Arizona Veg IPM Update: Aphid ID, lettuce wilt diseases, soil-applied herbicides

Arizona Veg IPM Update: Aphid ID, lettuce wilt diseases, soil-applied herbicides

Winged (alate) aphids are appearing on desert lettuce and cole crops. The proper identification of winged aphid species found on leafy vegetables is important for cost effective pest management.

The Arizona Vegetable Integrated Pest Management Update from the University of Arizona (UA) Cooperative Extension in Yuma, Ariz. released Oct. 30, 2013.

Aphid identification in leady vegetables

By John Palumbo, UA research scientist and Extension specialist

We are beginning to observe winged (alate) aphids appearing on desert lettuce and cole crops, but the proper identification of winged aphid species found on leafy vegetables in the desert is important for cost effective pest management.

Most of the important aphid species found on local crops (Yuma, Ariz. area) do not over-summer here due to high temperatures and the lack of viable hosts.

Thus, winged aphids typically begin to migrate onto desert crops beginning in late October to early November; often blown in with gusting winds.

My experience over the past 20 years suggests this is due in part to cooler weather more favorable for aphid behavior and development, plus changes in prevailing winds which begin to blow into the area from the north and west.

Get the  latest agricultural news each day to your Inbox. Click here for the free Western Farm Press Daily e-mail newsletter. [4]

Consequently, once the aphids reach the desert valleys, the aphids typically move from crop to crop until a suitable host is found to feed and colonize. It is common to find winged aphids on lettuce or broccoli that are specific pests of small grains (i.e., corn leaf aphid) or alfalfa (i.e., pea aphid).

Since these aphid species will not colonize lettuce, it is important to distinguish the aphids from the key aphid pests commonly found on lettuce which do colonize and require management to prevent problems at harvest (green peach aphid, foxglove aphid, and lettuce aphid).

It is common to find the cowpea aphid in lettuce as it can be common in alfalfa at this time. However, experience has shown that although small cowpea aphid colonies may be found on lettuce that the populations rarely increase in lettuce crops.

The bottom line is proper aphid identification can save a pest control adviser time and money, and can prevent unnecessary insecticide applications.

A simple pictorial key provides information that can assist PCAs with identifying winged aphids [5] important in lettuce and other leafy vegetables.

Listen to John [6].

“Remember, when in doubt - scout.”

Contact Palumbo: (928) 782-3836 or [email protected] [7].

Comparing lettuce wilt diseases

By Mike Matheron, UA Extension Plant Pathologist

Fusarium wilt of lettuce, caused by the fungus Fusarium oxysporum, f. sp. lactucae, was first detected on lettuce in Arizona during the 2001-2002 growing season. It continues to be found in lettuce fields from mid-October through early January.

There is another wilt disease of lettuce called Verticillium wilt and caused by the fungus Verticillium dahliae. This disease has occurred in the Salinas Valley since 1995 but has not yet been found in Arizona.

The primary symptoms of each disease are similar and consist of internal discoloration of the root cortex and plant wilting followed by death. The internal root discoloration ranges from green, brown to black in plants infected with Verticillium and reddish-brown to black in plants infected with Fusarium.

Since symptoms of the wilt diseases are similar, true disease identity only can be achieved by bringing symptomatic lettuce plants to the University of Arizona Yuma Agricultural Center YAC). There, the causal pathogen can be isolated from infected root tissue and identified by microscopic examination.

Both wilt pathogens are soil inhabitants which can persist there for many years. The pathogens also can be seed-borne.

The lettuce Fusarium pathogen can only infect and cause disease on lettuce, although it may sustain itself on roots of other plants without causing disease symptoms. Verticillium dahliae, in comparison, can infect and cause disease on numerous crops other than lettuce.

Management strategies for diseases caused by Fusarium oxysporum and Verticillium dahliae are similar. When available, genetic resistance in host crop plants can provide effective disease control.

Soil fumigation and soil solarization can reduce disease levels by lowering viable populations of both pathogens in soil.

On the other hand, no known fungicides applied after planting have provided consistent and effective control of diseases caused by Fusarium oxysporum or Verticillium dahliae.

Click link to Mike's Update [8]

Contact Matheron: (928) 726-6856 or [email protected] [9].

Hazards from soil-applied herbicides to rotation-grown crops

By Barry Tickes, UA area agriculture agent

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

Determining the potential for crop injury from herbicides used on previous crops can be difficult.

The 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 can be variable within the same field.

Rotational crop restrictions on product labels must often cover many diverse conditions and geographic regions and are frequently much longer than needed.

This link [10] is a chart that contains the crop interval for the major crops and herbicides used in the desert, plus usual soil persistence for each product.

Click link to listen to Barry [11].

Contact Tickes: (928) 580-9902 or [email protected] [12].

Area wide insect trapping network

By John Palumbo, UA research scientist and Extension specialist

The results of pheromone and sticky trap catches from Oct. 25-29 can be viewed here (check out the graphs) [13].

Corn earworm: Flight activity has declined significantly in all growing areas. However, larvae can be found on pre-cupping stage lettuce at the YAC.

Tobacco budworm: Numbers continue light in all areas so far this fall with very low numbers caught in traps in Wellton.

Cabbage looper: Flight activity was down in all areas over the last two weeks with the exception of the Roll and south Yuma Valley areas. Larval populations remain high on untreated lettuce at YAC.

Beet armyworm: Moths remain active in most areas; particularly in Wellton and the south Yuma Valley. Eggs masses and small larvae can readily be found in untreated lettuce at the YAC.

Whitefly: Overall, sticky traps indicate that adult movement is decreasing in most areas, but peak populations were trapped in the north Gila Valley last week.

Thrips: Numbers remain low in most trap locations and are highest in the Roll area. Numbers on untreated lettuce at the YAC are moderate and increasing.

Aphids: Winged aphids were trapped in low numbers in sticky traps in the Yuma Valley last week, consistent with the winds and cooler weather. A few cabbage aphid alates were identified on traps, but so far no alates or colonies have been found on untreated lettuce at YAC.

Listen to John [14].

“Remember, when in doubt - scout.”

Contact Palumbo: (928) 782-3836 or [email protected] [7].

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