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

Leafminer management on fall lettuce

By John Palumbo, UA Research Scientist and Extension Specialist

Although the hot weather is beginning to break in the desert, pest control advisers should not be complacent in leafminer management. Leafminer numbers have been light over the past few years. We have recently observed unusually high numbers (adults and damage) on melons and lettuce particularly in the Yuma Valley.

Yellow sticky traps used for monitoring whiteflies throughout the area have been almost covered with leafminer adults over the last few weeks. These numbers have been generated from melons and cotton.

With temperatures forecast in the mid-80s and 90s for next week, leafminers adults will be looking for a good host plant. Many leafy vegetables serve as ideal host this time of the year.

If left uncontrolled, leafminer populations can rapidly reproduce under these weather conditions and cause significant damage to older, pre-harvest lettuce by feeding and damaging wrapper leaves of head lettuce and romaine. Baby leaf lettuces, spring mixes, and spinach are susceptible to larval feeding (mining) on tender growing leaves.

This can be effectively controlled with currently available products. Radiant, Coragen, and Voliam Xpress applied at higher rates can effectively kill developing larvae in the leaves before causing significant damage. Similarly, Agri-Mek at 8 ounces or greater can provide long residual control.

For more information on leafminer biology and management, click on this link.

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

Comparison of two lettuce wilt diseases

By Mike Matheron, UA Extension Plant Pathologist

Fusarium wilt of lettuce, caused by the fungu Fusarium oxysporum, f. sp. lactucae, was first detected on lettuce in Arizona during the 2001-2002 growing season. Fusarium wilt continues to be found in lettuce fields from mid-October through early January.

On the other hand, Verticillium wilt of lettuce, caused by the fungus Verticillium dahliae, was first detected on lettuce in 1995 in the Salinas Valley of California and continues to spread in that region. The disease has not yet been found in Arizona lettuce fields.

The primary symptoms of each disease are similar and consist of internal discoloration of the root cortex and plant wilting followed by death. The internal root discoloration ranges from green to brown to black in plants infected with Verticillium and reddish-brown to black in plants infected with Fusarium.

Since symptoms of both wilt diseases are similar, true disease identity only can be achieved by bringing symptomatic lettuce plants to the University of Arizona Yuma Agricultural Center where the causal pathogen can be isolated from infected root tissue and identified by microscopic examination.

Both wilt pathogens are soil inhabitants which can persist in the soil for many years. Both pathogens also can be seed-borne.

The lettuce Fusarium pathogen, Fusarium oxysporum, f. sp. lactucae, can only infect and cause disease on lettuce, although it may sustain itself on roots of other plants without initiating disease. Verticillium dahlia can infect and cause disease on numerous crops other than lettuce.

Management strategies for diseases caused by Fusarium oxysporum and Verticillium dahliae are similar. Genetic resistance, when available in host crop plants, can provide effective disease control. Soil fumigation and soil solarization can reduce disease levels by lowering viable populations of both pathogens in the soil.

No fungicides are known to provide effective control of diseases caused by Fusarium oxysporum or Verticillium dahlia.

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

Generic herbicides in lettuce

By Barry Tickes, UA Area Agriculture Agent

A proprietary use patent is granted for 17 years to the company that first develops a herbicide. All of the active ingredients in the herbicides commonly used on vegetables are now off patent. Most have been off patent for more than 20 years.

Some companies have developed proprietary formulations that contain off patent active ingredients. Examples include Prowl H2O, GoalTender, Select Max and Dacthal Flowable.

Click on this link to see a list of the herbicides, the year of first registration, and the original registrant for lettuce, cole crops, and melons.

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