Two common soil-borne diseases found in lettuce production are also found in Coachella Valley lettuce fields — lettuce necrotic stunt virus and tomato bushy stunt virus (TBSV).

The primary symptom of these soil-borne diseases is severe plant stunting. Stunting typically occurs after thinning when the plant moves into the rosette stage. Stunting can actually occur at any developmental stage. Plant maturity can cease in infected plants after the 8-10 leaf stage.

Extensive yellowing generally occurs in the outermost leaves followed by small areas of brown leaf issue necrosis in and between the veins. The tissue eventually dies.

Researchers suspect the viruses persist in plant debris, soil, and water and likely are spread by irrigation water, flooding, and infested soil and mud, Aguiar says. Plant dieback is more severe in fields which leach poorly and remain saturated for long periods of time.

“In the Coachella Valley, these diseases are likely related to poorly working leach lines.”

Romaine cultivars have the most pronounced and serious symptoms of lettuce dieback.

Leaf and butterhead cultivars are also susceptible. Most modern crisphead cultivars (iceberg and head lettuce) are resistant.

Some new romaine cultivars have more tolerance to TBSV.

Okra is another important valley crop which covers almost 900 acres. Some growers experience heavy infestations of the cotton mealybug, Phenacoccus solenopsis. Feeding causes stunted plants and reduced yields.

Management guidelines for cotton mealybug in okra include:

  1. Disk and plow under crop residue in infested fields. Crop residue provides food and shelter year round.
  2. Harvest mealybug-infested fields after non-infested fields.
  3. Mealybug crawlers get on workers’ clothes so wash clothes every day. Workers become a “vector” of sort during the crawler stage.
  4. Control weeds around the field perimeter.
  5. Sanitize equipment before moving from an infected field to a non-infected field.
  6. Ants protect mealybug so control the ants in and around the field.

Zyi Mendel, an entomologist from Israel, scouted Coachella okra fields and found two effective wasp parasitoids — Aenasius arizonensisand Pseudleptomastix squammulata.

Aguiar adds that high populations of these wasps would be required for effective mealybug control.

The economic value of Riverside County agriculture in 2011 totaled $1.2 billion, the 12th largest in California.