Phytophthora In Disease Suppressive Soils

by Nathan Harman, Staff Consultant at Advancing Eco Agriculture

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 on well drained. Mainstream agriculture has very limited management options, offering these standard suggestions:


  • Prevent it before it starts. (Okay… how?)

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

  • Correct water management and drainage. (Right. Because phytophthora spreads in water, and wet roots are not healthy roots. But even well-drained fields can still flood, and spores 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 these applications before acknowledging that they have not changed Phytophthora pressure?)

  • Soil sterilization. (Difficult. Expensive. Impractical for most growers, and moreover, Phytophthora is known to thrive when introduced to previously sterilized soils.)

  • Variety selection. (The best of the above options. But Phytophthora resistance or tolerance is not yet available for most crops. It usually covers only one or two races of the disease.)

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


I was working with a grower in Kentucky at the time and we had a powerful experience showing the value of healthy, regenerated soil. The farm grows melon crops and mixed produce along with grain acres and has had a strong AEA-based program for several years. The grower routinely uses Planter Solution™, in-season drip and foliar applications, cover crops, and has worked to balance bulk soil ratios. The real backbone of the farm though, is biology. Applications of Rejuvenate™ and Spectrum™ are made every fall. SeaShield™ and Biogenesis™ are added on more challenged soils. All of the seeds are treated with BioCoat Gold™ for mycorrhizae, and Pepzyme Clear™ to ensure optimum conditions for inoculants.


AEA had a field walk scheduled at the farm, which was nearly canceled as the very wet weather had made cultivation difficult and no one wants to show off a weedy field.


But there was still great interest in the walk. Growers had questions about Phytophthora specifically, as there had been devastating losses in the neighborhood, but this farm had zero Phytophthora problems. The grower harvested 100 high-quality bins per acre, despite experiencing the exact same spring flooding conditions and planting the same varieties as neighboring farms. They wanted to know why.


So we met that day and talked 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 most farmers and crop advisors, I used to think 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.


Also, oomycetes’ nutritional needs are very similar to those of proper plants, 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 hyphae feeding activity, 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 ineffective or just temporary suppressants. Most fungicides act on chitin-based organisms and are formulated not to damage cellulose-based plants (though some certainly do). Phytophthora is largely unaffected by this mode of action. Most pathogenic fungi are decomposers, and in healthy soil assist in digesting weak/dead tissue and crop residues into stable humus. Phytophthora has a minimal decomposing role; it simply feeds when it can on living plants, 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. Don’t jump to conclusions, but you’d have a decent chance of being right if you guess that a suddenly devastated crop has succumbed to some kind of 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 resting 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 up 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. Think soaking rains and hurricanes and you see why this is a global phenomenon.


Phytophthora reproduces on an utterly 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 already native to where you are. It’s all-natural, highly adaptable, and loves to travel. We’re not getting rid of it. Be skeptical of anyone who tells you otherwise. 


In the Kentucky 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 symptoms can be successfully and reliably inhibited by common soil fungi and bacteria. This primary line of defense is what is lacking when crops are overtaken. Phytophthora mycelium 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 microorganisms destroy that which plants are powerless against. After all, if the plant dies, the microbial community dependant on that plant 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. The plant is mayor, contractor, refinery, factory, bank, grocery, water company, pharmacy and Internet provider combined. You and the trillions of other diverse residents are all housed and employed by this Watermelon Plant. You’ve invested your life here, harvested the metals to build it. It’s all you want or need.

One afternoon at work, just sucking on some sugary sap-soda and mining manganese, you come across something strange. Ooo… oh, uh… 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 interlopers, 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 it! 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. 

Okay, this cartoon is meant to pound in the point that in the life-and-death struggles of the rhizosphere, 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 physically living outside the roots can 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.


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.

The Kentucky grower used proven and effective products like Rejuvenate™, SeaShield™, Biogenesis™, Spectrum™, BioCoat Gold™ and Pepzyme Clear™ to make a thriving soil ecosystem. We didn’t hold Phytophthora at bay. But some combination of soil microbes did. And paid dividends.