The latest Arizona Vegetable Integrated Pest Management Update from the University of Arizona (UA) Cooperative Extension in Yuma, Ariz. released Oct. 20, 2011.
Bagrada bugs finally arrive
By John Palumbo, UA Research Scientist and Extension Specialist
Bagrada bug adult numbers were relatively low at the beginning of this winter vegetable season in September in the Yuma and Dome valleys. However, over the past 7 to 10 days pest control advisers from the Yuma, Coachella, and Imperial valleys have reported heavy Bagrada bug activity in cole crops.
I am not sure why they showed up later this season since last fall the insect began migrating into fields in heavy numbers in early September.
Control of the Bagrada bug is necessary under these population pressures and warm growing conditions.
Research conducted this fall shows that seedling plants (cotyledon to 2 to 3 leaf stages) are very susceptible to damage. In one trial, untreated broccoli direct-seeded on Sept. 14 sustained about 15 percent damage (blind plants/multi-terminals) at thinning under what I perceived to be light Bagrada pressure (on average we estimated ~1 adult per three row foot).
With the numbers now being reported, it will be important to control the adults on young stands. This includes chemigation with pyrethroids, and using contact insecticides (pyrethroids, Lannate, Lorsban) by air or ground.
Trials this fall suggest that about 4 to 5 days residual can be expected with these treatments. We are also finding feeding signs (star-burst shaped lesions) on the terminal leaves of older plants (>6 leaf stage), but it is unknown at this time whether Bagrada bugs infesting larger plants will result in yield losses.
If large numbers of Bagrada adults are present on older plants, it is recommended that sprays be applied to reduce the numbers.
Dinotefuron (Venom/Scorpion) applied in alternation or combination with pyrethroids/OPs/carbamates has been shown to provide good protection from Bagrada bug feeding.
Finally, we are observing a strong correlation between adults found on or under plants, and the presence of fresh feeding signs on the terminal leaves in infested plots. This suggests that when scouting for the Bagrada bug in fields, inspect young leaves for fresh feeding signs, plus for adults on plants and the soil surface.
Contact Palumbo: (928) 782-3836 or [email protected].
Fusarium wilt on lettuce
By Mike Matheron, UA Extension Plant Pathologist
Since the first detection of Fusarium wilt on lettuce in Arizona during the 2001-02 growing season, the disease has been found yearly in lettuce fields from mid October through early January.
This year is no exception as the first confirmed appearance of Fusarium wilt on lettuce for this growing season was recorded last week (week of Oct. 10).
The initial visual suggestion of the disease is the yellowing of one or more older leaves followed by wilting and necrosis. The external root surface is unaffected. However, a brown to black necrosis of the internal taproot and crown tissue will be apparent.
Disease incidence can range from a few plants up to large areas or zones of infected plants within a field. Plants can become infected and display symptoms at any age, ranging from very young plants just after thinning to those ready for harvest.
The symptoms of Fusarium wilt resemble two other lettuce disorders; ammonia toxicity and the early stages of lettuce drop. To confirm disease identity, it is necessary to bring plant samples to me at the Yuma Agricultural Center for analysis.
Confirmation of disease identity is achieved by isolation and identification of the causal fungus of Fusarium wilt of lettuce, Fusarium oxysporum, f. sp. lactucae, from symptomatic root tissue.
Disease development is strongly affected by planting date and the type of lettuce grown. The main determinant of disease severity with respect to planting date is soil temperature.
Experimental data demonstrates that lettuce planted in early September results in high levels of Fusarium wilt. Plantings in the same naturally-infested field in mid-October or early December sustain moderately low and trace levels of disease, respectively.
Of the many crisphead and romaine cultivars tested, crisphead cultivars generally are significantly more susceptible to Fusarium wilt than romaine lettuce.
The lettuce Fusarium wilt pathogen can survive in the soil for many years. Minimizing the spread of infested soil within and especially between fields is of paramount importance.
Two comprehensive research reports concerning disease development and management of Fusarium wilt of lettuce are available. Please contact me and I will email these reports to you.
Contact Matheron: (928) 726-6856 or [email protected].
Evaluating potential herbicide injury to rotational crops
Evaluating potential herbicide injury to rotational crops
By Barry Tickes, UA Area Agriculture Agent
Herbicides with residual soil activity are very useful in the low deserts where weed seeds continue to germinate with year-round irrigation. These herbicides can also be hazardous when sensitive crops are planted into soil where they are still active.
Determining the potential for crop injury from herbicides used on previous crops can be difficult. Injury potential is related to several interrelated factors including soil type, irrigation practices, tillage, environmental conditions, organic matter, and other conditions. Injury can vary from field to field and year to year, and be variable within the same field.
Rotational crop restrictions on product labels must often cover many diverse conditions and geographic regions and are frequently much longer than needed. This link, "Recroping Interval", contains a chart that has the labeled and likely rotational crop interval for major crops and herbicides used in the deserts plus the usual soil persistence for each product.
Tests can be conducted prior to planting to evaluate the potential for crop injury. Many times soil samples are taken and sent to a lab for analysis. The tests are usually done using a High Performance Liquid Chromatograph or a Gas Chromatoaph and can be costly, time consuming, and difficult to interpret.
A simpler and more direct test can be done by using a bioassay or growing sensitive plants in pots containing soil from the questionable field. This technique is less expensive, requires little equipment, and can be done by anyone.
Bioassays are often conducted by growing species of plants known to be sensitive to a specific herbicide or class of herbicides. It is reasonable, however, to use the crop that is to be planted.
If, for instance, lettuce will be grown, the seed used in the bioassay should be from the variety and lot number that will be used. Bioassays can be more accurate than more sophisticated lab tests in predicting potential crop injury. These tests are only good if the soil sample collected is representative of what is in the field.
Sample collection is very important and small amounts should be taken from several areas of the field. Herbicide concentration often varies within fields and separate samples should be collected and labeled to indicate in which part of the field they came from.
Samples are normally taken from the top 2 to 4 inches from the surface. Soil must also be collected from similar soil types in the same vicinity not treated for comparison.
We are conducting a bioassay program this season to help growers evaluate potential herbicide injury in greenhouses at the Yuma Agricultural Center. Contact me, or Marco Pena at (928) 782-5871, for guidelines on how to collect and drop off soil samples.
Contact Tickes: (928) 580-9902 or [email protected].