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

Whiteflies, CYSDV in desert melons

By John Palumbo, UA Research Scientist and Extension Specialist

Fall melon planting is closely approaching and so is the potential for cucurbit yellow stunting disorder virus (CYSDV) to cause problems.

The virus was first identified in desert melons in the fall of 2006 when widespread infection in cantaloupes, honeydews, and other melons cost growers significant crop losses.

Recent USDA statistics suggest that CYSDV reduced yields by 30 percent or more in Arizona in 2008 and 2009.

Over the last four years the UA has conducted considerable research to better understand the epidemiology of the virus and its impact on melons. Also, new information has been generated on the control of the vector, Bemisia whitefly adults.

Whitefly numbers this spring and summer have been relatively light. Coupled with the low incidence of virus on spring melons and the aggressive management programs that PCAs and growers are now using, it will be interesting to see how CYSDV impacts melons this fall.

UA research suggests that the area-wide incidence of CYSDV may be light this fall. Based on what we don't know about the virus-vector relationship it is difficult to predict anything.

To view a preliminary report of one of our research projects that details our findings on the “Area-wide Incidence of Whiteflies and CYSDV in Desert Melons,” click on this link: http://extension.arizona.edu/sites/extension.arizona.edu/files/resourcefile/resource/marcop/Areawide%20CYSDV%20-%20preliminary%20report%20July%2028_2010.pdf.

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

Physical barriers to plant diseases

By Mike Matheron, UA Extension Plant Pathologist

Plants can be resistant to plant diseases, but the question is how is resistance accomplished?

One method is by employing physical defense mechanisms. Just as our skin provides a physical barrier which prevents the movement of microbes into our bodies, the cuticle or surface covering on plants serves the same function.

Many plant pathogens must adhere to the plant surface to penetrate the plant and cause infection. This ability to colonize plants may be disrupted by the amount of wax present and the quality of the cuticle that covers the plant.

Waxes prevent the formation of a film of water on plants which is essential for the deposition or growth of bacterial and fungal pathogens on plants. Abundant plant hair can perform a similar water-repellent function.

Cuticle thickness and toughness of epidermal cell walls play an important role in the resistance of plants to several pathogens. This form of disease resistance can be circumvented by wounds.

Many pathogenic bacteria and fungi enter plants only through stomata. The structure and pore size when open can greatly impact the ability of some pathogens to invade these structures.

These physical plant attributes which are present before exposure to potential plant pathogens play an important role in a plant’s ability to resist many diseases.

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

Evaluating the potential for herbicide injury

By Barry Tickes, UA Area Agriculture Agent

Herbicides with residual soil activity are very useful in the low desert where weed seeds germinate with each irrigation year round. These herbicides can also be hazardous, however, when sensitive crops are planted into the soil where herbicides are still active.

Determining the potential for crop injury from herbicides applied to 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, year-to-year, and even be variable within the same field.

Rotational crop restrictions listed on product labels cover many diverse conditions and geographic regions. Tests conducted prior to planting can evaluate the potential for crop injury.

Many times soil samples are taken and sent to a lab for analysis. The tests are usually conducted using a High Performance Liquid Chromatograph (HPLC) or a Gas Chromatoaph (GC) and can be costly, time-consuming, and difficult to interpret.

A simpler and more direct test uses a bioassay or growing sensitive plants in pots containing soil from the questionable field. This technique is less expensive, requires little equipment, and can conducted 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 to use the crop to be planted. If, for instance, lettuce will be grown, the seed used in the bioassay should be from the variety and lot number to be planted.

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. Small amounts should be taken from several areas of the field. Herbicide concentration often varies within fields. Separate samples should be collected and labeled to indicate which part of the field each one came from.

Samples are normally taken from the top 2-4 inches from the soil surface. Soil must be collected from similar soil types in the same vicinity not treated for comparison.

We hope to start a pilot bioassay program this season to help growers evaluate potential herbicide injury using the new greenhouses at the Valley Agriculture Center. We would like to determine if such a program is needed and would be effective in identifying potential injury problems.

To have a field bioassayed for potential herbicide injury, contact Barry Tickes at (928) 580-9902 or Marco Peña at (928) 782-5871 for collection guidelines and how to drop off the soil samples.

Contact Tickes: btickes@ag.arizona.edu.

11-52-0 fertilizer: Best choice for desert soils

By Kurt Nolte, UA agriculture agent, Yuma County

As world urea and phosphate prices inch back to 2007 levels, fertilizer prices for the upcoming 2010 vegetable season should be a bargain compared to the unprecedented prices seen during recent years.

Current mid-July UAN32 prices hover around the $260/ton mark compared to $830/ton in the summer of 2008. With the current price of 11-52-0 ranging around $450/ton, down from over $1,000/ton two years ago, pre-plant phosphate applications should approach near normal rates.

Despite the comparable prices for 11-52-0 (monoammonium phosphate - MAP) and 18-46-0 (diammonium phosphate - DAP), 11-52-0 is the preferred choice as a pre-plant phosphorus fertilizer for desert vegetables.

Below is an 11-52-0 pre-plant fertilizer refresher:

1 - Soil solution differences: When 11-52-0 is applied, the soil solution pH surrounding the granule ranges from an acid pH of 3.5 - 4.2. However, the initial pH around the 18-46-0 granule will be alkaline with a pH of 7.8 - 8.2.

The high pH soil solution in combination with high pH soils, and extra ammonia added to 18-46-0, can result in zones of free ammonia which can cause seed germination problems, seedling injury, and potentially interfere with root development.

2 - Phosphorus uptake: P is taken up from soil solutions by roots in two forms: H2PO4 and HPO4. Research has shown a trend that plants take up H2PO4 more rapidly than HPO4.

This factor is important in the MAP-DAP comparison since the acid soil solution in 11-52-0 favors the formation of H2PO4; thus more potential P uptake.

3 – Micronutrient effects: Plants impact the plant’s availability of micronutrients manganese, iron, and zinc which usually increase in acid soil solution environments. The acidic zone (pH 4.0) created by 11-52-0 enhances micronutrients availability while the alkaline zone created by 18-46-0 (pH 8.0) decreases the availability of these micronutrients.