The Battle Against Crops’ Invisible Enemy: Nematodes

Reducing nematode populations would result in more food production around the world, and thus represent a large step toward fighting crop losses, ensuring ample food supply, and stimulating farmers’ economic growth. 
 

What Is a Nematode? 
 

The diverse organisms that comprise the Nematoda phylum inhabit a wide range of different environments. And while most growers know of them, many do not realize the scale of their destruction. Nematodes – also known as roundworms – are the most numerous multicellular animals on earth, with more than 28,000 known species. With soil samples often containing in excess of millions of individual nematodes per square meter, they account for roughly 80% of all individual animals on earth. 
 

Nematodes resemble slender worms with a mouth on one end — complete with teeth to help them feed — and a tail that secretes an adhesive to help it remain in place while feeding. A thick and rigid cuticle covers the body of the nematode, helping it move and protecting it against predators. 
 

A variety of nematodes exist, both harmful and beneficial. The good nematodes include those that feed on bacteria harmful to plants. Another type of beneficial nematode includes those that fight fungus by puncturing the cell walls of the fungi and feeding on the internal contents, thus destroying it. There are also nematodes that attack various damaging insects. On the opposite end of the spectrum, predatory nematodes exist that feed on plants. 
 

The Impact of Nematodes 
 

Nematodes can be found just about anywhere. The amount of biological damage they cause to the plants upon which they feed, and the resulting economic damage, is cause for serious concern. Besides reducing crop yields, nematodes can affect the external appearance of the harvested crops. Nematode-infected potatoes appear lumpy and unattractive, carrots can appear forked and thus unsalable, and tomato plants will have withered leaves, leaving the tomato crop to get scalded by the sun. In the case of turf grass, nematode-affected areas simply can appear as withered and brown patches in an otherwise green, lush lawn. In addition to the direct damage they cause to crops, nematodes, even in low populations, enable easy penetration of other soil diseases to plants’ roots. 

According to one researcher, Dr. Saad Hafez of the University of Idaho, the McDonalds fast-food chain has a zero-tolerance policy for the presence of nematode damage in the potatoes they use for their French fries. So, nematodes not only cause damage to foodstuffs purchased by average families every day, but they also affect the businesses that buy vegetables and other products in mass. As a result of the continued damage caused by nematodes, there is a dire need to develop more successful alternatives to fighting them. 
 

Why Are Nematodes So Hard to Fight? 
 

Because these organisms are unseen, they can appear to be less of a threat than they really are. But the large number of nematodes in our environment makes them hard to fight. Couple that fact with the shrinking arsenal of fumigants and nematicides — the farmer’s main defense against them — and growers face a continual uphill battle. 
 

Nematodes typically do most of their work and damage, beneath the soil. They often act invisibly within the roots and cannot be seen even when pulling the plant out of the ground. Undetected, the fields the nematodes infest often become sick as the nematodes damage the roots, weaken the plants, and make them more susceptible to disease. 
 

Because nematodes are often microscopic and evasive in nature, growers have a difficult time identifying them. Some growers may identify decreasing yield, mineral deficiencies, stress or secondary infections that are a result of weakened plants due to nematode activity. It may take several years of these types of observations before bringing in an expert for consultation or sending soil samples for analysis to identify nematodes as the culprit. By this time, the nematode population in the soil will likely have built up to vast levels and much of the damage has already been done. 
 

For the past 50 years, growers turned to fumigants like methyl bromide to battle nematodes, but the chemical’s alleged harmful effects on the ozone layer led the U.S. and other governments to phase out its use. Other pesticides in use today are specific nematicides, typically highly toxic organophosphates or carbamates. Due to these products’ toxicity and environmental effects many of them are being phased out and there are many fewer nematicides available today than even a year ago. Those that are still available require heavy Personal Protective Equipment (PPE) for proper administration, and labor intensive placement of fields under plastic coating to prevent environmental damage after use. 
 

According to Dr. Billy Crow of the University of Florida, the nematicides currently available are not effective enough to battle certain nematodes. “These types of nematodes — endoparasites — are very difficult to manage, because most of our products now are contact [nematicides],” Crow said, referring to some nematodes that live within the roots of the turf grass that he deals with on a daily basis. 
 

Worse yet, most current nematicides are only nematostatic, meaning that they do not kill them, so when it rains, the nematodes come back to life as the pesticide is washed away, requiring growers to retreat the field. Add to that the buffer zone required when applying nematicides (due to their harmful nature), and the PPE that must be worn while applying it, and the hassle and cost to farmers is evident. Thus, the need for a simpler and more effective nematicide is stronger than ever. 
 

What Can Growers Do to Fight Nematodes? 
 

If a grower knows of a nematode problem, there are several things that can be done to help overcome nematodes besides the existing nematicides, each with its own advantages and drawbacks.

 

One way to combat the spread of nematodes from crop to crop involves specific sanitation methods, including purchasing nematode-free plants and not moving plants and soil from the nematode-affected area to another location. Also, when irrigating, growers must ensure that water runoff from infected areas does not flow into uninfected areas. They should also sanitize equipment used in soil infected with nematodes, a process which is extremely difficult to do with large agricultural equipment. Also, when done effectively this merely stops the spread of nematodes, and does not retard their growth on the already-affected fields. 
 

Another method used to fight nematode infection includes using varieties of plants resistant to the damage that nematodes cause. These are most often genetically modified crops, and resistant varieties are still not available for many types of crops. 
 

Other types of crops such as cabbage, cauliflower, mustard and rapeseed naturally contain chemicals that can suppress nematodes, and these can be rotated between crops that are susceptible to them. This is in many cases not a practical solution. Furthermore, often the nematodes remain in the ground even after the suppressing crop has been harvested. 
 

Certain organic and biological materials such as chicken and cow manure can be used to suppress nematodes, but the amounts necessary and the required pre-treatment of the manure makes this a highly unwieldy solution. 
 

Soil solarization, which involves warming soil to a point that kills off the invading nematodes, is also a possibility for fields destined for shallow-rooted crops. This process includes moistening the soil and then covering the field with a plastic tarp over a four-to-six-week period during the hottest part of the summer. By raising the temperature of the soil, the population of nematodes and their eggs in the top layer of soil is greatly reduced. However, this does not provide long-term protection for woody plants, vines, or fruit trees, and might change the population dynamics of other pests. 
 

In short, solutions for ridding fields of harmful nematodes exist, but they are time consuming, complicated, and not necessarily effective. 
 

As most nematicides are being phased out due to their toxicity and harmful effects on the environment, there is an urgent growing need for creating new alternatives that are effective against nematodes, easy for growers to apply, and have minimum environmental impact. The introduction of such a new nematicide could revitalize many growers affected by nematodes.