The latest Arizona Vegetable Integrated Pest Management Update from the University of Arizona (UA) Cooperative Extension in Yuma, Ariz. released April 17, 2013.

Whitefly management on spring melons

By John Palumbo, UA research scientist and Extension specialist

It is that time of the year to think about management of whitefly nymphs on spring melons. I have had a couple of reports from pest control advisers (PCAs) that whitefly adults are showing up on cantaloupes. Adults can be found on pre-bloom melons at the Yuma Agricultural Center in Yuma.

As temperatures increase, feeding damage from whitefly nymphs should be a concern on all melon types. Honeydew and sooty mold contamination on the fruit of cantaloupes, mixed melons, and watermelons can significantly reduce quality and marketability.

Although whitefly numbers have been low up to now, PCAs should not be complacent in monitoring and sampling. With the warmer weather, numbers are likely to increase rapidly over the next few weeks.

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UA research has shown that a foliar insecticide treatment should be applied when a threshold of two adult whiteflies per leaf is exceeded to prevent melon yield and quality losses.

By timing sprays based on the adult threshold, immature populations should be starting to colonize. Applying foliar sprays at this stage has been shown to significantly reduce the chance of yield and quality loss during harvest.

This threshold applies for the insect growth regulators (Vetica, Courier, Knack, Oberon), and foliar-applied neonicotinoids (Assail, Venom, Scorpion).

For more information, go to these documents on whitefly management, thresholds, and insecticide efficacy.

Be aware of pollinators in or around melon fields. Carefully read labels to determine the product’s bee safety information before applying pesticides in melon fields, particularly when bees are foraging.

Cucurbit yellow stunting disorder virus (CYSDV) is not generally known to limit yield on spring melons. However, research suggests fall melons may be at greater risk of CYSDV infection when planted in areas where CYSDV symptoms were found on late spring melons.

When practical, keep whitefly populations low on spring melons. This will also prevent potential dispersal to cotton in June and July.

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“Remember, when in doubt - scout.”

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

Differences between disease resistance and tolerance

By Mike Matheron, UA Extension Plant Pathologist

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

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

Resistance can be considered 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.

Resistance focuses on infection prevention. 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 – immunity - through degrees of useful resistance, and finally to the lowest level when a plant is highly susceptible to a particular pathogen.

In addition, 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 which lead to disease resistance and tolerance are many and varied, but usually are biochemical or structural in nature.

The successful suppression of pathogen activity by a plant is tied to how a particular pathogen gains entrance into a plant to initiate disease, plus how a plant defends itself from the infection.

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 any input by the grower.

On the other hand, disease management products, including fungicides, often must be applied several times over the entire growth period of the plant when disease is expected.

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

Building disease resistance or tolerance into plants is an ongoing activity of plant breeders who use classic and modern genetic manipulation techniques to achieve this goal.

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

Herbicide volatility

By Barry Tickes, UA area agriculture agent

Some of the oldest and most effective broadleaf herbicides are growth regulators.

These include: 2,4-D, including 2,4-DB, Butyrac, and Butoxone; MCPA, including Rhomene Rhonox, Chiptox, and Battleship; Dicamba, including Clarity, Banvel, Clorpyrolid ( Stinger), and Triclopyr (Garlon and others); and many products containing two or more of these.

The principle use of these products in the low-desert growing region is on grain, bermudagrass, alfalfa, cole crops, and sugarbeets.

The product utility is very limited in the region due to the volatility and potential to move and cause injury to sensitive crops.

Volatility is very different than spray drift. Spray drift is the airborne movement of the herbicide to non-target sites. It is often related to physical conditions including wind speed, nozzle type and pressure, application technique, and other factors.

Volatility involves movement after the herbicide has evaporated into the air as a gas. It is affected by physical conditions, but more importantly by characteristics of the herbicide including chemical properties of the active ingredient - especially the vapor pressure and formulation type.

All growth regulators have high vapor pressure and are volatile.

These herbicides can be formulated differently. Formulation can significantly affect volatility. The amine formulations are much less volatile than the ester formulations of these products. The amount of product used can vary with formulation. There is generally more volatilization potential with higher use rates.

The type of salt in amine formulations is also influential. Formulations of Dicamba which use sodium salt (Distinct, Celebrity Plus, and Northstar), or diglycolamine salt (Clarity) are less volatile than formulations using the DMA salt used in Banvel.

The volatility of these growth regulators is always higher during the hot summer temperatures. Use is restricted in some California counties from March 15 to Oct. 15.

Questions come up every year at this time about how long after application these products can volatilize and how far the products can move. Available information addresses how safe the products are after drying and the products which can volatilize 60 days after application.

Many people have seen products not move at all while others claim it can move several miles. This is likely since volatility is affected by many factors. It will always be difficult to predict.

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

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