The Dirt on White Rot / Garlic and Onions

Montana State University confirmed white rot on a garlic sample from our ASUM Community Gardens in 2019. I spoke with their horticulture specialist and their plant disease diagnostician to better understand how worried we should be, and most importantly, what it means for community gardeners and the future of our garlic growing.

About White Rot

The pathogen persists as small, dormant structures (sclerotia) in the soil. Sclerotia remain dormant in the absence of a suitable host (garlic, onion, or other Allium crops), and can survive in the soil for over 20 years. Sclerotia can spread throughout a field, or from field to field, via flood water, equipment, or plant material. Disease severity depends on soil temperature and the amount of sclerotia in the soil at planting. Disease development is favored by cool, moist soil conditions, which coincidentally is also perfect growing conditions for onions and garlic.

The Good

The good news is that white rot typically isn’t found in Missoula soils, and this is the first time we’ve had white rot diagnosed at a community garden. However, there have been numerous cases brought up this year. Eva Grimme, the MSU plant disease diagnostician, believes this is due to the high levels of moisture, humidity and rains this year, which are ripe growing conditions for molds and fungi.

If we all do our part to prevent and manage these disease, and the weather next year isn’t as cool and wet as this year, white rot should not be a big problem for ASUM Garden moving forward. Since we can’t predict the weather, please follow these prevention guidelines below.

Prevention and Management

• Sanitation is the key to control the disease spreading: pull and destroy affected plants (aka throw them away in the garbage)

• Crop rotation: don't plant garlic or other plants in the onion family in the same spot for several years

• Clean all tools between garden sections to avoid spreading the fungus

• Don't plant infected garlic cloves in the garden.

  • Do not plant garlic cloves previously grown in ASUM Garden because the sclerotia may be present on the garlic even if not visible. Instead, please buy your garlic seed from other local farmers or friends who do not have rot. (To clarify: garlic seed does not need to be certified mold or fungus free).

What You Didn’t Know About Growing Garlic

• Remove the mulch - In the fall, garlic is typically mulched with 6 inches of straw or leaves. Mulch helps regulate soil temperature, keeps the soil moist, and suppresses weeds. However, Cheryl Moore-Gough, the MSU horticultural specialist, recommends removing the mulch in early summer when the bulbs begin to size, which is typically at the beginning of warm summer temperatures. This is the time when garlic is most susceptible to rot.

• Avoid over-watering and cut back watering in early summer (when the bulbs begin to size).

• Harvest

  • Hard-neck garlic is usually ready to harvest when the scapes begin to straighten

  • Soft-neck garlic is usually ready to harvest when the first 3 leaves from the bottom of the plant brown

• Harvest early! If you cannot reduce the water on your garlic, you will not see the leaves brown, so check you garlic heads by pulling back the soil to see their size. If the size is to your liking, then feel free to harvest! This is a good practice for all garlic growers…when in doubt, pull back the soil, and check. Harvesting early is better than garlic sitting in the soil too long and developing molds or disease.

For more on growing garlic in Montana, read this guide by the MSU Extension below.

Read more about white rot with the recommended links below.