By Dr. Thomas T. Yamashita

Many plant diseases have counterparts in human ailments. For example, the disfiguring disease crown gall. Caused by a bacterium that affects more than 90 families of plants, the disease is both physically and physiologically similar to cancerous carcinomas in humans.

Because of this, our knowledge of plant diseases can oftentimes be enlightened by drawing analogies to similar human ailments. This blog post focuses on some model analogies that will hopefully shed light on diseases that commonly impact the agricultural industry.

Bacterial Canker vs. Skin Infections

People often contract infections through minor cuts and abrasions to the skin. This is why antiseptic ointments are used—to minimize the opportunity for bacteria to penetrate these openings in the skin and create an infection. The bacteria which commonly cause such infections, such as Staphylococcus aureus, Pseudomonas aeruginosa, and Streptococcus salivarius are all part of the normal microflora commonly residing on the skin.

Under most conditions, these bacteria are harmless. However, when an open wound is created, rich food materials are made available for their proliferation. The rapid rate of growth of most bacteria, combined with their production of various byproducts, can result in serious infections and deleterious effects to the wound site, in severe cases resulting in septicemia. Infections can be particularly serious with bacteria that produce toxins. For instance, Staphylococcus aureus is known to produce both an alpha toxin and an enterotoxin, which affect nervous and general body tissues.

But a critical point to consider is that the intact skin represents a formidable obstacle for the bacteria. Without a wounding of the epidermis, infection cannot occur.

This is also the case for bacterial canker of stone fruit, pome fruit, and nut trees. The organism Pseudomonas syringae is a common inhabitant on the surface of plant tissues. Such surface-dwelling bacteria are often referred to as ‘epiphytes,’ and are harmless as long as they don’t penetrate plant tissues.

However, infections are common on plants that are afflicted with nematodes and in general poor health. Such plants have tissues of low integrity, which succumb to freezing damage with relative ease. The fissures and cracking of epidermal tissues adjacent to and at the base of buds, frequently arising from freezing damage, are common ports of entry for the bacteria. Freezing damage is oftentimes followed by the leaking of cellular contents, rich media which serves as ideal food for the rapidly growing bacteria.

On weakened trees blossom tissues are also of low integrity and in certain cases, the bacteria are capable of direct entrance through the nectaries, glands that secrete nectar. Leaf scars can also serve as a port of entry. If freezing damage has not already induced the leaking of cell contents, the initial colonization and byproducts of the bacteria will cause further loss in the membrane integrity of cells, resulting in the release of cell contents.

Further, once in a growth mode, these bacteria are known to produce two toxins, syringotoxin and syringomycin (counterparts to S. aureus’s aforementioned toxins). These low molecular weight peptides can be moved to more distant areas of the tree where they cause further death of tissues.

An interesting note is that the causal organism, Ps. syringae, hosts a polysaccharide coat which serves as a nucleation point for ice formation. When this epiphytic bacterium colonizes the surface of host tissues, freezing can occur at temperatures 4 to 5 degrees higher than would occur in their absence.

The important commonality to remember between bacterial skin infections and bacterial canker of trees is that both diseases require a wound or area of surface weakness for bacterial infection to take hold. In man this occurs through an accidental laceration or abrasion. For trees, this is facilitated by freezing or poor tissue development as a result of suboptimal plant health.

Thus, addressing factors that contribute to maximum health can minimize or entirely prevent bacterial canker. Many growers have also gained a degree of added protection with the use of late fall and/or dormant sprays of copper. Similar to using disinfectants on or around a skin wound, the sprays reduce the bacterial populations on the tissue surface.

Phytophthora Root Rot vs. Gangrene

In the human condition gangrene, a prolonged depletion of blood flow to an extremity (ischemia) results in the build-up of toxic metabolic by-products, deficiency in needed salts and nutrients, and a deficiency in life-giving oxygen. This reduction of blood flow may originate from maladies including thrombosis, arteriosclerosis, an embolism, diabetes, freezing, or a severe burn. Whatever the cause, extended oxygen deficiency result in the death of surrounding tissues.

The affected skin and encompassing tissue darken while surrounding areas, partially traumatized, begin to leak fluids. The fluids are rich in nutrients and are easily colonized by bacteria. If infection by bacteria is avoided, the affected tissue may dry, thus the name ‘dry gangrene’ given to this scenario. The more severe form of the disease, however, involves infection by anaerobic bacteria of the genus Clostridium. This infection, sometimes referred to as “wet gangrene,” is extremely serious, as the bacteria produce deadly toxins which can permeate surrounding tissues, inciting further necrosis and spread of gangrene.

This is much the same as how prolonged depletion of oxygen to the root systems of plants results in debilitation of typical physiological functions. In almost all cases, the oxygen tension is induced by excess moisture or standing water.

