Basics of Plant Diseases: Part Three

In part one of my series on plant diseases, I described the process of diagnosing a plant problem that might have been caused by a biotic agent (pathogen) or some abiotic factor, such as an environmental stress of some sort.

Assuming the causal agent was some pathogen, determining what organism was involved is critical to take the proper corrective action. The most likely pathogens would be fungi, bacteria, viruses or nematodes.

Knowing the biology of the pathogen allows one to choose the best remedy and time its application to be the most effective. In this article, I will briefly describe the management options a grower might consider.

Curing Plant Diseases: Modifications in Cultural Practices

In most cases where plant diseases occur, regardless of the causal agent, there are always things that the grower can and should do to minimize the occurrence or reoccurrence of the disease.

First, recall the disease triangle that describes the need for three elements to be present to have a disease: the host plant, the pathogen and a conducive environment for the disease.

Since the disease triangle conditions have been met, the management approach might be as simple as asking the question: where did this pathogen come from, how is it spread and how can it be prevented?

Recall the pathogen types. If the causal agent is a virus, then you know that the pathogen is resident in plant cells of an infected plant and can only be transmitted by contact or by normal propagation from an infected plant. Touching the infected plant and then touching healthy plants could transmit the pathogen.

Some virus diseases are transmitted by insects from infected to healthy plants during their feeding. Once the virus is inside the plant cells, it will multiply and spread from cell to cell, eventually causing visible symptoms in the foliage.

Bacterial pathogens might also be moved around from a source plant to healthy plants by hands, insects and water splashes. Bacterial diseases usually require a lengthy period of free water on the plant leaves for infection to occur.

Irrigating to avoid free water on the foliage would reduce the chances for bacterial pathogen spread. The same could be said for many aerial fungal pathogens that sporulate on infected plants and the spores are transmitted to healthy leaves by water splash or air movement.

Some fungal pathogens, such as the causal agent of powdery mildew, are not favored by free water, and their spores move from plant to plant by air movement. The powdery mildew pathogen is an obligate pathogen.

In other words, it only lives on living host plant tissue and produces the white masses of spores on infected leaves that can quickly spread to healthy leaves. Detection and removal of early infections before extensive sporulation can be helpful.

Another way for viruses, bacteria or fungi to be transmitted from infected to healthy plants is on pruning shears. Tools can be sanitized by soaking in bleach or other chemical agents, but heat is the only real sure-fire method to kill any of the pathogens.

Too often, quick dipping tools in bleach is performed and this might be too short a time frame for pathogen elimination. Soaking tools for up to 10 minutes or more might be required.

For soil-borne pathogens, consider that the pathogen source could be the grow medium, container, cutting, seed or irrigation water. However, pathogens can be introduced by other means, such as insects carrying the pathogen in from outside sources, hose nozzles touching contaminated soil or splashing water from soil to potting media.

If the medium is already contaminated, such as from a previous crop, the best method to eradicate the pathogen is by heat. Moist heat is better than dry heat. If the soil volume is relatively small, heating it in a microwave to 145°F for 30 minutes will pasteurize the medium.

The same heat treatments will also work to sanitize re-used containers. Don't rely on chemical drenches, since most are not fungicidal, only fungistatic (i.e., they reduce the growth of the pathogen but do not kill it).

Also, keep insect infestations in check, since fungus gnats and shore flies can carry fungal pathogens into the production areas and lay eggs in the grow medium.

Fungal pathogens might also reside in infected plant residues or on a wide range of surfaces, especially wooden surfaces, so removing infected residues and sanitizing surfaces could reduce or remove the sources of some pathogens. Contaminated water might also be a source of foliage or root pathogens.

Well water is not likely to be contaminated, but any run-off water used for irrigation could contain pathogens. Chlorinating such water is one way to rid it of pathogens, but again, concentration of the chemical and exposure times are critical.

The last source of the pathogen is the cutting or seeds of plants. Of course, this depends on where you have gotten the seed and what crops are involved. Most commercial seed is certified to be pathogen-free. Some are bred to have resistance to certain diseases. If you collected and processed your own seed, it might be contaminated. Seeds can be sanitized by soaking in chemical agents.

If you previously have not had damping-off of seedlings, seed treatment might be unnecessary. Cuttings taken from production plants might have been exposed to pathogen contamination and might need to be sanitized to avoid transmitting pathogens to the new plants.

