In periods of pervasive wet and rainy weather, growers must be prepared for the possible ravages of Phytophthora and Pythium infections. Phytophthora fungi are commonly known as ‘water molds.’ But despite the innocuous name, they can be some of the most devastating plant pathogens known to man.
In the mid-19th century, more than one million citizens of Ireland succumbed to starvation due to the failure of the island’s potato crop, and another 1.5 million immigrated to the United States. The epidemic that devastated the Irish potato crop was ‘late blight,’ caused by Phytophthora infestans.
Such plant epidemics are not restricted to the distant past. In the 1970s, Australians experienced a near annihilation of jarrah eucalyptus—a highly prized lumber—due to dieback caused by P. cinnamoni. In the late 1990s, years of heavy rains devastated California almond growers, with some losing more than 25% of their orchards to Phytophthora. Even today, cocoa growers routinely lose crops due to a disease called ‘black pod,’ caused by P. palmivora.
Worldwide, there are more than 40 well-recognized species of Phytophthora known to inflict serious diseases on a variety of hosts. In the California Central Valley, we are concerned with about 12 species. Within each species there are different races which host slightly different characteristics. As the specific description of individual species is beyond the scope of this article, I will attempt to provide general characterizations that review salient features more germane to our agricultural concerns for mounting preventative programs.
Main Characteristics of Phytophthora
- Promoted by abundant moisture.
- Infective unit is generally the motile, swimming zoospore. The zoospore has a tinsel and whiplash flagellum. The tinsel is anterior, the whiplash posterior.
- The asexual spore sac, the sporangium, is where the zoospores are differentiated. In Pythium, the sporangium gives rise to an appendage vesicle and there differentiates the zoospore.
- The zoospore is influenced by many environmental conditions, one of which is the attraction to cell contents leaking from the root.
- The sporangia, once matured and giving rise to zoospores, can again form another sporangium and continue repeated sporangial production. This condition is termed ‘indeterminate growth,’ as opposed to ‘determinate growth. A close relative, downy mildew, for example, has determinate growth and once a sporangium matures and releases zoospores, does not give rise to successive waves of sporangia.
- The biochemical make-up differs from that of most terrestrial forms of fungi. The cell wall constitution is that of cellulose and beta-glucans. This differs from most fungal pathogens in that cell walls of representative fungi such as the powdery mildews, brown rot, Aspergillus and Polyporus species contain chitin.
- Species of Phytophthora cannot synthesize steroidal compounds, the ‘mycosterols,’ whereas production is common to most other species of fungi. This is the reason why ‘sterol-inhibiting’ fungicides such as Bayleton, Rally, Funginex and Rubigan are not toxic to Phytophthora
Nutritional Requirements of Phytophthora
Basically, a fungus such as Phytophthora is a plant that lacks chlorophyll. A fungus, thus, lacks the ability to make its own food. Rather, Phytophthora must secure carbon from preformed, organic sources. Phytophthora takes most of the minerals and moisture it needs from the surrounding environment, and the balance from host plant tissue and sap.
Simple mono- and disaccharide carbohydrates are their preferred source of carbon, foremost of which are glucose, fructose and sucrose. While organic acids (e.g. citric, oxalic, malic, alpha ketoglutaric, etc.) can be used, most Phytophthora species experience difficulties with these as sole carbon sources. Carbohydrates lightly supplemented with organic acids, however, are known to support vigorous growth.
Amino acids and ammonic forms of nitrogen provide the primary source of nitrogen. Concentrations of amino acids between 100 to 1000 ppm appear to be desirable. Few species of Phytophthora can utilize NO3 directly, the majority of nitrogen being secured from organic sources (amino acids, nucleic acids, etc.). Some unsaturated fatty acids are known to provide a rich source of carbon. For example, the complex lipid, lecithin, is stimulatory to growth. While sterols do not provide a direct nutritional benefit to Phytophthora, they do enhance the ability of the species to form spores. Additional mineral nutrients are generally secured from the soil or plant sap.
