The latest Arizona Vegetable Integrated Pest Management Update from the University of Arizona (UA) Cooperative Extension in Yuma, Ariz. released May 30, 2012.

Bagrada bug impact on desert cole crops in 2010-2011

By John Palumbo, UA Research Scientist and Extension Specialist

The Bagrada bug, Bagrada hilaris, has been a major problem in desert cole crops over the last two fall-growing seasons.

In 2010, widespread infestations were reported throughout the desert growing area in September and October. Stand losses and yield/quality reductions to broccoli, cauliflower, cabbage, and other Brassica crops were considered economically significant in some growing areas.

In 2011, Bagrada populations did not appear until early October but certainly caused crop losses and required control with insecticides.

In an attempt to document the impact of these outbreaks on desert production, a survey of growers and pest control advisers (PCAs) from the Yuma and Imperial Valley areas was recently conducted to estimate the severity of Bagrada bug infestations on direct-seeded and transplanted cole crops during 2010.

A summary of the survey results can be found at Impact of the Bagrada Bug on Desert Cole Crops: A Survey of PCA/Growers in 2010 and 2011.

In general, the results of surveys in 2010 and 2011 showed that more than 93 percent of direct-seed cole crop acreage (e.g., broccoli) was treated for Bagrada adult infestations with insecticides compared to transplanted cole crops (e.g., cauliflower) where about 86 percent of the acreage was reported treated.

Similarly, estimates showed that direct-seeded cole crops sustained greater stand losses and plant injury from Bagrada feeding than transplanted crops.

When averaged over both years, estimated stand losses and plant injury caused by Bagrada bug feeding exceeded 10 percent in direct-seeded crops.

PCAs also provided information on insecticides that provided effective control through chemigations and foliar sprays. In general, they reported that products that have contact activity (i.e., Pyrethroids, OP/Carbamates) appeared to provide the most effective control against Bagrada adults on both direct-seeded and transplanted cole crops.

Overall, the results of the PCA survey are consistent with results obtained in research trials conducted at the Yuma Agricultural Center last year.

In addition, access to the 1080 database showed that insecticide usage on cole crops has nearly doubled since Bagrada outbreaks first occurred in the fall of 2010.

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

Disease resistance and tolerance

By Mike Matheron, UA Extension Plant Pathologist

An extremely valuable weapon in the battle to manage plant diseases may reside within the genetic composition of the plant.

This plant genetic disease management tool is commonly referred to simply as disease resistance or tolerance. These names are often used interchangeably; however, the definitions of each term denote a significant difference.

Resistance is the ability of a plant to exclude or overcome the effect of a plant pathogen. Tolerance is the ability of a plant infected by a pathogen to grow without dying or sustaining serious injury or yield loss.

Therefore, resistance focuses on infection prevention whereas tolerance permits the plant to grow without serious injury or yield loss after infection.

Disease resistance and tolerance are not all or nothing conditions. For example, resistance can range from its highest level – referred to as immunity - through degrees of useful resistance, and finally to its lowest level when a plant is highly susceptible to a particular pathogen.

Also, resistance and tolerance usually are specific to one or at most a few diseases and not a broad range of plant ailments. The mechanisms within plants that create disease resistance and tolerance are many and varied.

Successful suppression of pathogen activity by a plant is tied to how a particular pathogen gains entrance into a plant to initiate disease, and how a plant defends itself from infection by one or more physiological (biochemical) or morphological (structural) changes.

One key advantage of strong genetic resistance or tolerance is that this disease management tool will be active for the life of the plant without an input by the grower. On the other hand, when products including fungicides are used to manage diseases, the products must be applied to be in place over the entire growth period of the plant when disease is expected.

Also, disease management provided by plant genetics often targets diseases for which no other known effective disease management tools are known.

Building disease resistance or tolerance into plants is an ongoing activity of plant breeders - using classical as well as modern genetic manipulation techniques to achieve this goal.

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

Summer fallow weed control for vegetables

By Barry Tickes, UA Area Agriculture Agent

It is much easier to kill weeds when there is no crop in the field. Now is a good time to reduce the seed bank of summer annual weeds in fallow fields.

Weed seeds are buried at variable depths in the soil, some have hard seed coats, and there are other variables that cause germination over a long period of time. If all came up at the same time the weeds would be much easier to control.

It takes time to repeatedly irrigate, germinate, and kill weeds with tillage or herbicides. UA-conducted trials suggest in most years summer annual weeds begin to germinate in February, peak in June, but continue germination into October.

Proper timing of tillage to kill weeds can be important with some species. Some weeds including common Purslane are very succulent and can remain viable for several days after cultivation or hoeing. The weed can re-root at the nodes and continue growing if allowed to grow too large before uprooting.

Growers sometimes allow early emerging weeds to get fairly large in an effort to germinate as many seeds as possible. Incorporating large amounts of organic matter into the soil can also have a negative effect on some pre-emergent herbicides used in vegetables.

Many of the root and shoot inhibitor herbicides including Trifluaralin, Pendimethalin, Benefin, DCPA, and others can bind to organic matter and be less available to kill weeds.

Tillage has the opposite effect on perennial weeds including nutsedge and Bermudagrass than on annual weeds. These weeds are spread vegetatively. Repeatedly irrigation and tillage will spread rather than kill these weeds.

Contact and systemic herbicides are used during fallow periods to control weeds. The contact herbicides include Paraquat (Gramoxone, Firestorm), Carfentrazone (Aim, Shark), Pyraflufen (ET), Pelegonic Acid (Scythe), and others.

Some of the advantages of these are quickness and no soil residual which allows crop planting soon after application. Disadvantages are the effective use primarily is on small weeds.

The most commonly used systemic herbicide for fallow ground is Glyphosate. It is broad spectrum and has no soil residual.

Many of the systemic herbicides registered for fallow use, including Oxyfluorfen (Goal, Galigen) or EPTC (Eptam), require at least 90 days before planting many vegetable crops. If used correctly, Eptam can be very effective in controlling nutsedge during summer fallow.

Only fumigants kill weed seeds including Chloropicrin, Methyl Bromide, Metam Sodium, Dazomet, Telone, and others.

Most pre-emergent herbicides only work after the seed has germinated. Pre-emergent herbicides are often used for fallow weed control only when at least 30 to 45 days or longer are available.

Fumigants are expensive, can be difficult to use, and are often used for disease or nematode control with the added benefit of controlling weeds. Soil solarization and flooding have become increasingly popular in recent years as techniques to control pests during summer fallow.

Few regions are as well suited for these techniques as the low desert. The techniques are used primarily to control diseases, but also have the benefit of controlling some summer annual weeds.

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