Some fungal or bacterial pathogens can contaminate the cuttings without causing any visible symptoms. The likelihood of this happening is reduced by other efforts to eliminate sources of the pathogen.

Chemical Applications of Pesticides, Fungicides, and Hericides

Chemical pesticides might be needed to manage many plant diseases—it is the predominant practice in commercial agriculture. Most chemical pesticides are not used by indoor or greenhouse growers mainly because of the human health risk and the potential damage to the environment.

However, chemical fumigant applications might be the only way to sanitize contaminated soil in outdoor plantings. Chemical applications are useful to manage foliar fungal pathogens since few chemicals are effective against bacterial pathogens (such as copper materials). There are no chemicals that are effective against viral pathogens other than insecticides to control insect virus vectors.

Fungicides are either contact/surface or systemic in their effectiveness. Some contact fungicides penetrate leaves but don’t leave the treated leaves. Systemic chemicals, on the other hand, move from the site of application to other plant parts. Some, if applied to the foliage, also move down into the roots.

Some, if drenched into the soil move up into the plant canopy. Some systemic chemicals also stimulate the plant’s natural disease resistance. Some biological agents discussed in detail later on also induce systemic resistance.

Most chemicals that target fungal pathogens are only fungistatic, not fungicidal, so don’t expect them to eradicate the pathogen, only keep it in check. They can be useful, however, in reducing pathogen sporulation.

Biological Controls for Pests in the Garden

Because of the potential risk of using chemical agents to human health and damage to the environment, alternative methods to treat pathogens are needed. Use of non-chemical, bio-based disease management strategies should always be in addition to modifications in cultural practices.

The strategy is to increase microbes that can suppress or kill pathogens before they initiate infections. Some commercial products that address problems with fungal, bacterial and nematode diseases are called bio-pesticides for purposes of registration, or just bio-control agents.

These agents must be delivered to the pathogen infection site to block infection. Some might attack the pathogen and kill it in place. Most agents produce inhibitory chemicals that prevent the growth of the pathogen, whether on roots or leaves. The discovery of potential bio-control agents is based on pathogen inhibition in vitro on agar plates due to production of some antibiotic chemicals.

For soil-borne fungal pathogens, such as the causes of root rot diseases, methods generally involve increasing the antagonistic potential of the grow medium. This involves amendment with potential antagonists (some are commercially available) or adding organic materials that will increase the microbial diversity of the grow medium.

For example, adding composts will increase the number of potential antagonists simply by virtue of the diverse microbes that were involved in the composting process.

Other products are commercial mixtures of a wide range of microbes, some of which might be antagonistic to specific pathogens. Antagonistic microbes are present in most all soils or composts, but are generally lacking in most soilless potting mixes.

The goal is to increase the numbers of antagonists that can target specific pathogens and block their development, thereby preventing disease. Some mycorrhizal fungal products include some potential antagonists.

The critical question is whether any of the amendments or treatments sufficiently increases the antagonistic potential in the rhizosphere soil to inhibit the pathogen. The practice of soil pasteurization with aerated steam at 145°F for 30 minutes is one that kills the pathogen but leaves potential antagonists that could tolerate the heat, such as spore-forming bacteria or Actinomycetes. That antagonism would not happen if there were no heat-tolerant microbes to begin with. That’s where amendment first with compost with diverse microbial composition might help.

Take-home Message

One should be able to manage plant diseases that might occur in an indoor or outdoor garden by properly diagnosing the problem as a disease caused by some biological agent (as opposed to an abiotic cultural problem), determining the type of causal agent (virus, bacterium, fungal or nematode) and then choosing the appropriate control or management strategy that best solves the problem.

The first steps to take are to make improvements in the cultural practices that might have contributed to the problem. This applies to every disease, regardless of the pathogen. Some diseases might best be controlled by chemical applications, assuming they are available and can be used in an indoor or outdoor garden setting.

For human risk and environmental damage potential, chemicals might not be the best way to go. There is currently a strongly increasing need and desire for bio-based disease or pest management options.

Some will involve just changing the cultural practices and grow medium to enhance the antagonistic potential, especially to suppress soil-borne pathogens.

Don't Miss:

The Basics of Plant Diseases Part One

The Basics of Plant Diseases Part Two