This condition is especially damaging if combined with warm temperatures, as temperature increases correspond with increased demand for oxygen. Tissues deprived of oxygen lose control of their membranes and begin to leak cell contents. These contents are sensed by zoospores of water mold fungi, which literally swim towards the food source. Zoospores attach to the leaking and weakened cells, and penetrate them with an infection peg to initiate infection. These fast-growing water mold fungi then continue their spread to surrounding tissues. In severe cases, the fungi aggressively move upwards into the crown and trunk of the tree, oftentimes expressed above-ground as profusely gumming trunk and limb cankers.

Preventative measures rely upon sound water management and avoiding excessive irrigation and standing water conditions. Other measures include reducing the inoculum levels of water mold fungi, and the utilization of systemic protectant chemicals.

Powdery Mildew vs. Ringworm

Ringworm is generally caused by 3 groups of fungi: Trichophyton, Microsporum, and Epidermophyton. These three fungal groups, being common agents of skin infections, are referred to as ‘dermatophytes.’ The names ‘ringworm’ and ‘tinea’ both refer to the characteristic central clearing which often occurs in dermatophyte infections of the skin.

The dermatophytes are commonly found among the normal microflora of the gut of humans, animals, and birds, the gastrointestinal tract being the main reservoir for these fungi. They display a preference for colonizing tissues hosting the protein keratin. Keratin contains all the common amino acids, but differs from other fibrous and structural proteins in its high cystine content. The primary infection of the skin, hair, and nails is not only dependent upon a degree of humidity or moisture, but also the presence of this preferred protein for its sustenance. This characteristic—and the tendency to grow in the superficial layers of the stratum corneum (the tough outermost layer of the skin)—is analogous to the biology of powdery mildew.

Powdery mildew is a broad term referring to various members of generally host-specific fungi of the fungal class Ascomycetes. The name ‘powdery mildew’ is a reference to the white, powder-like appearance of the fungus during formation of spores (sporulation). Like ringworm, powdery mildew fungi typically limit their colonization to the superficial tissue layers of the plant host.

But, in the presence of high humidity (but not free moisture), the germinating spores sends forth thread-like growths called ‘hyphae.’ The hypha produces lobed organs, the ‘appresoria,’ which are used as a point of stabilizing attachment and leverage, which allows the fungus to send forth an infection peg. Once penetrated into the living cell, the infection peg becomes multilobed to increase its absorptive surface.

Whereas ringworm fungi prefer tissues high in keratin, powdery mildew fungi display a need for sterols. A chemical group the public is familiar with primarily thanks to cholesterol, sterols are commonly found in the nervous and fat tissues of animals and humans. The sterols form an integral part of the cell wall of these fungi.

This fact is why newer fungicides for controlling powdery mildews are sterol inhibitors (e.g. triadimefon and myclobutanil). Both ringworm and powdery mildew infections can be minimized by maintaining sound integrity and health of the superficial tissues (with an emphasis on nutrients promoting solidity of epidermal tissues), lowering the opportunity for exposure to inoculum, and preventative treatments (e.g. bathing for humans, preventative sprays with fungicides for plants).

Verticillium Wilt vs. Valley Fever

Valley fever, or coccidioidomycosis, is a disease of rodents, other animals, and humans. It is caused by the soil-dwelling fungus, Coccidioides immitis. The fungus appears to be indigenous to hot, arid regions with characteristically short, wet seasons, such as Central California, Texas, Arizona, New Mexico, Honduras, Nicaragua, Colombia, Argentina, and Paraguay (areas known as the “lower Sonoran life zone”).

Typically, following showers or bursts of rainfall, the fungus is activated and produces an abundance of spores, known as ‘arthrospores.’ These spores are easily dispersed into wind currents, and infections naturally occur through inhalation. Individuals who work in the fields and are exposed to spore-laden dust are commonly infected. In the presence of a humid, warm, and food-rich environment, the inhaled spores initiate growth. The rich internal environment of the human make-up is conducive to enhanced growth, giving rise to large, rounded spores, ‘spherules,’ which are transported throughout the body via the blood stream. Those native to the area and who have been repeatedly exposed generally host natural resistance to valley fever. However, individuals with natural resistance can contract valley fever when their health is weakened, or while experiencing unusual physiological stress.

On the other hand, natives of regions home to valley fever will oftentimes test positive for C. immitis, but remain asymptomatic. But again, these same asymptomless people with developed resistance can become severely infected if compromised by an auto-immune deficiency disorder, diabetes, physiological stress brought on by excessive activity without sufficient rest, improper nutrition, or coinciding illnesses.

Valley fever finds its plant-based counterpart in verticillium wilt, a soil-borne disease which affects many crops grown in temperate regions of the world. Cotton, tomatoes, peppers, melons, pistachios, almonds, and grapevines are among hosts commonly afflicted with this often-devastating disease. In nature, the fungus gains entrance into the host through the root system.

