Whitefly management in fall vegetables

By John Palumbo, UA Research Scientist and Extension Specialist, Yuma

To date, whitefly numbers on fall melons in Yuma County have varied from very low (1-2 per leaf) in the Yuma Valley to extremely high (more than 100/leaf) in the Dome Valley-Wellton area.

The differences in whitefly abundance can be attributed largely to the crop landscape in these areas. Melon fields near or adjacent to cotton and alfalfa clearly are the most heavily infested. Last week I observed very high numbers of adults steadily moving out of a cotton field in Dome Valley that reminded me of the "whitefly clouds" of the early 1990s.

Does this mean fall crops in 2010 will experience heavy whitefly pressure? I do not know, but pest control advisors should start thinking about whiteflies and talking with growers concerning control strategies.

The key to effective whitefly management in fall crops is to prevent adult and immature populations from colonizing small plants. It is recommended that growers apply a soil neonicotinoid on early lettuce and cole crops planted near cotton, alfalfa, or in areas that historically have high whitefly numbers.

Local research suggests that imidacloprid (Alias, 16 oz/acre; or 0.25 lbs. active ingredient (AI)/acre) applied at planting provides less residual control of nymphs today than it did 10 years ago. This is not surprising considering the heavy neonicotinoid usage on fall and spring crops over the past 17 years.

The insecticide still provides short-term control (21-28 days), but will likely require additional foliar treatments to prevent damage, particularly on cole crops. Growers may consider using higher rates of imidacloprid (0.375 lb AI/ac), or switching to an alternative soil-applied product including Venom or Durivo.

Several foliar insecticides are available that can provide effective control of adult and immature whiteflies.

Contact Palumbo: (928) 782-3836 or [email protected].

Successfully managing lettuce drop

By Mike Matheron, UA Extension Plant Pathologist, Yuma

The successful management of any plant disease is achieved by focusing control efforts on one or more vulnerable stages in the disease cycle of the pathogen.

For lettuce drop caused by the fungi Sclerotinia minor and  S. sclerotiorum , this point of attack is the sclerotial stage of each pathogen. At crop maturity, sclerotia produced on infected plants are incorporated into the soil along with crop debris when land is prepared for planting of the next crop.

For Sclerotinia, sclerotia serve the same purpose that seeds do for plants; allowing the organism to carry over in soil in a dormant state until conditions are favorable for germination and growth. Sclerotia are the target for control of lettuce drop.

Over the past 10 years of research trials, the traditional application of fungicides to the lettuce bed surface after thinning has provided at best a 50 percent to 60 percent reduction in dead plants, compared to plots without a fungicide treatment.

Application of fungicides to the bed surface prevents germination of sclerotia at or near the soil surface, but has little effect on sclerotia deeper in the soil profile.

Experiments conducted for the last three years demonstrated that physically incorporating a fungicide (Endura) into the bed to a depth of two inches increased the level of disease control over that observed in plots where the same fungicide was applied to the bed surface.

Over the three separate field trials, the average level of disease control achieved through soil incorporation compared to the soil surface application of Endura was 68 percent compared to 59 percent in the presence of S. minor, and 62 percent compared to 53 percent in  S. sclerotiorum .

Similar results were obtained for Rovral in plots containing  S. sclerotiorum . However in two of three trials, incorporation of Rovral into the soil resulted in less disease control compared to soil surface application in plots containing  S. minor .

Soil incorporation consistently improved the efficacy of Endura. However since each fungicide has unique chemical properties that impact movement and behavior in soil, the usefulness of this application technique for other fungicides needs to be evaluated in the future.

Contact Matheron: (928) 726-6856 or [email protected].

Diagnosing herbicide injury in lettuce

By Barry Tickes, UA Area Agriculture Agent, Yuma

Lettuce is in the composite or sunflower family which is the largest family of vascular plants. Thirty-four of the 156 weeds in the “Illustrated Guide to Arizona Weeds” by Kittie Parker are in this family. It is remarkable that we are able to control almost all weeds in this and other plant families selectively in lettuce with a minimal amount of crop injury.

