The latest Arizona Vegetable Integrated Pest Management Update from the University of Arizona (UA) Cooperative Extension in Yuma, Ariz., released May 2, 2012.

Insect and weed interactions in vegetables

By John Palumbo, UA Research Scientist and Extension Specialist

Effective weed management is critical for the profitable production of vegetable crops in the desert southwest for obvious reasons. However, weed management is also essential for another important but often overlooked reason.

Several common weed species found in and around vegetable crops can serve as host plants to many insect pests that can later infest nearby crops.

Although flowering weeds can provide a reservoir for natural enemies and as a source of nectar and pollen for pollinators, these same weedy refuges can serve as host sources for many key insect pests that cause economic damage to vegetable crops.

Weeds found on field margins and ditch banks can provide insect pests with suitable resources needed for rapid population growth which subsequently can lead to insect infestations occurring in adjacent vegetable crops.

In addition, many weed species can provide insects with host plants that serve as a bridge between cropping seasons when vegetables crops are not in production (May through August).

Volunteer melons and cotton can also be considered weeds (“a plant out of place”). If not controlled in a timely manner, these weedy volunteer plants can sustain large numbers of insect pests, plus many plant viruses that are transmitted by insect vectors that can migrate onto newly planted fields.

Finally, weeds can serve as impediments to insecticide applications.

Dense weed foliage in vegetable and melon fields can negatively influence foliar spray applications by intercepting spray droplets before reaching the target crop, which can result in less insecticide deposition and unacceptable crop damage.

Soil applied insecticides (e.g., imidacloprid) can also be impacted by unmanaged weed growth.

Weeds growing unchecked during stand establishment can compete with the seedling plants for water and fertilizer, but can also compete with crop plants for soil insecticides. Excessive weed densities can significantly intercept insecticides in the soil profile and reduce the amount available for uptake by the target crop.

For more information, please check out this report Interactions between Insects and Weeds in Vegetable Crops.

Remember: “When in doubt - Scout.”

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

Managing powdery mildew on melons

Managing powdery mildew on melons

By Mike Matheron, UA Extension Plant Pathologist

Maximum control of powdery mildew on melons requires initiation of a fungicide application program when environmental conditions favor disease development but before the first visible detection of disease. Less than optimal but good levels of disease control can also be achieved by waiting to begin fungicide application until no later than the very first sign of disease in the field.

Early initiation of fungicide treatment on susceptible melon varieties is essential due to the rapid development and spread of powdery mildew from initial invisible infection sites within the crop. Application of a newly registered active ingredient usually is effective on virtually all of the individual pathogen spores or colonies developing from spores.

However, the very small number of individuals not killed or inhibited by the fungicide will become an increasingly larger proportion of the pathogen population as the use of the same active ingredient increases. This is how resistance to a particular fungicide becomes established.

The melon powdery mildew fungus Podosphaera xanthii has developed significant resistance to some fungicides in the past. An important strategy to delay development of fungicide resistance is to alternate among or mix products with different modes of action.

Previous research demonstrated that fungicide application sequences, containing a highly efficacious fungicide alternated with a product of moderate to low efficacy, provided a final level of disease control equivalent to that achieved by continuous application of the highly effective material.

Data from these trials support the notion that high levels of disease control and resistance management can be realized with fungicide alternation programs containing different modes of action of only highly effective chemistries as well as application programs incorporating products with high efficacy along with those of moderate and low effectiveness.

The most recently completed cantaloupe powdery mildew fungicide evaluation trial in 2011 reveals the relative efficacy against this disease of fungicides currently registered as well as those in development.

Click on this link to see the Comparison of fungicides for management of powdery mildew on muskmelon, 2011.

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

Band applications of herbicides

Band applications of herbicides

By Barry Tickes, UA Area Agriculture Agent

Band applications of herbicides are less common than band applications of insecticides since weeds need to be controlled in the entire field; not just in or on the plant area. Banding herbicides in row crops is not uncommon and has several advantages.

The primary advantages are reduced cost and herbicide carryover in the soil. Banding over the crop row is often combined with cultivation. Cultivation is cost effective with immediate gratification.

Determining the rate of application when making band applications is sometimes misunderstood and-or abused. The application rate should be the same for band and broadcast applications but the treated acreage will differ depending on the band width.

If a 50 percent band is applied on a 10-acre field. Five acres will be treated. The amount of water and herbicide should be calculated for five acres. This is fairly straight forward but often misunderstood.

It can be tempting, when the herbicide is inexpensive and easily accounted for, to apply higher than labeled rates to increase weed control. A good example is the use of glyphosate on Roundup Ready cotton. Although crop safety is well known with this herbicide, there are some instances where other herbicides can cause injury at higher than labeled rates.

More important, however, is that this practice increases the possibility of herbicide resistance. Selection pressure for resistant biotypes increases with higher than labeled rates.

The rapid increase in the number of glyphosate resistant weeds in the Midwest has been alarming. One of the primary reasons for this has been the progressive increase in application rates to control tough weeds. Increasing rates is a short term solution that can cause long term problems.

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