Transport & Spread of Phytophthora
Cosmopolitan distribution of organisms such as Phytophthora can be attributed in part to increased activity of travel and transport between continents. However, the ubiquity of a pathogen is also attributed to the organism’s lending itself to wind transport. Most propagules of Phytophthora are similar in diameter to a large piece of silt, and can thus be transported not only across continents, but between them.
In previous articles I have spoken of the coffee rust epidemic which plagued Brazil in 1970. Plant pathologists knew that coffee rust (Hemileia vastatrix) was indigenous to Asia, Africa and the Pacific Islands, but not to the South American continent. Investigations ultimately traced the Brazilian epidemic rust spores to an outbreak suffered the previous year in African coffee plantations. Spores were lifted with updrafts into a jet steam which carried the rust spores more than 3,500 miles across the Atlantic, depositing them in Brazilian coffee-growing regions.
We have experienced mild to severe dust storms in April and May in which Central Valley residents became the recipients of peat dust lifted from the Delta regions near Stockton and Sacramento. Older Central Valley residents remember a particularly severe example of the power of the jet stream, when Washington’s Mount St. Helens erupted in 1980, with ash was deposited as far south as Fresno days later.
In the past, one of my seasonal chores was the cleaning of my home’s rain gutters, freeing them of leaves and deposited soil. Each year, out of curiosity, I would take samples of the soil and examine them for organisms. I would not only find Pythium and Phytophthora, but nematodes as well.
Indeed, wind transport of Phytophthora offers a formidable means of distribution. Phytophthora, which can also breed in reservoirs or canals, can thus be introduced through irrigation water or flooding. Passively, Phytophthora can hitch a ride on equipment, tires, shoes, infected plants, and as we learned with greenhouse infections, on hair and clothing. Awareness of its potential ease of distribution can partly explain why even nurseries with pristine, sanitary methodology have experienced occasional outbreaks of Phytophthora infections.
Routes of Initial Infection by Phytophthora
Zoospores are attracted by the exudates of tree and plant rootlets, particularly when the organic compounds are indicative of moist conditions. For instance, P. cinnamomi zoospores are attracted to ethanol, which is commonly produced in roots under anaerobic, water-logged conditions.
Thus, factors which minimize root leaking or crown leaking will tend to minimize Phytophthora infections. High moisture levels are known to encourage such infections, as these conditions not only supports the motility of zoospores, but oxygen tension—which temporarily or permanently disrupts the control mechanisms of membranes—allows them to retain cell contents. For example, many of us have at one time or another examined the roots of trees following water-logging and found the lenticels (breathing pores located on the bark) to be swollen. A swollen lenticel is the equivalent of a gasping mouth, increasing membrane surface exposure in an attempt to increase oxygen uptake. However, in increasing the membrane surface area for oxygen harvest, the susceptibility to infection by Phytophthora is increased as well.
Environmental Factors Influencing Infection
We have discussed the importance of moisture in promoting phytophthora infections. However, a measurement of relative moistness can be represented through ‘water potential.’ This measurement is generally expressed in bars of pressure, where one bar is equivalent to nearly one atmosphere of pressure. For example, field capacity represents 0 bars of pressure, while the permanent wilting point is -15 bars. The range between 0 bars and -15 bars represents the amount of available moisture.
But water potential is comprised of 3 components:
- Solute Potential: The amount of dissolved salts
- Pressure Potential: Hydrostatic pressure such as the negative pressure from transpiration on the xylem vessels
- Metric Potential: The adhesive retention pressure of water on the soil’s surface
It should be understood that the interaction of the components of water potential must account for the availability of free water to both host plants and the pathogen. For example, let’s set the ideal water potential for infection by Phytophthora at 0 bars. If dissolved salts (solute potential) create a net water potential at field capacity of -1 bars, water-logging does not necessarily equate to ideal conditions for infection. The realistic set of factors involved in Phytophthora infections are an interaction of parameters.
One must invariably consider the soil temperature as major player. Most Phytophthora species, for example, operate best between 60 to 80 degrees Fahrenheit. In considering soil temperatures, one must be aware of physical and biochemical laws. For example, at 45 degrees, water can hold about 3 times more oxygen than at 70 degrees (24 mg/L vs. 8 mg/L). Furthermore, the metabolic rate of the plant and its concomitant demand for oxygen is 2.5 to 3 times less at 45 degrees versus 70 degrees.
