With the great majority of diseases afflicting crops, we are told to pay very close attention to moisture on flowers, fruit, and shoots, as well as moisture in the soil. This is because it’s understood that most disease organisms thrive under wet or rainy conditions. This seems logical, after all. Fungi, for instance, require a period of wetness for spores to absorb water, germinate, and grow. The same is true for bacteria, which require free moisture to live and rely upon water as a medium of transport into hosts via natural openings or wounds.
As a result of this understanding, the natural reactions of farmers, pest control advisors, consultants and others in production agriculture are:
- If the wetness occurs on aerial parts of the plant, provide protection with fungicides or bactericides.
- If the wetness occurs in the soil environment, either try to anticipate and manage the water, or apply protective fungicides.
These responses are backed by a rich, diverse and effective arsenal of various chemicals which can meet the challenge. The use of these fungicides and bactericides are largely responsible for the superior and unmatched wealth of food in the United States. However, the farming community has had to accommodate an intense and growing nationwide concern for public environmental safety. Over the past 40 years this focus has incited a wave of responses:
- A careful defining of chemical toxicity and persistence
- Both enforced and voluntary removal of agricultural chemicals from public use
- Tighter restrictions on pesticide use
- Replacing broad-spectrum products with specific, targeted pesticides
- An overall reduction in the variety of fungicides and bactericides available for use
This trend isn’t going to stop, even though standards for produce quality will only continue to increase. Thus, farmers and PCAs must use the old school reactive approach of anticipating wet weather, projecting what diseases are likely, and reacting accordingly with chemicals. Chemical treatments have become too narrow and specific for this shotgun approach. Instead, it’s necessary for growers to educate themselves on pathogen biology, and the events that lead to plant disease. This article is designed to be a first step towards understanding disease management in terms of how the natural mechanisms of disease resistance in plants can be leveraged, to the grower’s advantage.
A highly predictable and universal model disease is helpful in explaining key concepts of plant disease management.
An effective example case that can be used to explain this approach is Verticillium wilt, a disease that almost every grower has encountered innumerable times. Verticillium wilt is a soil-borne, fungal disease that afflicts hosts by entering through the vascular tissues of plants, causing both direct and immune reaction-induced plugging of conductive channels. As the responsible pathogen, Verticillium dahliae, damages root tissues, visible symptoms appear in the form of wilting of the leaves and branches directly connected to affected vascular channels.
Once the V. dahliae pathogen is established, it produces a dark, compact mass of tissue that look like black soil particles. These tiny particles, which measure less than 1/60th of a millimeter, are known as ‘microsclerotia’ and act as the seeds of the fungus. When the roots of an appropriate host plant approach a microsclerotia particle, the particle germinates and invades the tissues. Typically, it will find its way in through the delicate root hairs, as well as natural openings and wounds in plant tissue. However, pathological infection that produces visible symptoms involves infections which progress past the outer root cylinder (called the ‘cortex’), into the inner conductive tissues, known as the xylem.
But this last, most damaging step can actually be interrupted and stopped in its tracks. When a microsclerotia germinates and sends out an infection thread into the host, the plant or other environmental factors sometimes act to prevent the pathogen from progressing into the xylem. Understanding how this occurs arms growers with the knowledge that allow us to develop programs which can optimize the effectiveness of conventional disease control protocols. Let’s look at a few different encounters with various plant species that can help illustrate the above.
Pistachio trees demonstrate how prioritizing fruit or nut growth can result in vulnerability to disease.
Many years ago, Sunburst Plant Disease Clinic conducted model studies with V. dahliae which helped to explain some key principles of disease development. The model hosts used in these studies were pistachio trees. Under the practices that were then standard, thousands of pistachio trees had been succumbing to Verticillium wilt. Many 15- to 16-year-old orchards we examined had replant rates of 60% to 70% for entire blocks.
Complicating this was the fact that commercial pistachio trees (Pistacia vera) had a notorious reputation for alternate bearing. That is, trees would produce bumper crops one year, then drop to a mere fraction of this production the next year. As it would turn out, this phenomenon provided insight as to how Verticillium wilt develops, and a glimpse into a salient feature of plant resistance to disease.
We started by measuring levels of V. dahliae microsclerotia in the soil. These levels are generally expressed by pathologists as ‘colony-forming units,’ or CFUs. The levels found in our selected orchards varied between 15 and 260 CFUs per gram of soil.
We then randomly marked 15 mature (11th leaf) trees for root examinations. In mid-April, roots of 2 to 5 millimeters in diameter were taken at a depth of 6 inches from four equally spaced sides of the tree, which were marked on the base of the trunk for later reference. Areas from which root samples were cut were painted over with a latex-fungicide mix before replacing soil. The five-inch sections of root were washed free of soil, gently bathed in a light detergent solution, and rinsed in sterile water.
The bark (cortex) was then stripped and separated from inner wood cylinder containing the xylem tissues, and the cortex and wood cylinder were cut into ½” sections and placed on individual sterile potato dextrose agar media and incubated for 7 days. In all 15 trees, of which more than 600 individual tissue samples were tested, samples of V. dahliae could be isolated. However, 29% of cortex tissues tested positive, while only 4% of xylem samples produced cultures.
