Field Scouting Guide: White Rot

Onion White Rot on bulb Photo by Koike_ FEATURE

Photo by Steven Koike

This month’s Field Scouting Guide concentrates on Sclerotium cepivorum (white rot). We’ve reached out to pathologists Dr. Jeremiah K.S. Dung, Oregon State University, and Steven T. Koike, TriCal Diagnostics, to learn how to spot and treat this fungus.

BASIC

  • Scientific name: Sclerotium cepivorum
  • Common name: White rot
  • Geographical Range: Worldwide, including Asia, Australia, Africa, Europe, Central and South America, North America (Canada and the U.S. states of California, Oregon, Washington, New York, plus other states).
  • Crops affected: It affects allium species, with onion and garlic the most susceptible. Sclerotia, the resting structure of the fungus, remain dormant and only germinate in response to sulfur-containing compounds released by Allium roots. Reports of a few ornamentals in the amaryllis family exist, but the reports are old and poorly documented.

DISEASE IMPACT

Dung: Here in central Oregon, seed garlic bulbs growers struggle with the disease. It can also be a problem for onion and garlic growers in certain regions of California, and I collaborate extensively with scientists in our neighboring state.

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If a field of seed garlic has white rot, some or part of it may be destroyed or used for dehydrated products, which is not as profitable as harvesting the garlic for seed. Additionally, the fungi can survive in soils for decades (at least 20 years), so infested fields are often taken out of Allium production completely.

Koike: The economic impact is significant. In the West, white rot is probably the No. 1 soilborne problem of onion and garlic. If infected, the plant will never be of acceptable quality and size, and most plants die before full maturity.

IDEAL CONDITIONS

Dung: The disease develops best in cool, moist soil conditions. The soil temperature range for infection is 50°F to 75°F, with optimum being 60°F to 65°F.

Koike: White rot is a chronic problem because the resting, survival structures (sclerotia) spread readily and can persist in soil for many years. In the West, we can find white rot in most places growing the susceptible crop.

IDENTIFICATION

Koike: White rot does not show on very young onion seedlings or garlic plants. Symptoms show up after the plants have been growing for a bit. It’s hard to give a “stage” to this.

As a result, identifying white rot can be confused with several other diseases. If the infected plant shows stunting, overall poor growth, wilting, decline, and drying of older leaves, decayed bulbs and lower stems, and eventual plant collapse, the problem could be caused by a number of other problems: Fusarium basal rot, pink root, Embellisia bulb canker, Southern blight, and stem and bulb nematode.

Dung: Scouts can readily diagnose the disease by pulling suspect plants and inspecting for fluffy white mycelial growth of the fungus and the small, poppy seed-like sclerotia that form on the bulbs.

RECOMMENDED TREATMENT

Sclerotium Cepivorum from white rot Photo by Koike

The fungus structure, sclerotia, can survive in soils for decades — at least 20 years. The number of sclerotia required to incite disease is very low. Approximately one sclerotium per quart of soil can cause economic losses.
Photo by Steven T. Koike, TriCal Diagnostics

Dung: The number of sclerotia required to incite disease is very low — approximately one sclerotium/quart of soil can cause economic losses. The long survival potential of sclerotia in soil makes controlling white rot difficult.

Exclusion is important to reduce the chances of introducing the pathogen into fields or a production area.

Once introduced, sanitation is important to reduce the chances of spreading the pathogen. Fungicides provide only partial control/suppression, especially when environmental conditions are favorable for disease. Growers often apply fungicides in-furrow during planting to protect the seedlings from infection.

Here are my tips for using Integrated Pest Management to protect against white rot:

  1. Plant pathogen-free cloves, sets, or transplants
  2. Sanitation: wash equipment, boots, etc. to avoid moving infested soil within and between fields
  3. Hot water treatment can help but is not entirely effective
  4. Rogue infested plants and surrounding soil for disposal.

For traditional chemistries, contact your local Extension agent for registered fungicide options for your crop, area, or state.

Koike: The most effective treatment is sanitation and field selection:

  • Do not spread the pathogen that is in mud and dirt on equipment or tires.
  • Do not plant Allium crops in known infested fields, since the pathogen survives in the field for many years).

Not sure how effective these traditional chemistries are, but I believe they are registered: tebuconazole, penthiopyrad, fludioxynil, and boscalid.

For white rot, I don’t think fungicides are important management options because they don’t work all that great. Others may have different experiences.

ADDITIONAL INFORMATION

Combined-effect-of-a-sclerotial-germination-stimulant-and-in-furrow-fungicide_left_white-rot-compared-to-a-non-treated-control_right_credit-Rob-Wilson,-UC-ANR

A collaborative study between University of California, Agriculture and Natural Resources, and Oregon State University trialed using fungicides in-furrow during planting. The treated plants are on the left, and untreated are to the right.
Photo by Rob Wilson, University of California, Agriculture and Natural Resources

Dung: Our current research project, funded the USDA-National Institute of Food and Agriculture Specialty Crop Research Initiative, is investigating a two-pronged approach to manage white rot.

The first approach begins with the use of sulfur-containing compounds, including Allium-based products and byproducts, to “trick” the sclerotia into germinating prior to planting. The germinating sclerotia have a limited amount of stored energy and will eventually die without a host.

This approach successfully reduced sclerotia populations by more than 90% in the past, but there are no commercial products currently on the market. We need new sources of germination, biostimulants.

Our second approach uses in-furrow fungicides to suppress symptoms and increase yields. This work is a collaborative effort with Robert Wilson, University of California, Agriculture and Natural Resources (UC, ANR), Tulelake, Tom Turini, UC, ANR, Fresno, and Dr. Michael Qian, Oregon State University.

White rot is just one aspect of our USDA-NIFA Specialty Crop Research Initiative funded research. The project titled “Development and Delivery of Integrated Management Packages for the Most Serious Pest and Diseases Threatening U.S. Allium Industries” also includes the evaluation and implementation of IPM tactics for the control of thrips and iris yellow spot virus (IYSV), the development and release of onion germplasm that is resistant to thrips and IYSV, and broad outreach and Extension efforts.

The project is led by Dr. Hanu Pappu at Washington State University and includes principal investigators from Cornell University, the College of Idaho, Oregon State University, New Mexico State University, and USDA-Agricultural Research Service. AVG