What Is Onion Pythium Rot: Treating Pythium Root Rot Of Onions

By: Mary H. Dyer, Credentialed Garden Writer

Pythium root rot of onions is a nasty fungal disease that can live in the soil for long periods of time, just waiting to take hold and attack onion plants when conditions are right. Prevention is the best defense, since onion pythium rot is difficult to control once it begins. What to do about onions with pythium rot? Read on for tips.

About Pythium Root Rot of Onions

Onion pythium root rot can infect onion plants any time soil is moist for extended periods, but it tends to be most severe in rainy weather when days are hot and nights are warm. The fungus also lives on plant debris and in weed roots, and may be spread by over-irrigation and splashing water.

Onion seeds may be killed before germination, or infection may show up a few weeks later. The disease also appears on other members of the allium family, including leeks and garlic.

Symptoms of Onion Pythium Root Rot

During early phases of the disease, plants with onion pythium rot appear yellow and stunted. They often wilt during the day and recover in the evening. Eventually, water-soaked lesions develop on lower stems and onion bulbs. A watery rot appears on the roots, which may also turn black.

Controlling Pythium Root Rot of Onions

Plant onions in well-drained soil. Consider planting onions in raised beds, which may minimize the impact of the disease. Similarly, consider growing onions in pots filled with commercial potting mix.

Discard infected plants in sealed bags or containers. Never place infected plant matter in compost.

Keep the planting area clean and free of plant debris. Control weeds, as pythium rot can live on weed roots.

Don’t use excessive nitrogen-based fertilizer. Nitrogen causes lush, tender growth that is more susceptible to infection.

Fungicides may be effective when applied every two or three weeks, or any time rain continues for more than two days. Look for products registered for use against pythium root rot of onions.
Use fungicides only when necessary; the pathogen may become resistant.

Clean shoe soles after walking on infected soil. Clean tools thoroughly after working in infected areas.

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How to Treat Pythium in Soil

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Various species of the fungal pathogen Pythium can cause root rot or blight on a wide range of food crops, ornamental plants and turfgrass species. Pythium, also known as water mold, cottony blight and grease spot, appears as a rot beginning in plant roots before sometimes moving up into stem tissue, potentially causing plant wilt, loss of vigor or even plant death. Fumigation is generally only feasible for commercial growers, as it is a last resort and most products require a permit for application. Soil solarization provides an effective, cost-efficient and chemical-free means to kill Pythium, as well as other pathogens, pests and weed seeds in soil.

Break up the top several inches of soil with a rototiller, spade, garden fork or other implement so that the soil is loose and friable with no large clods or debris remaining on the surface.

Level and smooth the soil surface with a leveling bar or garden rake. Remove any large rocks or sticks on the soil surface.

Water the soil in the entire treatment area slowly and deeply, or lay a soaker hose or dripline over the soil where you will spread out a plastic tarp.

Stretch sheets of clear, UV-stabilized plastic over the soil surface. Various sheet thicknesses are available thinner sheeting costs less but tears more easily than thicker sheeting. Implementing two sheets of plastic with a thin layer of air in between can increase the effectiveness of the solarization. If multiple sheets are needed to cover the area, provide several inches to a few feet of overlap where sheets meet to ensure the creation of a solid plastic layer.

Cover the edges of the plastic tarp with 5 or 6 inches of soil or large, round rocks, or otherwise secure the edge of the sheeting tightly to the ground just beyond the edge of the treatment area.

Turn on the soaker hose or drip line occasionally to keep the soil under the plastic constantly moist.

Inspect the sheeting for holes or tears regularly and patch them with duct tape to avoid heat loss. Sweep or hose off any debris or dust that accumulates on top of the sheeting.

Remove the sheeting after four to six weeks during hot summer weather or six to eight weeks during cooler weather in spring or fall.

What to Do If Your Seedlings Get Damping Off

There is no cure for damping off, once it occurs. The tiny seedlings die so quickly, you probably would not have time to help them if you could. That's why it is important to try and avoid the problem altogether, with the following prevention practices.

Damping off spreads rapidly.   If you should spot signs of damping off, remove those seedlings immediately and apply one of the homemade fungicides described below, to all remaining seedlings.

Treating Root Rot in a Hydroponics System

A lot of what you can do to treat root rot will be the same as what you can carry out for prevention. But, there are a couple of things you can do to immediately tackle the problem is there are only a few plants which are infected.

If plant leaves are showing signs of dead matter, all this should be removed and discarded away from your growing room. You can remove your plants and physically clean the root system. If you do this over a sink, you can remove anything that is dead or slimy from the roots.