Many years ago, I examined asparagus seedlings from a nursery that were under 3 feet of water for more than 35 days. The nurseryman was very anxious to determine whether he had to call this a complete loss. However, close examination and testing of tissues revealed the absence of water mold diseases. It was later determined that the water had been consistently around 42 degrees Fahrenheit. While the water potential was 0 bars, the increased oxygen-holding capacity due to the water’s temperature, coupled with the lowered rate of plant metabolism, combined for a saving grace. All the plants were unscathed, and the nurseryman was able to recover all his seedlings.
How Plant Nutrition & Tissue Stability Impact Infection
I once conducted a test in which tomato seedlings were grown in sterile, hydroponic cultures in ½ strength Hoagland Solution. The calcium levels were varied: 0, 100, 200, 300, and 400 ppm. At about the 5th leaf stage, the root systems began to manifest symptoms.
In examining the roots of the tested seedlings, we found that calcium levels had a profound impact on the amount of root necrosis that was apparent:
- 400 ppm calcium – No necrosis
- 300 ppm calcium – No or minimal necrosis
- 200 ppm calcium – Mild necrosis
- 100 ppm calcium – Moderate necrosis
- 0 ppm – Complete necrosis
Roots from tomato seedlings grown on hydroponic media deficient in calcium took the appearance of water mold infections of the root tips. An increase in ECw indicated that cell contents were leaking from root tissues. As calcium is an integral part of cell wall and membrane integrity, it came as no surprise that the low to absent calcium levels contributed to cell leakage and natural attrition of roots. While many more factors are involved, the exudation of cell contents and natural attrition of tissues invariably present a predisposition to water mold attack. Indeed, one of the most overlooked factors in water mold management is the maintenance of natural resistance to infection by building high root tissue integrity.
For example, most deciduous and some evergreen trees and vines initiate root flush in mid to late January. When the roots emerge, however, they enter an inhospitable, cold soil environment. The significance of this event is that key elements necessary to imparting high root tissue integrity and health—such as calcium, potassium, phosphorus and boron—are tied up even in warm soils. Thus, the chances of extracting these elements in cool soils is even more remote. But the tree or vine has been programmed for growth and perpetuation of its kind. The root tissues continue to form, despite the lack of minerals needed to support tissue integrity. Thus, it is common for root tissues formed during their sojourn in cool soil to be akin to tissue paper in durability, versus the cardboard-esque hardiness produced during more favorable mineral harvests.
Soil Microbial Ecology’s Influence on Phytophthora Root & Crown Rot
A microbially rich soil not only contributes to the natural suppressing of Phytophthora, but also promotes various growth parameters of the soil:
- Increased release of tied-up minerals
- Increased water infiltration rates
- Enhanced degradation of man-made and natural occurring toxicants
- Improved humus content and cation exchange capacity
Thus, encouraging microbial activity in the soil is commensurate with improving nutrition and securing natural resistance of the root system via increased tissue integrity. Some of the mechanisms by which beneficial microorganisms combat Phytophthora are:
- Promoting the various growth parameters of the soil needed to impart natural resistance to the host.
- Production of byproducts such as antibiotics or waste products of metabolism which are toxic to pathogens such as Phytophthora species (e.g. Bacillus pumilus, a bacterium known for its production of antibiotics used in antiseptic ointments, and Gliocladium virens, a common soil fungus which produces an antibiotic known to be toxic to strains of Rhizoctonia, Phytophthora and Sclerotinia).
- Direct feeding and/or parasitization (e.g. Trichoderma viride, a common soil fungus that is an aggressive mycoparasite which preys upon fungi, including Phytophthora species; and a frequently isolated nematode, Aphelenchus avenae, which feeds on fungi, including Phytophthora).
