Time to Rethink Your Lettuce Drop Management Plan?

A field of healthy lettuce.

Many growers fight an ongoing battle with Sclerotinia sclerotiorum and are looking to revisit their lettuce drop management plan to help prevent future outbreaks.

At its worst in the 2010-2011 season, Sclerotinia destroyed about 3 percent of the romaine and iceberg lettuce crops in Yuma County, Ariz., which totaled nearly $8 million in losses.

Rescue fungicide applications can help to slow or stall disease progression once symptoms are spotted, but prevention is the best way to maintain lettuce quality and avoid crop losses, says Martin Reid, retail account manager, DuPont Crop Protection. “Now is a good time to assess the effectiveness of your lettuce drop management strategies this season and consider additional control options for next year.”

Understand the disease cycle

The two fungi that cause lettuce drop — Sclerotinia minor and Sclerotinia sclerotiorum — have become common in the soils of major lettuce-growing areas of the Southwest, where about 90 percent of the U.S. winter lettuce crops is grown. “Once the disease is present, it becomes an ongoing challenge to control,” explains Reid.

Developing an effective strategy for managing Sclerotinia drop in lettuce first requires understanding its disease cycle. Both fungi species carry over in fields between lettuce crops in the form of sclerotia, explains Reid. “The sclerotia are dormant in the soil until moisture and temperature conditions are right for germination, often just after lettuce seedlings emerge.”

Research shows that sclerotia lay dormant in the top 2 inches of soil and require moisture and soil temperatures between 46 and 61 degrees Fahrenheit to germinate. Even though the sclerotia will naturally decay over time, enough will remain to infect the next susceptible crop, including lettuce, cabbage or cauliflower.

Sclerotinia minor affects only the leaves and stems in contact with the soil. Once lettuce plants are infected, leaves wilt and drop to the soil surface, with plant decay eventually destroying plant crowns. Sclerotinia sclerotiorum produces spores that can also be spread by air, infecting the upper plant and potentially spreading across an entire field. This species of the disease was largely responsible for the rapid spread of the 2010-2011 outbreak, notes Reid.

Lay down a barrier

Fungicides play an important role in managing against Sclerotinia species in lettuce. University of Arizona fungicide evaluation trials have demonstrated that one or two fungicide applications to the bed surface at seeding or just after transplant typically provide 50 to 60 percent reduction in dead lettuce plants compared to untreated plots.

Carefully timed fungicide applications can improve that control, adds Reid. “The idea is to create a chemical barrier between the soil surface and young lettuce seedlings. An application of DuPont™ Fontelis® fungicide at the 24-fluid ounce rate, made just after plant thinning, creates that kind of barrier to shield plants from fungal spores. A second Fontelis® application made 7 to 14 days later extends the protection at this very critical stage.”

In addition to Sclerotinia drop in lettuce, Fontelis® provides reliable control of a variety of leafy vegetable diseases, including Cercospora leaf spot (Cercospora spp.) powdery mildew, early blight (Cercospora apii) and late blight (Septoria apicola). Fontelis® delivers preventive protection against foliar diseases, for fast-acting, long-lasting control under light and heavy disease pressure. A single active ingredient makes Fontelis® ideal as part of a resistance management plant.

Use other control measures

Research shows that a variety of control measures can help reduce the presence of Sclerotinia fungi in the soil or lessen its impact on crops. Some options include:

  • Moisture management. Well-drained soils and careful timing of irrigation are some of the best ways to avoid overly wet soil conditions that can foster germination and growth of fungi.
  • Preplant soil flooding. Research at the University of Arizona’s Yuma Agriculture Center demonstrated that flooding fields for a three-week period in the summer destroys all sclerotia of both Sclerotinia minor and Sclerotinia sclerotiorum species.
  • Crop rotation. When possible, avoid planting infected fields with lettuce two years in a row and rotate to less-susceptible crops such as onions, corn or small grains.

Always read and follow all label directions and precautions for use.

DuPont Fontelis® is not registered in all states. Contact your DuPont representative for details and availability in your state.

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