Can mycorrhiza protect crops from root rot, wilt and nematodes?

The root's enemies: soil-borne diseases and nematodes

Two groups of soil organisms do most of the damage to roots.

Soil-borne fungi and fungus-like organisms. Fusarium and Verticillium enter through the roots and block the vessels that carry water up the plant, so it wilts. Pythium, Rhizoctonia and Macrophomina rot seedlings and roots, causing damping-off in nurseries and root rot or dry root rot in the field.

Plant-parasitic nematodes. These microscopic worms feed on roots. Root-knot nematodes (Meloidogyne) cause the familiar galls on tomato, brinjal, okra and banana roots, and cyst nematodes (Heterodera) form tiny cysts. Both settle in one place and turn a few cells in the root's central core into permanent feeding cells. Lesion nematodes (Pratylenchus) and the burrowing nematode of banana (Radopholus) keep moving through the root and leave dead, brown tissue behind.²

Mycorrhizal fungi live in the same roots. Arbuscular mycorrhizal (AM) fungi, the type in most crops and in most VAM products, grow in the root's outer tissue, the cortex. They never cross into the central core where the water vessels run.³ For the types of mycorrhiza and what they do for nutrients and soil carbon, see types of mycorrhiza and why they matter in farming.

How mycorrhiza protects roots: four ways

1. A better-fed, stronger plant

Mycorrhizal threads bring the plant phosphorus, zinc and copper from soil its roots cannot reach. A well-fed plant with a strong root system can lose some roots to disease or nematodes and still grow well. Scientists call this tolerance.^{4, 2}

2. Getting there first

Once established, the fungus occupies part of the root cortex and uses a share of the sugars the plant sends to its roots. That leaves fewer free places and less food for disease organisms that arrive later.⁴ The evidence for this is mostly indirect. And because the fungus stays in the cortex, it cannot crowd root-knot or cyst nematodes out of their feeding cells in the core; against them, any competition is for the plant's food, not for space.²

3. Priming the plant's defences

Mycorrhiza puts the plant's defences on alert, mainly through the jasmonic acid pathway. A primed plant does not run its defences at full strength all the time; it responds faster and more strongly when it is attacked. Scientists call this mycorrhiza-induced resistance.⁵ Helpful bacteria that gather around mycorrhizal roots add to the effect.⁶

In tomato attacked by Phytophthora, a root-rot organism, mycorrhizal roots strengthened their cell walls where the attacker tried to enter, built up protective phenolic compounds and made extra enzymes, such as chitinases and glucanases, that break down the walls of disease organisms. They also made more antioxidant enzymes, which help cells cope with the stress of attack. The protection reached even parts of the root system that the mycorrhizal fungus had not colonised, so it is partly a whole-plant response.^{7, 8}

Priming works against root-knot nematodes too: in tomato, defence genes were switched on more strongly in mycorrhizal roots when nematodes attacked, and fewer nematodes infected them.⁹

4. Changing what the roots release

Roots release sugars, acids and other compounds into the soil, and nematodes follow them to find the root. Mycorrhiza changes this mix. In tomato, fewer root-knot nematodes entered mycorrhizal roots: most stayed in the soil around them, and the liquid released by mycorrhizal roots temporarily paralysed the nematodes.¹⁰

What the research shows

On average, it helps. A review of 144 studies found that plants with mycorrhiza grew better than plants without it when attacked by disease-causing fungi or by nematodes.¹¹

The fungus strain matters most. A review of 106 studies found that the choice of mycorrhizal fungus made a large difference to the level of protection, while the kind of disease organism did not. Nitrogen-fixing crops such as pulses, which are usually heavily colonised, were better protected against fungal diseases than other broad-leaved crops.¹²

Nematodes are less predictable. One overview found that mycorrhiza reduced root-knot nematodes more than cyst nematodes, while migratory nematodes, such as lesion nematodes, were sometimes more numerous on mycorrhizal plants.¹³ The review of 144 studies found no such difference between nematode types,¹¹ so the picture is still unsettled.