Examination of the root systems of crops will often indicate the presence of the fungus within the cortex, the outer fleshy layer between the bark and vascular tissue. However, it is usually not until the onset of a stress event that invasion of the vascular tissue takes place. Such stress events include water shortages, nutrient imbalances and deficiencies, heat stress, excessive crop loads, and natural events such as flowering or crop maturation.

In our research, we have found that a common denominator of all the above stress events is the subsequent reduction in metabolic rate that occurs. The verticillium fungus perceives these physiological states as an opportunity to infect, as natural resistance is closely related to the rate at which the infected host can effectively wall off the intruder. Hosts which are weak and have a low rate of metabolism cannot mount a successful defense and the pathogen gains foothold. Many of varieties and rootstocks with resistance to verticillium wilt are not endowed with unique sets of toxic combatants, but rather characteristics of high vigor and a high rate of metabolism.

The end result is that a stressed host is prevented from effectively mounting resistance due to a lowered metabolic rate. Similarly, people with natural resistance to valley fever whose physiology becomes stressed lose the ability to ward off the pathogen, and subsequently become infected. The basic epidemiology and mechanics of valley fever and verticillium wilt infections are very closely related.

Lesion Nematodes of Plants vs. Hookworms

In moist, warm climates, hookworms remain one of the most serious parasites to infect humans. The World Health Organization estimates that the hookworm, a type of nematode parasite, affects more than 700 million people worldwide, most of them in poor, developing countries.

However, the Southeastern United States has been known to report occasional hookworm infestations. The two species of hookworm known to infect people are the Old World hookworm (Ancylostoma duodenale), and the New World hookworm (Necator americanus).

The life cycle of these parasites begins with the passing of eggs with human feces. Under favorable conditions, the eggs will hatch within 24 to 48 hours and begin feeding on nearby bacteria. Within 7 to 10 days, the larvae mature into the invasive, ‘filariform’ stage. These larvae will perch themselves vertically with head swaying back and forth, waiting for a suitable host to pass by. Groups of these perched larvae give the appearance of a wheat field blowing in the wind. They are capable of penetrating various parts of the skin, but typically invade by penetrating bare feet. The larvae make their way into the venous bloodstream and are carried to the right heart, lungs, up the respiratory tree and over the epiglottis, where they are swallowed. They finally settle in the small intestine, where they attach themselves to the inner mucosa where they begin taking a blood meal. Heavily infested individuals with burdens of approximately 400 worms may lose more than half a pint of blood per day.

Rashes and irritation occur at the site of entrance into the skin, with unproductive coughing and pneumonialike symptoms appearing as the larvae break through the capillaries into the breathing sacs of the lungs. Ultimately, symptoms escalate into abdominal pain, swelling of the face, and in advanced cases, mental dullness followed by death. The intensity of the disease is most severe when hosts have poor nutrition.

The plant-infecting counterpart to the hookworm is the lesion nematode, genus Practylenchus. Like the hookworm, the lesion nematode is a member of the class Secernentea. Various stages of the parasite are capable of invading plant root systems, as they are armed with a robust spear known as a ‘stylet.’ The parasite uses the stylet to puncture the soft epidermal tissues of young, developing roots.

Unlike the dagger, ring, pin, and spiral nematodes, which feed from the surfaces of roots, lesion nematodes enter into the root tissue, and puncture adjoining cell walls to migrate and feed throughout the root system. The stylet is hollow, and is used to inject digestive enzymes into cells. The liquified cell contents are then extracted out through the stylet.

This process of feeding invariably kills the pierced and adjoining cells. The lesion nematode migrates away from the lesions which are formed, which then become infected by bacteria and fungi and exacerbate the effects of the nematodes. Water mold fungi are commonly associated with predisposing infestations of lesion nematodes. In cases of severe infestations, we have found more than 5,000 nematodes per 250 cc of soil, and more than 10,000 per 250 grams of root tissue.

While an infestation cannot be directly observed, above-ground symptoms typically include severe stunting, reductions in crop yield and quality, early flowering, premature defoliation, more intensive pest infestations (e.g. insects and mites), and enhanced susceptibility to frost and disease.

Symptoms are more severe under conditions of poor nutrition and suboptimal plant health. The lesion nematode is notoriously aggressive, and is considered by many nematologists to be one of the most devastating plant-parasitic nematode species known to man.

In both plant and human infections, they are worsened by pre-existing poor nutrition, and remedied by corrections to said nutrition.

In our many years of research, we have found that many severe plant diseases, even some considered to be incurable, can be overcome—or better yet, prevented—with the application of specially tailored nutritional programs. To learn more about how Fusion 360’s line of agricultural products can be used to return your crops to full health, contact us today.