The productive farmland in Southern California’s Coachella Valley in Riverside County is nestled between the San Bernardino Mountains to the west and the Salton Sea, the state’s largest lake, to the east.

The Coachella Valley’s cornucopia of crops and livestock include the top five-ranked commodities – nursery stock, dairy, table grapes, hay, and bell peppers, respectively.

Like other low-desert farming valleys, a plethora of pests and diseases in vegetable crops can have producers sleeping with one eye open. A new problem found in Coachella Valley bell pepper fields in 2012 caught producers off guard.

The first hint of the problem came when a bell pepper producer contacted the University of California Cooperative Extension (UCCE) in Indio in Riverside County to explain a perplexing problem found in three-quarters of a field.

“This problem threw us for a loop,” says Jose Aguiar, UCCE vegetable crops farm advisor in Indio. “The bell pepper had a silvering appearance on the fruit exterior. It was not found inside the fruit. It was strictly a cosmetic issue.”

Riverside County is the largest bell pepper producer in California. The Coachella Valley has about 5,000 acres of bell peppers with a farm gate value of about $90 million.

UCCE farm advisor Richard Smith of Monterey County has found a similar problem in red pepper fields in the Salinas Valley. After testing, Smith’s first guess is the problem could be caused by the fruit rubbing against a branch. There are no holes in the fruit which eliminates the idea of insect damage.

Silvering can affect fruit marketability. 

Aguiar discussed the bell pepper phenomena and other Coachella Valley pest and disease issues during the 2012 Fall Desert Crop Workshop in El Centro, Calif., in November. The workshop was conducted by UCCE Imperial County and was sponsored by Western Farm Press.

Commercial sponsors of the 23rd annual workshop included: Platinum Level – BASF and Bayer CropScience; Gold Level – Dow AgroSciences, Oro Agri, Westbridge Agricultural Products, and Syngenta; and Silver Level – FMC and Valent.

An ongoing problem for Coachella Valley bell pepper growers is plant damage from root knot nematodes (RKN), Meloidogyne incognita. Mini bell peppers are especially susceptible to the microscopic roundworms.

RKN is the most serious nematode parasite for California bell peppers, according to UC IPM Online. RKN damage in bell peppers in the Coachella Valley is widespread. Among the visual signs of insect damage is plant yellowing caused by galls formed on the roots.

In the Coachella Valley, producers typically use pre-plant and post-harvest soil fumigants with Metam-sodium (Vapam) or 1,3-dichloropropene (Telone) applied by drip irrigation. Aguiar says the application does not provide complete RKN control.

“The likely reasons for ongoing RKN problems are related to the valley’s sandy soils, pepper monocropping, hot soil temperatures during summer, and constant moisture from drip,” Aguiar said. “These factors allow the root knot nematode population to build despite fumigation.”

The RKN economic damage threshold is 5-25 nematodes per 100 grams of soil.

Aguiar, pest control adviser Juan Armendariz, and UCR nematologists Antoon Ploeg and Oli Bachi scouted 13 bell pepper locations in the east valley with a history of nematodes. Early soil sampling revealed 3-13 nematodes per 100 grams, a low population.

The group returned one month later and found increased yellowing in newer plant growth. Soil samples revealed an exploding population — 30,000 to 300,000 roundworms per 100 grams of soil. Root galling was present but not severe.

In one heavily-infested field, the grower applied Vapam as a post-harvest treatment. One week later, sampling indicated 500-600 live nematodes per 100 grams of soil. The roots contained 4,000-175,000 live nematodes.

“We are not getting a good kill,” Aguiar told the crowd.

Another issue which contributes to the problem is Coachella Valley growers do not rotate out of peppers due of limited acreage. Peppers are grown year after year.

Bachi, Ploeg, Armendariz, and Aguiar are working on this issue. The California Pepper Commission is providing financial support.

TBSV

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.

cblake@farmpress.com