A second round of samples were gathered in an identical fashion four months later in August and examined as described above. It should be noted that it is at this time of the year that pistachio nut clusters reach their peak demand for nutrition. The nuts are filling out with oils, carbohydrates, and protein, all of which drain the tree of photosynthates and minerals. In fact, this demand is so great that leaves near maturing nut clusters will turn a brilliant yellow, giving the appearance of fall coloration. Like many fruit-bearing plants, the first priority of pistachios is to mature the nuts, with all effort being directed towards this goal. Pistachio trees are particularly overzealous about this, and thus provide an ideal model host for the bacteria responsible for Verticillium wilt.
In the second round of samples, positive isolations of V. dahliae were found in 31% of cortex samples, a proportion similar to that found in the first round. However, 17% of xylem samples produces V. dahliae cultures, a significant increase over the 4% found in the first round of testing.
At the time, we attributed this increase to lowered natural resistance resulting from the pistachio trees shifting from the normal growth processes exhibited in April to the nut-maturation-above-all-else approach in August.
We have used cotton and tomato plants to demonstrate how substandard lighting conditions can also exacerbate vulnerability to disease.
In another test, cotton plants were established in 6” diameter pots. In the media of each pot, 10,000 microsclerotia had been evenly mixed in. The plants were grown for 5 weeks under identical light, temperature, and fertilizer programs, with the plants reaching a height of 5”.
The plants were then separated and placed under varying light regimes, designated at 100%, 75%, 50%, 25%, and 0% light. The 100% lighting regime comprised a cycle of 16 hours of light and 8 hours of dark. Accordingly, 75% light had 12 hours of light, 50% had 8 hours of light, and so on.
The cotton plants were irrigated identically to maintain 80% field capacity moisture, and received an identical nutrient solution with each irrigation. The light regimes were maintained for a period of 8 days, before all plants were returned to the normal 100%, 16 hours of light and 8 hours of dark regime. Plants were maintained for another 3 weeks under these conditions before being evaluated for disease.
All plants placed under 75% or 100% lighting were free of Verticillium wilt. Of the plants placed under 50% lighting, 40% were positive for Verticillium wilt. All plants placed under 25% or 0% lighting succumbed to Verticillium wilt.
Later, a similar test with Rutger tomato plants was conducted, with V. dahliae replaced by Fusarium oxysporum f.sp. lycopersici, the organism responsible for Fusarium wilt. All tomato plants placed under 75% or 100% lighting were free of wilt. Of the tomato plants placed under 50% lighting, 60% came down with wilt. All plants placed under 25% or 0% lighting were infected with wilt.
The three experiments described above demonstrate how the natural resistance of plants can stave off infection, unless plants are compromised by environmental stress factors.
We have conducted many tests examining the relationship between stress and disease in plants, but the three described above are more than adequate for formulating a theory of pathogen-initiated disease, and the mechanisms of host resistance.
Botanists have shown that all plants have an ability to resist disease, some more so than others. One of the primary ways in which resistance expresses itself is the production of various ‘walling-off compounds,’ which are used to block the further advance of a disease-causing organism. For example, if a pathogen colonizes part of a leaf, it may be stopped by the depositing of walling-off compounds at the point where the leaf stem meets the shoot. This produces an abscission layer, and the leaf drops, taking with it the isolated pathogens.
There are a number of walling-off compounds, including cork and callose tissues, cellulose and hemicellulose materials, gums, lignin, and phenolic compounds. While these differ in their composition, they all require the production and utilization of proteins or enzymes, carbon molecules which provide the structural skeleton of these compounds, and energy.
Resistance responses require the production of these compounds in a timely manner to ward off oncoming pathogens. This in turn requires a variety of plant materials, all of which much be synthesized, transported, and deposited in a coordinated fashion. But if the plant’s machinery is being directed towards the production of nuts or fruits, it does not have the capacity to create these walling-off compounds. The plant is essentially fatigued, much like the parent of an infant who neglects their own health in order to assure their child’s well-being. Consequently, the plant’s inability to mount a resistance response allows the disease to progress.
What can growers do about this? Growers have long sought to relied on massive applications of nitrogen fertilizer in December and February to carry their trees through the year. But instead, we have found that much better results can be produced by applying 65% to 75% of yearly nitrogen fertilizer in late summer or early fall, with the remaining 25% to 35% applied during the post-bloom to early summer period. By applying nitrogen fertilizer during more periods of intense growth and crop development, trees have the massive amounts of energy necessary to both mature crops, and resist disease.
There is one other observation we have made, with which we will close this article. There is a canker disease, known as Eutypa canker or Eutypa dieback, which primarily afflicts grapevines and apricot trees. This disease, caused by the fungus Eutypa lata, is especially severe on trees and vines which have incurred heavy pruning cuts. Disease severity is particularly pronounced on plants where cuts have not healed over in a timely fashion. In vineyards, Eutypa dieback is especially severe in plantings where vines are purposefully stressed to achieve “vintage quality” wine grapes.
In this example, again we see where a tree or vine with diminished vigor due to stress is especially susceptible to infections. But when this same pruning is performed on trees or vines that are healthy and vigorous, at a time when sap is flowing and protective callus tissues can be quickly formed, the incidence of disease is greatly minimized.
The takeaway is that, by adjusting our agricultural interventions to ensure that the health and vigor of plants is not compromised, but instead bolstered, we can often rely on plants’ native resistance responses, rather than chemical interventions.