The use of pelargonic acid as a herbicide

The Vegetable IPM Team asked Barry Tickes what is one of oldest acids used as a non-selective herbicide. His answer is in the following video: