Phytophthora In Disease Suppressive Soils

by Nathan Harman, Staff Consultant at Advancing Eco Agriculture

Spring and early summer of 2016 was very wet in much of the Midwest, with deluged fields, delayed planting, cultivation, compaction, and missed applications. The rain brought problems, and one of the more challenging was the scourge Phytophthora.


In Greek, phytón means “plant” and phthorá means “destruction.” “Plant destruction” sums up a Phytophthora infection quite well. Outbreaks occur primarily with wet, warm weather, and more readily on compacted soils than well drained. Mainstream agriculture has very limited management options, offering these standard suggestions:


  • Prevention before it starts. (Great idea, how can we accomplish it?)

  • Sterilize all equipment after working soil with any history of Phytophthora. (When and how is this practically done? Should I sterilize my tractor right at the edge of the field, or in the barn after driving by three other fields that I should be working today?)

  • Correct water management and drainage. (Always a good idea, Phytophthora spreads in water, and wet roots are not healthy roots. However, drain tiled fields can still flood after intense rain and spores can blow in from elsewhere.)

  • Crop rotation. (Don’t most published articles say Phytophthora spores survive 10 or more years in soil and that no rotation schedule has yet proven effective?)

  • Frequent preventive contact fungicide applications with multiple modes of action. (How many years do we repeat the same applications before acknowledging they are not performing?)

  • Soil sterilization. (Difficult and impractical for most growers. Moreover, Phytophthora is known to thrive when introduced to sterilized soils.)

  • Variety selection. (Phytophthora resistance or tolerance is not yet available for most crops and usually covers only one or two species and races.)


Paul Weaver’s farm, Montgomery Acres in Kentucky is a powerful testimony of the value of healthy, regenerated soil. He grows melon crops and mixed produce along with row crop acres and has had a strong AEA-based program for several years. Paul routinely uses Planter Solution™, keeps plants topped up with in-season drip and foliar, cover crops extensively and has worked to balance bulk soil ratios, but the real backbone of the farm is biology. Paul applies Rejuvenate™ and Spectrum™ every fall. He adds SeaShield™ and Biogenesis™ where needed on challenged soils. All of his seeds are treated with BioCoat Gold™ for mycorrhizae and he adds Pepzyme Clear™ to ensure optimum conditions for inoculants.


We had looked forward to showing the excellent results of intensive biological management. However, Paul jumped on an opportunity to plant, and then harvest, his main melon and watermelon crops three weeks earlier than planned. Only one small field, which he was not proud to show because cultivation and care had been so limited by weather, would still be growing a crop by the tour date. We decided to cancel a scheduled field walk.


Then Paul called just before the date, saying farmers still wished to attend and had many questions about Phytophthora, which had caused devastating losses there and other prime melon areas in the Midwest. He hadn’t had any problems and actually harvested 100 high-quality bins per acre, despite experiencing the same flooding conditions and planting the same varieties. Plus, this was on a field that had been in Roundup™-ready corn and beans. Knowing biology would be weak and soils tight, he gave the ground a strong Rejuvenate™/Spectrum™ application in the fall, cover cropped over winter, and applied Rejuvenate™ again in spring before planting with Planter Solution™.


So we met that day as a small group, mostly talking about Phytophthora. What follows is a recap of the message I delivered.


What is Phytophthora? It looks like a fungus, behaves like a fungus and is treated with fungicides. So, like many farmers and crop advisors, I thought it was a fungal disease. Not so. 


Phytophthora is in an altogether different, newly defined kingdom: Chromista. This kingdom also includes brown algaes, like kelp (from which AEA’s SeaStim™ is produced) and diatoms (from which diatomaceous earth is produced). Within this kingdom, Phytophthora is of a genus of organisms called Oomycetes (oh-uh-my-seed-ees), or water molds. There are plenty of important physiological and reproductive differences to distinguish Oomycetes from true fungi, but for those of us without microbiology backgrounds, one critical thing to know is this: Oomycetes cell walls are primarily cellulose, just like plants, whereas fungal cell walls are made primarily of chitin.


Oomycetes’ nutritional needs are similar to those of proper plants, (O, H, P, N, S, K, Ca, Mg, micronutrients and an organic carbon source), but since they lack the ability to photosynthesize, they feed on the tissue of living plants, gathering nutrients and energy with structures called hyphae. This, and the fact that they can reproduce with spores is why they were classified as Fungi until recently.


Physiological similarities to algae explain the diet and water-dependence of Phytophthora; differences from fungi explain why so many fungicides are either totally ineffective or only temporary suppressants. Most fungicides act on chitin-based organisms and are formulated not to damage cellulose-based cell wall plants (though some certainly do). Phytophthora is unaffected by this mode of action. Most pathogenic fungi are decomposers, and in healthy soil assist in digesting weakened tissues and crop residues into stable humus. Phytophthora has a minimal decomposing role; it simply feeds when it can, destroys indiscriminately, reproduces, and moves on, leaving true fungi to do the cleanup.


One Phytophthora, many names. With at least 100 known species, numerous races of each, and hundreds more predicted to be discovered, Phytophthora is abundant and wily. Some strain exists to attack nearly every plant type, be it crop, wild, annual, perennial, woody or not, and can attack every plant part from root to fruit.


Red stele in strawberries, late blight in tomatoes, black shank in tobacco, root rot in soybeans and blueberries, various colors and sizes of leaf spot, blights, crown, root or fruit rots, stem lesions and sudden wilts all have the same causative agent. The Irish potato famine was caused by Phyt. infestans, and various infestans strains are still major issues on potato, tomato, pepper and eggplant. Phyt. capsici is the usual suspect on all cucurbits, but also infects solanaceas. Phyt. nicotianae for onions, Phyt. sojae for soybeans, etc, not to mention ornamental and wild plants. Damping-off of seedlings is often caused by Phytophthora, but can also be Pythium. While we shouldn’t jump to conclusions, you’d have a decent chance of being right if you point to a suddenly devastated crop and say, “Ah, I see you’ve got Phytophthora…”


How did it get here, how do I get rid of it? Phytophthora is native to most soils globally. It survives up to a decade as inactive spores resistant to drought and freezing, waiting for the right conditions. In the short term, it resides as mycelium on undigested plant debris. 


Disease development starts when rain pools on soil. After several hours of saturation, zoospores activate and literally swim, hunting for a host. Further mobility is achieved by flowing across slopes, along waterways, in wind-driven rain, on wheels, equipment, boots, hooves and paws. 


There is also a phase of sporulation whereby millions of oospores release above ground and spread anywhere the breeze will blow.


Phytophthora reproduces on massive scale, both sexually and asexually. Thus, new genetic potentials are constantly bred anew, and copies of successful races are preserved. In short, it is here, there and everywhere. It’s native, natural, highly adaptable, and loves to travel. We’re not getting rid of it and be skeptical of anyone who tells you otherwise.


In Paul Weaver’s field, there is no doubt the inoculum was present and conditions were right. So it’s not a matter of whether the organism was present, but of what else was present.


Real solutions: You’ve heard this from AEA before, and it’s true as ever: The best disease prevention is through microbiology and mineral nutrition. Phytophthora can be successfully and reliably inhibited by common soil fungi and bacteria which destroy its oospores. This primary line of defense is what is lacking when crops are overtaken. Phytophthora mycelia grow quite well in sterilized soil with no microbial competition, but are strongly inhibited in diverse, living soils. Why do microbes seek and destroy Phytophthora? It could be that the microbes use Phytophthora as a food source, but there is a far more important story here. 


Disease suppressive soils: Some micro-organisms destroy that which plants are powerless against. After all, if the plant dies, the microbial community suffers, so It is very much in their interest to promote the sustenance of the whole.

Allow me to anthropomorphize. Imagine you: a bacteria. Content, fed, employed. Raising a very, very quickly growing family in a new neighborhood on the bustling outer root hairs of your city, Watermelon Plant. This plant is mayor, contractor, refinery, factory, bank, grocery, water company, pharmacy and Internet provider combined. It is relied on by the trillions of diverse residents in your beloved busy city, who are all housed and employed by it. You’ve invested your life here, harvested the metals to build it, and it’s all you want or need.

One afternoon at work, just sucking on sap-soda and mining manganese, you come across something strange. Ooo… oh, uh… Phytophthora oospore! Never actually seen one before. But the description fits. Instinctually, your gut (more or less your entire body) churns uneasily. You imagine it’s like the fleeting glimpse of a snake underfoot for a human. Recoiling, you call out for help, but no one is near.

It looks harmless and still, but these things are dangerous! Ruthless terrorists, eager and able to take down a whole city’s infrastructure in a flash! The work of many generations may come to naught if this little gremlin is allowed to get wet. It is the defining moment of your little bacterial life. Into the fray. Destroy that thing and be a hero! Then you can die happily of old age (in a day or two), knowing this story will pass to your thousands of grandchildren, and the city can go on growing and feeding its citizens. 

Yes, cartoonish projection. But the point to pound is that in the life-and-death struggles of the rhizosphere, most organisms can play multiple roles; they have many subtle partnerships, pay-offs and competitions. 


This is exactly what is meant by disease suppressive soil. The makeup of the soil actively works in plants’ favor: hunting, policing and taking action. Beneficials outside the roots may contact pathogens before they can reach plant tissue. The more diverse and densely populated the beneficials, and the better they are supplied with energy from strongly photosynthesizing plants, the more territory they can mine-sweep.

Microbial benefits thus go beyond merely munching away to supply fertility. Much remains to be learned about what balances of organisms create the most disease suppressive soils, strains that excel in specific environments, and how farmers can best guide the process. In the rhizosphere, organisms play multiple roles; they have subtle and multi-faceted partnerships, pay-offs and competitions. With so many variables, it seems daunting. The research work being done in labs and fields across the world must be celebrated and furthered.

Much of agriculture is just beginning to see the potential, but AEA has been laser focused on it and will continue to bring the best innovations in the field directly to the field. Meanwhile, don’t let the perfect be the enemy of the good! Make progress now. Our biological inoculants introduce diverse and healthy microbiology when it is absent. Rejuvenate™ and SeaShield™ support that diverse and healthy microbiology, both native and inoculated. Our consultants have the experience and knowledge to put it to use for you. The results are proven and replicated on thousands of acres.

Paul Weaver, like many other growers, used powerful microbial stimulants like Rejuvenate™, SeaShield™, Biogenesis™, Spectrum™, BioCoat Gold™ and Pepzyme Clear™ to make a thriving soil ecosystem that successfully held Phytophthora at bay and paid dividends. You can too. Get in touch with AEA today.