As any farmer knows, preventing disease and parasites is a constant battle. Boron application is well-known for its support of plant health from root to flower; it also may be a powerful defense against certain parasites.

Plasmodiophora brassicae are nasty little beasts of uncertain origin. Their parasitic way of life is to attack vegetables of the brassica family, causing the debilitating clubroot disease. Now, evidence is emerging that boron might play an important role in keeping the disease in check.

Clubroot’s economic effect

The brassica family contains about 3,700 different species which are enormously important globally. Arguably, they are second only to cereals in their contribution to human diet and welfare. They include:

• Cabbages, including Chinese cabbage
• Cauliflowers
• Broccoli
• Brussels sprouts
• Kohlrabi
• Radish
• Rutabaga and swedes

Not only are these important food crops, but much of the world’s supply of vegetable oil comes from rape (canola) and mustard seed, and many are used as animal fodder crops.

With such widespread and varied use, a soilborne parasite such as P. brassicae, can have dire consequences. Clubroot is known to occur in more than 60 countries.¹ In clubroot disease, the plant roots are distorted by massive galls, which inhibit water and nutrient uptake. The grossly deformed roots sap carbohydrates from the leaves and deprive developing flowers. The foliage turns bluish-green, then yellow, then wilts.

Clubroot disease is responsible for drastic crop losses and poor-quality yield. Some estimate that clubroot results in a 10–15% reduction in yields on a global scale.²

Clubroot control and elimination

It is virtually impossible, certainly in intensively-farmed regimes, to eradicate P. brassicae. In addition, climate change is improving the parasite’s living conditions in temperate climates which leads to its increased occurrence.³

When P. brassicae spores germinate in the soil, the tiny organisms swim around and as soon as they meet a root hair they attach and inject their own cell contents into the root. The genetic material multiplies inside the plant. It’s believed that this presence upsets the host’s hormone metabolism and leads to uncontrolled cell growth—almost like a plant cancer. Once established and mature, the parasites release billions of new spores back into the soil. It is a very robust lifecycle that is difficult to break.

There are clues that P. brassicae may incorporate DNA from the host—perhaps a reason why biological control methods or genetically-induced protection methods have not yet been found. The traditional ways of controlling P. brassicae are:

• Heavy liming to manage pH (adding quantities of calcium carbonate)
• Wider crop rotations
• Better soil drainage
• Using resistant cultivars
• Developing practices that limit resting spores from being carried to non-infected areas³

Currently, there are very few chemical treatments to eliminate or control P. brassicae. There are limited registered fungicides and even those are restricted in many countries. Using these products can also be costly considering the amount of material needed for clubroot control.

Are there other options?

Researchers continue to study beneficial microorganisms as a way to control the parasite. And, some scientists have investigated boron since it’s well-known that boron-healthy plants are better able to resist disease-causing organisms. In the case of brassica, the important thing is to give the plant a healthy head start.

Let’s take a look at some of the research.

Demonstrating how boron supports brassica crop health

A 15-year long series of experiments conducted in the United Kingdom demonstrated that a specific application of boron to the seedlings at transplanting does indeed reduce the onset of clubroot symptoms and hence protects crop yields to a significant degree.

The researchers started out with three possible ideas:

• Does boron somehow reduce the potency of the clubroot invader directly in the soil?
• Might it encourage the growth and activity of soil microbes, which then prey on the P. brassicae before they attack?
• Does it actually fight the invasion or its effects within the plant itself?

According to the study boron and, less strongly, calcium seem to reduce the rate at which P. brassicae mature inside the root and turn into secondary sporangiophores—the ones that cause the damage. Boron apparently doesn't stop the initial invasion, but puts the harmful metamorphosis into slow motion.⁴

Studies of boron and clubroot in canola

One study looked at boron application, including Solubor, in canola. Researchers found that there was a reduction of clubfoot rate and severity in field trials and sand culture. The study found that low rates of boron application early on in the plant growth suppressed primary infection and development of clubroot symptoms, and the effect increased with increasing concentration.⁵

Another study how clubroot spreads in Canadian canola looked at different concentrations of boron and other micronutrients in soil. Boron affected clubroot development on canola where P. brassicae had yet to established, but did not have a noticeable effect once the parasite had taken hold of a field.⁶

Combatting P. brassicae

Whether boron is altering the biochemical environment inside the root to make it P. brassicae-unfriendly, or is encouraging the plant to retaliate is not yet clear. But the effect is the same. With boron, crops are given more time to mature and establish effective roots before clubroot tumors cause damage.

Species by species, brassicas vary in their susceptibility to boron deficiency, but generally they are rated as vulnerable to low boron levels for general growth and health; that means boron supplementation is important anyway.

In the right amount and at an early stage, boron can keep clubroot in check.

Contact us

When you have questions whether a boron supplementation is right for your field, soil, and crop, contact your local agronomist or a U.S. Borax agriculture representative.

Resources

• Crop guides: Vegetables
• Crop guides: Tuber and root crops
• Crop guide: Canola
• Solubor