Arizona Veg IPM Update: insect-weed interaction, Sclerotinia lettuce drop, Kerb and lettuce

Arizona Veg IPM Update: insect-weed interaction, Sclerotinia lettuce drop, Kerb and lettuce

Weed management is very important in preventing insect buildups in fall crops. This is the perfect time to perform pre-plant soil flooding in fields which had high levels of Sclerotinia drop this past season. It is important to use the three approved preemergent herbicides in lettuce at the right place at the right time to be the most effective.

The Arizona Vegetable Integrated Pest Management Update from the University of Arizona (UA) Cooperative Extension in Yuma, Ariz. released Jul 10, 2013.

Insect and weed interactions in vegetable crops

By John Palumbo, UA research scientist and Extension specialist

In a recent update, I discussed how sanitation following spring melons is important to prevent whitefly buildups during the summer. With melons now finished, growers are preparing the ground for fall produce crops.

Another sanitation practice - weed management - is very important in preventing insect buildups in fall crops.

Effective weed management is critical for the profitable production of vegetable crops in the Desert Southwest. Weed management is also essential for another important but often overlooked reason.

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Several common weed species found in and around vegetable crops can serve as host plants to many insect pests which can later infest nearby crops. Flowering weeds can provide a reservoir for natural enemies and as a source of nectar and pollen for a 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 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 which serve as a bridge between cropping seasons when vegetables crops are not in production (i.e. July-August).

Since most key insect pests have the ability to move relatively long distances to find new food sources, weeds which grow unchecked in fallow fields during the summer often serve as a key source of insect infestations for fall vegetable and melon plantings.

For example, pale-striped flea beetle and beet armyworm populations will commonly develop on common purslane allowed to grow in fallow fields prior to fall vegetable and melon plantings.

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 plant viruses transmitted by insect vectors, which 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. This 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 seedling plants for water and fertilizer. Weeds 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 this link: Interactions between Insects and Weeds in Vegetable Crops.

And, click this link to listen to John.

“Remember, when in doubt - scout.”

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

Summer pre-plant soil flooding as a management tool for Sclerotinia lettuce drop

By Mike Matheron, UA Extension Plant Pathologist

Lettuce disease management is likely the last thing on the minds of pest control advisers and growers during the this, the hottest part of the year in the Desert Southwest region of Arizona.

However, this is the perfect time to perform pre-plant soil flooding in fields which had high levels of Sclerotinia drop this past season.

One might wonder how a soil flooding treatment can help manage a disease which will not be a problem for several more months.

First, the two fungi that cause lettuce drop, Sclerotinia minor and Sclerotinia sclerotiorum, carry over in fields between crops of lettuce as black structures called sclerotia. These fungal propagules function like seeds and remain dormant until germination in cool moist soil and infect lettuce plants.

Many of these sclerotia decay naturally over time. However, sufficient numbers can remain in a field after one or more years to cause lettuce drop when a planting is established.

If virtually all sclerotia in a field could be destroyed, then a field would no longer be a source of the Sclerotinia lettuce drop pathogens.

This is where summer pre-plant soil flooding comes in. Research conducted at the University of Arizona’s Yuma Agricultural Center showed a three-week period of flooding in the summer destroyed all sclerotia of S. minor and S. sclerotiorum present in soil.

Some growers in the Yuma area have used this soil treatment technique to successfully manage Sclerotinia lettuce drop in fields chronically affected by this disease.

Click link to listen to Mike's Update.

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

Kerb (Pronamide) use rates on lettuce

By Barry Tickes, UA area agriculture agent

Kerb SC is registered for use in lettuce at rates between 2.5-5 pints per acre. These rates do not vary for weed species or soil type but for the irrigation method.

It is important that all three preemergent herbicides used in lettuce be at the right place at the right time to be effective. The right place is around the germinating weed seeds. The right time is during germination.

Unlike Prefar (Bensulide) or Balan (Benefin), Kerb moves readily with high amounts of irrigation water. Prefar and Balan adhere better to the soil.

When lettuce germinates with furrow irrigation and Kerb is applied after planting and before the first irrigation, water moves laterally and upward (subbed) and dissolves, but does not move the herbicide downward.

Higher rates are required for effective weed control since much of the herbicide stays on the surface.

When the herbicide is applied after planting before the first irrigation and sprinkler irrigation is used, Kerb is moved with the overhead water further into the soil. Less herbicide is needed and rates drop by about one third.

Depending upon the weed species and timing, some of the herbicide can move deeply into the soil and below the germinating weed seeds. This is why delayed applications by chemigation are often used with Kerb.

Chemigation is the most efficient means of concentrating Kerb where and when a grower needs it. The rates are therefore half of the recommended rates for furrow irrigated lettuce.

Unfortunately, the most effective techniques to concentrate Kerb around weed seeds also concentrate it around the lettuce seed. For this reason, do not use higher rates than necessary for good weed control.

Click link to listen to Barry.

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

More great reads from Western Farm Press:

Orchard sanitation key to Navel orangeworm control

Herbicide drift: How to avoid it

Fire ant control important at almond harvest

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