Southern Blight

At the initial stages of the disease, outer leaves turn yellow and wilt. The bulb and bases of the leaves are affected by a soft rot. White mycelium is present on bulbs and in soil. As the infection progresses, infected plants wilt and eventually die.

Accurate diagnosis is critical for effective control. Misdiagnosis as white rot is common, especially where southern blight has not previously occurred. Plants with southern blight can be diagnosed by the presence of small (0.04 to 0.08 inch, or 1 to 2 mm), extremely numerous, tan-to-reddish-brown sclerotia in and on bulbs, as well as in the soil. The sclerotia are white initially, but become brown as they mature. Fanlike growth of the white mycelium may also be apparent on the bulb and the surrounding soil surface. To diagnose this disease in the absence of these signs in the field, incubate the affected tissue in a moist chamber for 5 to 7 days and look for the rapid, fanlike growth of the white mycelium.

Southern blight is not a common disease of onions, but is a persistent problem in Kern County, and more recently has been detected in Lake County. The disease is often not noticed until it is widespread in a field. Within a season or two, it can become serious enough to cause the loss of an entire field. It can progress very rapidly under warm conditions (86–95°F) and is more common in unusually hot and wet years.

Disease incidence and severity depend on the number of sclerotia in the soil, which can be high if previous crops were affected, since each infected plant can produce tens of thousands of sclerotia. Sclerotia also survive for long periods (at least five years) in the soil. The pathogen has a host range of over 500 plant species, so rotation is not a very effective management tactic. However, avoiding highly susceptible hosts can be useful (see Cultural Control).

The fungus may spread via infected bulbs and infested soil. Although higher temperatures generally favor the development of the disease, the fungus cannot survive at extremely high temperatures (105–120°F).

Proper management should include the following actions:

• Keep the tops of the beds dry to reduce incidence of the disease.
  • Use subsurface drip irrigation if feasible.
  • Avoid alternating between wet and dry periods.
• In fields where subsurface irrigation is not used, deep plow to bury plant residue. Sclerotia buried more than 6 inches deep are usually parasitized by other microbes and are killed over time.
• Use good sanitation to prevent moving inoculum from infested fields into noninfested fields. Clean boots and equipment before moving from one field to another.
• Rotate to crops that are poor hosts of the pathogen, such as corn, sorghum, rice, or small grains (wheat, millet, or oats), for at least two years after detecting the disease. This will reduce the inoculum in the soil.
  • If fumigation is not an option, consider rotating with a mustard cover crop. Mustards can suppress southern blight.
  • Avoid highly susceptible rotation crops, such as tomatoes, potatoes, beans, peas, cucurbits, peppers, hairy vetch, alfalfa, and carrots.

Solarization can be used in combination with fungicides to manage the disease in inland areas, where temperatures are higher. To kill the sclerotia near the soil surface, the soil must reach temperatures between 105°F to 120°F for 2 to 4 weeks. Solarization alone, as well as the combination of solarization and biological control, are not considered viable management strategies. Solarization is most effective when combined with a fungicide.

Biological control by other microbes reduces inoculum that is buried more than 6 inches deep. Fungi such as Trichoderma harzianum also have antagonistic effects on this pathogen and are available in organically certified pesticide products.

Use cultural controls and furrow application of Trichoderma harzianum in an organically certified crop.

Scout the fields during the summer months to determine if southern blight is present. This will enable you to take action before the sclerotia become too numerous.

With their narrow, easily penetrated canopy, onion and garlic are some of the only crops where fungicide control is highly effective. Since the pathogen is only active in the top 3 to 4 inches of the soil, and only infects the neck and upper bulb from the soil line, the fungicide must completely cover the base of the plant and penetrate into the soil. Since thorough coverage of the neck is necessary, subsurface drip chemigation does not provide effective control.

Preplant fumigation with metam sodium can reduce the amount of sclerotia in the soil. However, this is unlikely to eliminate the inoculum completely unless applied through the sprinklers, which is restricted in many counties.