Symptoms and Signs
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.
Comments on the Disease
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.
Organically Acceptable Methods
Use cultural controls and furrow application of Trichoderma harzianum in an organically certified crop.
Monitoring and Treatment Decisions
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.
|Common name||Amount per acre||REI‡||PHI‡|
|(Example trade name)||(hours)||(days)|
|Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are listed at the top of the table. When choosing a pesticide, consider information relating to the pesticide's properties, and application timing, honey bees, and environmental impact. Always read the label of the product being used.|
|(Vapam HL)||Label rates||See label||NA|
|COMMENTS: Fumigants such as metam sodium are a source of volatile organic compounds (VOCs) but are minimally reactive with other air contaminants that form ozone. Most effective when applied through sprinklers, which is not allowed in some counties. If sprinkler application is not allowed, shank in to apply. Shank applications are not as effective, and fields may still suffer losses when this type of application is used.|
|(K-Pam HL)||Label rates||See label||NA|
|COMMENTS: Fumigants such as metam potassium are a source of volatile organic compounds (VOCs) but are minimally reactive with other air contaminants that form ozone. Most effective when applied through sprinklers, which is not allowed in some counties. If sprinkler application is not allowed, shank in to apply. Shank applications are not as effective, and fields may still suffer losses when this type of application is used.|
|(Luna Experience)||12.8 fl oz||12||7|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Succinate-dehydrogenase inhibitor (7) and demethylation inhibitor (3)|
|B.||TRICHODERMA HARZIANUM AND TRICHODERMA VIRENS|
|(RootShield PLUS Granules)#||5–12 lb||See label||—|
|MODE-OF-ACTION GROUP NAME (NUMBER1): biological|
|COMMENTS: Registered for use in onion, garlic, leek, and shallot. Apply from a granular applicator in-furrow at seeding, or to established crops as a sidedress, and incorporate into soil. Use prior to disease onset.|
|1||Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of action. Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode-of-action group numbers 1, 4, 7, 9, 11, or 17 before rotating to a fungicide with a different mode-of-action group number; for fungicides with other group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode-of-action group number.|
|‡||Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.|
|#||Acceptable for use on organically grown produce.|
|*||Permit required from county agricultural commissioner for purchase or use.|
|§||Do not exceed the maximum rates allowed under the California Code of Regulations Restricted Use Materials Requirements, which may be lower than maximum label rates.|