The latest Arizona Vegetable Integrated Pest Management Update from the University of Arizona (UA) Cooperative Extension in Yuma, Ariz., released May 16, 2012.

Insecticide usage in head lettuce

By John Palumbo, UA Research Scientist and Extension Specialist

The annual Lettuce Crop Losses Workshop was held in April and the results of the surveys reveal some interesting trends in insecticide usage on desert head lettuce.

In general, the most commonly used insecticides in fall and spring lettuce correspond directly to the key pests that typically occur during these growing periods.

When compared by the class of chemistry using the IRAC mode of action classification system, the pyrethroids, applied as foliar sprays and chemigations, were by far the most commonly used insecticide class. This makes sense because pyrethroids are one of the few inexpensive, broad-spectrum insecticides still available for the effective control of beetles, crickets, and plant bugs.

Nonetheless, over the past few years, pyrethroid usage has steadily declined as has the use of organophosphates and carbamates; where Lannate and Orthene are the primary compounds used in desert lettuce.

The spinosyns remain the second most commonly-used class of insecticides. More than 90 percent of the lettuce acreage was treated with Radiant and Success in 2011-2012. The activity of these products against lepidopterous larvae and thrips are a good fit in desert lettuce.

The third most common class of chemistry in fall and spring lettuce is the neonicotinoids driven primarily by at-plant soil uses for sucking insects. Estimates this season showed that pest control advisers (PCAs) used generic imidacloprid and Admire Pro on more acres compared to last year.

Estimates of Diamide usage (Coragen, Voliam Xpress, and Vetica) suggest that PCAs applied more of this chemistry in 2011-2012 than the previous season. Estimates further suggest that growers are slowly beginning to incorporate at-planting soil uses of Coragen into the programs.

Ketoenol usage (Movento) on fall lettuce was down compared to 2010, but usage as an aphicide on spring lettuce remains about the same. From an integrated pest management perspective, the industry has made great strides in minimizing environmental impacts in lettuce production by continuing to incorporate the newer insecticides into insect management programs.

For the second season in a row, PCAs treated a greater percentage of the acreage with selective, reduced-risk products than with broadly toxic, older chemistries (pyrethroids, organophosphates, carbamates).

To view an estimated insecticide usage summary by chemical class, plus the 15 most commonly used insecticides on head lettuce during the past two growing seasons, click on this link: Insecticide Use in Arizona Head Lettuce.

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

Fusarium wilt on melons

By Mike Matheron, UA Extension Plant Pathologist

Fusarium wilt can occur in Yuma, Ariz. area melon fields and has already been detected in a melon planting this season.

Symptoms of Fusarium wilt on melons are similar to Fusarium wilt diseases on other plants including initial yellowing and wilting on one side of the plant or on one runner followed by runner collapse. Internal discoloration of the xylem tissue at the base of the plant can occur as well.

As the disease progresses, other runners show symptoms and collapse eventually leading to plant death.

Fusarium wilt on cantaloupes and watermelons is caused by a specific form of the fungal pathogen Fusarium oxysporum. For cantaloupes and other melons classified as Cucumis melo, the relevant pathogen is Fusarium oxysporum f. sp. melonis; whereas the pathogen for watermelon (Citrullus lanatus) is Fusarium oxysporum f. sp. niveum.

In general, Fusarium wilt severity increases when plants are by temperature extremes, heavy fruit loads, or other plant growth stress factors.

Resistant cultivar use is a useful disease management tool. The performance of a resistant cultivar can be affected by the inoculum level of the pathogen in soil.

According to published articles, the rotation out of melons for from three to 10 years, depending on the report, will significantly reduce but not eliminate the inoculum load of the pathogen in soil.

There are numerous different forms of the Fusarium wilt pathogen. Each form has the capability of initiating disease on one or at most a few closely related types of plants. There is no reason to worry about planting melons in a former lettuce field with Fusarium wilt. The Fusarium wilt pathogen of lettuce, Fusarium oxysporum f. sp. lactucae, will not cause the disease on melons.

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

How, where to drop off samples for weed identification, herbicide injury evaluation

By Barry Tickes, UA Area Agriculture Agent

It is difficult to solve a problem without first identifying the cause. We recognize that helping people identify problems is an important part of our jobs at the UA and we spend much time collecting and analyzing samples. Most samples received are for weed identification or herbicide injury evaluation.

Although there are probably less than 50 common weeds in this region, hundreds more occasionally turn up. We have identified weeds for many years but there are few weeks that go by when we don’t see one we are not sure about. Even common ones can look significantly different when growing in different areas under different conditions.

Weeds growing in the irrigated desert can have characteristics that complicate identification.

Annual weeds are normally classified as those living during the summer or winter and then die.

This distinction is often inaccurate in the low deserts of Arizona when weeds normally classified as summer annual weeds survive into the winter months under mild winter temperatures and frequent irrigations. It is not uncommon for some summer annual weeds to live for two years as biennials or even longer as weak perennials.

Similarly, winter annual weeds can sometimes survive through the summer where the weeds are shaded by a crop and receive frequent irrigation.

One of the best resources for identifying weeds is by Joe DiTomaso; two volumes and 900 pages which contain more than 750 species and cost $80. Click on this link for more information on Weeds of California and Other Western States.

The UA also receives many herbicide injury samples. These can be more difficult to accurately identify. There are more than 100 different active ingredients contained in herbicides used in this region. None of these produces crop symptoms which cannot be produced by many other factors including diseases, insects or other pests, nutritional problems, environmental stresses, and others.

Sometimes, symptoms and patterns in the field or the herbicide-use history make diagnosis easy, although often the only conclusive method to identify the problem is through chemical analysis. This can be costly and time consuming but many samples are sent to laboratories every year. Processing weed identification and herbicide injury samples are an important part of the UA’s responsibilities.

Download the sample ID form or fill it out at the Yuma Agriculture Center when dropping off samples. Contact Barry Tickes or Marco Peña if you plan to drop off samples. Plans are to use a refrigerator for sample drop off when no one is around to accept the samples.

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