We are able to accomplish this with three herbicides: Prefar, Kerb and Balan, which have been used for the last 45 years. Injury does occasionally occur. When this happens it can be difficult to diagnose.

These herbicides are mitotic inhibitors that are used pre-emergence to inhibit root growth in developing seedlings. The herbicides are often used in combination and produce the same injury symptoms.

Symptoms of crop injury include marginal chlorosis, stunting, irregular development of the cotyledon and first true leaves, and the inhibition of the primary and lateral roots. Many other factors can cause the same symptoms and crop injury is difficult to predict and diagnose.

Thousands of dollars are spent every season to have laboratories analyze soil and tissue samples that are difficult to interpret. Laboratories commonly use a chromatograph to run samples which cost from $100-$200/sample to run. Herbicide levels as low as 0.1 parts per million can be detected using this technology. Results are very precise but can be difficult to interpret.

We conducted a project to establish a correlation between detected levels of Kerb, Balan, and Prefar in soil and lettuce tissue and potential crop injury. Quantitative guidelines using the amount detected in the plant tissue and soil were established.

A less precise but more direct technique used to predict and diagnose herbicide injury is to conduct a herbicide bioassay in the greenhouse. The crop or other sensitive plants are grown in pots containing soil from the questionable field. Although this technique is less precise, it can be more accurate in predicting and diagnosing crop injury.

We will be using the new greenhouses at the Valley Agriculture Center to conduct herbicide bioassays for anyone wishing to use this technique to predict or diagnose injury. There will be no charge for this service.

Contact us for instructions on how to collect and drop off soil samples.

Contact Tickes: (928) 580-9902 or [email protected].

Soil fumigant pesticides now subject to new regulatory measures

By Kurt Nolte, UA Agriculture Agent, Yuma 

Within the next four months, the U.S. Environmental Protection Agency (EPA) will provide final approval of all soil fumigant labels; implementing the next phase of new soil fumigant mitigation measures.

With these new regulations, producers using soil fumigants must abide by stronger restrictions and mitigation regulation, all of which will be noted on the pesticide label.

In Yuma County, most soil fumigants are used in spring and fall melon crops. As melon ground preparation begins immediately following produce, it is important to understand the new requirements before the upcoming spring melon season.

The soil fumigants that fall under the new EPA regulations include: chloropicrin, dazomet, 1,3-dichloropropene, iodomethane, metam sodium/potassium, methyl bromide, and methyl isothiocyanate (MITC).

One notable change is that all soil fumigants will be classified as restricted-use pesticides (RUPs). This includes metam sodium and dazomet which previously were not classified as RUPs.

This also means that any applicator in Arizona who wishes to use soil fumigants will need to become certified before applying the products.

Other new mitigation measures include: additional worker protection measures, written fumigant management plans, registrants’ stewardship and training programs, mandatory good agricultural practices, buffer zones and buffer posting, and emergency preparedness and response measures.

Details of the new regulatory measures will be discussed at the Preseason Vegetable Workshop in Yuma on Aug. 31 at the UA Cooperative Extension office in Yuma.

A special soil fumigant workshop which addresses each new requirement is planned for this fall.

Contact: Kurt Nolte, (928) 726-3904 or [email protected].

Systemic insecticide evaluation

By Marco Peña, UA Research Specialist, Yuma

Systemic insecticides are absorbed by the roots, bark, or leaves into the tissue of a plant and kill or repel insects that feed upon it. The insecticides translocate or move through all plant parts so the entire plant is protected from the attack of insects.

The products have a long-term control since once absorbed the insecticides are not exposed to rain, irrigation, and sunlight which can diminish the persistence of conventional insecticides.

Recent insect losses workshops and insecticide usage surveys conducted by the UA suggest the products are an important tool in vegetable IPM programs.

The efficacy of new systemic insecticides and different application rates are being evaluated to increase the arsenal for combating the current pest complex.

Contact: Marco Peña, (928)-726-6848 or [email protected].