- Competitive displacement of the pathogen by the antagonist (e.g. a common soil bacterium, Pseudomonas fluorescens, which rapidly colonizes the perimeters of otherwise susceptible roots, displacing Phytophthora and preventing it from colonizing root exudates, and a strain of Bacillus thuringiensis bacterium , which we have isolated, and hosts such a rapid rate of growth that when placed in a soil with Phytophthora species, can outcompete the fungus for carbon and mineral sources).
In almost all instances of pathogen antagonism by beneficial microorganisms, there are multiple processes at work. The biological control of pathogens should not be conceptualized as we do fumigants or pesticides. That is, most post-WWII pesticides hosted broad spectrum, quick-acting processes whereby the applicator could essentially implement it as a “magic bullet.” A methyl bromide-based pesticide produced for growers in New Jersey could be anticipated to perform comparably in California fields.
However, when we temporarily step away from pesticides and enter the world of disease and pest control with soil microbiological systems, we must also walk away from the straightforward, magic bullet approach. Rather, use of beneficial soil microbiology must necessarily encompass a meticulous approach of unique site characterization and correspondingly specific programs. Beneficial microbes do not allow for the eradication of a targeted pathogen or pest, but instead require long-term commitment and the consideration of many factors.
In my many years of experience with beneficial soil microbiology, I have found that the most efficacious approach places 85% of the emphasis on plant nutrition, and 15% on pest and disease control. The latter is automatically addressed when one properly initiates soil microbiology programs—you must have the right mindset. You aren’t sending microbes into battle against diseases and then forgetting about them. This requires a long-term approach. You’re farming the microbes, much as you do your crops.
In addition, emphasizing one or two microbe strains that are narrowly tailored for your immediate needs may seem like the right idea. But the ultimate microbe program involves a concerted ecology of various beneficial microbes, including protozoa, free-living nematodes, tardigrades, rotifers, annelids, actinomycetes, bacteria, fungi, algae and many more. Programs which we find most effective involve a variety of indigenous, beneficial microbes, which are initially supplemented with proven antagonistic strains as ‘insurance’ to curb the early surge of pathogens resulting the addition of microbially assimilable nutrients.
Rootstocks May Also Offer Some Protection
Certain rootstocks host some tolerance or resistance to Phytophthora root and crown rot. Except for P. parasitica, all the commonly encountered species of Phytophthora will attack apricot, peach and sweet cherry rootstocks. Japanese plum, Myrobolan plum, prune and sour cherry rootstocks host a degree of tolerance or resistance to most encountered species of Phytophthora. However, P. cactorum and P. cambivora are known to be aggressive on most rootstocks. It should be noted that sound nutrition and soil microbiology can impact the resistance factors in any rootstocks, for better or worse.
An Ideal Program for Minimizing Losses to Phytophthora
Late Summer & Early Fall:
- Maintain active photosynthesis in trees and vines for as long as possible going into cooler temperatures.
- Apply 50 to 60% of nitrogen fertilizers in late summer. Do not apply large amounts of nitrogen in the winter and early spring months. Provide balanced nutrition, not nitrogen alone.
- Activate the soil in late summer or fall with selected microbiology to establish natural suppressiveness and impart mineral release characteristics.
- Apply incremental quantities of calcium to soil with calcium-based fertilizer (e.g. CN-9, no more than 40 to 60 units of nitrogen per application).
- Include booster application of soil microbiology.
- When leaves push, apply systemic fungicide protection (e.g. salts of phosphorous acid).
- Secure soil samples and examine plant-parasitic nematode and water mold levels and complete soil mineral analysis (plus chlorine).
- Increment nitrogen, phosphorus, potassium, calcium and boron during the season.
- Supplement with foliar nutrition.
- Apply booster applications of microbiology 3 to 4 times during the season (i.e. farm the microbes).
- Apply calcium, boron and nitrogen-phosphorus-potassium (N-P-K) sprays ahead of or during heat waves.
- Maintain stress-free irrigation regimes
- If indicated by soil analyses, reduce Phytophthora inoculum levels with fungicide, reduce nematode pressures, and/or restore necessary mineral balances and levels.
Please note: On weaker varieties and in susceptible regimes provide special, individualized programs.