White Rot – Sclerotium Cepivorum

This summary is based on a talk by Professor Fred Crowe from Oregon State University. The transcript can be viewed here. The original sound recording is here.

White Rot: Description

White Rot is a fungus that prefers cold weather (it won’t grow above 20-22 degrees C). It doesn’t have functional spores, and it only can spread by being physically moved around, surviving in the soil as small, round, seed-like structures known as sclerotia. These sclerotia can survive in the soil for decades. White rot can also be carried around as active mold inside a plant.

White rot can only grow a centimeter away from the sclerotium or root. It then has to find a root or a bulb to grow on in order to continue growing. White Rot appears on Alliums as a fluffy white growth, which develops at the base of the bulb. It generally it’s found in the first two feet of soil.

————————

Prevention, Treatment and Control

Below are some methods that might work to prevent, treat and control white rot.

1. Hot Water Bath (before planting)
   The fungus is vulnerable at temperatures above 45 degrees C, thus dipping seed garlic in hot water will greatly reduce the amount of pathogen and is a good preventative measure, although it may not completely eradicate the fungus. Also, temperatures above 49 degrees C may kill the garlic, so careful temperature control is essential.
2. Diluted Bleach or Alcohol Bath (before planting)
   Similarly, a 10:1 water and bleach solution was mentioned. Dilute alcohol and hydrogen peroxide might also work. Soaking too long may kill the garlic and these methods should be checked with certifying bodies.
3. Root Exudate Solution (before planting)
   A solution to stimulate the sclerotia to germinate in the soil can be made using onion or garlic juice using culls from your crop. Once the sclerotia are stimulated and germinate they then die due to lack of nutrient reserves because the garlic is not actually present. The optimum conditions for germination of sclerotia occur when soil temperatures are between 15 degrees to 18 degrees C. Garlic works better because it’s stronger than onions. An effective dilution for garlic is one to a thousand parts garlic and water. The idea is to somehow distribute it over the area that you’re going to crop in the future getting this juice stimulant concoction across the area and down into the soil. It needs to get it as deep as you’ve tilled it. It usually takes 6 months before the sclerotia are receptive. Garlic powder that’s tilled in at 250lbs/acre has also been used.
4. Cessation of irrigation (when disease is observed in the soil)
   When the disease has been observed, drying out he soil as much as possible has been shown to be beneficial.
5. Compost tea
   Compost tea has been shown to be effective in disease prevention causing the plants to be more resilient to disease.
6. Roguing
   If you see the leaf flagging you’re going to remove it no matter what’s causing it. By the time you see the leaf flagging you usually see a fair amount white fluffy growth present on the bulb or roots, Dig these out with a shovel and remove all that soil around there also. Roguing isn’t perfect because although 99.9% of the sclerotia form on bulbs there’s a few that form on roots, and that’s one reason why it is encouraged to actually dig soil up also, and even then you’re probably not going to get every single sclerotium.
7. Flooding
   Dyking fields up around affected areas and flooding them from May to November can be effective. White rot doesn’t like it wet for long periods of time. It’s a heroic measure but you could actually do it in your home garden. If you dyked up the area that had white rot and flooded it for an entire season and even into the winter if you could, you would probably do as well as if you had applied methyl bromide. Flooding will probably kill over 99% of the sclerotia. It might not kill them in the dyke itself, but could take them back to the point where they could be eliminated in the future
8. Solarization
   Solarization is achieved by covering (mulching, tarping) the soil with transparent polyethylene during the hot season, thereby heating it and killing the sclerotia. Solarization can kill white rot, but it can also live deeper than the temperature will penetrate and it might survive below the solarization level.
9. Crop rotation
   Finally, a 4 year rotation can be used to mitigate white rot damage. 3 years is probably ok and 5 is probably more than you need. The 4 year timeline is the basis that big industry uses as it’s standard for rotation.

Other Resources

1. “Diseases and Pests of Vegetable Crops in Canada” – The Canadian Phytopathological Society
2. “Compendium of Onion and Garlic Diseases” – American Phytopath Society