Nitrogen and Water Management for Coastal Cool-Season Vegetables, Publication 21581. During the months of December 1998 and January 1999, I have sent out a number of copies of this new publication. I have tried to send one free copy to every vegetable growing operation in northern Santa Barbara and southern San Luis Obispo Counties. If your farming operation has been overlooked, please contact me for your free copy. If you have received a copy, but would like additional copies for members of your management staff, additional copies can be purchased for $10.00 plus tax. Please contact me at 805/934-6240 with your requests.
APHIDS
As temperatures increase and the coastal production season gears up, aphids problems can be anticipated. On leafy vegetables, the aphid complex will consist primarily of green peach aphid, Myzus persicae, potato aphid, Macrosiphum euphorbiae, (minor problem in most seasons), and turnip aphid, Lipaphis erysimim. On cole crops, one can also expect to find cabbage aphids, Brevicoryne brassicae. Because of their ability to contaminate harvestable plant parts, preventing aphids from colonizing plants is critical. Cultural management tactics and natural enemies can reduce the impact of aphids, but control with insecticides is usually required to prevent economic damage to crops. Provided below is information summarized from research conducted over several years that outlines approaches for managing aphids in leafy vegetables.
IDENTIFICATION
Identifying aphid species can be intimidating, but not impossible. Provided below is a table of the comparative and distinguishing characteristics for aphids found colonizing leafy vegetables.
| Distinguishing Characteristics | Cabbage Aphid | Green Peach Aphid | Potato Aphid | Turnip Aphid |
| Colors | green-gray with heavy wax coating | pale green-yellow, red forms occur | shiny pink-green, eyes are red | gray-green, with light wax coating |
| Antennae | not as long as body | as long as body | not as long as body | not as long as body |
| Antennal Tubercles | diverging | converging | diverging | diverging |
| Canda | short and triangular | pale, short | long, shiny | short and blunt |
| Cornicles | short and dark | pale | long and dark at distal end | pale, swollen at distal end |
| Other Hosts | brassicas only | multiple hosts | solanaceous crops | other brassicas |
| Damage | contamination | contamination | contamination | rare; except on seedlings |
MONITORING
Because aphids can disperse onto crops at any time and reproduce rapidly, it is important that fields be monitored regularly. How aphids are distributed within plants is an important consideration when checking fields for this insect.
Fields should be checked at least twice a week after aphid activity is first noted. Ideally, a sample of 10-15 plants in each quadrant of a field should be sampled. Aphids tend to be prevalent along upwind field borders and next to other leafy vegetable crops or weeds, so initial sampling should be focused in these areas. Because aphid populations are generally clumped within fields, each field should be uniformly sampled.
Sticky Traps
The use of yellow sticky traps can be used to monitor aphid movement into fields. Research conducted in lettuce over the past years has consistently shown that colonies of wingless aphids on plants were observed immediately following sharp increases in the number of winged aphids caught on traps. Used properly, yellow traps placed within fields near upwind edges, can provide an early indication of when economic colonization by aphids is beginning. However, proper identification of aphid species is important because many aphid species are dispersing along with vegetable pests (pea aphid, blue alfalfa aphid, greenbug, etc.).
Management Alternatives
Several predators and parasitoids attack aphids on leafy vegetables. However, natural enemies rarely provide adequate control of high field populations. Consequently, control with insecticides is often the only viable alternative to preventing aphids from contaminating harvested products. Below is a summary of management alternatives for aphids in leafy vegetables based on replicated research trials:
Prophylactic Soil Treatment Approach
Responsive Foliar Approach
Information adapted from: Yuma County Farm Notes, December 1998 issue.
Bacterial leafspot has severely damaged much of the celery that was still in the field at the time of the December 1998 freeze. The freeze alone caused significant damage in the form of cracked petioles and blistered epidermis on the petioles. Once the epidermis was compromised, bacteria had easy access to interior leaf and petiole tissues, causing severe leafspot and petiole collapse.
Bacterial leafspot of celery is caused by Pseudomonas syringae pv. apii. Symptoms on leaves are first expressed as yellow spots, which enlarge and develop brown centers with a yellow halo. Tissues at the margins of spots also appear water-soaked. Affected petioles develop a gray-green color as the bacteria invade vascular tissues.
Celery bacterial leafspot has been most severe on the Central Coast following freezing weather. Although not definitely confirmed, it appears that freezing temperatures cause microscopic wounds to the leaf and petiole epidermis. These injuries allow the bacterium to enter the leaf or petiole and colonize the exposed tissues. If moist (heavy dew) weather follows the freezing temperatures, bacterial activity can be rapid and crop injury severe.
Copper compounds can be used to provide some suppression of this disease. Copper materials applied in anticipation of freezing weather would be the most useful in terms of reducing the injury caused by this bacterium. Coppers act both to reduce bacterial numbers and to reduce ice nucleation on the surface of plant tissues.