The next stage is to soak the root bed in a sterilizing agent up to a maximum of 12 hours. One product which is ideal for this is Physan 20. It should be noted, this product doesn’t know the difference between bad bacteria or good bacteria. This can also be an excellent time to begin sterilizing any growing equipment you have.

The addition of root builders can also be beneficial in helping roots grow stronger. These are packed full of good bacteria and help to aerate your nutrient solution. Many growers also use this as an addition to their regular feeding schedule as a way to boost plant growth and claim impressive results.

One other type of compound which can be added are microbial inoculant mixtures, these also help with new growth in the rooting system, and also aid in the eradication of diseases. The bacteria in these lead to the breakdown of what is causing the root rot.

Both the root builders or the microbial inoculants can be added as a prevention rather than a cure. However, these methods should not be relied on as the overall way of preventing root rot, this will come down to many other factors which will need your attention.

If you are looking for a natural remedy for root rot rather than using any of the above chemicals. The following natural recipe was devised by Heisenberg, who is a member of the rollitup forum. The following methods help breed beneficial microbes in DWC systems.

The following recipe needs to be added after you have performed the previous root sterilization, and system clean with Physan 20. This formulation isn’t added directly to your nutrient solution but formulated as a tea, which you then add as required. To make this tea, you do need to purchase a few ingredients, some of which are discontinued so we will provide alternatives.

Hydroguard or any solution that contains the bacteria Bacillus genus can be used. Hydroguard has plenty of good reviews, but the Hydroguard solution requires use within six months of opening, so it is better to order the smallest bottle required.

Great White comes from the same company as the root builder specified above, it delivers explosive root growth and contains mycorrhizal fungus that is well suited for a variety of plants.

Ancient Forest consists of 100% pure forest humus and contains a high diversity of microorganisms. This can be replaced by any earthworm casting product, but this is produced by General Hydroponics who are well renowned for superior products.

Before proceeding with the formula, there was an edit to the post. Both the Hydroguard and the Great White solutions can be replaced by Mycogrow soluble as a cheaper alternative.

Heisenberg Natural Formula Steps (Edited)

  1. Add 2 gallons of non-chlorinated water to a clean bucket, and add two air stones. For this to be effective, you need as much air as possible.
  2. Now, add 15-30ml of Hydroguard and about 1/4 to 1/2 scoop of the Great White powder (these are approximate, just don’t go overboard).
  3. Take an old pair of stockings or pantyhose and place 2 handfuls of Ancient Forest (or EWC alternative) inside.
  4. Tie off and place over one of the air stones in the solution. You can also put one air stone inside the stocking to give more stimulation. This method is more straightforward than straining two gallons of tea if you add the Ancient Forest directly to the bucket.
  5. Add one tablespoon of molasses. This wakes the microbes and gives them something to eat. Never add molasses to your nutrient tank. The beneficial bacteria will die in the tank due to starvation, but you will be replacing these, so it is okay.
  6. Let your tea solution bubble for 48-hours at room temperature. You can use it after 24, but it is more effective at 48. If you are using EWC, the water will foam, this is normal.
  7. After 48 hours, you can store your tea in the refrigerator where it can stay fresh for up to 10 days. If it begins to go bad, you can smell a bad odor. If you smell anything like rotting or sweaty socks, throw it away and make a new batch. Your fresh tea can smell of earth or slightly mushroomy.
  8. To start, add 1 cup to your nutrient tank for every gallon of water.
  9. Add 1 cup to the tank at 3-day intervals.
  10. You can drizzle a little of the tea at the base of your plant stalks. This helps inoculate the root crown (plant dependent). The solution can become cloudy, but your roots will remain white and highly stimulated.

When you multiply microbes with this method, your products will last longer. Once you have ridden your roots of slime build-up from root rot, you can add 1 cup for every 10 gallons at one-week intervals to help prevent future outbreaks.

Species Data

    • creeping bentgrass
    • all
    • patches (4 inches to greater than 3 feet)
      Figure 1, Figure 2, Figure 3, Figure 4, Figure 5, Figure 6, Figure 7, Figure 8
    • dieback from leaf tip
    • orange, yellow
    • roots tan and lacking root hairs
      Figure 9
    • none
Figure 1. Pythium root dysfunction stand symptoms. Figure 2. Pythium root dysfunction stand symptoms. Figure 3. Pythium root dysfunction stand symptoms. Figure 4. Pythium root dysfunction stand symptoms. Figure 5. Pythium root dysfunction stand symptoms. Figure 6. Pythium root dysfunction stand symptoms. Figure 7. Pythium root dysfunction stand symptoms. Figure 8. Pythium root dysfunction stand symptoms. Figure 9. Pythium root dysfunction root / crown symptoms.

Watch the video: How to Cure Root Rot With Hydrogen Peroxide

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