Early colonisation pays. In a laboratory study, banana plantlets colonised by a mycorrhizal fungus before they met the burrowing nematode had 60% fewer nematodes and 56% less root rot than plantlets without it.¹⁴ In nematode experiments, protection appeared when the fungus was well established before the nematodes arrived, but not when both were added at the same time.²

Most of the evidence comes from pots and greenhouses. The benefits have been shown mainly under controlled conditions; field results are fewer and more variable.¹⁵

Mycorrhiza with Trichoderma, Bacillus and other bio-inputs

Many bio-products combine mycorrhiza with Trichoderma, Bacillus or Pseudomonas, hoping for several layers of protection. Sometimes the partners help each other; sometimes they get in each other's way. In one experiment, Trichoderma harzianum reduced root colonisation by a mycorrhizal fungus, and the mycorrhizal fungus in turn held back the Trichoderma in the soil.¹⁶ A combination is worth using when it has been tested together, preferably on your crop. For a tested combination of friendly microbes with biochar against a crop disease, see biochar and Trichoderma against brinjal blight.

For mycorrhiza with nutrient bacteria such as PSB, Azotobacter and Rhizobium, see mycorrhiza and friendly bacteria.

Chitin-rich amendments, such as crab and prawn shell meal, feed soil microbes that break down chitin, and some of these microbes attack nematode eggs. Such amendments have reduced root-knot nematodes in trials;¹⁷ the rate matters, so follow the recommended dose. Chitosan, made from chitin, works differently: it switches on the crop's own defences. See chitosan for plants.

What weakens the protection

1. High doses of soluble phosphorus. A plant with plenty of easily available phosphorus sends less carbon to the fungus. Across field studies, phosphorus fertiliser reduced mycorrhiza by 32% on average.¹⁸
2. Some fungicides. Effects differ from product to product and from study to study, but some fungicides reduce mycorrhiza.¹⁹ In one study, fungicides sprayed on tomato plants lowered mycorrhiza in the roots.²⁰
3. Arriving late. If disease or nematodes are already in the roots, mycorrhiza added now is unlikely to save the crop.²
4. Crops that form no mycorrhiza, such as the mustard, cabbage and beet families. They gain no protection from it, and they leave fewer of the fungi for the next crop (see types of mycorrhiza).
5. Dead or weak inoculum. A product protects only if its spores are alive when they reach the root: see why many VAM products give uneven results.

How to use mycorrhiza for healthier roots

1. Start early. Apply mycorrhiza in the nursery, as a seed treatment or in the furrow at sowing, so the roots are colonised before disease and nematodes build up. For transplanted crops such as tomato, chilli, brinjal and tissue-culture banana, treat the seedlings in the nursery.^{2, 14}
2. Choose live, named fungi. Look for named species, such as Rhizophagus irregularis or Rhizophagus intraradices (both once sold as Glomus intraradices) or Funneliformis mosseae (formerly Glomus mosseae), a stated spore or propagule count and an expiry date. The fungus strain strongly affects how much protection you get.¹²
3. Go easy on soluble phosphorus at the start. Apply phosphorus according to a soil test, so the crop is fed without switching off the partnership.¹⁸
4. Check fungicides first. Do not mix mycorrhiza with a fungicide seed treatment or drench unless the label says they can be used together, and avoid fungicides the crop does not need.¹⁹
5. Keep the other controls. Healthy seed and seedlings, crop rotation, resistant varieties and good drainage come first. In fields with heavy wilt or nematode problems, mycorrhiza alone will not be enough; use it as one part of integrated pest management.⁴

Expect less damage, not none. Mycorrhiza tilts the balance in the root's favour, and it does most when it is already in the roots before trouble arrives.

Editor's note: this article is based on a technical note by Dr. Abhishek Mathur. NanoKheti's editors rewrote it in plain language, checked each statement against the published research listed under Sources, and added the summary, questions and links.